High food abundance permits the evolution of placentotrophy: evidence from a placental lizard, Pseudemoia entrecasteauxii.

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High Food Abundance Permits the Evolution of Placentotrophy: Evidence from a Placental Lizard, Pseudemoia entrecasteauxii . Author(s): James U. Van Dyke, Oliver W. Griffith, and Michael B. Thompson Source: The American Naturalist, Vol. 184, No. 2 (August 2014), pp. 198-210 Published by: The University of Chicago Press for The American Society of Naturalists Stable URL: http://www.jstor.org/stable/10.1086/677138 . Accessed: 26/09/2014 09:31 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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vol. 184, no. 2

the american naturalist

august 2014

High Food Abundance Permits the Evolution of Placentotrophy: Evidence from a Placental Lizard, Pseudemoia entrecasteauxii James U. Van Dyke,* Oliver W. Griffith, and Michael B. Thompson School of Biological Sciences, A08 Heydon-Laurence Building, University of Sydney, New South Wales 2006, Australia Submitted January 23, 2014; Accepted April 21, 2014; Electronically published July 8, 2014 Dryad data: http://dx.doi.org/10.5061/dryad.52n61.

abstract: Mechanisms of reproductive allocation are major determinants of fitness because embryos cannot complete development without receiving sufficient nutrition from their parents. The nourishment of offspring via placentas (placentotrophy) has evolved repeatedly in vertebrates, including multiple times in squamate reptiles (lizards and snakes). Placentotrophy has been suggested to evolve only if food is sufficiently abundant throughout gestation to allow successful embryogenesis. If scarcity of food prevents successful embryogenesis, females should recoup nutrients allocated to embryos via abortion, reabsorption, and/or cannibalism. We tested these hypotheses in the placentotrophic southern grass skink Pseudemoia entrecasteauxii. We fed females one of four diets (high constant, high variable, low constant, and low variable) during gestation and tested the effects of both food amount and schedule of feeding on developmental success, cannibalism rate, placental nutrient transport, offspring size, and maternal growth and body condition. Low food availability reduced developmental success, placental nutrient transport, offspring size, and maternal growth and body condition. Cannibalism of offspring also increased when food was scarce. Schedule of feeding did not affect offspring or mothers. We suggest that high food abundance and ability to abort and cannibalize poor-quality offspring are permissive factors necessary for placentotrophy to be a viable strategy of reproductive allocation. Keywords: life-history evolution, viviparity, matrotrophy, reproduction, cannibalism, parent-offspring conflict.

Introduction The evolution of reproductive allocation mechanisms is central to both life-history evolution and population ecology (Dunham et al. 1989). Reproduction is among the most important biological functions because it is the only mechanism that allows organisms to contribute genes and offspring to future generations. Offspring must receive suf* Corresponding author; e-mail: [email protected]. Am. Nat. 2014. Vol. 184, pp. 198–210. 䉷 2014 by The University of Chicago. 0003-0147/2014/18402-55234$15.00. All rights reserved. DOI: 10.1086/677138

ficient nutrients from their parents in order to survive until their first independent meal. Animals allocate nutrients to developing embryos using two general strategies: lecithotrophy and matrotrophy (Blackburn 2014). Lecithotrophy is the allocation of nutrients prior to ovulation, usually as yolk produced during vitellogenesis and deposited in ovarian follicles (Wallace 1985). Matrotrophy is the allocation of nutrients to developing embryos after ovulation, for example, via a placenta, oviducal secretions, oophagy, and adelphophagy (Wourms 1981; Blackburn 1992; Dulvy and Reynolds 1997). In either strategy, nutrients allocated to offspring can be derived from either recently ingested food (income) or body stores (capital; Wheatley et al. 2008; Van Dyke et al. 2012). Whereas lecithotrophy is ancestral to all vertebrates, matrotrophy has evolved repeatedly in viviparous (live-bearing) vertebrate taxa, including mammals, squamate reptiles (lizards and snakes), amphibians, and both cartilaginous and bony fishes (Dulvy and Reynolds 1997; Reznick et al. 2002; Blackburn 2014; Wake 2014), but the selective pressures favoring its evolution remain poorly understood. Matrotrophy has been suggested to evolve either as a response to ecological selection pressures (Trexler and DeAngelis 2003) or as a result of parent-offspring conflict (Crespi and Semeniuk 2004). Life-history facilitation has also been suggested to drive the evolution of matrotrophy, but this hypothesis was recently found to have little support in fish (Bassar et al. 2014). Matrotrophy may have advantages over lecithotrophy because it allows females to ovulate small eggs and provide the balance of nutrients necessary for development during gestation (Trexler and DeAngelis 2003). By reducing egg size, females can theoretically ovulate more eggs at a time and potentially increase litter sizes (Trexler and DeAngelis 2003). Critically, food resources must remain abundant throughout gestation for the female to replace nutrients not allocated to eggs prior to ovulation (Trexler and DeAngelis 2003). If


Evolution of Vertebrate Placentotrophy 199 food resources are too limited to allow successful development of all embryos in a litter, Trexler and DeAngelis (2003) predicted that females should selectively abort, reabsorb, and/or cannibalize poor-quality embryos and reallocate their nutrients to other offspring in the litter. The Trexler and DeAngelis (2003) model has been tested several times in fish, in which food limitation during gestation causes reductions in matrotrophic provisioning, offspring size, and maternal mass in matrotrophic species but affects only maternal mass in lecithotrophic species (Marsh-Matthews and Deaton 2006; Banet et al. 2010; Pollux and Reznick 2011; Riesch et al. 2013). Food restriction rarely causes abortion or cannibalism of offspring in either matrotrophic or lecithotrophic fish (Banet and Reznick 2008; Banet et al. 2010; Pollux and Reznick 2011; but see Riesch et al. 2013). In lecithotrophic reptiles, food limitation during gestation does not affect offspring size or developmental success (Gregory and Skebo 1998; reviewed by Lourdais et al. 2002; Hare and Cree 2010), likely because placental nutrient transport is minimal in these taxa and embryos rely almost entirely on yolk for nutrition (Van Dyke and Beaupre 2012). Instead, food restriction during gestation in these species reduces maternal body condition (Lourdais et al. 2004; Gignac and Gregory 2005) and may even cause maternal starvation (Bonnet et al. 2002). Notably, the predictions of the Trexler and DeAngelis (2003) model have not been directly tested in any matrotrophic amniote vertebrate. In contrast, the parent-offspring-conflict hypothesis suggests that matrotrophy evolves as a result of iterative adaptations in embryos to gain greater control over nutrient supply from the mother and countering mechanisms in the mother to maintain control over nutrient provisioning (Crespi and Semeniuk 2004). An inherent prediction is that the evolution of placentas favors increasing resource allocation to current embryos, potentially at the expense of future reproductive success (Crespi and Semeniuk 2004). Although parent-offspring conflict is expected to occur in all viviparous organisms, several factors are expected to increase conflict, including multiple paternity and embryonic regulation of provisioning mechanisms (Crespi and Semeniuk 2004). Although matrotrophy relies on maternal mechanisms to transport nutrients from maternal blood to offspring across several cell layers (Biazik et al. 2009; Moore 2012; Griffith et al. 2013a), these mechanisms could be induced by embryonic cues to maternal tissues via embryonic hormones. Notably, the Trexler-DeAngelis and parent-offspring-conflict hypotheses are not mutually exclusive (Banet et al. 2010). Parent-offspring conflict may be the mechanism that drives the evolution of matrotrophy, but matrotrophy may only be a viable strategy in environments where food abundance is consistently high during gestation or in species capable of

aborting, reabsorbing, or cannibalizing poor-quality offspring when food is scarce. In eutherian mammals and some viviparous squamate reptiles, matrotrophy is accomplished via placental tissues that allow direct contact between maternal and embryonic tissues and actively transport nutrients from mother to offspring (i.e., placentotrophy). Among vertebrates, eutherian mammals exhibit the greatest reliance on placentotrophy because all species ovulate microlecithal eggs and provide more than 99% of embryonic nutrition via a placenta. In contrast, viviparous squamates vary widely in their reliance on placentotrophy (Ramirez-Pinilla 2006; Thompson and Speake 2006; Van Dyke and Beaupre 2012). Evidence for placentotrophy is typically interpreted from calculations of matrotrophy indices (MIs), or the ratio of offspring dry mass to egg dry mass (Blackburn 1992, 1994). In squamates, MIs of ∼0.7 are typical of lecithotrophic species because they catabolize 25%–30% of yolk mass for energy during development (Blackburn 1994; Thompson et al. 2000). Matrotrophy indices between 0.7 and 1.0 indicate that incipient placentotrophy may replace catabolized yolk mass during gestation. Matrotrophy indices greater than 1.0 are interpreted as clear evidence of placentotrophy (Blackburn 1994) and have only been reported in six genera of the family Scincidae (Flemming and Blackburn 2003; Thompson and Speake 2006; Blackburn and Flemming 2012). For example, MIs of skinks in the genus Mabuya exceed 40,000 (Vitt and Blackburn 1991; Ramirez-Pinilla 2006), while MI of Pseudemoia entrecasteauxii is only 1.6 (Stewart and Thompson 1993), and MI of Niveoscincus metallicus is only 0.91 (Thompson et al. 1999a). However, MI is usually calculated from females fed high food-availability diets in captivity (e.g., Stewart and Thompson 1993; Thompson et al. 1999b, 2001), and the potential effects of low food abundance during pregnancy on MIs of placentotrophic squamates have been ignored. The Australian southern grass skink P. entrecasteauxii is one of the only extant amniotes that allocates substantial quantities of nutrients to offspring via both yolk and a welldeveloped placenta (Stewart and Thompson 1993; Adams et al. 2005). Thus, it represents one of the few “intermediates” between full reliance on lecithotrophy and full reliance on placentotrophy. In addition, the placental tissues of P. entrecasteauxii are among the most studied of any nonmammal and are homologous in origin with those of eutherian mammals (Stewart and Thompson 1996, 2009; Adams et al. 2005; Griffith et al. 2013b; reviewed by Van Dyke et al. 2014a). Multiple paternity, which reduces relatedness between siblings and increases the potential for parent-offspring conflict, is high in P. entrecasteauxii (Stapley et al. 2003). These traits make P. entrecasteauxii an


200 The American Naturalist outstanding model species for testing hypotheses regarding the evolution of placentotrophy among amniote vertebrates. We tested two hypotheses for the evolution of placentotrophy in P. entrecasteauxii. By comparing the effects of food abundance and stochasticity of feeding schedule during gestation on developmental success, offspring size, and placental nutrient transport, we tested the hypothesis that matrotrophy (in this case, placentotrophy) is a successful reproduction-allocation strategy only in environments with high food abundance during gestation (Trexler and DeAngelis 2003). We also examined the effects of food abundance and stochasticity of feeding schedule on maternal growth and nutrient composition, because mothers might maximize nutrient allocation to offspring regardless of food limitation and possibly as a result of offspring control over placental nutrition (e.g., Banet et al. 2010). If this occurred, food limitation may not affect offspring but would instead reduce maternal growth and body composition. The second hypothesis we tested was that food-limited pregnant females were more likely to cannibalize offspring than were pregnant females with abundant food, which we tested by comparing the effects of food abundance and stochasticity of feeding schedule on cannibalism rate. Viviparous squamates are not capable of reabsorbing ovulated eggs or embryos prior to parturition (Blackburn et al. 2003) and so are unlikely to reallocate nutrients among offspring within a single litter, as predicted in the Trexler and DeAngelis (2003) model. Thus, our experiment tests a logical extension of this prediction, that food-limited females are more likely to cannibalize offspring to regain nutrients for future growth or reproduction than females that are not food limited. Methods Animal Collection and Husbandry We collected 50 pregnant Pseudemoia entrecasteauxii just after ovulation in October 2012 from Kanangra-Boyd National Park, New South Wales, Australia. Most females ovulate in late September or early October in this population (Murphy et al. 2006). Immediately after capture, we measured snout-vent length (SVL), tail length (TL), regenerated tail length (RTL), tail base width (TBW), and live mass of each female. We also estimated tail volume (TVOL) because lizards often use their tails to store fat and protein (Clark 1971). We assumed that the tail was cone-shaped, where the diameter of the cone base was TBW, and the height of the cone was TL. Shortly after capture, 10 females were euthanized via intraperitoneal injection of sodium pentobarbital (100 mg/kg body mass) for collection of freshly ovulated eggs and preexperimental tissue-content analyses.

The remaining 40 females were housed individually in glass aquaria with a 25-W basking light over one end to provide a thermal gradient. Skinks had access to a basking/hide rock formation and ad lib. access to water. We housed skinks in a climate-controlled room at 18⬚C with a 12L : 12D photoperiod. Thermochron temperature data loggers were placed in cages at the hot and cool ends of each cage. Throughout the experiment, the hot sides of cages alternated between a mean nighttime low of 18.8⬚ Ⳳ 0.1⬚C and a mean daytime high of 30.1⬚ Ⳳ 0.2⬚C, while the cool sides alternated between 17.4⬚ Ⳳ 0.1⬚C and 22.5⬚ Ⳳ 0.6⬚C. We randomly divided females into four feeding treatment groups and ensured that mean body size was similar among all treatments. All feeding treatments were sustained until a female gave birth (∼2.5 months): high constant (HC) females were fed 3 crickets every day; high variable (HV) females were fed 0–5 crickets per day in a randomized schedule that summed to 30 crickets in each 10-day period; low constant (LC) females were fed 1 cricket per day; and low variable (LV) females were fed 0–5 crickets per day in a randomized schedule that summed to 10 crickets in each 10-day period. In this way, both constant and stochastic treatments received the same total amount of food over a given 10-day period, and only the feeding schedules differed. These feeding regimes were continued for the whole of pregnancy until the females gave birth (∼75 days). We inspected females daily to ensure that they had consumed all of the crickets offered. Once females began to give birth, we inspected cages at least twice daily for offspring. Female P. entrecasteauxii usually consume all extraembryonic and extraplacental membranes immediately after parturition so the presence of offspring was our only indication that females had given birth. Once females gave birth, we euthanized the mother and offspring with sodium pentobarbital. We then measured SVL, TL, RTL, TBW, and mass of the mother and her offspring and the length of the right hind limb (RHL) of each offspring. If a female did not apparently give birth, we euthanized her 2 weeks after the last female in her treatment gave birth. Tissue Harvesting Females euthanized early in the season for initial egg and body contents were dissected to remove all ovulated eggs. Eggs were weighed and frozen at ⫺20⬚C. We counted corpora lutea in each female to verify that we had collected all of the eggs that were ovulated. We then weighed female carcasses and froze them at ⫺20⬚C. Euthanized offspring were weighed and frozen at ⫺20⬚C. Euthanized mothers were dissected to ensure that they had given birth to all offspring. If females had consumed undeveloped eggs or offspring after birth, these


Evolution of Vertebrate Placentotrophy 201 were dissected from the gut, weighed, and frozen at ⫺20⬚C. We also removed, weighed, and froze any undeveloped eggs or unborn offspring still present in the uterus. We counted corpora lutea to ensure that we had collected all eggs/concepti that the female had ovulated. Female carcasses were weighed and frozen at ⫺20⬚C. Tissue Analysis Tissue analyses largely followed methods employed in past studies of placentotrophy in Pseudemoia spp. (Thompson et al. 1999b, 1999c). We lyophilized all tissues to asymptotic mass and stored them in desiccators at room temperature. We homogenized all samples using mortars and pestles. A single egg or offspring from each female was randomly chosen for each analysis, including bomb calorimetry, lipid extraction, or mass spectrometry, because individual eggs and offspring were too small to subsample for all three analyses. Female carcass homogenates were subsampled for each analysis. We measured energy contents of each sample using a Phillipson microbomb calorimeter (Gentry Instruments, Aiken, SC; Thompson et al. 1999b). Dried samples were pressed into pellets, which were weighed and combusted in 100% oxygen atmospheres. The calorimeter was calibrated with benzoic acid after every 10th sample. Total lipid was extracted from each sample by homogenization in an excess of chloroform-methanol-water (1 : 1 : 1, by volume) in a Dounce homogenizer. The homogenate was filtered through a Bu¨chner funnel and deposited in a 50-mL graduated cylinder. The chloroform layer carrying extracted lipids was allowed to separate from the methanol-water layers for at least 10 min, and the methanol-water layer was removed via aspiration. The amount of total lipid in each sample was determined gravimetrically after evaporation of the remaining chloroform. We estimated total protein content by measuring total nitrogen content using a Delta V isotope-ratio mass spectrometer (IRMS; Thermo Fisher Scientific, Waltham, MA) at the Analytical Facility of the University of Sydney Faculty of Agriculture and Environment. Nitrogen content (%) was multiplied by 6.25 to calculate protein content (%) in each sample (Dumas method; Jung et al. 2003). Percent protein was then multiplied by the total dry mass of the sample to determine total protein mass. Statistical Analysis We compared developmental success and cannibalism rate among feeding treatments using generalized linear mixed models (PROC GLIMMIX) with binomial error structures. We defined developmental success as a live offspring. We defined cannibalism as a female consuming either an undeveloped egg or a developed offspring, because it was im-

possible to determine from gut-content analysis whether a cannibalized, fully developed offspring was born alive or dead. Thus, offspring found in gut-content examinations may not have completed development in some way that was not visually obvious. We do not report body sizes or nutrient contents of offspring found in the gut to avoid potential biases caused by partial digestion. We report dry nutrient contents, but not body sizes, of unborn yet apparently fully developed offspring and included them in our comparisons with live offspring. We assume that these offspring had not completed development but had received ample opportunity for placental nutrient transport because we waited until 2 weeks after the last female in a treatment group gave birth before dissecting females who did not give birth. All unborn offspring were from low-food treatments, so if unborn offspring did not receive sufficient nutrients to complete development in the same amount of time as those on high-food treatments, then low food during pregnancy reduced placental nutrient transport relative to gestation time. Litter was the experimental unit in both analyses. We used multivariate analysis of covariance (MANCOVA) to compare SVL, TL, TVOL, and RHL of live newborn skinks among feeding treatments. We included initial maternal SVL as a covariate and included maternal identity as a random effect to account for litter effects. We compared the dry masses of newborns and freshly ovulated eggs using ANOVA, with maternal identity included as a random effect. We also calculated mean matrotrophy indices for each treatment by dividing the mean of offspring dry mass of each treatment by the mean of egg dry mass from freshly ovulated females (Blackburn 1992, 1994). As these were composite estimates of MI for each entire treatment, we could not statistically compare MI among treatments and only report them for comparison with prior studies of matrotrophy in squamates. We compared offspring composition (total energy, protein, and fat contents) among feeding treatments using MANOVA. We also included eggs collected at ovulation in the analysis to compare the composition of newborn skinks to that of freshly ovulated eggs. We did not include offspring size as a covariate because we were specifically interested in comparing total energy, protein, and lipid contents rather than densities. Litter was the experimental unit in these analyses because we measured energy, protein, and fat contents in a single egg or offspring per litter. We did not include undeveloped eggs or cannibalized offspring in these analyses but did include fully developed unborn offspring. We estimated female growth during the experiment by subtracting the initial measurements of SVL, TL, RTL, and TVOL from the final measurements of each female. We did not examine growth in mass because the loss of offspring


202 The American Naturalist Table 1: Reproductive milestones and endpoints of females assigned to each feeding treatment Treatment No. females assigned to treatment No. females that ovulated No. females that gave birth to live offspring No. females not observed to give birth No. females not observed to give birth but contained undeveloped eggs No. females not observed to give birth but contained unborn, fully developed dead offspring in uterus No. females not observed to give birth or contain undeveloped eggs/offspring No. females not observed to give birth confirmed to have eaten entire litter

High constant

High variable

Low constant

Low variable

10 9 9 0 0

10 9 9 0 0

10 8 2 6 0

10 10 2 8 1

0 0 0

0 0 0

2 4 0

3 3 1

Note: Numbers in boldface indicate litters that were included in analyses of offspring size. Numbers in italics indicate litters that were included in analyses of offspring composition. The analysis of developmental success did not include females that were not observed to give birth or contain undeveloped eggs/offspring because we could not determine whether eggs successfully developed. We did include females that contained unborn, dead offspring because these offspring were not successfully born alive. The single female that was confirmed to have eaten her entire litter in the LV treatment was included in the analysis of developmental success because dissection revealed that her gut contained an entire litter of undeveloped eggs.

0.42 eggs, females in the HV treatment ovulated 3.00 Ⳳ 0.24 eggs, females in the LC treatment ovulated 3.38 Ⳳ 0.26 eggs, and females in the LV treatment ovulated 2.80 Ⳳ 0.33 eggs. Not all females in the experiment appeared to give birth, but two females from the LC treatment and three from the LV treatment contained dead, fully developed offspring (table 1). By the end of the experiment, only two females from each of the LC and LV treatments gave birth to live offspring that were not cannibalized (table 1). Therefore, we could not statistically compare newborn body sizes among all 4 treatments, and only means are presented for the measurements of offspring from low-food treatments.

mass during parturition would bias the result. We then compared growth among treatments using MANCOVA. Initial SVL was included in the model as a covariate to account for differences in initial body size. We compared maternal composition (dry mass, total energy, protein, and fat content) among feeding treatments using MANOVA. We also included females that were sacrificed at ovulation in the analysis to compare the composition of females in feeding treatments to those at ovulation. In all statistical analyses, except the binomial tests, we assessed univariate normality and homoscedasticity of variance using normal probability plots and Shapiro-Wilk tests. We followed all multivariate analyses with univariate ANOVAs that tested for among-treatment differences within each measurement. For ANCOVAs, all measurements were log transformed to linearize the relationships between growth estimates and SVL. We used post hoc Tukey’s tests to examine pairwise differences between treatments. Sample sizes were not large enough to test for violations of multivariate normality, so we used Pillai’s trace as a test statistic for all multivariate tests because it is robust to violations of multivariate normality (Scheiner 2001). All statistical tests were performed using SAS 9.3 (SAS Institute, Cary, NC) and were judged at a p 0.05. All means are presented Ⳳ1 SE. Data reported in the results are deposited in the Dryad Digital Repository: http://dx.doi.org/10.5061/dryad.52n61 (Van Dyke et al. 2014b).

Both developmental success and cannibalism rate are presented as the mean percent of each female’s litter that developed successfully and/or was cannibalized (fig. 1). Developmental success differed among feeding treatments (GLIMMIX F3, 25 p 6.11, P p .003). Developmental success was higher in both high-food treatments than in both low-food treatments (post hoc least squares means differences tests, P ≤ .009; fig. 1). Cannibalism rate also differed among feeding treatments (GLIMMIX F3, 32 p 4.52, P p .009), and was higher in both low-food treatments than in both high-food treatments (P ≤ .020; fig. 1).

Results

Offspring Body Size

Throughout the experiment, all females consumed every cricket that was offered. After parturition, dissection revealed that four females (HC: 1, HV: 1, LC: 2) did not ovulate, and these females were removed from subsequent analyses. Females in the HC treatment ovulated 3.11 Ⳳ

Comparisons between HC and HV treatments showed that offspring body size was not affected by the interaction between feeding treatment and log-transformed maternal SVL (Pillai’s trace p 0.406, F5, 9 p 1.23, P p .370). Offspring body size was also not affected by feeding treatment

Developmental Success and Cannibalism


Evolution of Vertebrate Placentotrophy 203

Figure 1: Litter developmental success and cannibalism rate in skinks fed diets differing in amount and schedule during gestation. Both developmental success and cannibalism differed among feeding treatments. Letters indicate significant pairwise differences among treatments within each measurement. Sample sizes differed between developmental success (high constant [HC]: 9, high variable [HV]: 9, low constant [LC]: 4, low variable [LV]: 7) and cannibalism rate (HC: 9, HV: 9, LC: 8, LV: 10) because some offspring and/or eggs, while cannibalized, could not be confirmed to have successfully developed. Columns represent mean percentage (%) of ovulated eggs that developed successfully or were cannibalized, and error bars represent Ⳳ1 SE.

(Pillai’s trace p 0.064, F5, 10 p 0.14, P p .980; table 2) or log-transformed maternal SVL alone (Pillai’s trace p 0.189, F5, 10 p 0.47, P p .793). Offspring from LC and LV treatments appeared to be smaller than those from HC and HV treatments in every category (table 2), but limited sample sizes, caused by developmental failure and cannibalism in LC and LV treatments, prevented statistical comparison. Offspring Composition Dry mass differed among feeding treatments and between offspring and freshly ovulated eggs (F4, 17 p 28.85, P ! .001). Dry mass was higher in HC and HV offspring than in LC and LV offspring and freshly ovulated eggs (P ≤ .001; table 3) but did not differ between eggs and offspring from both low-food treatments (P ≥ .936; table 3). Notably, mean dry masses of offspring from high-food treatments were nearly double those of offspring from lowfood treatments (table 3). Matrotrophy index, the ratio of mean offspring mass to mean masses of eggs from females at ovulation, was twice as high in offspring from both high-food treatments than in offspring from both lowfood treatments (table 3). Nutritional composition differed among feeding treatments and eggs (Pillai’s trace p 1.404, F12, 57 p 4.18, P ! .001). Specifically, total energy (F4, 23 p 6.38, P p .002), total protein (F4, 23 p 11.27, P ! .001), and total lipid contents (F4, 23 p 3.81, P p .020) all differed among feed-

ing treatments between offspring and eggs. Newborn skinks from both high-food treatments contained more energy than did newborn skinks from both low-food treatments (P ≤ .027; table 3). Energy contents of freshly ovulated eggs only showed statistically significant differences from the energy contents of LC offspring (P p .008; table 2). Newborn skink protein content was higher in individuals from both high-food treatments than in those from low-food treatments and eggs (P ≤ .009; table 3) but did not differ between newborn skinks on both low-food treatments and eggs (P ≥ .189; table 3). Newborn skink lipid content was higher in HC offspring than in both low-food offspring (P ≤ .022; table 3), but lipid content of HV offspring was only higher than that of LC offspring (P p .049; table 3). Offspring from LC diets also contained less lipid than did freshly ovulated eggs (Tukey’s P p .009; table 3), but egg lipid content did not differ from any other treatment (P ≥ .055; table 3). Maternal Growth Maternal growth was not affected by the interaction between feeding treatment and initial SVL (Pillai’s trace p 0.368, F12, 66 p 0.77, P p .680), but the effects of feeding treatment (Pillai’s trace p 0.778, F12, 75 p 2.19, P p .021) and SVL (Pillai’s trace p 0.505, F4, 23 p 5.87, P p .002) were both statistically significant. Post hoc univariate ANCOVAs revealed that SVL growth differed among feed-


204 The American Naturalist Table 2: Least squares mean (Ⳳ1 SE) body size measurements of newborn skinks born to females maintained on different diets during gestation Treatment High constant (N p 9) High variable (N p 9) Low constant (N p 2) Low variable (N p 2)

SVL (mm)

TL (mm)

TVOL (mm3)

RHL (mm)

23.1 Ⳳ .4 23.5 Ⳳ .5 20.3 20.5

29.7 Ⳳ .8 29.7 Ⳳ .2 20.8 23.9

23.8 Ⳳ 1.6 24.3 Ⳳ 2.0 9.8 11.1

9.9 Ⳳ .1 10.1 Ⳳ .2 9.1 8.9

Note: Standard errors are not available for measurements of newborns in the low constant and low variable diets because of sample size limitations (N p 2). SVL p snout-vent length, TL p tail length, TVOL p tail volume, RHL p length of right hind limb.

ing treatments (F3, 30 p 4.51, P p .011) and decreased with increasing initial SVL (F1, 30 p 12.44, P p .002). Growth in SVL was higher in skinks fed both high-food diets than in LV skinks (P ≤ .020; fig. 2), and HV skinks grew more than LC skinks (P p .022; fig. 2). Post hoc univariate ANCOVA revealed that neither growth in overall TL nor in RTL differed among treatments (TL: F3, 30 p 1.76, P p .180; RTL: F3, 30 p 0.90, P p .455; fig. 2) or with initial SVL (TL: F3, 30 p 0.17, P p .688; RTL: F3, 30 p 1.34, P p .258). However, growth in TVOL differed among treatments (F3, 30 p 6.76, P p .002; fig. 2) and decreased with increasing initial SVL (F3, 30 p 4.54, P p .043). While TVOL increased in skinks fed both highfood diets, it decreased in skinks fed both low-food diets (P ≤ .009; fig. 2). Maternal Composition Maternal body composition was strongly affected by feeding treatment (Pillai’s trace p 1.199, F16, 164 p 4.39, P ! .001). Post hoc univariate ANOVA showed that dry mass (F4, 45 p 9.96, P ! .001), total energy (F4, 45 p 22.50, P ! .001), total protein (F4, 45 p 17.76, P ! .001), and total lipid contents (F4, 45 p 8.66, P ! .001) were all affected by feeding treatment. Dry mass did not differ between females from both high-food treatments and females at ovulation (P ≥ .129; table 4). Dry mass of females on LV diets was smaller than the dry mass of females from both high-food treatments and females sampled at ovulation (P ≤ .049; table 4). Females on LC diets were smaller in dry mass than females on both high-food treatments (P ≤ .011; table 4) but were not smaller than females at ovulation (P p .557; table 4). Total energy content was higher in females from both high-food treatments than in females on both low-food treatments and females at ovulation (P ≤ .034; table 4). Total energy content was lower in LV treatments than in females at ovulation (P ! .001; table 4). Total protein content did not differ between females from both high-food treatments and females at ovulation (P ≥ .284; table 4),

but females from both low-food treatments contained less total protein than either females on high-food diets or females at ovulation (P ≤ .045; table 4). Total lipid content was higher in HV females than in females from both lowfood treatments and in females at ovulation (P ≤ .007; table 4) but was only higher in HC females than in LV females (P p .002; table 4). Females from both low-food treatments and from the HC treatment did not differ in total lipid content from females at ovulation (P ≥ .110; table 4).

Discussion In the placentotrophic southern grass skink Pseudemoia entrecasteauxii, food availability during gestation is an important determinant of reproductive success. Females with access to abundant food during gestation exhibit greater developmental success and produce larger offspring that contain more energy, protein, and lipid than do females with limited food. The ability of female P. entrecasteauxii to substantially increase placentotrophic provisioning when food is abundant shows that placentotrophy is facultative in placentotrophic reptiles (Stewart 1989; Swain and Jones 2000). Thus, previous measurements of matrotrophy index, which were calculated from captive females with abundant food (e.g., Stewart and Thompson 1993; Thompson et al. 1999b, 2001), may not be representative of placentotrophy in wild populations. Instead, individuals likely vary widely in placentotrophic provisioning depending on their foraging success (and body temperature; Itonaga et al. 2012b) during gestation. Our data clearly show that differences in offspring size due to differing feeding regimes in placentotrophic skinks (e.g., Shine and Downes 1999; Itonaga et al. 2012b) are caused by differences in placental transport of protein, lipid, and energy during gestation. In particular, offspring from high-food treatments contained nearly twice as much protein as did freshly ovulated eggs. Our results contrast with previous observations that food availability during gestation does not affect devel-


Evolution of Vertebrate Placentotrophy 205 Table 3: Mean (Ⳳ1 SE) body composition measurements of newborn skinks born to females fed different diets during gestation and of eggs at ovulation Treatment High constant (N p 9) High variable (N p 9) Low constant (N p 4) Low variable (N p 5) Ovulated (N p 10)

Dry mass (mg) A

49.88 55.88A 30.50B 27.55B 30.63B

Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ

MI

Energy (kJ)

% change

2.19 1.63 .88 Ⳳ .09 2.16 1.82 1.06A Ⳳ .09 2.82 1.00 .52B Ⳳ .10 3.29 .90 .62BC Ⳳ .12 2.26 ... .80AC Ⳳ .08 A

... ... ⫺35.26 ... ...

Protein (mg) A

35.38 35.88A 21.62B 22.57B 18.71B

Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ

2.48 2.48 2.78 3.21 2.10

% change 89.03 91.76 ... ... ...

Lipid (mg) A

10.48 6.71AB 4.02C 4.08BC 8.33AB

Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ

% change 1.49 1.49 1.67 1.93 1.26

... ... ⫺51.71 ... ...

Note: Percent change is the percent difference between the mean contents of newborns within a treatment and the mean contents of freshly ovulated eggs. Thus, positive percent change indicates a net gain of energy or material during gestation, via placentotrophy, while negative percent change indicates a net loss of energy or material during gestation, via catabolism. Percent change is not presented if the mean estimate in the given treatment was not different from the mean estimate in freshly ovulated eggs. Letters indicate significant pairwise differences between treatments within each measurement, as judged via Tukey’s tests. Matrotrophy indices (MIs) greater than 1 indicate net gains of dry mass between ovulated eggs and newborn offspring, while MIs of 0.7 are typical in lecithotrophic species.

opmental success or offspring size in lecithotrophic viviparous reptiles (Gregory and Skebo 1998; reviewed by Lourdais et al. 2002; Hare and Cree 2010). Lecithotrophic species allocate all nutrients to offspring prior to ovulation, so offspring should not suffer negative effects of food restriction during gestation. Many species of lecithotrophic viviparous reptiles reduce foraging effort and prey consumption and may even fast completely during pregnancy, with few negative effects on offspring (Shine 1980; Gregory et al. 1999; Lourdais et al. 2002, 2004). In contrast, the placentotrophic P. entrecasteauxii feeds throughout the year, even while pregnant (Brown 1988), and our data show that resource availability during gestation is a critical determinant of reproductive success. Foraging success during gestation is likely even more important in placentotrophic skinks of the genus Mabuya, which ovulate at juvenile body size at 3–4 months of age and grow to adult size during gestation of their first litter (Vitt and Blackburn 1983; Ramirez-Pinilla et al. 2002). Resource availability is also a major determinant of degree of matrotrophy in matrotrophic fish (Trexler 1985; Riesch et al. 2013). A critical component of the Trexler and DeAngelis (2003) hypothesis is that females can abort, reabsorb, or cannibalize offspring for reallocation to other offspring if food becomes limited during gestation. Although viviparous reptiles cannot reabsorb offspring nor reallocate nutrients to siblings within a litter as Trexler and DeAngelis (2003) suggested for matrotrophic fish, our data show that female P. entrecasteauxii with limited food are more likely to abort embryos and more likely to cannibalize both undeveloped eggs and fully developed offspring after parturition. Therefore, instead of reallocating nutrients among offspring within a clutch, female P. entrecasteauxii can use abortion to reduce the number of offspring to which they allocate nutrients within a litter and may even abort entire litters to save more nutrients for themselves. We could not determine whether abortion and canni-

balism are beneficial to the mother when food is scarce, but we infer that females save and recoup energy and nutrients using both strategies. When food is limiting, females provide less energy, protein, and lipid to offspring, which should allow them to better maintain their own energy, protein, and lipid reserves. Undeveloped eggs and unborn offspring do not increase in mass relative to freshly ovulated eggs, so abortion likely allows females to save nutrients that would otherwise have been allocated to production of small, potentially poor-quality offspring (reviewed by Sinervo 1990; Banet and Reznick 2008). Furthermore, each offspring or egg contains nearly 10% of an adult female’s energy content, approximately 7% of a female’s protein content, and 10–20% of a female’s lipid content, so cannibalism allows females to recoup substantial quantities of nutrients quickly after reproduction. In matrotrophic fish, food limitation during gestation causes reductions in matrotrophic provisioning to offspring (Marsh-Matthews and Deaton 2006; Banet et al. 2010; Pollux and Reznick 2011). Unlike P. entrecasteauxii, matrotrophic fish usually do not abort or cannibalize offspring when food availability is limited during gestation (Banet and Reznick 2008; Banet et al. 2010; Pollux and Reznick 2011; but see Riesch et al. 2013). Two key differences may explain why a placentotrophic reptile is more likely to abort offspring than matrotrophic fish. First, the offspring of matrotrophic fish are very small; Poeciliopsis prolifica offspring mass averages less than 0.2% of maternal mass (Banet and Reznick 2008). In P. entrecasteauxii, offspring dry mass ranges from 5.8% to 6.1% of maternal dry mass, depending on food abundance. Thus, placentotrophic reptiles allocate proportionally greater amounts of nutrients to each of their offspring than do matrotrophic fish. As a result, a poor-quality offspring may represent a greater potential loss of resources in reptiles than in fish. The larger size of reptile offspring also means that cannibalism will recoup larger amounts of energy and nutri-


206 The American Naturalist

Figure 2: Growth in female skinks fed diets differing in amount and schedule during gestation. Growth in snout-vent length (SVL) and tail volume (TV) differed among feeding treatments, but growth in tail length (TL) and regenerated tail length (RTL) did not. Growth in SVL, TL, and RTL are measured by the left Y-axis (mm), while growth in TV is measured by the right Y-axis (cm3). Letters indicate significant pairwise differences among treatments within each measurement. Columns represent least squares means, and error bars represent Ⳳ1 SE.

ents. Second, many matrotrophic fish exhibit superfetation, or the ability to carry multiple litters at different developmental stages (Pires et al. 2010). Superfetation allows matrotrophic fish to produce multiple litters repeatedly throughout the active season, so the potential fitness costs of producing poor-quality offspring in any single litter should be low. In contrast, viviparous reptiles, regardless of lecithotrophy or placentotrophy, rarely produce more than a single litter each year (Tinkle and Gibbons 1977), so the potential fitness costs of poor-quality offspring in any single litter are relatively high. Pseudemoia entrecasteauxii may be an extreme example because females reproduce only once per year and produce relatively small litters (3–5 offspring). Based on these lines of reasoning, we suggest that if food is limited during gestation, then placentotrophic reptiles may receive greater fitness and energy benefits via abortion and cannibalism than do matrotrophic fish. In addition to reproductive effects, female P. entrecasteauxii with abundant food also grow larger and retain greater reserves of protein, energy, and lipid after birth than do females with limited food. Although no females died of starvation during our study, it is possible that starvation could have occurred if females were maintained on low-food treatments after parturition. In lecithotrophic reptiles, starvation has been reported as an extreme result of fasting during reproduction (Bonnet et al. 2002), and less extreme effects, such as reduced muscle performance, may be common (Bauwens and Thoen 1981; Lourdais et

al. 2005). Females with greater body condition prior to vitellogenesis and/or ovulation may be better able to buffer the effects of food restriction during pregnancy on both reproductive effects and maternal growth or starvation. Unfortunately, we could not test this hypothesis because P. entrecasteauxii have never reproduced in captivity and frequently reabsorb ovarian follicles if captured prior to ovulation. Controlling access to food in wild populations is also not logistically possible, and replications of this experiment testing the effects of interannual variation in preovulatory body condition on placental nutrient transport rely on stochastic changes in food availability in the wild. Thus, our experiment was necessarily limited to investigating the effects of food restriction on placentotrophy in a single reproductive bout. In addition to total food abundance, Trexler and DeAngelis (2003) suggested that the consistency of food abundance (i.e., temporal variations in food availability independent of overall food abundance) may influence reproductive success in matrotrophic vertebrates. In contrast, we did not detect any differences caused by feeding schedule within either the high- or low-food abundance treatments. Therefore, the effects of foraging on reproductive success, offspring size, and maternal growth appear to be driven by total food abundance over the period of gestation rather than stochasticity in availability during gestation. However, we note that our sample sizes may have been too small to detect effects of stochasticity or interactions between stochasticity and food amount, par-


Evolution of Vertebrate Placentotrophy 207 Table 4: Mean (Ⳳ1 SE) body composition estimates of adult female skinks maintained on different diets during gestation and at ovulation Dry mass (mg)

Treatment High constant (N p 9) High variable (N p 9) Low constant (N p 8) Low variable (N p 10) Ovulation (N p 10)

838.6A 817.1A 604.3BC 533.4C 696.5AB

Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ

Energy content (kJ)

42.6 42.6 45.3 4 0.4 40.4

13.42A 13.06A 7.65BC 6.39B 10.28C

Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ

.67 .67 .71 .64 .64

Protein content (mg) 516.4A 507.7A 346.3B 283.2B 448.1A

Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ

24.7 24.7 26.2 23.5 23.5

Lipid content (mg) 63.9AB 73.9A 45.1B 26.5C 41.4BC

Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ

6.5 6.5 6.9 6.2 6.2

Note: Letters indicate pairwise significant differences between treatments within each measurement.

ticularly in the low-food treatments in which offspring developmental success was reduced. The effects of parent-offspring conflict are likely to be heightened under food limitation because the availability of resources to both mother and offspring are restricted (Banet et al. 2010), and increased provisioning to offspring is more likely to negatively affect the mother and vice versa. The dry masses of lecithotrophic neonatal squamates are typically 20%–30% lower than ovulated eggs due to embryonic catabolism of yolk resources (Robert and Thompson 2000; Thompson and Speake 2006; Van Dyke and Beaupre 2011). We did not observe similar decreases in embryos on the low-food treatment. Thus, although there was no net gain in nutrient content during gestation, placentotrophy provided enough nutrients to replace those catabolized by offspring during development even when food was scarce. Specifically, protein contents of offspring from both low-food treatments did not decrease during development as has been observed in lecithotrophic species (Thompson and Speake 2006). This result is noteworthy because squamate embryos typically catabolize a substantial portion of yolk protein for energy during embryogenesis (Thompson and Speake 2003). Thus, a lack of change in offspring protein content indicates that mothers transported marked quantities of protein to offspring across the placenta even when food abundance was low. Mothers on low-food treatments also exhibited significant decreases in protein content after pregnancy, presumably because these mothers allocated protein to offspring at their own expense. Direct tests of this conclusion would require comparisons of postpartum maternal protein content to that of nonpregnant females maintained on a similar diet, which we were unable to do. Regardless, the observation that offspring protein content is maintained while maternal protein content decreases suggests that mothers either allocate stored protein to offspring or allocate dietary protein directly to offspring. If the latter, then mothers also necessarily keep less dietary protein for themselves. In either case, mothers experience a net loss in protein available for allocation to maternal body condition or future reproduction.

Maintenance of offspring protein in preference to maternal protein could occur for one of two reasons. Either embryos can manipulate mothers to allocate nutrients across the placenta even when resources are limited or mothers value their current bout of reproduction over their own condition or future uncertain reproductive events and preferentially allocate dietary protein directly to offspring. The latter explanation seems unlikely given the high rates of abortion and cannibalism we observed when food was limited. Furthermore, pregnant female P. entrecasteauxii are more likely to retain nutrients when highly stressed and allocate more nutrients to offspring when less stressed, regardless of food abundance (Itonaga et al. 2012a). Thus, we suggest that parent-offspring conflict occurs in P. entrecasteauxii, with embryos exerting some control over maternal provisioning and mothers ultimately reclaiming those nutrients from poor-quality offspring via cannibalism if food becomes scarce. These findings support Crespi and Semeniuk’s (2004) prediction that the evolution of placentas favors investment in current embryos, potentially at the expense of maternal lifetime reproductive success. Taken together, our results provide the first support for the Trexler and DeAngelis (2003) model for the evolution of placentotrophy in amniote vertebrates. Pseudemoia entrecasteauxii is an ideal model in which to test hypotheses regarding the evolution of placentotrophy in amniotes because it is one of the few extant species that allocates nutrients to offspring via both placentotrophy and lecithotrophy, and its placenta is both well-studied and homologous to those of eutherian mammals. In P. entrecasteauxii, high food abundance during gestation is critical for reproduction to be successful. In contrast, the unwillingness of many lecithotrophic species to forage during gestation may fundamentally limit opportunities for placentotrophy to evolve. Parent-offspring conflict may drive the evolution of placentotrophy, but we suggest that high food abundance and the ability to abort and cannibalize offspring are, at a minimum, permissive factors necessary for placentotrophy to be a viable strategy of reproductive allocation in amniotes.


208 The American Naturalist Acknowledgments We thank M. Brandley, J. Herbert, F. Horswell, G. Manning, J. McKenna, and M. Vo Hoang for logistical support. We thank C. Keitel for performing all mass spectrometry. Early drafts of the manuscript were considerably improved by comments from M. Brandley, C. Friesen, J. Herbert, M. Laird, F. van den Berg, and C. Whittington, and we thank two anonymous reviewers and J.-M. Gaillard for additional critical comments. J.U.V. was supported by an international fellowship from the National Science Foundation’s International Research Fellowship Program (grant 1064803), and some of the work was supported by an Australian Research Council Discovery grant to M.B.T. All animals were collected under NSW Scientific Licence number SL100401, and all procedures were approved by the University of Sydney Animal Ethics Committee.

Literature Cited Adams, S. M., J. M. Biazik, M. B. Thompson, and C. R. Murphy. 2005. Cyto-epitheliochorial placenta of the viviparous lizard Pseudemoia entrecasteauxii: a new placental morphotype. Journal of Morphology 264:264–276. Banet, A. I., A. G. Au, and D. Reznick. 2010. Is mom in charge? implications of resource provisioning on the evolution of the placenta. Evolution 64:3172–3182. Banet, A. I., and D. N. Reznick. 2008. Do placental species abort offspring? testing an assumption of the Trexler-DeAngelis model. Functional Ecology 22:323–331. Bassar, R. D., S. K. Auer, and D. Reznick. 2014. Why do placentas evolve? a test of the life-history facilitation hypothesis in two clades in the genus Poeciliopsis representing two independent origins of placentas. Functional Ecology. doi:10.1111/1365–2435.12233. Bauwens, D., and C. Thoen. 1981. Escape tactics and vulnerability to predation associated with reproduction in the lizard Lacerta vivipara. Journal of Animal Ecology 50:733–743. Biazik, J. M., M. B. Thompson, and C. R. Murphy. 2009. Lysosomal and alkaline phosphatase activity indicate macromolecule transport across the uterine epithelium in two viviparous skinks with complex placenta. Journal of Experimental Zoology B: Molecular and Developmental Evolution 312:817–826. Blackburn, D. G. 1992. Convergent evolution of viviparity, matrotrophy, and specializations for fetal nutrition in reptiles and other vertebrates. American Zoologist 32:313–321. ———. 1994. Standardized criteria for the recognition of embryonic nutritional patterns in squamate reptiles. Copeia 1994:925–935. ———. 2014. Evolution of vertebrate viviparity and specializations for fetal nutrition: a quantitative and qualitative analysis. Journal of Morphology. doi:10.1002/jmor.20272. Blackburn, D. G., and A. F. Flemming. 2012. Invasive implantation and intimate placental associations in a placentotrophic African lizard, Trachylepis ivensi (Scincidae). Journal of Morphology 273: 137–159. Blackburn, D. G., K. K. Weaber, J. R. Stewart, and M. B. Thompson. 2003. Do pregnant lizards resorb or abort inviable eggs and em-

bryos? morphological evidence from an Australian skink, Pseudemoia pagenstecheri. Journal of Morphology 256:219–234. Bonnet, X., O. Lourdais, R. Shine, and G. Naulleau. 2002. Reproduction in a typical capital breeder: costs, currencies, and complications in the aspic viper. Ecology 83:2124–2135. Brown, G. W. 1988. The diet of Leiolopisma entrecasteauxii (Lacertilia, Scincidae) from southwestern Victoria, with notes on its relationship with the reproductive-cycle. Australian Wildlife Research 15: 605–614. Clark, D. R. 1971. The strategy of tail-autotomy in the ground skink, Lygosoma laterale. Journal of Experimental Zoology 176:295–302. Crespi, B., and C. Semeniuk. 2004. Parent-offspring conflict in the evolution of vertebrate reproductive mode. American Naturalist 163:635–653. Dulvy, N. K., and J. D. Reynolds. 1997. Evolutionary transitions among egg-laying, live-bearing and maternal inputs in sharks and rays. Proceedings of the Royal Society B: Biological Sciences 264: 1309–1315. Dunham, A. E., B. W. Grant, and K. L. Overall. 1989. Interfaces between biophysical and physiological ecology, and the population ecology of terrestrial vertebrate ectotherms. Physiological Zoology 62:335–355. Flemming, A. F., and D. G. Blackburn. 2003. Evolution of placental specializations in viviparous African and South American lizards. Journal of Experimental Zoology A: Comparative Experimental Biology 299:33–47. Gignac, A., and P. T. Gregory. 2005. The effects of body size, age, and food intake during pregnancy on reproductive traits of a viviparous snake, Thamnophis ordinoides. Ecoscience 12:236–243. Gregory, P. T., L. H. Crampton, and K. M. Skebo. 1999. Conflicts and interactions among reproduction, thermoregulation and feeding in viviparous reptiles: are gravid snakes anorexic? Journal of Zoology 248:231–241. Gregory, P. T., and K. M. Skebo. 1998. Trade-offs between reproductive traits and the influence of food intake during pregnancy in the garter snake, Thamnophis elegans. American Naturalist 151: 477–486. Griffith, O. W., B. Ujvari, K. Belov, and M. B. Thompson. 2013a. Placental lipoprotein lipase (LPL) gene expression in a placentotrophic lizard, Pseudemoia entrecasteauxii. Journal of Experimental Zoology B: Molecular and Developmental Evolution 320:465–470. Griffith, O. W., J. U. Van Dyke, and M. B. Thompson. 2013b. No implantation in an extrauterine pregnancy of a placentotrophic reptile. Placenta 34:510–511. Hare, K. M., and A. Cree. 2010. Incidence, causes and consequences of pregnancy failure in viviparous lizards: implications for research and conservation settings. Reproduction, Fertility, and Development 22:761–770. Itonaga, K., S. M. Jones, and E. Wapstra. 2012a. Do gravid females become selfish? female allocation of energy during gestation. Physiological and Biochemical Zoology 85:231–242. ———. 2012b. Effects of maternal basking and food quantity during gestation provide evidence for the selective advantage of matrotrophy in a viviparous lizard. PLoS ONE 7:e41835. Jung, S., D. A. Rickert, N. A. Deak, E. D. Aldin, J. Recknor, L. A. Johnson, and P. A. Murphy. 2003. Comparison of Kjeldahl and Dumas methods for determining protein contents of soybean products. Journal of the American Oil Chemists’ Society 80:1169– 1173. Lourdais, O., X. Bonnet, and P. Doughty. 2002. Costs of anorexia


Evolution of Vertebrate Placentotrophy 209 during pregnancy in a viviparous snake (Vipera aspis). Journal of Experimental Zoology 292:487–493. Lourdais, O., F. Brischoux, D. DeNardo, and R. Shine. 2004. Protein catabolism in pregnant snakes (Epicrates cenchria maurus Boidae) compromises musculature and performance after reproduction. Journal of Comparative Physiology B: Biochemical Systemic and Environmental Physiology 174:383–391. Lourdais, O., F. Brischoux, R. Shine, and X. Bonnet. 2005. Adaptive maternal cannibalism in snakes (Epicrates cenchria maurus, Boidae). Biological Journal of the Linnean Society 84:767–774. Marsh-Matthews, E., and R. Deaton. 2006. Resources and offspring provisioning: a test of the Trexler-DeAngelis model for matrotrophy evolution. Ecology 87:3014–3020. Moore, T. 2012. Parent-offspring conflict and the control of placental function. Placenta 33(suppl.):S33–S36. Murphy, K., S. Hudson, and G. Shea. 2006. Reproductive seasonality of three cold-temperate viviparous skinks from southeastern Australia. Journal of Herpetology 40:454–464. Pires, M. N., J. Arendt, and D. N. Reznick. 2010. The evolution of placentas and superfetation in the fish genus Poecilia (Cyprinodontiformes: Poeciliidae: subgenera Micropoecilia and Acanthophacelus). Biological Journal of the Linnean Society 99:784–796. Pollux, B. J. A., and D. N. Reznick. 2011. Matrotrophy limits a female’s ability to adaptively adjust offspring size and fecundity in fluctuating environments. Functional Ecology 25:747–756. Ramirez-Pinilla, M. P. 2006. Placental transfer of nutrients during gestation in an Andean population of the highly matrotrophic lizard genus Mabuya (Squamata: Scincidae). Herpetological Monographs 20:194–204. Ramirez-Pinilla, M. P., V. H. Serrano, and J. C. Galeano. 2002. Annual reproductive activity of Mabuya mabouya (Squamata, Scincidae). Journal of Herpetology 36:667–677. Reznick, D., M. Mateos, and M. S. Springer. 2002. Independent origins and rapid evolution of the placenta in the fish genus Poeciliopsis. Science 298:1018–1020. Riesch, R., R. A. Martin, and R. B. Langerhans. 2013. Predation’s role in life-history evolution of a livebearing fish and a test of the Trexler-DeAngelis model of maternal provisioning. American Naturalist 181:78–93. Robert, K. A., and M. B. Thompson. 2000. Energy consumption by embryos of a viviparous lizard, Eulamprus tympanum, during development. Comparative Biochemistry and Physiology A: Molecular and Integrative Physiology 127:481–486. Scheiner, S. M. 2001. MANOVA: multiple response variables and multispecies interactions. Pages 99–115 in S. M. Scheiner and J. Gurevitch, eds. Design and Analysis of Ecological Experiments. Oxford University Press, New York. Shine, R. 1980. “Costs” of reproduction in reptiles. Oecologia (Berlin) 46:92–100. Shine, R., and S. J. Downes. 1999. Can pregnant lizards adjust their offspring phenotypes to environmental conditions? Oecologia (Berlin) 119:1–8. Sinervo, B. 1990. The evolution of maternal investment in lizards: an experimental and comparative analysis of egg size and its effects on offspring performance. Evolution 44:279–294. Stapley, J., C. M. Hayes, and S. J. Keogh. 2003. Population genetic differentiation and multiple paternity determined by novel microsatellite markers from the mountain log skink (Pseudemoia entrecasteauxii). Molecular Ecology Notes 3:291–293. Stewart, J. R. 1989. Facultative placentotrophy and the evolution of

squamate placentation: quality of eggs and neonates in Virginia striatula. American Naturalist 133:111–137. Stewart, J. R., and M. B. Thompson. 1993. A novel pattern of embryonic nutrition in a viviparous reptile. Journal of Experimental Biology 174:97–108. ———. 1996. Evolution of reptilian placentation: development of extraembryonic membranes of the Australian scincid lizards, Bassiana duperreyi (Oviparous) and Pseudemoia entrecasteauxii (Viviparous). Journal of Morphology 227:349–370. ———. 2009. Parallel evolution of placentation in Australian Scincid lizards. Journal of Experimental Zoology B: Molecular and Developmental Evolution 312:590–602. Swain, R., and S. M. Jones. 2000. Facultative placentotrophy: halfway house or strategic solution? Comparative Biochemistry and Physiology A: Molecular and Integrative Physiology 127:441–451. Thompson, M. B., and B. K. Speake. 2003. Energy and nutrient utilisation by embryonic reptiles. Comparative Biochemistry and Physiology A: Molecular and Integrative Physiology 133:529–538. ———. 2006. A review of the evolution of viviparity in lizards: structure, function and physiology of the placenta. Journal of Comparative Physiology B: Biochemical Systemic and Environmental Physiology 176:179–189. Thompson, M. B., B. K. Speake, J. R. Stewart, K. J. Russell, and R. J. McCartney. 2001. Placental nutrition in the Tasmanian skink, Niveoscincus ocellatus. Journal of Comparative Physiology B: Biochemical Systemic and Environmental Physiology 171:155–160. Thompson, M. B., B. K. Speake, J. R. Stewart, K. J. Russell, R. J. McCartney, and P. F. Surai. 1999a. Placental nutrition in the viviparous lizard Niveoscincus metallicus: the influence of placental type. Journal of Experimental Biology 202:2985–2992. Thompson, M. B., J. R. Stewart, and B. K. Speake. 2000. Comparison of nutrient transport across the placenta of lizards differing in placental complexity. Comparative Biochemistry and Physiology A: Molecular and Integrative Physiology 127:469–479. Thompson, M. B., J. R. Stewart, B. K. Speake, K. J. Russell, and R. J. McCartney. 1999b. Placental transfer of nutrients during gestation in the viviparous lizard, Pseudemoia spenceri. Journal of Comparative Physiology B: Biochemical Systemic and Environmental Physiology 169:319–328. Thompson, M. B., J. R. Stewart, B. K. Speake, K. J. Russell, R. J. McCartney, and P. F. Surai. 1999c. Placental nutrition in a viviparous lizard (Pseudemoia pagenstecheri) with a complex placenta. Journal of Zoology 248:295–305. Tinkle, D. W., and J. W. Gibbons. 1977. The distribution and evolution of viviparity in reptiles. Miscellaneous publication no. 154. Museum of Zoology, University of Michigan, Ann Arbor. Trexler, J. C. 1985. Variation in the degree of viviparity in the sailfin molly, Poecilia latipinna. Copeia 1985:999–1004. Trexler, J. C., and D. L. DeAngelis. 2003. Resource allocation in offspring provisioning: an evaluation of the conditions favoring the evolution of matrotrophy. American Naturalist 162:574–585. Van Dyke, J. U., and S. J. Beaupre. 2011. Bioenergetic components of reproductive effort in viviparous snakes: costs of vitellogenesis exceed costs of pregnancy. Comparative Biochemistry and Physiology A: Molecular and Integrative Physiology 160:504–515. ———. 2012. Stable isotope tracer reveals that viviparous snakes transport amino acids to offspring during gestation. Journal of Experimental Biology 215:760–765. Van Dyke, J. U., S. J. Beaupre, and D. L. Kreider. 2012. Snakes allocate amino acids acquired during vitellogenesis to offspring: are capital


210 The American Naturalist and income breeding consequences of variable foraging success? Biological Journal of the Linnean Society 106:390–404. Van Dyke, J. U., M. C. Brandley, and M. B. Thompson. 2014a. The evolution of viviparity: molecular and genomic data from squamate reptiles advance understanding of live birth in amniotes. Reproduction 147:R15–R26. Van Dyke, J. U., O. W. Griffith, and M. B. Thompson. 2014b. Data from: High food abundance permits the evolution of placentotrophy: evidence from a placental lizard, Pseudemoia entrecasteauxii. American Naturalist, Dryad Digital Repository, http://dx .doi.org/10.5061/dryad.52n61. Vitt, L. J., and D. G. Blackburn. 1983. Reproduction in the lizard Mabuya heathi (Scincidae): a commentary on viviparity in new world Mabuya. Canadian Journal of Zoology 61:2798–2806. ———. 1991. Ecology and life history of the viviparous lizard Ma-

buya bistriata (Scincidae) in the Brazilian Amazon. Copeia 1991: 916–927. Wake, M. H. 2014. Fetal adaptations for viviparity in amphibians. Journal of Morphology doi:10.1002/jmor.20271. Wallace, R. A. 1985. Vitellogenesis and oocyte growth in nonmammalian vertebrates. Pages 127–177 in L. W. Browder, ed. Developmental biology: a comprehensive synthesis. Plenum, New York. Wheatley, K. E., C. J. A. Bradshaw, R. G. Harcourt, and M. A. Hindell. 2008. Feast or famine: evidence for mixed capital-income breeding strategies in Weddell seals. Oecologia (Berlin) 155:11–20. Wourms, J. P. 1981. Viviparity: the maternal-fetal relationship in fishes. American Zoologist 21:473–515.

Associate Editor: Jean-Michel Gaillard Editor: Susan Kalisz

Late-stage fetal southern grass skink (Pseudemoia entrecasteauxii). Photo credit: Oliver W. Griffith.


Evolution: A lizard that generates heat.

A stem acrodontan lizard in the Cretaceous of Brazil revises early lizard evolution in Gondwana.

The evolution of placental mammals.

Evidence for the role of thyroxine as a hormone in the physiology of a lizard.

Evidence of adaptive evolution of alpine pheasants to high-altitude environment from mitogenomic perspective.

Evolution of the primate lentiviruses: evidence from vpx and vpr.

The evolution of pandemic influenza: evidence from India, 1918-19.

The Evolution of Functionally Redundant Species; Evidence from Beetles.

Molecular evolution: Warm and wild lizard sex changes.

Cationic isotachophoresis separation of the biomarker cardiac troponin I from a high-abundance contaminant, serum albumin.

Predicting reproductive success from hormone concentrations in the common tern (Sterna hirundo) while considering food abundance.

Evidence of nitrification and denitrification in high and low microbial abundance sponges.

Evidence that the systematic analysis of bile cytology permits monitoring of hepatic allograft rejection.

Enter the Dragon: The Dynamic and Multifunctional Evolution of Anguimorpha Lizard Venoms.

Biogeography and evolution of a widespread Central American lizard species complex: Norops humilis, (Squamata: Dactyloidae).

Poverty, inequality, and increased consumption of high calorie food: Experimental evidence for a causal link.

The first iguanian lizard from the Mesozoic of Africa.

Evolution of modern humans: evidence from nuclear DNA polymorphisms.

Predation Risk Perception, Food Density and Conspecific Cues Shape Foraging Decisions in a Tropical Lizard.

Packing of ribosomes in crystals from the lizard Lacerta sicula.

Food restriction affects maternal investment but not neonate phenotypes in a viviparous lizard.

Evidence for a testosterone binding macromolecule in human placental cytosol.

A New Eocene Casquehead Lizard (Reptilia, Corytophanidae) from North America.

Unprecedented evidence for high viral abundance and lytic activity in coral reef waters of the South Pacific Ocean.