Evolution of the hippocampus in reptiles and birds.

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Evolution of the Hippocampus in Reptiles and Birds Georg F. Striedter* Department of Neurobiology & Behavior and Center for the Neurobiology of Learning and Memory, University of California, Irvine, Irvine, California 92697-4550

ABSTRACT Although the hippocampus is structurally quite different among reptiles, birds, and mammals, its function in spatial memory is said to be highly conserved. This is surprising, given that structural differences generally reflect functional differences. Here I review this enigma in some detail, identifying several evolutionary changes in hippocampal cytoarchitecture and connectivity. I recognize a lepidosaurid pattern of hippocampal organization (in lizards, snakes, and the tuatara Sphenodon) that differs substantially from the pattern of organization observed in the turtle/archosaur lineage, which includes crocodilians and birds. Although individual subdivisions of the hippocampus are difficult to homologize between these two patterns, both lack a clear homolog of the mammalian dentate gyrus. The strictly trilaminar organization of the ancestral amniote hippocampus was gradually lost in the lineage leading to birds, and birds expanded the system of intrahippocampal axon collater-

als, relative to turtles and lizards. These expanded collateral axon branches resemble the extensive collaterals in CA3 of the mammalian hippocampus but probably evolved independently of them. Additional examples of convergent evolution between birds and mammals are the loss of direct inputs to the hippocampus from the primary olfactory cortex and the general expansion of telencephalic regions that communicate reciprocally with the hippocampus. Given this structural convergence, it seems likely that some similarities in the function of the hippocampus between birds and mammals, notably its role in the ability to remember many different locations without extensive training, likewise evolved convergently. The currently available data do not allow for a strong test of this hypothesis, but the hypothesis itself suggests some promising new research directions. J. Comp. Neurol. 000:000–000, 2015. C 2015 Wiley Periodicals, Inc. V

INDEXING TERMS: dentate gyrus; cortex; sauropsid; lizard; homology

In contrast to the contentious debates about the evolution of the neocortex (see present issue of The Journal of Comparative Neurology; also Medina et al., 2013), debates about hippocampus evolution are relatively tame. The tentative consensus seems to be that the function of the hippocampus has been highly conserved across the vertebrates, despite some minor structural differences. Especially among the amniotes (reptiles, birds, and mammals) the hippocampus is said to be “functionally homologous” (Colombo and Broadbent, 2000), implying that its functions are highly conserved. Comparative neuroanatomists likewise emphasize the conserved features of hippocampal homologs. However, most previous analyses have compared birds directly with mammals, which obscures the possibility that birds and mammals evolved similar hippocampi independently of one another. The few studies that have compared the hippocampus of lizards with its mammalian homolog C 2015 Wiley Periodicals, Inc. V

have largely ignored birds, thus neglecting the possibility that the hippocampus changed significantly within reptiles and birds (i.e., within sauropsids; Fig. 1). To address these limitations, I here review hippocampus structure and function in a variety of sauropsids. Due to space constraints, I assume that the reader is somewhat familiar with the mammalian hippocampus (Andersen et al., 2007). In addition to reviewing the literature, I present my own detailed illustrations of the hippocampus in six sauropsid species from key lineages. Contrasting these images reveals substantial species differences and reinforces the proposed homologies. The review begins with structural

*CORRESPONDENCE TO: Georg Striedter, 2205 McGaugh Hall, Department of Neurobiology & Behavior, University of California, Irvine, Irvine, CA 92697-4550. E-mail: [email protected]. Received March 12, 2015; Revised April 17, 2015; Accepted April 29, 2015. DOI 10.1002/cne.23803 Published online Month 00, 2015 in Wiley (wileyonlinelibrary.com)

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G.F. Striedter

Figure 1. Amniote phylogeny. The position of turtles is based on Hedges (2012), Field et al. (2014), and Green et al. (2014). Divergence times are approximate and based on a variety of sources. In this paper, I recognize two principal patterns of hippocampal organization among the sauropsids: one pattern is found in lepidosaurs (shaded orange); the other pattern is seen in the lineage containing turtles and archosaurs (shaded blue).

comparisons and then considers functional data. I conclude that several features of hippocampal structure and function have diverged substantially between mammals and sauropsids and that a few key similarities evolved convergently in mammals and birds.

STRUCTURAL COMPARISONS: TWO CYTOARCHITECTURAL PATTERNS The hippocampus of rodents lies inside the telencephalon’s caudal pole, whereas the primate hippocampus lies deep within the medial temporal lobe. Because of this species difference in position, one cannot be sure where to expect the hippocampal homolog in nonmammals. However, the mammalian hippocampus always develops in the telencephalon’s dorsomedial sector and remains at that location in both marsupials and monotremes, the two most primitive mammalian lineages (Elliot Smith, 1910). Therefore, one can predict that the hippocampal homolog in non-mammals should likewise occupy a dorsomedial position, at least during the early stages of development. Indeed, that is where subsequent research has located the hippocampus in all reptiles and birds, amphibians, and cartilaginous fishes. Only in the ray-finned fishes does the hippocampal homolog occupy a dorsolateral position, which it adopts because the embryonic telencephalon in these species does not evaginate but, instead, everts (Gage, 1893; Striedter and Northcutt, 2006). Within sauropsids, the hippocampal homologs differ considerably in cytoarchitectural appearance and in

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their number of subdivisions, causing substantial disagreements in the literature about homologies and terminology. However, once we take sauropsid phylogeny into account (Fig. 1), two fundamentally different patterns of hippocampus organization become discernible. One pattern is seen in lepidosaurs, which include lizards, snakes and Sphenodon (the common name of which is “tuatara”). The second pattern is evident in turtles, crocodilians, and birds, all of which are closely related to one another (Fig. 1). Crocodilians have long been recognized as the closest living relatives of birds and, collectively, birds and crocodilians constitute the archosaurs. In contrast, the phylogenetic position of turtles has been controversial. Turtles have often been considered to be either the sister group of all other reptiles or the sister group of lepidosaurs (Laurin and Reisz, 1995; deBraga and Rieppel, 2008; Northcutt, 2013). However, recent analyses of diverse molecular data, ranging from microRNA and thousands of individual genes to whole genome analyses (Chiari et al, 2012; Fong et al., 2012; Lu et al., 2013; Schaffer et al., 2013; Field et al., 2014; Green et al., 2014; Thomson et al., 2014), consistently support the hypothesis that turtles are the closest living relatives of archosaurs (Fig. 1; see also Hedges, 2012; Gilbert and Corfe, 2013). This conclusion seems at odds with the observation that the telencephalon’s dorsal ventricular ridge (DVR) is smaller and cytoarchitecturally simpler in turtles than in most other reptiles, suggesting that “turtles are primitive.” However, a large and complex DVR may well have evolved independently in some lepidosaurs (Northcutt, 1978) and archosaurs, implying that turtles retained their simple DVR as a primitive character. In light of these considerations, I refer to the second pattern of hippocampus organization as the turtle/archosaur pattern. Should subsequent research reject the turtle/archosaur grouping, then some of my conclusions here will have to be revised.

The lepidosaur pattern The hippocampal homolog in lizards and snakes (i.e., squamates) is called the medial cortex (Fig. 2A). It is strictly trilaminar in its cytoarchitecture, meaning that it has three very distinct principal layers. The middle layer contains almost all of the neuronal cell bodies, whereas the flanking plexiform layers contain only a few cell bodies. The cell body layer is thinner in the dorsolateral sector of the squamate medial cortex than in the ventromedial sector, and the cell bodies in the dorsolateral sector are larger (on average) and more widely spaced. Accordingly, the dorsolateral and ventromedial subdivisions are called large-celled medial cortex (lM) and small-celled medial cortex (sM), respectively. Deep to

The Journal of Comparative Neurology | Research in Systems Neuroscience

Evolution of the sauropsid hippocampus

Figure 2. Cytoarchitecture of the medial cortex in lepidosaurs. Shown here are regularly spaced transverse sections through the medial cortex of the lizard Gekko gekko (A) and the tuatara Sphenodon punctatus (B). In both species the medial cortex is strictly trilaminar and divisible into two major parts, called the small-celled and large-celled divisions (sM and lM, respectively). In lizards the cell-dense layer is thinner in lM than in sM and contains larger cell bodies (on average) that are more widely spaced. In Sphenodon, lM and sM are less distinct from one another but, just as in lizards, the cell bodies in the cell-dense layer are less densely packed in lM than in sM. A distinct cluster of neurons, called the cell plate of Unger (CPU), lies deep to lM in lizards but not in Sphenodon. D, dorsal cortex; DVR, dorsal ventricular ridge. Brain sections courtesy of the R. Glenn Northcutt collection. Scale bar 5 0.5 mm in A; 1 mm in B.

the cell body layer of lM (i.e., between that layer and the ventricular surface) lies a small group of neurons, called the cell plate of Unger (1906); it is generally considered part of the squamate dorsal cortex and, therefore, thought to be homologous to part of the mammalian neocortex. The closest living relative of squamates is the tuatara Sphenodon punctatus, a heavily protected “living fossil” that looks like a large lizard and is endemic to New Zealand. Its medial cortex is similar to that of lizards and snakes, except that lM is not as thin (Fig. 2B). Sphenodon also lacks the cell plate of Unger.

The turtle/archosaur pattern Turtles comprise two ancient lineages, namely, cyptodire and pleurodire turtles (Crawford et al., 2012). In both lineages the medial cortex is divisible into three major divisions, rather than the two divisions seen in lepidosaurs. This difference has been neglected by most previous authors, who tend to recognize just two subdivisions in the turtle medial cortex and refer to them using the nomenclature developed for lepidosaurs (Desan, 1988; Ulinski, 1990). To avoid this confusion, here I use the older terminology of Riss et al. (1969) and refer to the three divisions of the turtle medial


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Figure 3. Cytoarchitecture of the medial cortex in turtles. Shown here are regularly spaced transverse sections through the medial cortex of the cryptodire turtle Pseudemys scripta (A) and the pleurodire turtle Podocnemis unifilis (B). In both species the medial cortex is divisible into three major parts, called zones 2–4 following Riss et al. (1969). Zone 3 differs from zones 2 and 4 in having its cell bodies more highly scattered through the depth of the cortex, causing a loss of strict trilaminarity. The cell-sparse plexiform layer between the celldense layer and the telencephalic ventricle is significantly thinner in cryptodire turtles than in pleurodire turtles (or lepidosaurs; see Fig. 2). DVR, dorsal ventricular ridge. Brain sections in A courtesy of Cosme Salas and Fernando Rodrıguez. Sections in B courtesy of the R. Glenn Northcutt collection. Scale bar 5 0.5 mm in A; 1 mm in B.

cortex as zones 2, 3, and 4. Importantly, zone 3 is cytoarchitectonically distinct from zones 2 and 4 in turtles because its cell bodies are more widely scattered through the depth of the medial cortex (Fig. 3). Because of this dispersion, the medial cortex is not as uniformly trilaminar in turtles as it is in lepidosaurs. An interesting difference between the medial cortex of cryptodires and pleurodires is that the deep plexiform layer is much thinner in cryptodires, meaning that

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most of the cell bodies are much closer to the ventricular surface than they are in pleurodires (or, for that matter, in lepidosaurs). However, the largest differences between cryptodire and pleurodire turtles exist outside of the medial cortex. In general, pleurodire turtles have a much larger telencephalon than cryptodire turtles, with correspondingly reduced telencephalic ventricles (Fig. 3B). Especially enlarged are zone 5 and the DVR. The former is presumed to be the turtle homolog of the



Figure 4. Cytoarchitecture of the medial cortex in archosaurs. Shown here are regularly spaced transverse sections through the medial cortex of the crocodilian Caiman crocodilus (A) and the black phoebe Sayornis nigricans (B), which is a suboscine passerine closely related to songbirds. The medial cortex of crocodilians is very similar to that of pleurodire turtles (Fig. 3B), which is why its three main subdivisions are named as in turtles: zones 2–4 (following Riss et al., 1969). The medial cortex homolog of birds is also divisible into three major parts, called the ventral (V), dorsomedial (DM), and dorsolateral (DL) divisions of the hippocampus (following Atoji and Wild, 2004). However, the cell bodies in all three avian subdivisions are scattered throughout the depth of the cortex, leading to an almost complete loss of trilaminar organization. Cbl, cerebellum; CDL, dorsolateral corticoid area; DVR, dorsal ventricular ridge; HA, apical hyperpallium. Caiman brain sections courtesy of the R. Glenn Northcutt collection. Scale bar 5 0.5 mm in A,B.


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Figure 5. Neuronal morphology in the hippocampus of lizards and birds. A: Composite of Golgi-impregnated neurons in the medial cortex of the lizard Podarcis hispanica. Medial is to the left and dorsal to the top. Axons have been highlighted in red. The inset shows an axon with en passant varicosities at higher magnification. B: Composite of Golgi impregnated neurons in the hippocampus of 28-day-old chickens (Gallus domesticus). Medial is to the left and dorsal to the top. Axons are highlighted in red. A is adapted from figures in Luis de la Iglesia and Lopez-Garcia, 1997. B is adapted from T€omb€ol et al., 2000. Scale bar 5 50 in A,B.

mammalian primary visual cortex (Hall and Ebner, 1970); the latter is probably homologous to either the lateral portion of mammalian neocortex or the claustroamygdaloid complex (depending on the author; see Dugas-Ford et al., 2012; Medina et al., 2013; Reiner, 2013). The hippocampus homolog of crocodilians is very similar to that of pleurodire turtles, except that zone 3 occupies a more dorsal position in crocodilians, and zone 4 extends further laterally (Fig. 4A). Crocodilians also differ from pleurodire turtles in having an even larger DVR and a thinner zone 5 (called general cortex by Crosby, 1917). The avian hippocampus resembles that of crocodilians in that it also contains three major divisions, but it is more complex (Fig. 4B). After examining the hippocampus in 16 different orders of birds, Craigie (1940) divided this region into four longitudinal zones, each containing several layers. Craigie also identified several subdivisions within a “parahippocampal area” that more recent authors consider to be part of the hippocampus. Contemporary studies of the avian hippocampus have focused mainly on pigeons (Erichsen et al., 1991; Krebs et al., 1991; Szekely, 1999; Kahn et al., 2003; Atoji and

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Wild, 2004). These studies vary widely in the number of proposed hippocampus subdivisions, in the boundaries of these divisions, and in the names assigned to them. However, I follow Kahn et al. (2003) and Atoji and Wild (2004) in recognizing three major divisions in the avian hippocampus: ventral, dorsomedial, and dorsolateral. These divisions are likely present in all birds, but some of the smaller subdivisions within those areas are indistinct or absent in some species. For example, a V-shaped layer can be identified in the ventral hippocampus of pigeons (Karten and Hodos, 1967) and songbirds, but this region is indistinct or absent in many bird species, including ostriches, chickens, parrots, and the closest relatives of songbirds, the suboscines (Fig. 4B).

STRUCTURAL COMPARISONS: NEURONAL CONNECTIONS Data on the intrinsic and extrinsic connections of the hippocampus in sauropsids come mainly from Golgi and experimental tract tracing studies, respectively. The following review focuses only on the densest projections and on those that are most relevant to a comparative analysis.



Figure 6. Principal connections of the hippocampal homologues in lizards (A), birds (B), and mammals (C). Based on multiple sources (see text). Abbreviations not listed in previous figures: DLA, dorsolateral anterior nucleus; DLAm, medial part of the dorsolateral anterior nucleus; HD, densocellular hyperpallium; HL, lateral hyperpallium; NFL, lateral frontal nidopallium; subic, subiculum.

Lepidosaurs Golgi studies on the medial cortex of lizards and snakes (Ulinski, 1977, 1990; Luis de la Iglesia and Lopez-Garcia, 1997) have shown that neurons that have their cell body in the cell-dense middle layer are similar to pyramidal neurons in the mammalian hippocampus. They extend long apical dendrites into the superficial plexiform layer (toward the brain surface) and short basal dendrites into the deep plexiform layer (toward the ventricular surface). The axons of these neurons typically divide into two major branches when they reach the deep plexiform layer (Fig. 5A). One branch courses ventrally and terminates in the septum. The other axon branch courses dorsomedially and appears to contact, through en passant synapses, the basal dendrites of projection

neurons in more dorsal portions of the medial cortex (Fig. 5A, inset). Importantly, sM neurons do not send axon collaterals into the superficial plexiform layer. Pedro Ramon y Cajal (1917; the brother of Santiago Ramon y Cajal) did report such ascending collaterals in chameleons, but the neurons he described were probably located in lM, rather than sM (Carlos Lopez-Garcia, personal communication; see also Berbel, 1988). Experimental tract tracing studies have shown that the two major subdivisions of squamate medial cortex, sM and lM, are reciprocally interconnected (Fig. 6A). Neurons in sM project to ispilateral lM, where they terminate on the distal portions of both apical and basal dendrites (Hoogland and Vermeulen-VanderZee, 1993). Conversely, neurons in lM project to sM, terminating in


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two narrow bands adjacent to the cell body layer (Lohman and Mentink, 1972); in some species this projection is bilateral (Hoogland and Vermeulen-VanderZee, 1993). Neurons in lM also project to contralateral lM (Lohman and Metink, 1972). Extrinsic inputs to sM and lM derive from several sources. Both regions receive input from the dorsal cortex (Lohman and Mentink, 1972; Ulinski, 1976) and the lateral cortex, which receives direct projections from the olfactory bulb (Heimer, 1969; Ulinski, 1990). Subcortical inputs to lizard medial cortex originate mainly in the dorsolateral nucleus of the dorsal thalamus (Bruce and Butler, 1984; Hoogland et al., 1998), which receives multimodal sensory input. Cholinergic inputs to the medial cortex are weak and probably originate outside of the septum, in the nucleus of the diagonal band (Hoogland et al., 1998). Both sM and lM project back to the dorsal cortex (Lohman and Mentink, 1972). In addition, both regions project bilaterally to the septum (Ulinski, 1975; Olucha et al., 1988; Martınez-Garcıa et al., 1990; Hoogland and Vermeulen-VanderZee, 1993), which has extensive projections to hypothalamic and midbrain regions (Font et al., 1998). According to most reports, the medial cortex of squamates does not project outside of the telencephalon (Olucha et al., 1988; Hoogland and Vermeulen-VanderZee, 1993).

Turtles and archosaurs Only one Golgi study has been published on the medial cortex of turtles (Davydova and Goncharova, 1979). According to this report, the dendrites of most medial cortex neurons in turtles are similar to those in lizards and snakes; ascending collaterals were not described. Golgi studies on the crocodilian hippocampus are lacking entirely. Tract tracing studies in turtles and crocodilians are similarly scarce. However, zones 2 and 3 in cryptodire turtles reportedly receive inputs from the dorsomedial anterior and dorsolateral anterior nuclei of the dorsal thalamus, respectively (Desan, 1988; Zhu et al., 2005). In caiman the dorsomedial anterior nucleus also projects to the hippocampal homolog, but axons from the dorsolateral anterior nucleus reportedly project mainly to dorsal cortex (Pritz, 2014). Golgi studies have shown that many neurons in the avian hippocampus have extensive axon collaterals that spread for a considerable distance in many different directions (T€ omb€ol et al., 2000; Fig. 5B). These collaterals are reported to be more extensive in pigeons than in chickens (T€ omb€ol et al., 2000), but this observation has not been quantified. Experimental studies on the connections of the avian hippocampus have been conducted only in the pigeon Columba livia (Szekely, 1999; Atoji et al., 2002; Kahn

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et al., 2003; Montagnese et al., 2003; Atoji and Wild, 2004, 2005; Liu et al., 2012; Shanahan, 2013). Based on these studies, one can conclude that the ventral, dorsomedial, and dorsolateral divisions of the pigeon hippocampus are reciprocally connected to one another (Fig. 6B). Extrinsic inputs to the pigeon hippocampus originate mainly in the hyperpallium (Reiner et al., 2004), which is homologous to part of the neocortex and provides the hippocampus with highly processed, multimodal sensory information. Additional inputs to the pigeon hippocampus come from parts of the nidopallium that process somatosensory and auditory information, as well as from the dorsolateral corticoid area, which receives inputs from the primary olfactory cortex and a large variety of other brain regions (Atoji and Wild, 2005). Extratelencephalic inputs to the pigeon hippocampus derive mainly from the dorsal thalamus, specifically the dorsal part of the dorsolateral anterior nucleus pars medialis (Atoji and Wild, 2004), and from the supramammillary region of the hypothalamus (Berk and Hawkin, 1985). Efferent projections from the pigeon hippocampus originate mainly from its dorsomedial and dorsolateral divisions and tend to reciprocate the major afferents. The dorsomedial division of the hippocampus also projects heavily to the septum; septal projections from the dorsolateral and ventral divisions tend to be weaker. Finally, the dorsomedial division of the pigeon hippocampus has direct projections to the lateral hypothalamus (Atoji and Wild, 2004).

STRUCTURAL COMPARISONS: HOMOLOGIES AND CHANGE Given that sauropsids exhibit two distinct patterns of hippocampal organization that differ in the number of subdivisions (two in lepidosaurs versus three in turtles and archosaurs), one may inquire about the homologies of those subdivisions. Which division in one pattern is homologous to which division in the other pattern? One may also ask which of the two patterns is primitive, and which is derived. That is, one may ask what evolutionary transformations have occurred. Here I use mainly the criteria of relative position (topology), cytoarchitecture, and neuronal connections to identify putative homologies, followed by the methodology of cladistics to test those hypotheses and identify the most parsimonious scenarios of evolutionary change (Northcutt, 1984; Nieuwenhuys, 1994; Striedter, 1998, 2005).

Homologizing subdivisions across the two sauropsid patterns As noted earlier, the lepidosaur pattern of hippocampal organization features a fairly uniform trilaminar



TABLE 1. Proposed Homologies for Subdivisions of the Hippocampus in Amniotes Mammals Subic CA1 CA3 Dentate

Sphenodon

Lizards/snakes

Cryptodire turtles

Pleurodire turtles

Crocodilians

Birds

lM

lM

sM

sM

?

?

Zone 4/D1 Zone 3 Zone 2 ?

Zone 4 Zone 3 Zone 2 ?

Zone 4 Zone 3 Zone 2 ?

DL DM V Vv?

CA1,3: cornu ammonis 1,3; D1, another name for zone 4; DL, dorsolateral hippocampus; DM, dorsomedial hippocampus; lM, large-celled division of the medial cortex; M, medial cortex; sM, small-celled division of the medial cortex; V, ventral hippocampus; Vv, ventral part of the ventral hippocampus.

organization and is divisible into sM and lM. In contrast, the turtle/archosaur pattern is characterized by three distinct zones, the middle one being much less trilaminar than the adjacent zones. These differences make it difficult to homologize the subdivisions to one another (Rose, 1923). Some authors have implied that zones 2 and 3 of turtles are homologous to sM and lM of lizards, respectively (e.g., Ulinski, 1990), but those homologies have not been proposed explicitly and would be difficult to justify. My own analysis suggests, instead, that zones 2 and 3 of turtles and crocodilians are collectively homologous to sM of lepidosaurs (the term “field homology” is sometimes used to denote such collective homology; see Puelles and Medina, 2002, but also Northcutt, 1999). This leaves zone 4 as the most likely homolog of lizard lM (Table 1). The avian hippocampus is cytoarchitecturally quite similar to that of crocodiles. Based on this similarity, I propose that the ventral, dorsomedial, and dorsolateral subdivisions of the avian hippocampus are homologous to zones 2, 3, and 4 of the crocodilian hippocampus, respectively. Testing this hypothesis will require additional data on the hippocampus of crocodilians and turtles, especially on their neuronal connections.

Evolutionary changes in hippocampal structure within the sauropsids The fact that the mammalian CA region of the hippocampus is uniformly trilaminar in organization suggests that the uniform trilaminarity of the lepidosaurian pattern is probably primitive for sauropsids. Supporting this hypothesis is the observation that Sphenodon, with its uniformly trilaminar hippocampus, is a “living fossil” in numerous other respects. When did uniform hippocampal trilaminarity evolve? Because amphibians are the outgroup to all amniotes and do not have a trilaminar hippocampus (their medial pallium), it is most parsimonious to conclude that the hippocampus became trilaminar with the origin of amniotes. The avian hippocampus is clearly less trilaminar than its turtle homolog and exhibits several small

subdivisions (such as the V-shaped region in pigeons and songbirds) that are not apparent in reptiles. Because birds are a relatively recent branch within the sauropsids (Fig. 1), the absence of trilaminar organization and increased structural complexity are almost certainly derived features for birds. Many hippocampal connections are conserved across the sauropsids, but some pathways have changed (Fig. 6A,B). Particularly interesting is that lizards have direct projections from primary olfactory cortex to the hippocampus, whereas birds and mammals do not. Because amphibians and other anamniotes resemble lizards in this respect (Heimer, 1969; Northcuttt, 1995), birds and mammals have likely lost the direct olfactory cortex projections into the hippocampus. The avian hippocampus does receive some olfactory input, but this input is indirect (coming through the dorsolateral corticoid area) and more multimodal. Similarly, the mammalian hippocampus receives olfactory inputs indirectly through the entorhinal cortex. Birds also differ from lizards in having a more extensive system of axon collaterals in their hippocampus (Fig. 5). Because extensive collaterals have not been reported in the hippocampus of turtles (Davydova and Goncharova, 1979) it seems likely that intrahippocampal collaterals expanded in the avian lineage. The extensive recurrent collaterals in CA3 of the mammalian hippocampus (Witter, 2007; Wittner et al., 2007; Ropireddy et al., 2010) probably evolved independently of the widespread axon collaterals in the avian hippocampus. However, it is possible that ancestral amniotes already had a fairly extensive system of axon collaterals in their hippocampus and that this system became more restricted in the common ancestor of lepidosaurs and turtles. The difference between these two hypotheses is that the mammalian condition is derived only in the first scenario. Additional data on the extent of axon collaterals in the hippocampus of amphibians, marsupials, and monotremes would help resolve the issue. Information flow within the mammalian hippocampus tends to be unidirectional, following the “trisynaptic pathway” from the entrorhinal cortex to the dentate


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gyrus, to CA3, to CA1, and then out of the hippocampus through the subiculum (Both et al., 2008; but see Scharfman, 2007, and Jackson et al., 2014, for data on “reverse” information flow within the hippocampus). In contrast, the connections between the major hippocampal divisions in reptiles and birds are consistently reciprocal, implying that information flow is probably more bidirectional. The avian hippocampus may harbor some more unidirectional circuits between its small subdivisions (Kahn et al., 2003), but this idea requires more testing. If it is confirmed, then the most parsimonious interpretation would be that intrahippocampal circuits became increasingly unidirectional in birds and mammals. In both sauropsids and mammals, the hippocampus receives inputs from the dorsal thalamus, projects to the large parts of the septum, and has reciprocal connections with the dorsal pallium (neocortex in mammals, dorsal cortex in lepidosaurs, visual Wulst in birds), but the proportional weights of these connections differ between the lineages. Whereas the septum is nearly as large as the dorsal cortex in lizards (Platel, 1980), it is less than 1/30 the size of the neocortex even in mammals with a small neocortex (Stephan et al., 1970; Finlay and Darlington, 1995). Considering that the septum appears to be larger than the dorsal pallium also in amphibians (Neary, 1990), we can infer that functional interactions between the hippocampus and subcortical structures became weaker in mammals, whereas neocortical interactions increased in strength.

Do sauropsids have dentate, CA, and subiculum homologs? Many authors have pondered where in the sauropsid hippocampus one might find a homolog of the mammalian dentate gyrus. Most have argued that a dentate homolog can be found in the medial cortex of lepidosaurs and the ventral hippocampus of birds. However, Santiago Ram on y Cajal had noted over 100 years ago that mossy fiber axons, which connect the mammalian dentate gyrus to CA3 and are distinctive in Golgi-stained material, cannot be seen in Golgi-stained material of the reptilian hippocampus (Ramon y Cajal, 1995). Therefore, he concluded that it is “pure speculation” (p. 690) to claim that reptilian medial cortex is homologous to the dentate gyrus. Yet some of Cajal’s sparring partners in this debate were not entirely convinced. Most notably, Elliot Smith argued that the ventral tip of the medial cortex may be “on the way to the differentiation of a true fascia dentata” (Elliot Smith, 1910, p. 149). This argument has continued, in diverse forms, up to today. The strongest argument in favor of a homology between the mammalian dentate gyrus and the lizard

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medial cortex, as well as the avian ventral hippocampus, is their similarity in relative (topological) position at the telencephalon’s extreme dorsomedial edge. This argument is not definitive, however, because an evaginating telencephalon must (by definition) have a dorsomedial edge, whereas a dentate homolog may or may not exist (i.e., we cannot stipulate its existence a priori). The finding that the ventral edge of the avian hippocampus expresses a similar set of genes as the dentate gyrus during embryonic development (Gupta et al., 2012; Abellan et al., 2014) strengthens the dentate homology hypothesis, but the dorsal midline of the evaginating telencephalon contains a molecular signaling center (the cortical hem) that may induce specific, conserved genes in neighboring regions regardless of their adult phenotype (Striedter, 1998; Mangale et al., 2008). Furthermore, the supposed dentate homologs in lizards and birds do not exhibit the remarkable longdistance migration of neuronal precursors away from the ventricular surface that is so uniquely characteristic of the mammalian dentate gyrus (Altman and Bayer, 1990; Barnea and Nottebohm, 1994; Nacher et al., 1996; Gupta et al., 2012). Another effort to identify a dentate homolog in sauropsids involves the distribution of zinc-positive axons, which can be visualized with a Timm’s stain. In rodents this stain labels the mossy fibers projecting from the dentate gyrus to CA3 (as well as several other, weaker projections; see Long et al., 1995). Timm’s staining in lizards and birds is much more widespread and diffuse (Olucha et al., 1988; Faber et al., 1989; Montagnese et al., 1993), but the neurons giving rise to the Timmpositive axons in lizards have their cell bodies in sM, supporting the hypothesis that this region is a dentate homolog (Olucha et al., 1988). Unfortunately, these sM neurons project to the septum as well as to more dorsal cortical areas, whereas the mammalian dentate gyrus does not project outside of the hippocampus. In pigeons the ventral tip of the ventral hippocampus does resemble the mammalian dentate in projecting only to other hippocampal areas (Atoji and Wild, 2004), but anterograde tracers have not yet been injected into this region, leaving open the possibility that it projects to areas outside of the hippocampus that have not been injected with retrograde tracers. A variety of other similarities between the mammalian dentate gyrus and parts of the sauropsid hippocampus are also uncertain indicators of homology. For example, adult neurogenesis is a prominent feature of the mammalian dentate gyrus, the lizard medial cortex (Lopez-Garcia et al., 1984; Font et al., 1991; Marchioro et al., 2005), and the avian ventral hippocampus (Barnea and Nottebohm, 1996), but adult neurogenesis is



found in many parts of the sauropsid telencephalon, even outside of the hippocampal region (Alvarez-Buylla et al., 1994; P erez-Ca~nellas and Garcıa-Verdugo, 1996). Similarly, Herold et al. (2014) have used the spatial distribution of neurotransmitters to homologize parts of the avian hippocampus to the mammalian dentate gyrus (as well as CA1 and CA3, but these data reveal a complex mix of similarities and differences rather than straightforward homologies. Given these arguments and counterarguments, it seems unlikely that the search for a dentate homolog in sauropsids will be settled (or abandoned) any time soon. Good scientists may disagree about which similarities convincingly support hypothesized homologies (as explained more fully in the next section). However, if we assume that the sauropsid condition is primitive, then we can conclude that mammals must have either evolved a dentate gyrus de novo or expanded it enormously in volume, modified its mode of development, and eliminated its projections to subcortical targets (Fig. 5A, inset); in light of such drastic changes, the mammalian dentate would be at least as “new” as the mammalian neocortex. Alternatively, if we assume that the sauropsid condition is derived, then the dentate gyrus would have shrunk considerably in sauropsids and we would have to extend our search for a dentate gyrus to the amphibians, where no such structure has ever been postulated. Aside from the dentate gyrus, the mammalian hippocampus contains the subiculum and the cornu ammonis (mainly CA1 and CA3). Those who find a dentate homolog in lizard medial cortex tend to argue that lizard dorsal cortex is homologous to CA and the subiculum (Nacher et al., 1996). A major problem with this argument is that it threatens to leave the lizard telencephalon without a homolog of the mammalian neocortex, even though a necortex homolog is generally thought to exist in all jawed vertebrates (Northcutt, 1995). Lizards do have a DVR, which some authors have homologized to parts of the mammalian neocortex (Karten and Shimizu, 1989). However, even advocates of this hypothesis homologize the DVR only to the lateral neocortex, excluding the primary visual cortex and several other cortical areas. As an alternative, I propose that lM of lepidosaurs may be homologous to the subiculum (Table 1) because both structures are located adjacent to the dorsal pallium (dorsal cortex and neocortex, respectively) and have reciprocal connections with it. One complication for this idea is that the mammalian subiculum projects strongly to the lateral septum, whereas lepidosaurian lM does so weakly at best (Ulinski, 1975). Still, if lM were homologous to the subiculum, then sM would

remain as the most likely homolog of the cornu ammonis. Because projections to the septal complex arise mainly from dorsal sM (Hoogland and VermeulenVanderZee, 1993; Font et al., 1995, 1997) and because CA3 lies topologically ventral to CA1, I hypothesize that dorsal and ventral sM may be homologous to CA1 and CA3, respectively. An obstacle for this hypothesis is that CA1 and CA3 project to the septum in rats (Swanson and Cowan, 1977; Risold and Swanson, 1997), whereas ventral sM does not. Additional data are clearly needed to shore up all these comparisons.

What to do with troublesome homologies? Debates about homology invariably raise questions about how to weight the various types of similarity in deciding what is homologous to what. Some authors have stressed the value of similarities in developmental gene expression (Striedter, 1997; Puelles et al., 2000; Medina et al., 2013). This approach tends to work well for homologizing brain regions, but ancient molecular mechanisms can acquire novel functions independently in multiple lineages (Hall, 2012a; Parker et al., 2013), in which case the similarities in gene expression fail to indicate homology. Other seekers of homology emphasize similarities in adult gene expression, connectivity, cytoarchitecture, or function. Different schools of thought tend to emphasize different types of similarity (Dugas-Ford et al., 2012; Medina et al., 2013; Reiner, 2013), but evolution can, in principle, tinker with all of them. Despite these debates, it seems reasonably clear that the similarities most useful for finding homologies will be those that uniquely define the character under consideration; far less useful are similarities that are shared not only between the putative homologs but also with other characters. As noted earlier, adult neurogenesis characterizes both the reptilian medial cortex and the mammalian dentate gyrus, but it is also found in other parts of the reptilian and avian telencephalon; this reduces its significance as a “dentate marker.” Similarly, neurons with zinc-containing axons are found in both the reptilian medial cortex (specifically sM) and the mammalian dentate, but some zinc-containing neurons (with projections to the septum) are also found in CA1, CA3, and the subiculum (Sørensen, 1993). Therefore, we cannot simply assume that any zinc-containing neurons in the reptilian cortex must be homologous to neurons in the mammalian dentate, especially if those neurons project to the septum (Olucha et al., 1988). Similar issues arise in studies that have looked for “neocortex markers” in avian brains (Medina et al., 2013). Of course, unique similarities between two or more characters may be rare and difficult to find. When no


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uniquely shared features are apparent, then homologies may be based on the confluence of many different similarities, none of which are by themselves uniquely shared. Such arguments can be valid (especially if the confluence is statistically unexpected), but we should keep in mind that some characters may represent true evolutionary innovations and, thus, have no strict homolog in other lineages (Striedter, 1999; Hall, 2012b). I suspect that the mammalian dentate gyrus is such an innovation. This statement does not imply that the dentate evolved “from nothing” and or arose in a single, giant evolutionary step (most intermediate forms are likely to have gone extinct). The point is simply that despite our eagerness to find homologies, novelty should remain an open possibility. If every biological feature had existed since the beginning of life, evolution would be drab indeed.

FUNCTIONAL COMPARISONS Given the complex mix of similarities and differences in hippocampal structure across the amniotes, how similar or different are the functions of the hippocampus across those lineages? As the following sections make clear, comparative data on hippocampal functions are even sparser than the comparative anatomical data, and they are focused almost exclusively on the role of the hippocampus in spatial memory. Nonetheless, some structure–function correlations can be discerned.

Lepidosaurs Among lepidosaurs, hippocampal functions have been experimentally explored only in Cnemidophorus inornatus. Day et al. (2001) trained these whiptail lizards on an analog of the Morris water maze task, which is commonly used to assess spatial memory in rodents (Morris et al., 1982). The lizards had to learn where in a circular arena filled with sand and several large rocks they could find a single heated rock on which to bask themselves. Bilateral lesions of the medial cortex increased the time it took the lizards to find the heated rock (even during early training trials) and reduced the rate of learning relative to sham-lesioned controls. However, lesions of the dorsal cortex had similar behavioral effects, and probe trials revealed that even the intact lizards did not use the large cues on the maze walls to find their way. In essence, the lizards seemed to employ a nonspatial search strategy that involved moving along the arena walls and periodically entering the arena’s interior. More recent work has shown that side-blotched lizards (Uta stansburiana) can learn to find food in a modified Barnes maze using extra-maze cues, as evidenced

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by their performance on trials in which those cues are rotated (Ladage et al., 2012). Unfortunately, hippocampal lesions have not yet been performed in this species. However, Ladage et al. (2009) have reported that male side-blotched lizards with large territories tend to have a larger dorsal cortex, relative to the remaining telencephalon, than males with small territories or none at all. The size of the medial cortex was not associated with territoriality, suggesting that its role in spatial processing is probably minor, at least in this lizard species.

Turtles One species of cryptodire turtle, namely, the redeared slider Pseudemys scripta, has been tested in a modified Morris water maze consisting of a water-filled tub with four platforms, only one of which was baited (Lopez et al., 2003). Over a series of trials, the turtles learned to swim from the center of the arena directly to the baited platform. Moving the baited platform randomly from trial to trial prevented the learning. Moreover, obscuring all extra-maze cues with a curtain eliminated the learning effect. Given these results, the turtles must have used extra-maze visual cues to learn where in the room the baited platform is located. Lopez et al. (2003) then made large bilateral electrolytic lesions in the medial cortex of the trained turtles. Five days after the surgery, the turtles with the medial cortex lesions took significantly longer to find the baited platform than the sham-lesioned turtles, suggesting that they had forgotten their training. Several days of additional training allowed the lesioned turtles to become as good at finding the goal location as they had been before the surgery and indistinguishable from sham-lesioned controls. However, the lesioned turtles differed from the sham-lesioned turtles in one important respect: they could not find the goal location if a curtain was used to block their view of extra-maze cues in the half of the room containing the goal. Therefore, they must have used a different strategy to solve the learning task. Indeed, it is possible to solve spatial memory tasks, including the Morris water maze, by means of several alternative strategies. For example, it has been demonstrated that ants and bees can find their way to a remembered location by continuously comparing their current view of the environment with a memory of what the world had looked like at the goal location (Cartwright and Collett, 1983; Judd and Collett, 1998). Turtles with hippocampal lesions may be able to use such “snapshot memories” to find the baited platform as long as they can see extra-maze cues in the half of the room containing the goal (i.e., behind the goal as they swim toward it); when this view is obscured by a



curtain, the snapshot memory strategy would be ineffective. Turtles with an intact hippocampus would not need to use this strategy because they can presumably use a “cognitive map” strategy, which involves the learning of spatial relationships between multiple landmarks (O’Keefe and Nadel, 1978). Most researchers agree that mammals with an intact hippocampus tend to use such a cognitive map to solve the Morris water maze and other, similar spatial memory tasks (but see Benhamou, 1996). Some sea turtles migrate great distances to lay their eggs where they themselves were born. To guide them on these journeys, the turtles use a variety of cues, including the orientation of the earth’s magnetic field (Lohmann et al., 2004; Collett and Collett, 2011; Fuxjager et al., 2014). Unfortunately, no studies have tested whether turtles need the hippocampus to learn and remember these geomagnetic cues.

Archosaurs Alligators can sense the earth’s magnetic field (Rodda, 1984), but the underlying mechanisms are totally unknown. More specifically, there are no studies on the functions of the crocodilian hippocampus. In contrast, the avian hippocampus has been studied extensively. Most of this research has focused on pigeons and songbirds, two widely separated lineages of birds.

Songbirds The earliest functional studies on the avian hippocampus focused on scatter-hoarding songbirds that hide small “caches” of food in multiple locations across their home range and then retrieve those caches days or even months later, when food is scarce. Most of these birds come from two songbird families, namely, the Paridae (titmice and chickadees) and Corvidae (crows, jays and their relatives). The most remarkable spatial memory is exhibited by a corvid called Clark’s nutcracker (Nucifraga columbiana). Birds of this species can remember thousands of cache sites, and they can remember some of them for at least 9 months (Balda and Kamil, 1992). Because these birds tend store their food in open areas whose surface features may change during the storage period (e.g., due to snowfall), they probably use relatively distant landmarks to locate their caches (Kamil et al., 2001). Because the distance to such distant landmarks is difficult to estimate with precision, the nutrackers are probably encoding cache location by remembering the bearings (e.g., compass directions) of multiple landmarks at each site (Kamil and Cheng, 2001). They can then find their caches by moving in the direction that

minimizes the overall difference between the remembered bearings and the currently apparent ones. The first hippocampal lesion study in birds was performed by Natasha Krushinskaya (1966, 1970), who scooped out either the hippocampus, the hyperpallium (visual Wulst), or part of the DVR in three groups of Eurasian nutcrackers (Nucifraga caryocatactes). After a recovery period of 20 days, Krushinskaya tested how well the birds could find some seeds that they themselves had buried in a 2 3 2-m patch of forest floor. The major finding was that the hippocampus-lesioned birds could remember cache locations for about 15 minutes, but could not retrieve caches made 1–3 hours earlier (Krushinskaya, 1970). In contrast, birds with a lesion outside of the hippocampus were not impaired compared with intact birds, which retrieved roughly 80% of their caches (Fig. 7A). Krushinskaya’s work involved a very small number of birds and relatively large brain lesions, but her findings were substantiated by subsequent research on chickadees. Bilateral lesions of the hippocampus severely impaired the ability of these birds to retrieve seeds that they had cached in an indoor aviary, even though rates of caching and attempted retrieval were unaltered (Sherry and Vaccarino, 1989); sham lesions or lesions of the visual Wulst (hyperpallium) did not have this effect (Fig. 7B). Hampton and Shettleworth (1996) extended these findings by showing that chickadees with hippocampus lesions were impaired at remembering a goal location on a computer screen, but had no trouble remembering a color cue. In all these studies, the lesions damaged axons passing through the hippocampus, but two more recent studies used ibotenic acid to make excitotoxic lesions that spare fibers of passage. Zebra finches (Taeniopygia guttata) with such lesions in the dorsomedial hippocampus were impaired in their ability to remember the location of seeds hidden in one of 50 possible compartments within a 10 3 20-cm tray (Patel et al., 1997); the lesioned birds were not impaired when the goal location was marked with a color cue. Similarly, zebra finches with excitotoxic hippocampal lesions exhibited a postlesion deficit in spatial learning, whereas song learning and song perception were unimpaired (Bailey et al., 2009). These data are interesting in part because zebra finches do not hoard food and, therefore, might not have to be spatial memory experts. Still, zebra finches live in Australia and often fly long distances in search of food and water; to the extent that these resources are predictable, the finches would benefit from having good spatial memory. Most hippocampus lesions in birds are permanent, but one study used lidocaine to temporarily inactivate


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G.F. Striedter

Figure 7. Results of lesioning the hippocampus in scatter-hoarding birds. A: Eurasian nutcrackers with hippocampal lesions were impaired in their ability to retrieve seed caches that they had made 1–3 hours earlier; control birds did not exhibit this deficit. B: Chickadees with large hippocampus lesions stored seeds in an indoor aviary as they had done before the lesion, but they became much less successful at finding those caches, visiting numerous sites where they had not stored food. Adapted from Krushinskaya (1966) and Sherry and Vaccarino (1989).

the chickadee hippocampus during a spatial memory task. Shiflett et al. (2003) reported that bilateral inactivation of the hippocampus during training trials impaired the acquisition of spatial memories, but did so only when the locations could not be identified by local color cues. Inactivation of the hippocampus during memory retrieval impaired the birds’ ability to find locations they had learned about 15 minutes earlier, but they did not impair memory retrieval 3 hours after learning. Another study showed that infusions of an N-methyl-D-aspartate (NMDA) receptor antagonist into the hippocampus of chickadees block the formation of long-term but not short-term spatial memories (3 hours vs. 15 minutes after learning; Shiflett et al., 2004). Collectively, these findings indicate that songbirds need their hippocampus to encode long-term spatial memories, but that the recall of those memories can become independent of the hippocampus after several hours.

Pigeons Pigeons are well known for the ability to return to their home loft after being displaced over long distances (up to 1,800 km in specially bred “homing pigeons”). To accomplish this feat, the pigeons learn where their home loft is relative to diverse spatial cues, including distant visual landmarks (e.g., a road or a fire lookout tower in the forest), the earth’s magnetic field, windinduced infrasounds, and wind-borne odorants (Able, 2000; Hagstrum, 2013; Holland, 2014; Phillips and Jorge, 2014; Wallraff, 2014). In terms of memory capacity, homing seems simpler than remembering the location of food caches because homing requires memory for just a single location. However, homing entails a

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different set of challenges, such as having to switch from using very distant landmarks to reliance on more local cues as the birds approach their home. Large bilateral lesions of the hippocampus impair the ability of pigeons to return to their home loft from both unfamiliar and familiar sites (Bingman et al., 1984). On release, the lesioned birds fly off in the correct homeward direction, but they rarely return home. In contrast, unlesioned control birds home successfully, as do birds with lesions of the visual Wulst. Even when the pigeons with hippocampal lesions are released very close to home, they fail to enter their home loft. However, this home loft recognition deficit disappears when the birds are allowed to sit in their home loft for 1 week after the lesion, unable to fly around but able to see, hear, and smell the local environment (Bingman et al., 1985). Curiously, homing pigeons with hippocampal lesions become homing-impaired when their sense of smell is blocked by anesthetizing the olfactory epithelium (Bingman et al., 1987). Such anosmic hippocampus-lesioned birds cannot determine the homeward direction even when they are released from sites that they had been familiar with before the lesion, and most of them never find home. Because it is unlikely that intact pigeons can smell their home loft from great distances, they are probably orienting relative to other odor sources, some of which may be quite far away (e.g., the Mediterranean in Italy, where most of these experiments were done). Alternatively, olfactory stimulation may simply activate the brain’s ability to process other kinds of spatial cues (Phillips and Jorge, 2014). Yet another possibility is that treatments aimed at the olfactory epithelium also disable magnetoreceptors, which seem to be located in that vicinity (Mora et al., 2004). Either way, we can



conclude that the pigeon hippocampus is involved in spatial navigation using nonolfactory cues. Additional experiments have shown that adult pigeons with hippocampal lesions can learn to home from a familiar site even if they are rendered anosmic, as long as they were given postlesion opportunities to fly around their loft (Bingman et al., 1988). Another study reported that hippocampus lesions in young, naive pigeons prevent later homing success from unfamiliar sites, but only if the birds were not allowed to fly around their loft (Bingman et al., 1990). These results are difficult to fit into a unified framework, but they imply that pigeons can use both hippocampusdependent and hippocampus-independent strategies for finding their way home. This conclusion fits well with the general finding that pigeons can employ a multitude of different guidance cues to find their way; if one of those cues is unavailable, the birds can use another one. Such functional redundancy complicates mechanistic analyses. Because the function of the pigeon hippocampus is difficult to study in the field, several investigators have studied it in under more controlled laboratory conditions (Watanabe, 1999; Colombo and Broadbent, 2000). Early studies by Good and Macphail (1994) showed that hippocampus lesions impair learning in a spatial version of the delayed match-to-sample task when the delays are long. Hippocampus lesions also impair place learning by pigeons in an open field task without impairing visual discrimination (Colombo et al., 1997). Similar results have been obtained using a dry version of the Morris water maze task, although the lesioned birds do eventually master this task (Fremouw et al., 1997). Overall, these data confirm that the hippocampus in pigeons is involved fairly selectively in spatial learning and memory, just as it is in songbirds, turtles, and mammals. That said, hippocampus lesions in pigeons also interfere with some nonspatial behaviors, most notably autoshaping and behavioral inhibition (Good and Macphail, 1994; Colombo and Broadbent, 2000; Scarf et al., 2014). This is interesting because hippocampus lesions in mammals cause similar deficits.

Which amniotes have a large hippocampus? Among songbirds, the families (or subfamilies) that store food for later retrieval tend to have a larger hippocampus, relative to total telencephalon volume, than birds that do not cache (Krebs et al., 1989). These results have been extensively discussed and extended (Garamszegi and Eens, 2004; Lucas et al., 2004; Smulders, 2006; Roth et al., 2010), but the basic result has stood the test of time: birds that store their food in multiple locations tend to have a proportionately large

hippocampus, relative to the rest of the telencephalon (or total brain or body size). What is less clear is whether the quantitative degree of food storing in a particular species correlates with hippocampus volume (Brodin and Bolhuis, 2008). Homing pigeons also have a disproportionately large hippocampus, as well as an enlarged olfactory bulb, relative to other pigeon breeds (Rehkamper et al., 1988). Intriguingly, young homing pigeons that are allowed to fly around their loft end up with a larger hippocampus than confined birds (Cnotka et al., 2008). This finding is reminiscent of an earlier study (Clayton and Krebs, 1994), which showed that young marsh tits (Parus palustris) end up with a significantly larger hippocampus when they are allowed to store some seeds than when they are deprived of storing opportunities. In the latter case at least, the increase in hippocampus volume seems to results from the experience-dependent growth of individual neurons, rather than adult neurogenesis. Correlative studies outside of songbirds and pigeons are rare and less clear-cut (e.g., Volman et al., 1997). However, hummingbirds have excellent spatial memory (Hurly, 1996; Flores-Abreu et al., 2013; Jelbert et al., 2014) and a disproportionately large hippocampus (Ward et al., 2012). Among lizards and snakes, the evidence for a correlation between hippocampus volume and spatial behavior is relatively weak or negative (Day et al., 1999; Roth et al., 2006; Ladage et al., 2009). However, the medial cortex was found to be selectively enlarged in Northern Pacific rattlesnakes that had been relocated and forced to home repeatedly over a 2month period; this finding suggests that the medial cortex of rattlesnakes grows in response to increased navigational demands (Holding et al., 2012).

STRUCTURE–FUNCTION SYNTHESIS: CONSERVATION AND CHANGE As reviewed above, the effects of hippocampus lesions on spatial memory are remarkably similar among birds, turtles, and mammals. Even goldfish exhibit similar effects when their hippocampal homolog is lesioned (Rodriguez et al., 2002a,b). Therefore, it is reasonable to suppose that the functions of the hippocampus are broadly conserved across the amniotes, if not across all vertebrates. The concept of “functional homology” may be philosophically dubious because traditional definitions of homology explicitly allow homologs to vary in both structure and function (Striedter and Northcutt, 1991; Striedter, 2002), but the available data do suggest, at least at first blush, that the hippocampus in amniotes evolved without substantial changes to its role in spatial memory. Even some


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G.F. Striedter

nonspatial functions of the hippocampus seem to be broadly conserved (Good and Macphail, 1994; Colombo, 2003). However, the available data are limited to just a few branches within the amniote lineage, and whether the hippocampal homolog of lizards plays a role in spatial memory remains unclear at best. Therefore, hippocampus-dependent spatial memory may well have evolved independently in mammals and the turtle/archosaur lineage (Fig. 1). If it evolved from very different (nonhomologous) ancestral traits, then one would call this kind of independent evolution “convergence.” In contrast, if hippocampus-dependent spatial memory evolved several times independently from a common (homologous) ancestral trait, then the process would be called “parallelism” (i.e., parallel homoplasy; see Striedter and Northcutt, 1991). In practice, it is often difficult to distinguish parallelism from convergence, and I do not formally distinguish between these two possibilities in this paper. In either case, this line of inquiry raises the question of how hippocampusdependent spatial memory might have evolved from one or more ancestral traits. One possible (but clearly speculative) scenario is based on data showing that the mammalian hippocampus is involved in novelty detection and response inhibition (McNaughton, 2006; Kumaran and Maguire, 2009; Maren, 2014). Assuming that these hippocampal functions are ancient, I suggest that ancestral amniotes might have used the hippocampus primarily to discriminate between familiar and unfamiliar stimuli and to suppress ongoing behaviors when faced with anything unfamiliar (i.e., novel). Both mammals and the last common ancestors of turtles and birds might then have modified the hippocampus so that it became involved in suppressing movements toward unfamiliar stimuli, a process that would tend to steer an individual toward familiar ground. From such humble beginnings, more complex forms of hippocampus-dependent spatial memory and navigation may have evolved. To test this hypothesis, it would help to know whether spatial memory in amphibians (Pasukonis et al., 2013) requires an intact hippocampus (i.e., the frog medial pallium). If ancestral mammals and the last common ancestors of turtles and birds did evolve hippocampus-dependent spatial memory independently of one another, then one must wonder what benefits those species would have derived from their innovation. In the case of turtles and birds, an improvement in spatial memory might have been linked to their invasion of the sea and air, respectively. In both niches, navigation by a constellation of distal cues is probably more useful than navigation by local cues, which are quickly dispersed by air or water

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currents. But why would mammals, whose ancestors were probably terrestrial, evolve an improved spatial memory? An interesting possibility is that ancestral mammals, being nocturnal, might have used spatial memory to relocate scarce resources that they were unable to see at night (or while moving through tunnels and underbrush). If this is true, then one would expect navigation by idiothetic path integration (which employs only self-generated signals) to play a larger role in mammals (McNaughton et al., 1996; Samsonovich and McNaughton, 1997) than in turtles or birds. Indeed, birds seem inferior to rodents in this kind of path integration (Able, 2000). Even if hippocampus-dependent spatial memory already existed in ancestral amniotes, some aspects of hippocampal function almost certainly evolved convergently within the amniotes, especially between mammals and birds. For example, it seems likely that the inputs to the hippocampus from other telencephalic areas became more complex (more highly preprocessed) in mammals and in birds, compared with other amniotes. This hypothesis would be consistent with both taxa having expanded their neocortex homologs and losing the direct projections from the olfactory cortex into the hippocampus. Similarly, the relative expansion of the hippocampus in birds and mammals suggests that spatial memory capacity expanded independently in those two lineages, at least to some degree. Another interesting possibility is that evolutionary enhancements of spatial memory preadapted some birds and mammals for the evolution of hippocampusdependent episodic (or episodic-like) memory, which so far has been described only in songbirds, pigeons, and mammals (Clayton and Dickinson, 1998; Feeney et al., 2009; Allen and Fortin, 2013; Meyers-Manor et al., 2014). Aside from convergent evolution, the hippocampus of amniotes exhibits some divergent evolution in both structure and function. For example, given that a postulated function of the mammalian dentate is the reduction of interference between similar memories (McNaughton and Morris, 1987; Treves and Rolls, 1994; Treves et al., 2008; Myers and Scharfman, 2011), the evolutionary addition (or dramatic expansion) of the dentate gyrus in mammals probably increased their ability to remember multiple spatial locations (or other experiences) without getting them confused. If this is true, then one must wonder how scatter-hoarding birds can remember so many different cache sites even though they lack a proper dentate. Presumably they have evolved a different, as yet unknown solution to the problem of memory interference. Research on this avian solution might reveal more general structure–



function principles of memory encoding and recall, and it might inspire new ideas about the dentate’s functions in mammals. Additional evidence for functional divergence between the hippocampus of mammals and birds comes from neurophysiological experiments. Particularly interesting is that the avian hippocampus does not seem to contain the kind of “place cells” that are are characteristic of the mammalian hippocampus (O’Keefe and Conway, 1978; O’Keefe and Speakman, 1987). Some neurons in the pigeon hippocampus do fire preferentially in specific locations, but those locations tend to be associated with rewards and are unstable over time even when the environment remains constant (Hough and Bingman, 2004, 2008; Siegel et al., 2005, 2006). I suspect that these species differences in spatial response properties may be related to the use of different spatial navigation strategies in birds and mammals. Indeed, if birds are less sensitive than mammals to landmark distance and instead rely on multiple bearings to find their goals (Kamil and Cheng, 2001), then one would expect significant differences in “place cell” activity between birds and mammals.

increase our understanding of hippocampal phylogeny. Moreover, it likely would provide some new ideas on how the hippocampus “works,” not just in rodents or humans but in a wide variety of species.

ACKNOWLEDGMENTS I thank Veronica Martınez Cerde~no and Stephen Noctor for inviting me to contribute an article to this special issue. Glenn Northcutt was generous enough to lend me sections through several rare reptilian brains, and John Guzowski was good enough to let me scan them at high resolution in his laboratory. Cosme Salas and Fernando Rodrıguez sent me the images of the Pseudemys hippocampus. Tim Allen, Norbert Fortin, Daniel Hoops, Carlos Lopez-Garcia, Glenn Northcutt, Martin Pyka, and Tom Smulders provided valuable feedback on the manuscript (without necessarily agreeing with my interpretations). I am also grateful to the MTL journal club in UCI’s Center for the Neurobiology of Learning and Memory for letting me test-drive my ideas, and to Aaron Wilber for useful input at an early stage.

CONFLICT OF INTEREST STATEMENT The author has no conflicts of interest.

CONCLUSIONS

LITERATURE CITED

A general tenet of comparative neurobiology is that “shared architectural features should map to shared functional properties while functional differences may be related to structural differences” (Hampton and Shettleworth, 1996). Given this principle, it is surprising to find that hippocampal function seems much more conserved across the amniotes than hippocampal structure. However, the functional similarities are based on sampling a relatively small number of species and may, therefore, reflect a significant amount of independent evolution. Especially intriguing is the possibility that both birds and mammals independently expanded the extent of axon collateralization within the hippocampus. In addition to convergent evolution, the amniote hippocampus underwent some structural divergence. Particularly interesting is the addition or expansion of the dentate gyrus in mammals, which probably increased the number of distinct memories that the mammalian hippocampus can store. Overall, the “story” of hippocampus evolution is more complex than previous discussions have implied. To say that the hippocampus is “functionally homologous” across the various amniote groups overemphasizes the species similarities and obscures the possibility of major evolutionary change. Additional experiments will be needed to test most of the hypotheses I have proposed, but this additional research would substantially

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