Why Is Sex Determined by Nest Temperature in Many Reptiles? Richard Shine

REVIEWS Why is sex determined by nest temperature in many reptiles? Richard Shine

nvironmental sex determi- Temperature-dependent sex determination preceding ones and seems the nation (ESD) occurs in (TSD) is widespread in reptiles but its most consistent with available both plants and animals, adaptive significance remains data3,9,16. However, it has not Eand has stimulated consid- controversial. The most plausible been widely appreciated that the erable research and speculation theoretical models for the evolution of TSD ‘differential fitness’ models actu- from evolutionary biologists1–3. rely upon differential fitness of male and ally include a considerable diver- Although some examples of ESD female offspring incubated under different sity of separate hypotheses that fit well with theory1,4–8, other cases thermal regimes. Several mechanisms advocate quite different mecha- remain an evolutionary enigma might generate these fitness differentials, nisms as to why sex and incu- and continue to be the focus and recent work provides empirical bation temperatures can have an of significant controversy3,6,9–12. support for some of them. interactive effect on organismal Temperature-dependent sex deter- fitness. Table 1 presents a classifi- mination (TSD) in reptiles pro- cation of these models and an vides such a case. Several alterna- Richard Shine is at the School of Biological Sciences, attempt to characterize assump- tive explanations for the nature of Building A08, The University of Sydney, NSW 2006, tions underlying each of them. Australia ([email protected]). the adaptive advantage conferred The six models listed in Table 1 by TSD have been proposed4,13,14. differ in significant ways. For ex- All of these hypotheses invoke a ample, the phenotypic variance series of assumptions, most of that is acted upon by selection is which had little empirical support at the time the hypoth- generated by fundamentally different mechanisms: by eses were generated. This situation has now changed con- maternal effects (egg size) in the ‘size-matching’ model siderably, with a spate of recent papers describing experi- (Table 1a); by seasonal shifts in hatching date in the ‘time- mental studies that clarify a fundamental underpinning of matching’ model (Table 1b); by incubation temperatures the theoretical models – reaction norms of reptilian embryos (with the sexes showing identical reaction norms) in the in response to the physical conditions under which they ‘nest-philopatry’ and ‘phenotype-matching’ models (Table are incubated. Hence, we can now look more closely at 1c,d); and by the interaction between sex and incubation competing hypotheses for the adaptive significance of TSD temperature (with the sexes following different norms of in reptiles. My aim is to identify the unique features of each reaction) in the ‘interaction’ models (Table 1e,f). The puta- of these hypotheses and evaluate the assumptions upon tive mechanisms by which organismal fitness is affected which they are based in the light of recent experimental also vary, and might involve differential mortality (either in studies. In particular, how do incubation temperatures the egg or in later life-history stages) or phenotypic modifi- influence organismal fitness in these animals? cations that translate into viability differences (again, at a range of life-history stages) between males and females Hypotheses for the adaptive significance of TSD from different incubation conditions. Although many authors have speculated on the evolu- All of the models posit links between incubation tem- tionary advantages of TSD in reptiles, the hypotheses fall perature and phenotype, and between phenotype and fit- into a few clear categories: ness. All rely on males and females differing in the overall • Phylogenetic inertia: TSD is an ancestral form of sex relationship between incubation temperature and fitness, determination with no current adaptive significance. This but this sex difference is suggested to emerge in different hypothesis is difficult to test but seems inconsistent with the ways. For some, the link that differs between the sexes is that phylogenetic lability of sex-determining systems in reptiles13. between phenotype and fitness (Table 1a–d), whereas oth- • Group-adaptation: TSD has evolved because it facilitates ers rely on sex differences in the effects of incubation tem- group fitness through sex-ratio skewing13. This hypothesis perature on phenotype (the interaction models, Table 1e,f). relies upon the onerous assumptions inherent in group selection15, is an argument for sex-ratio bias rather than Validity of the assumptions any specific sex-determining system and is not consistent How realistic are the assumed mechanisms linking with available data on population structure of TSD reptiles14. incubation temperature to male versus female fitness in • Inbreeding avoidance: by resulting in single-sex clutches these models? This empirical question can be asked as eas- from natural nests, TSD reduces deleterious effects of ily of a species with genetic sex determination (GSD) as inbreeding among siblings13. This hypothesis is inconsist- one with TSD. Indeed, it is notable that the adaptationist ent with the prevalence of TSD in long-lived iteroparous hypotheses outlined in Table 1 bear strong similarities in organisms (such as crocodilians and many turtles), where logical structure to analogous hypotheses for sex-ratio offspring from many annual cohorts interbreed14. manipulation in GSD species. In many cases, the hypoth- • Differential fitness: if incubation temperatures differen- eses based on GSD sex-ratio manipulation have substantial tially affect the fitness of male and female offspring, TSD empirical support. For example, the size-matching model can enhance maternal fitness by enabling the embryo to (Table 1a, sex-ratio adjustment relative to offspring size) develop as the sex best-suited to those incubation condi- bears a strong similarity to explanations for the ability of tions1. This hypothesis is logically more robust than the female invertebrates (e.g. hymenopterans17) to adjust

Table 1. An overview of ‘differential fitness’ models for the adaptive significance of temperature-dependent sex determination (TSD) in reptiles a

Source of phenotypic Why does incubation temperature Role of incubation temperature variation that engenders affect fitness differently in Model (apart from sex determination) fitness variation males and females? Refs

(a) Matching sex to egg size None: simply enables female to Maternal effects (egg size) Egg size affects fitness differently 12 determine sex ratio of her offspring in males and females, and TSD enables mother to adjust sex ratio relative to offspring size. (b) Matching sex to time of hatching Correlate (predictor) of seasonal Seasonal timing of hatching Hatching time affects fitness 8 timing of hatching differently in males and females, and TSD enables mother to match offspring sex ratio to time of hatching. (c) Nest philopatry Influences offspring fitness Incubation temperature Because of sex differences in philopatry, 44 (e.g. survival, growth and viability) daughters derive more advantage from independent of sex a ‘good’ nest (which they eventually use themselves) than do sons. (d) Matching sex to phenotype Induces changes to phenotype, Incubation temperature Incubation temperature affects 3 independent of sex phenotype, and phenotypic determinants of fitness differ between males and females. (e) Sex by temperature interaction Influences survival of eggs Interaction between Differential mortality of males and 42 for offspring survival or hatchlings, but differently incubation temperature females from different incubation in males and females and sex temperatures. (f) Sex by temperature interaction Induces changes to phenotype, Interaction between Thermally induced phenotypes affect 16,25 for offspring phenotypes but differently in males and females incubation temperature fitness, with the relationship between and sex phenotype and fitness identical in both sexes.

aNote that references are examples only and do not necessarily refer to the first mention of the model in published literature.

offspring gender based on the probable size of their off- for example, the occurrence of sex-specific philopatry spring at birth. The time-matching model (Table 1b, sex- (nest-philopatry model, Table 1c). The following assump- ratio adjustment relative to date of hatching) resembles tions are of particular interest. adaptive explanations for the correlation between hatching date and offspring sex ratio in GSD organisms as Phenotype at hatching affects organismal fitness diverse as birds18 and fig wasps2. The nest-philopatry A link between hatchling traits and lifetime reproduc- model (Table 1c, sex-ratio adjustment in relation to sex dif- tive success (LRS) is assumed by many adaptationist mod- ferences in the degree of philopatry and consequently in els, including all those listed in Table 1. (Model e, the the probability of ‘inheritance’ of the nest site) is analo- sex-by-temperature-interaction’ model, which relies upon gous to sex-ratio manipulations in some primate societies, mortality prior to hatching, offers a possible exception whereby the nondispersing sex (daughter) is overpro- because differential incubation-dependent fitness is duced by high-status females that can transfer their status already expressed at hatching and need not occur later in to their nondispersing offspring19. The phenotype-matching the life history. However, the issue is a matter of definition, model (Table 1d, overproduction of the sex that and whether one is prepared to accept ‘hatches versus derives the greatest benefit from the available phenotypic does not hatch’ as a phenotypic trait of the embryo.) conditions at hatching) is similar to a situation in verte- Several studies have documented phenotypic correlates brates in which offspring sex ratio is manipulated relative of survival rates in hatchling reptiles under field condi- to maternal (and thus offspring) condition19,20. It is more tions10,21–24 but the generality of such effects remains difficult to find parallels with the ‘interaction’ models unknown. Although the models listed in Table 1 generally (Table 1e,f), because they invoke a direct interaction focus on phenotypic traits that are potentially measurable between sex and incubation temperature. Importantly, at hatching, the models do not require that all phenotypic the models listed in Table 1 do not provide mutually exclu- traits affecting LRS are expressed early in ontogeny. sive explanations, and it is theoretically possible for all Traits that were masked until maturation (e.g. the degree six of these models to apply simultaneously to the same of elaboration of secondary sexual characteristics) would population. suit the models just as easily as traits such as offspring I focus on differential-fitness (Charnov–Bull) models size or shape25. rather than alternative approaches (such as group adap- One phenotypic trait of particular interest is time at tation and sib-avoidance), for reasons already mentioned. hatching – the central feature of the time-matching model In the following sections, I consider the models listed (Table 1b). Field studies on many kinds of organism have in Table 1 and discuss the empirical support available revealed strong fitness advantages associated with hatch- for assumptions that are invoked by each of them. Any ing early in the season26–28. The most convincing empirical adaptationist hypothesis is based upon a massive array support for adaptive advantages to TSD involves time: of assumptions about the natural world and the efficacy of early-hatching fish (Menidia menidia) grow larger before natural selection, and consequently I can deal with only breeding, and the consequent size-induced increase in LRS a small subset of them. Hence, I will highlight assumptions as a result of early hatching can be higher for females than concerning the ways in which incubation temperatures for males7,8. Under these conditions, the observed produc- might influence organismal fitness in reptiles, rather than, tion of females early in the season (in response to water

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temperatures at that time) might offer a fitness advantage Male and female phenotypes at hatching are affected to reproducing females that exhibit TSD rather than GSD differently by incubation temperature (Refs 7,8). Similar fitness advantages from early hatching This assumption is inherent in only two of the models are plausible for many reptilian systems29. It will often be listed in Table 1: the sex-by-temperature-interaction mod- true that hotter nests hatch earlier than cooler nests, at els (e and f). Somewhat ironically, the best data in support least in those species with a single clutch per year, and of this assumption come from studies on GSD snakes and that ‘early’ and ‘late’ nests will differ thermally in species lizards. Burger and Zappalorti42 demonstrated sex-specific with multiple clutches. Hence, covariation between hatch- mortality of snake eggs at different incubation tempera- ing date and nest temperature is likely to be common in tures. The phenotypic responses of hatchling scincid lizards field populations of reptiles. (Bassiana duperreyi) to incubation temperature indicate that males and females have different thermal optima Phenotype at hatching affects organismal fitness for incubation16. In one TSD species of gekkonid lizard differently in males and females (Eublepharis macularius), growth rates are affected by both The first four models listed in Table 1 make this sex and incubation temperature25. assumption. Field studies provide abundant evidence that phenotypic traits contribute differently to LRS in males Prospects and females, and hence that selection pressures on pheno- Overall, there are several plausible pathways by which typic traits can differ substantially between conspecific differential-fitness models might apply to reptilian popu- males and females30. However, I do not know of any empiri- lations and favor the evolution of TSD. Although many cal data on reptiles to link hatchling phenotypes to event- assumptions of these models remain untested, the general ual LRS. Also, theoretical models that posit an adaptive sig- impression from recent work is that incubation tempera- nificance of TSD require that this relationship differs not tures can substantially affect organismal traits that are only between the sexes, but also in such a way that TSD likely to be related to LRS; and that the nature of this effect will maximize offspring fitness. It remains possible that could differ between the sexes. These results are encour- incubation conditions affect phenotypes and that pheno- aging for those who advocate an adaptive significance for types affect LRS – but that the direction(s) of those influ- TSD in reptiles. ences do not result in maternal fitness being maximized by Stronger tests among competing models will require the particular pattern of sex ratio versus temperature data sets on the nature of the link between incubation envi- generated by TSD. ronment and LRS. Two problems that precluded such work have recently been solved. First is the discovery of increas- Incubation temperature affects the phenotype at ing numbers of model systems that are more amenable to hatching experimentation than those that have been used tradition- This assumption is clearly part of three models in Table 1 ally. Initial research on the adaptive significance of TSD (c, nest-philopatry; d, phenotype-matching; and f, the sex- focused primarily on crocodilians and turtles. The delayed by-temperature-interaction model that deals with pheno- maturation and long reproductive lifespan of these animals typic traits). An additional model (the sex-by-temperature- (with other logistical constraints) effectively precluded interaction model that relies upon differential mortality experimental studies of the ways that incubation condi- prior to hatching, Table 1e) also fits into this category tions might influence organismal fitness. The discovery of because the differential mortality is presumably a result of TSD in shorter-lived organisms6,25,43 overcomes this ob- some unmeasured component of the phenotype. Recent stacle. The second problem is the confounding effect of sex work supports the generality of incubation effects on and incubation temperature (i.e. some temperatures pro- hatchling phenotypes in many reptilian taxa, including duce males, others produce females), making it difficult to lizards, snakes, crocodilians and turtles. These studies separate temperature effects from gender effects. Hor- include work on both GSD species31–35 and TSD species36–38. monal manipulation can break this link3,25 and thus provide Most of the data for TSD species can be challenged on the an opportunity for direct experimentation to test the cen- grounds that apparent effects of incubation temperatures tral tenet of adaptationist models in this field: the notion are (or might be) a secondary consequence of sex. How- that a female who can adjust her offspring’s sex to their ever, recent work has used hormonal manipulation to incubation temperature can thereby enhance her own override thermal cues for sex determination and clearly genetical fitness. demonstrate direct effects of incubation temperatures on growth rates of juvenile turtles in a TSD species3. Acknowledgements Nonetheless, many questions have yet to be resolved. I thank P. Harlow and M. Elphick for discussions and the For most of these systems, we do not know if biologically Australian Research Council for financial support. meaningful levels of phenotypic variation are generated by the range of nest temperatures that the organisms experi- References ence under natural conditions39–41. Natural nests might not 1 Charnov, E.L. and Bull, J.J. (1977) When is sex environmentally vary enough to induce long-lasting phenotypic effects on determined? Nature 266, 828–830 traits that actually influence LRS. Testing this assumption 2 Charnov, E.L. (1982) The Theory of Sex Allocation, Princeton remains a major challenge for field biologists. University Press Thermally induced differences in survival rates of eggs 3 Rhen, T. and Lang, J.W. (1995) Phenotypic plasticity for growth in natural nests might also constitute an important source in the common snapping turtle: effects of incubation of temperature-induced differentials in organismal fitness. temperature, clutch, and their interaction, Am. Nat. 146, 726–747 However, the magnitude of variance in egg survival intro- 4 Bull, J.J. (1980) Sex determination in reptiles, Q. Rev. Biol. 55, 4–21 duced by temperature could be minor compared with the 5 Bull, J.J. and Charnov, E.L. (1988) How fundamental are Fisherian variance introduced by other factors (e.g. soil moisture sex-ratios? in Oxford Surveys in Evolutionary Biology (No. 5) and predation). Further data on the determinants of hatch- (Harvey, P.H. and Partridge, L., eds), pp. 96–135, Oxford ing success in natural nests are needed. University Press

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