Current Biology
Dispatches reflect marginal Utility. Curr. Biol. 24, 2491– 2500. 10. Nakahara, H., Itoh, H., Kawagoe, R., Takikawa, Y., and Hikosaka, O. (2004). Dopamine neurons can represent context-dependent prediction error. Neuron 41, 269–280. 11. Eshel, N., Bukwich, M., Rao, V., Hemmelder, V., Tian, J., and Uchida, N. (2015). Arithmetic and local circuitry underlying dopamine prediction errors. Nature 525, 243–246. 12. Sutton, R.S., and Barto, A.G. (1998). Reinforcement Learning: An Introduction (Cambridge, MA: The MIT Press). 13. Heeger, D.J. (1992). Normalization of cell responses in cat striate cortex. Vis. Neurosci. 9, 181–197.
14. Louie, K., Grattan, L.E., and Glimcher, P.W. (2011). Reward value-based gain control: divisive normalization in parietal cortex. J. Neurosci. 31, 10627–10639. 15. Omelchenko, N., and Sesack, S.R. (2009). Ultrastructural analysis of local collaterals of rat ventral tegmental area neurons: GABA phenotype and synapses onto dopamine and GABA cells. Synapse 63, 895–906. 16. Tan, K.R., Yvon, C., Turiault, M., Mirzabekov, J.J., Doehner, J., Laboue`be, G., Deisseroth, K., Tye, K.M., and Lu¨scher, C. (2012). GABA neurons of the VTA drive conditioned place aversion. Neuron 73, 1173–1183. 17. van Zessen, R., Phillips, J., Budygin, E., and Stuber, G. (2012). Activation of VTA GABA
neurons disrupts reward consumption. Neuron 73, 1184–1194. 18. Cohen, J.Y., Haesler, S., Vong, L., Lowell, B.B., and Uchida, N. (2012). Neuron-typespecific signals for reward and punishment in the ventral tegmental area - Supplementary Information. Nature 482, 85–88. 19. Watabe-Uchida, M., Zhu, L., Ogawa, S.K., Vamanrao, A., and Uchida, N. (2012). Whole-brain mapping of direct inputs to midbrain dopamine neurons. Neuron 74, 858–873. 20. Tian, J., and Uchida, N. (2015). Habenula lesions reveal that multiple mechanisms underlie dopamine prediction errors. Neuron 87, 1304–1316.
Evolution: Big Bawls, Small Balls John L. Fitzpatrick and Stefan Lu¨pold Computational and Evolutionary Biology, Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, UK Correspondence: [email protected] (J.L.F.), [email protected] (S.L.) http://dx.doi.org/10.1016/j.cub.2015.09.060
Males must carefully allocate the energy they devote to sex. A new study of howler monkeys shows that males who use vocalizations to ward off rivals invest less in producing large numbers of sperm. When it comes to sex, you can’t have it all. Animal reproduction is about compromising between the competing demands of finding a mate and successfully fertilizing eggs. Males, who are the sex that typically competes for access to mates, are particularly sensitive to this reproductive balancing act. In many animals, males show off ornate sexual colouration, ear-splitting vocalizations, and even dances to attract a female’s attention. While catching a female’s interest is a good first step toward successful reproduction, access to a receptive female often requires contending with, and outcompeting, rival males. Consequently, competition between males over access to females has in many species led to the evolution of male sexual weaponry (such as horns or antlers) or to massive divergence in body size between males and females. These conspicuous male sexual displays and weapons were instrumental in Darwin’s [1] formulation of sexual selection. We now know, however, that females of most species mate with multiple males
[2], which means that male–male competition over a female is followed by competition among their sperm to fertilize her egg(s) [3]. Competition among rival males before and after mating imposes a host of evolutionary constraints. Males have to allocate limited resources to both securing mates and producing ejaculates that are better at fertilizing eggs than rival males’ sperm. Thus, investment in whatever helps a male get a mate should limit investment in ejaculate traits crucial during sperm competition, and vice versa [4]. However, we know surprisingly little about how males balance their investment in sexual traits. In a study published recently in Current Biology, Dunn et al. [5] address this gap in our understanding of the evolution of male sexual traits by comprehensively examining the evolution of male vocalizations and testes in howler monkeys. Dunn et al. [5] provide compelling evidence that male howler monkeys investing more in vocalizations produce less sperm. Their study capitalizes on the many characteristics that make
howler monkeys a particularly apt model for assessing evolutionary patterns of investment in vocalization. These primates produce extraordinarily loud calls using a specialized larynx with an enlarged hyoid bone, which contains a sound-amplifying air sac. The exact function of the howler monkey roar is unclear, but it almost certainly plays a role in male–male competition and demarcating territories. But while all howler monkey species howl, they vary considerably in hyoid bone size. And that’s not all: howler monkeys live in varying group sizes and environments and show remarkable sexual dimorphism in body size and wide variation in testicular investment among species (Figure 1). In short, these primates represent an evolutionary biologist’s dream for testing hypotheses about how selection shapes sexual traits. Armed with this extreme variation in sexual traits, Dunn et al. [5] used laser surface scanning to generate threedimensional representations of howler monkey hyoid bones to critically test the
R1084 Current Biology 25, R1070–R1091, November 16, 2015 ª2015 Elsevier Ltd All rights reserved
Current Biology
Dispatches recent prediction [4] that traits important in sexual selection before and after mating should exhibit evolutionary tradeoffs. They first demonstrated that males have a considerably larger hyoid than females, suggesting that sexual selection acts to increase hyoid size in males, relative to females, via an advantage during pre-mating competition. Next, the authors examined the number of males in a social group, a good proxy for the species-specific strength of sperm competition, given that females tend to copulate with multiple males when living in multi-male groups. Consistent with expectations, hyoid bone size was negatively related with the number of males in a social group, while species with more males in groups had relatively larger testes, a particularly well-known response to sperm competition. While hyoid bone and testes size were each associated with the pre-mating and post-mating strength of selection, respectively, Dunn et al. [5] importantly also found direct evidence for an evolutionary trade-off between the two: male investment in testes size decreases as their investment in hyoid bone size increases. That sexual selection is the most likely candidate for explaining the differential investment in hyoid bones and testes among howler monkeys was convincingly shown by excluding a competing hypothesis, namely that living in dense, sound-absorbing vegetation selects for lower frequency calls [6]. Dunn et al. [5] found no evidence that hyoid bone size is related with habitat structure, which sharpens their argument that sexual selection best explains their results. By providing the first evidence that vocal characteristics may trade off against investment in sperm production, Dunn et al.’s [5] findings demonstrate the degree to which males can creatively allocate their energetic budget to maximize reproductive success. Previous attempts to link investment in mate acquisition with investment in ejaculate traits have focused on sexual traits that are directly used in physical combat among males. For example, male pinnipeds (seals, sea lions and walruses) investing in larger body size to win fights between harem-holding males have smaller testes and genitals than closely related species that forgo such costly contest competition
Figure 1. A howler monkey in the Brazilian Amazon. A male Spix’s red-handed howler monkey (Alouatta discolor) with clearly visible testes descending a tree in the south-eastern Amazon in Brazil. A recent study [5] sheds light on the evolutionary link between male investment in testes and vocalizations in howler monkeys. (Photo: Charles Cruze, Wikimedia Commons.)
[7]. Similarly, across cetaceans (dolphins, porpoises and whales), species with the most prominent sexual dimorphism in size, teeth, tusks, and singing invest significantly less in testes size [8]. Such apparent trade-offs between investment in pre-mating traits and testes size (but not sperm size [9]) are sensitive to the degree to which males can monopolize access to females [10]. Males relax investment in testes size in species where male investment in sexual weaponry effectively monopolizes females. In contrast, when male investment in sexual weapons does not prevent females from mating with multiple males, males invest in both testes and weapons. In light of the sliding scale of positive to negative relationships observed between pre-mating weapons and testes size depending on the degree of female monopolization observed in a diverse array of taxa [10], Dunn et al.’s [5] demonstration of a negative relationship between vocal-tract and testicular dimensions in howler monkeys suggests investment in loud calls may effectively help monopolize females in these primates. Dunn et al. [5] also used an MRI-based approach to examine the vocal anatomy of howler monkeys and bioacoustic
methods to determine if hyoid volume influences the acoustic properties of a male’s howl. These analyses revealed that howler monkeys produce sound at a similar frequency to animals like tigers or reindeers that are more than an order of magnitude larger. The secret to their larger-than-life roars lies in the size of the voice-generating anatomy (hyoid bone and larynx). The overall effect of the howler monkeys’ highly modified vocal anatomy is that males appear acoustically larger than they actually are. While humans may be used to the idea of telling a white lie on a first date, male howler monkeys do something far more extreme — their vocalizations offer a deceptive advertisement of their body size. But while other species (and researchers!) may be fooled, it may not actually matter to other howler monkeys. Across species Dunn et al. [5] found hyoid volume to be independent of body size, but within species these two traits are positively related. Thus, at least within a species, male vocalization conveys honest, albeit exaggerated, information about relative male size. While the negative relationship between vocal and testes dimensions
Current Biology 25, R1070–R1091, November 16, 2015 ª2015 Elsevier Ltd All rights reserved R1085
Current Biology
Dispatches appears to be an evolutionary trade-off [5], it remains unclear how this works mechanistically. Howling may be so energetically expensive that males are unable to invest as much in testes. Alternatively, howling may represent an efficient means of deterring rival males and lowering sperm competition risk, and with it selection to increase testes size. Resolving these alternative mechanistic hypotheses is challenging, particularly at the macroevolutionary scale where varying energy budgets among species make it hard to contrast patterns of energy allocation with sexual traits. However, Dunn et al.’s [5] study pushes the door wide open to future studies assessing potential trade-offs among a wide range of sexually selected traits. Dunn et al. [5] are not the first to realize that something special is going on in howler monkeys, but they are in good company. Darwin [5] was fascinated by the sexual dimorphism in the ‘vocal organs’ of black howler monkeys and
how their calls ‘‘make the forest resound.with their overwhelming voices’’. Had Darwin realized he could gain insights into male investment in testicular tissue from the strength of these vocal organs, he would surely have enjoyed the howler monkeys’ resounding forest all the more for it. REFERENCES 1. Darwin, C. (1871). The Descent of Man and Selection in Relation to Sex (London: John Murray). 2. Birkhead, T.R., Hosken, D.J., and Pitnick, S. (2009). Sperm Biology: An Evolutionary Perspective (San Diego: Academic Press). 3. Parker, G.A. (1970). Sperm competition and its evolutionary consequences in the insects. Biol. Rev. 45, 526–567. 4. Parker, G.A., Lessells, C.M., and Simmons, L.W. (2013). Sperm competition games: a general model for pre-copulatory male-male competition. Evolution 67, 95–109. 5. Dunn, J.C., Halenar, L.B., Davies, T.G., Cristobal-Azkarate, J., Reby, D., Sykes, D.,
Dengg, S., Fitch, W.T., and Knapp, L.A. (2015). Evolutionary trade-off between vocal tract and testes dimensions in howler monkeys. Curr. Biol. 25, 2839–2844. 6. Morton, E.S. (1975). Ecological sources of selection on avian sounds. Am. Nat. 109, 17–34. 7. Fitzpatrick, J.L., Almbro, M., Gonzalez-Voyer, A., Kolm, N., and Simmons, L.W. (2012). Male contest competition and the coevolution of weaponry and testes in pinnipeds. Evolution 66, 3595–3604. 8. Dines, J.P., Mesnick, S.L., Ralls, K., MayCollado, L., Agnarsson, I., and Dean, M.D. (2015). A trade-off between precopulatory and postcopulatory trait investment in male cetaceans. Evolution 69, 1560–1572. 9. Lu¨pold, S., Simmons, L.W., Tomkins, J.L., and Fitzpatrick, J.L. (2015). No evidence for a trade-off between sperm length and male premating weaponry. J. Evol. Biol. http://dx. doi.org/10.1111/jeb.12742. 10. Lu¨pold, S., Tomkins, J.L., Simmons, L.W., and Fitzpatrick, J.L. (2014). Female monopolization mediates the relationship between pre- and postcopulatory sexual traits. Nat. Commun. 5, 3184.
Neurobiology: What Drives Flies to Sleep? Rozi Andretic Department of Biotechnology, University of Rijeka, R. Matejcic 2, 51000 Rijeka, Croatia Correspondence: [email protected] http://dx.doi.org/10.1016/j.cub.2015.08.062
The number of wake-promoting neurons in the Drosophila brain is relatively small, and only some of them have the unique ability to promote robust recovery sleep following wakefulness. In this issue of Current Biology, Seidner et al. [1] describe neurons in the Drosophila brain that likely hold clues about sleep regulation in general. We transition daily between sleep and wake, but how does our brain know when to fall asleep and when to wake up? The neurobiology of sleep regulation is an area of intense scientific research, and advances in this field will be important for managing problems that we all face daily — staying awake and attentive when we need to, and getting restorative sleep to function better. The current model of sleep regulation proposes two major sleep influences: homeostatic and circadian [2]. The
circadian clock regulates the speciesspecific optimal time to sleep; most humans fall asleep easier and sleep better at night. Homeostatic regulation causes sleep drive to increase during wakefulness and dissipate during sleep; i.e., staying awake makes us sleepy. While the principles of circadian regulation of sleep are fairly well understood, the neurobiology behind homeostatic regulation of sleep is mostly missing [3]. With analogy to the circadian clock, which is located in the suprachiasmatic nucleus of mammals, researchers refer to a sleep homeostat, which measures sleep pressure and triggers restorative sleep. However,
evidence is very sparse regarding its location and cellular and molecular mechanism. Recently, use of advanced transgenic techniques in Drosophila has led to new insights about homeostatic sleep regulation, and the new work by Seidner et al. is adding to this growing knowledge. Their work shows that not all wakepromoting neurons can trigger rebound sleep after extended wakefulness. Only certain neurons have the ability to respond to increased sleep need, such as the group of cholinergic neurons that they identify in this report. The authors show that the homeostatic sleep these neurons regulate is needed to remember better,
R1086 Current Biology 25, R1070–R1091, November 16, 2015 ª2015 Elsevier Ltd All rights reserved
Dispatches reflect marginal Utility. Curr. Biol. 24, 2491– 2500. 10. Nakahara, H., Itoh, H., Kawagoe, R., Takikawa, Y., and Hikosaka, O. (2004). Dopamine neurons can represent context-dependent prediction error. Neuron 41, 269–280. 11. Eshel, N., Bukwich, M., Rao, V., Hemmelder, V., Tian, J., and Uchida, N. (2015). Arithmetic and local circuitry underlying dopamine prediction errors. Nature 525, 243–246. 12. Sutton, R.S., and Barto, A.G. (1998). Reinforcement Learning: An Introduction (Cambridge, MA: The MIT Press). 13. Heeger, D.J. (1992). Normalization of cell responses in cat striate cortex. Vis. Neurosci. 9, 181–197.
14. Louie, K., Grattan, L.E., and Glimcher, P.W. (2011). Reward value-based gain control: divisive normalization in parietal cortex. J. Neurosci. 31, 10627–10639. 15. Omelchenko, N., and Sesack, S.R. (2009). Ultrastructural analysis of local collaterals of rat ventral tegmental area neurons: GABA phenotype and synapses onto dopamine and GABA cells. Synapse 63, 895–906. 16. Tan, K.R., Yvon, C., Turiault, M., Mirzabekov, J.J., Doehner, J., Laboue`be, G., Deisseroth, K., Tye, K.M., and Lu¨scher, C. (2012). GABA neurons of the VTA drive conditioned place aversion. Neuron 73, 1173–1183. 17. van Zessen, R., Phillips, J., Budygin, E., and Stuber, G. (2012). Activation of VTA GABA
neurons disrupts reward consumption. Neuron 73, 1184–1194. 18. Cohen, J.Y., Haesler, S., Vong, L., Lowell, B.B., and Uchida, N. (2012). Neuron-typespecific signals for reward and punishment in the ventral tegmental area - Supplementary Information. Nature 482, 85–88. 19. Watabe-Uchida, M., Zhu, L., Ogawa, S.K., Vamanrao, A., and Uchida, N. (2012). Whole-brain mapping of direct inputs to midbrain dopamine neurons. Neuron 74, 858–873. 20. Tian, J., and Uchida, N. (2015). Habenula lesions reveal that multiple mechanisms underlie dopamine prediction errors. Neuron 87, 1304–1316.
Evolution: Big Bawls, Small Balls John L. Fitzpatrick and Stefan Lu¨pold Computational and Evolutionary Biology, Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, UK Correspondence: [email protected] (J.L.F.), [email protected] (S.L.) http://dx.doi.org/10.1016/j.cub.2015.09.060
Males must carefully allocate the energy they devote to sex. A new study of howler monkeys shows that males who use vocalizations to ward off rivals invest less in producing large numbers of sperm. When it comes to sex, you can’t have it all. Animal reproduction is about compromising between the competing demands of finding a mate and successfully fertilizing eggs. Males, who are the sex that typically competes for access to mates, are particularly sensitive to this reproductive balancing act. In many animals, males show off ornate sexual colouration, ear-splitting vocalizations, and even dances to attract a female’s attention. While catching a female’s interest is a good first step toward successful reproduction, access to a receptive female often requires contending with, and outcompeting, rival males. Consequently, competition between males over access to females has in many species led to the evolution of male sexual weaponry (such as horns or antlers) or to massive divergence in body size between males and females. These conspicuous male sexual displays and weapons were instrumental in Darwin’s [1] formulation of sexual selection. We now know, however, that females of most species mate with multiple males
[2], which means that male–male competition over a female is followed by competition among their sperm to fertilize her egg(s) [3]. Competition among rival males before and after mating imposes a host of evolutionary constraints. Males have to allocate limited resources to both securing mates and producing ejaculates that are better at fertilizing eggs than rival males’ sperm. Thus, investment in whatever helps a male get a mate should limit investment in ejaculate traits crucial during sperm competition, and vice versa [4]. However, we know surprisingly little about how males balance their investment in sexual traits. In a study published recently in Current Biology, Dunn et al. [5] address this gap in our understanding of the evolution of male sexual traits by comprehensively examining the evolution of male vocalizations and testes in howler monkeys. Dunn et al. [5] provide compelling evidence that male howler monkeys investing more in vocalizations produce less sperm. Their study capitalizes on the many characteristics that make
howler monkeys a particularly apt model for assessing evolutionary patterns of investment in vocalization. These primates produce extraordinarily loud calls using a specialized larynx with an enlarged hyoid bone, which contains a sound-amplifying air sac. The exact function of the howler monkey roar is unclear, but it almost certainly plays a role in male–male competition and demarcating territories. But while all howler monkey species howl, they vary considerably in hyoid bone size. And that’s not all: howler monkeys live in varying group sizes and environments and show remarkable sexual dimorphism in body size and wide variation in testicular investment among species (Figure 1). In short, these primates represent an evolutionary biologist’s dream for testing hypotheses about how selection shapes sexual traits. Armed with this extreme variation in sexual traits, Dunn et al. [5] used laser surface scanning to generate threedimensional representations of howler monkey hyoid bones to critically test the
R1084 Current Biology 25, R1070–R1091, November 16, 2015 ª2015 Elsevier Ltd All rights reserved
Current Biology
Dispatches recent prediction [4] that traits important in sexual selection before and after mating should exhibit evolutionary tradeoffs. They first demonstrated that males have a considerably larger hyoid than females, suggesting that sexual selection acts to increase hyoid size in males, relative to females, via an advantage during pre-mating competition. Next, the authors examined the number of males in a social group, a good proxy for the species-specific strength of sperm competition, given that females tend to copulate with multiple males when living in multi-male groups. Consistent with expectations, hyoid bone size was negatively related with the number of males in a social group, while species with more males in groups had relatively larger testes, a particularly well-known response to sperm competition. While hyoid bone and testes size were each associated with the pre-mating and post-mating strength of selection, respectively, Dunn et al. [5] importantly also found direct evidence for an evolutionary trade-off between the two: male investment in testes size decreases as their investment in hyoid bone size increases. That sexual selection is the most likely candidate for explaining the differential investment in hyoid bones and testes among howler monkeys was convincingly shown by excluding a competing hypothesis, namely that living in dense, sound-absorbing vegetation selects for lower frequency calls [6]. Dunn et al. [5] found no evidence that hyoid bone size is related with habitat structure, which sharpens their argument that sexual selection best explains their results. By providing the first evidence that vocal characteristics may trade off against investment in sperm production, Dunn et al.’s [5] findings demonstrate the degree to which males can creatively allocate their energetic budget to maximize reproductive success. Previous attempts to link investment in mate acquisition with investment in ejaculate traits have focused on sexual traits that are directly used in physical combat among males. For example, male pinnipeds (seals, sea lions and walruses) investing in larger body size to win fights between harem-holding males have smaller testes and genitals than closely related species that forgo such costly contest competition
Figure 1. A howler monkey in the Brazilian Amazon. A male Spix’s red-handed howler monkey (Alouatta discolor) with clearly visible testes descending a tree in the south-eastern Amazon in Brazil. A recent study [5] sheds light on the evolutionary link between male investment in testes and vocalizations in howler monkeys. (Photo: Charles Cruze, Wikimedia Commons.)
[7]. Similarly, across cetaceans (dolphins, porpoises and whales), species with the most prominent sexual dimorphism in size, teeth, tusks, and singing invest significantly less in testes size [8]. Such apparent trade-offs between investment in pre-mating traits and testes size (but not sperm size [9]) are sensitive to the degree to which males can monopolize access to females [10]. Males relax investment in testes size in species where male investment in sexual weaponry effectively monopolizes females. In contrast, when male investment in sexual weapons does not prevent females from mating with multiple males, males invest in both testes and weapons. In light of the sliding scale of positive to negative relationships observed between pre-mating weapons and testes size depending on the degree of female monopolization observed in a diverse array of taxa [10], Dunn et al.’s [5] demonstration of a negative relationship between vocal-tract and testicular dimensions in howler monkeys suggests investment in loud calls may effectively help monopolize females in these primates. Dunn et al. [5] also used an MRI-based approach to examine the vocal anatomy of howler monkeys and bioacoustic
methods to determine if hyoid volume influences the acoustic properties of a male’s howl. These analyses revealed that howler monkeys produce sound at a similar frequency to animals like tigers or reindeers that are more than an order of magnitude larger. The secret to their larger-than-life roars lies in the size of the voice-generating anatomy (hyoid bone and larynx). The overall effect of the howler monkeys’ highly modified vocal anatomy is that males appear acoustically larger than they actually are. While humans may be used to the idea of telling a white lie on a first date, male howler monkeys do something far more extreme — their vocalizations offer a deceptive advertisement of their body size. But while other species (and researchers!) may be fooled, it may not actually matter to other howler monkeys. Across species Dunn et al. [5] found hyoid volume to be independent of body size, but within species these two traits are positively related. Thus, at least within a species, male vocalization conveys honest, albeit exaggerated, information about relative male size. While the negative relationship between vocal and testes dimensions
Current Biology 25, R1070–R1091, November 16, 2015 ª2015 Elsevier Ltd All rights reserved R1085
Current Biology
Dispatches appears to be an evolutionary trade-off [5], it remains unclear how this works mechanistically. Howling may be so energetically expensive that males are unable to invest as much in testes. Alternatively, howling may represent an efficient means of deterring rival males and lowering sperm competition risk, and with it selection to increase testes size. Resolving these alternative mechanistic hypotheses is challenging, particularly at the macroevolutionary scale where varying energy budgets among species make it hard to contrast patterns of energy allocation with sexual traits. However, Dunn et al.’s [5] study pushes the door wide open to future studies assessing potential trade-offs among a wide range of sexually selected traits. Dunn et al. [5] are not the first to realize that something special is going on in howler monkeys, but they are in good company. Darwin [5] was fascinated by the sexual dimorphism in the ‘vocal organs’ of black howler monkeys and
how their calls ‘‘make the forest resound.with their overwhelming voices’’. Had Darwin realized he could gain insights into male investment in testicular tissue from the strength of these vocal organs, he would surely have enjoyed the howler monkeys’ resounding forest all the more for it. REFERENCES 1. Darwin, C. (1871). The Descent of Man and Selection in Relation to Sex (London: John Murray). 2. Birkhead, T.R., Hosken, D.J., and Pitnick, S. (2009). Sperm Biology: An Evolutionary Perspective (San Diego: Academic Press). 3. Parker, G.A. (1970). Sperm competition and its evolutionary consequences in the insects. Biol. Rev. 45, 526–567. 4. Parker, G.A., Lessells, C.M., and Simmons, L.W. (2013). Sperm competition games: a general model for pre-copulatory male-male competition. Evolution 67, 95–109. 5. Dunn, J.C., Halenar, L.B., Davies, T.G., Cristobal-Azkarate, J., Reby, D., Sykes, D.,
Dengg, S., Fitch, W.T., and Knapp, L.A. (2015). Evolutionary trade-off between vocal tract and testes dimensions in howler monkeys. Curr. Biol. 25, 2839–2844. 6. Morton, E.S. (1975). Ecological sources of selection on avian sounds. Am. Nat. 109, 17–34. 7. Fitzpatrick, J.L., Almbro, M., Gonzalez-Voyer, A., Kolm, N., and Simmons, L.W. (2012). Male contest competition and the coevolution of weaponry and testes in pinnipeds. Evolution 66, 3595–3604. 8. Dines, J.P., Mesnick, S.L., Ralls, K., MayCollado, L., Agnarsson, I., and Dean, M.D. (2015). A trade-off between precopulatory and postcopulatory trait investment in male cetaceans. Evolution 69, 1560–1572. 9. Lu¨pold, S., Simmons, L.W., Tomkins, J.L., and Fitzpatrick, J.L. (2015). No evidence for a trade-off between sperm length and male premating weaponry. J. Evol. Biol. http://dx. doi.org/10.1111/jeb.12742. 10. Lu¨pold, S., Tomkins, J.L., Simmons, L.W., and Fitzpatrick, J.L. (2014). Female monopolization mediates the relationship between pre- and postcopulatory sexual traits. Nat. Commun. 5, 3184.
Neurobiology: What Drives Flies to Sleep? Rozi Andretic Department of Biotechnology, University of Rijeka, R. Matejcic 2, 51000 Rijeka, Croatia Correspondence: [email protected] http://dx.doi.org/10.1016/j.cub.2015.08.062
The number of wake-promoting neurons in the Drosophila brain is relatively small, and only some of them have the unique ability to promote robust recovery sleep following wakefulness. In this issue of Current Biology, Seidner et al. [1] describe neurons in the Drosophila brain that likely hold clues about sleep regulation in general. We transition daily between sleep and wake, but how does our brain know when to fall asleep and when to wake up? The neurobiology of sleep regulation is an area of intense scientific research, and advances in this field will be important for managing problems that we all face daily — staying awake and attentive when we need to, and getting restorative sleep to function better. The current model of sleep regulation proposes two major sleep influences: homeostatic and circadian [2]. The
circadian clock regulates the speciesspecific optimal time to sleep; most humans fall asleep easier and sleep better at night. Homeostatic regulation causes sleep drive to increase during wakefulness and dissipate during sleep; i.e., staying awake makes us sleepy. While the principles of circadian regulation of sleep are fairly well understood, the neurobiology behind homeostatic regulation of sleep is mostly missing [3]. With analogy to the circadian clock, which is located in the suprachiasmatic nucleus of mammals, researchers refer to a sleep homeostat, which measures sleep pressure and triggers restorative sleep. However,
evidence is very sparse regarding its location and cellular and molecular mechanism. Recently, use of advanced transgenic techniques in Drosophila has led to new insights about homeostatic sleep regulation, and the new work by Seidner et al. is adding to this growing knowledge. Their work shows that not all wakepromoting neurons can trigger rebound sleep after extended wakefulness. Only certain neurons have the ability to respond to increased sleep need, such as the group of cholinergic neurons that they identify in this report. The authors show that the homeostatic sleep these neurons regulate is needed to remember better,
R1086 Current Biology 25, R1070–R1091, November 16, 2015 ª2015 Elsevier Ltd All rights reserved
