Fitness disadvantages to disrupted embryogenesis impose selection against suboptimal nest-site choice by female grass snakes, Natrix natrix (Colubridae)

Phenotypic traits of hatchling reptiles are strongly influenced by incubation regimes (e.g. of temperature and moisture), suggesting that maternal choice of suitable nest-sites should be under intense selection. Our laboratory incubation of 209 eggs (17 clutches) from wild-caught Swedish grass snakes (Natrix natrix) showed that scale abnormalities (half-scales on one side of the body, often reflecting lateral asymmetry in the number of ribs) occurred more frequently if eggs were incubated under cooler conditions. Especially at low incubation temperatures, individuals with scale asymmetries took longer to hatch than did symmetric conspecifics, were smaller in body length at hatching and were slower in trials of locomotor speed. Anti-predator tactics also covaried with scale asymmetry. These patterns suggest that individuals with asymmetric scales should have lower fitness and hence should rarely survive to adulthood in the wild. We tested this prediction by examining 201 field-collected snakes from museum collections. As predicted, scale asymmetries were seen primarily in small snakes, and rarely in larger animals. We interpret these data to suggest that scale asymmetries in this species offer an index of developmental instability and that fitness disadvantages to disrupted embryogenesis impose selection against suboptimal nest-site choice by females.     

Effects of sex, body size, temperature, and location on the antipredator tactics of free-ranging gartersnakes (Thamnophis sirtalis, Colubridae)

Gartersnakes (Thamnophis sirtalis parietalis) in southern Manitoba are subject to intense predation (primarily by crows) duringtheir spring breeding season. The huge numbers of snakes providea unique opportunity to quantify behavioral traits. We simulatedpredator attacks by "pecking" more than 500 free-ranging snakes,to explore the determinants of snake response. Snakes respondedto a human finger in the same way as they did to a more realisticstimulus (a model crow). A snake's response to attack dependedon several factors, which interacted in complex ways. The primaryinfluences on response were body temperature (warmer snakes tended to flee, whereas colder snakes remained cryptic or flattenedand/or gaped and struck) and sex (males were more likely toflee). Responses also depended on microhabitat (i.e., insidethe winter den versus in adjacent grassland) and on the snake'sprior activity (e.g., courting snakes often ignored our closeapproach). These factors interacted in significant ways; for example, snakes outside the den were smaller and warmer thanthose inside, male snakes were smaller and warmer than females,and mean body temperatures were higher in larger snakes withineach sex. Thus, a snake's body size and its location affectedits defensive response indirectly (via their influence on bodytemperature). Our results differ from those of previous studiesand suggest that antipredator responses in these animals dependin a flexible and complex way upon biotic and abiotic variables.Interactions among these variables also must be consideredbefore we can identify underlying causal processes.     

Repeatability and heritability of reproductive traits in free-ranging snakes

The underlying genetic basis of life-history traits in free-ranging animals is critical to the effects of selection on such traits, but logistical constraints mean that such data are rarely available. Our long-term ecological studies on free-ranging oviparous snakes (keelbacks, Tropidonophis mairii (Gray, 1841), Colubridae) on an Australian floodplain provide the first such data for any tropical reptile. All size-corrected reproductive traits (egg mass, clutch size, clutch mass and post-partum maternal mass) were moderately repeatable between pairs of clutches produced by 69 female snakes after intervals of 49-1152 days, perhaps because maternal body condition was similar between clutches. Parent-offspring regression of reproductive traits of 59 pairs of mothers and daughters revealed high heritability for egg mass (h2= 0.73, SE=0.24), whereas heritability for the other three traits was low (< 0.37). The estimated heritability of egg mass may be inflated by maternal effects such as differential allocation of yolk steroids to different-sized eggs. High heritability of egg size may be maintained (rather than eroded by stabilizing selection) because selection acts on a trait (hatchling size) that is determined by the interaction between egg size and incubation substrate rather than by egg size alone. Variation in clutch size was mainly because of environmental factors (h2=0.04), indicating that one component of the trade-off between egg size and clutch size is under much tighter genetic control than the other. Thus, the phenotypic trade-off between egg size and egg number in keelback snakes occurs because each female snake must allocate a finite amount of energy into eggs of a genetically determined size.     

Parental Influences on Pathogen Resistance in Brown Trout Embryos and Effects of Outcrossing within a River Network

Phenotypic plasticity can increase tolerance to heterogeneous environments but the elevations and slopes of reaction norms are often population specific. Disruption of locally adapted reaction norms through outcrossing can lower individual viability. Here, we sampled five genetically distinct populations of brown trout (Salmo trutta) from within a river network, crossed them in a full-factorial design, and challenged the embryos with the opportunistic pathogen Pseudomonas fluorescens. By virtue of our design, we were able to disentangle effects of genetic crossing distance from sire and dam effects on early life-history traits. While pathogen infection did not increase mortality, it was associated with delayed hatching of smaller larvae with reduced yolk sac reserves. We found no evidence of a relationship between genetic distance (W, F_ST) and the expression of early-life history traits. Moreover, hybrids did not differ in phenotypic means or reaction norms in comparison to offspring from within-population crosses. Heritable variation in early life-history traits was found to remain stable across the control and pathogen environments. Our findings show that outcrossing within a rather narrow geographical scale can have neutral effects on F₁ hybrid viability at the embryonic stage, i.e. at a stage when environmental and genetic effects on phenotypes are usually large.     

Behavioral thermal tolerances of free-ranging rattlesnakes (Crotalus oreganus) during the summer foraging season

Increasing temperature due to climate change is one of the greatest challenges for wildlife worldwide. Behavioral data on free-ranging individuals is necessary to determine at what temperatures animals modify activity as this would determine their capacity to continue to move, forage, and mate under altered thermal regimes. In particular, high temperatures could limit available surface activity time and time spent on fitness-related activities. Conversely, performance, such as feeding rate, can increase with temperature potentially having positive fitness effects. Here, we examine how the hunting behaviors of free-ranging Northern Pacific Rattlesnakes (Crotalus oreganus) associate with air temperature and body temperature. We continuously recorded snakes in the field using videography, capturing behaviors rarely considered in past studies such as movements in and out of refuge and strikes on prey. We found that as mean daily air temperature increased, hunting activity and the likelihood of hunting at night decreased, while the number of movements and distance moved per day increased. Snakes typically retreated to refuge before body temperatures reached 31 °C. Body temperatures of snakes hunting on the surface were lower compared to temperatures of non-hunting snakes in refuge in the morning, while this relationship was inverted in the afternoon. Snake body size influenced the disparity of these temperatures. Finally, strike initiation and success occurred across a wide range of body temperatures, indicating hunting performance may not be strongly constrained by temperature. These results on the temperatures at which free-ranging rattlesnakes exhibit fitness-related behaviors could be valuable for understanding their vulnerabilities to future climates.     

Developmental success, stability, and plasticity in closely related parthenogenetic and sexual lizards (Heteronotia, Gekkonidae)

The developmental trajectory of an organism is influenced by the interaction between its genes and the environment in which it develops. For example, the phenotypic traits of a hatchling reptile can be influenced by the organism's genotype, by incubation temperature, and by genetically coded norms of reaction for thermally labile traits. The evolution of parthenogenesis provides a unique opportunity to explore such effects: a hybrid origin of this trait in vertebrates modifies important aspects of the genotype (e.g., heterozygosity, polyploidy) and may thus impact not only on the phenotype generally, but also on the ways in which incubation temperature affects expression of the phenotype. The scarcity of vertebrate parthenogenesis has been attributed to developmental disruptions, but previous work has rarely considered reaction norms of embryogenesis in this respect. We used closely related sexual and asexual races of the Australian gecko Heteronotia binoei, which include those with multiple origins of parthenogenesis, to explore the ways in which reproductive modes (sexual, asexual), incubation temperatures (24, 27, and 30 degrees C), and the interaction between these factors affected hatchling phenotypes. The hatchling traits we considered included incubation period, incidence of deformities, hatchling survivorship, body size and shape, scalation (including fluctuating asymmetry), locomotor performance, and growth rate. Developmental success was slightly reduced (higher proportion of abnormal offspring) in parthenogenetic lineages although there was no major difference in hatching success. Incubation temperature affected a suite of traits including incubation period, tail length, body mass relative to egg mass, labial scale counts, running speed, growth rate, and hatchling survival. Our data also reveal an interaction between reproductive modes and thermal regimes, with the phenotypic traits of parthenogenetic lizards less sensitive to incubation temperature than was the case for their sexual relatives. Thus, the evolution of asexual reproduction in this species complex has modified both mean hatchling viability and the norms of reaction linking hatchling phenotypes to incubation temperature. Discussions on the reasons why parthenogenetic organisms are scarce in nature should take into account interactive effects such as these; future work could usefully try to tease apart the roles of parthenogenesis, its hybrid origin (and thus effects on ploidy and heterozygosity, etc.), and clonal selection in generating these divergent embryonic responses.     

Natural selection and genetic variation for reproductive reaction norms in a wild bird population

Many morphological and life-history traits show phenotypic plasticity that can be described by reaction norms, but few studies have attempted individual-level analyses of reaction norms in the wild. We analyzed variation in individual reaction norms between laying date and three climatic variables (local temperature, local rainfall, and North Atlantic Oscillation) of 1126 female collared flycatchers (Ficedula albicollis) with a restricted maximum likehood linear mixed model approach using random-effect best linear unbiased predictor estimates for the elevation (i.e., expected laying date in the average environment) and slope (i.e., adjustment in laying date as a function of environment) of females' reaction norms. Variation in laying date was best explained by local temperature, and individual females differed in both the elevation and the slope of their laying date-temperature reaction norms. As revealed by animal model analyses, there was weak evidence for additive genetic variance of elevation (h2 +/- SE = 0.09 +/- 0.09), whereas there was no evidence for heritability of slope (h2 +/- SE = 0.00 +/- 0.01). Selection analysis, using a female's lifetime production of fledglings or recruits as an estimate of her fitness, revealed significant selection for a lower phenotypic value and breeding value for elevation (i.e., earlier laying date at the average temperature). There was selection for steeper phenotypic values of slope (i.e., greater plasticity in the adjustment of laying date to temperature), but no significant selection on the breeding values of slope. Although these results suggest that phenotypic laying date is influenced by additive genetic factors, as well as by an interaction with the environment, selection on plasticity would not produce an evolutionary response.     

The genetics of phenotypic plasticity. IX. Genetic architecture, temperature, and sex differences in Drosophila melanogaster

Abstract.— We examined the genetic architecture of plasticity of thorax and wing length in response to temperature in Drosophila melanogaster . Reaction norms as a function of growth temperature were analyzed in 20 isofemale lines in a natural population collected from Grande Ferrade near Bordeaux (southern France) in two different years. We found evidence for a complex genetic architecture underlying the reaction norms and differences between males and females. Reaction norms were negative quadratics. Genetic correlations were moderately high between traits within environments. Among characteristic values, the magnitudes of genetic correlations varied among traits and sexes. We hypothesized that genetic correlations among environments would decrease as temperatures became more different. This expectation was upheld for only one trait, female thorax length. For males for both traits, the correlations were large for both very similar and very different temperatures. These correlations may constrain the evolution of the shape of the reaction norms. Whether the extent of independence implies specific regulatory genes or only a specific allelic regulation of trait genes can not be decided from our results.     

An empirical test for a zone of canalization in thermal reaction norms

Theoretical models on the evolution of phenotypic plasticity predict a zone of canalization where reaction norms cross, and genetic variation is minimized in the environment a population most frequently encounter. Empirical tests of this prediction are largely missing, in particular for life‐history traits. We addressed this prediction by quantifying thermal reaction norms of three life‐history traits (somatic growth rate, age and size at maturation) of a Norwegian population of Daphnia magna and testing for the occurrence of an intermediate temperature (T_m) at which genetic variance in the traits is minimized. Size at maturation changed relatively little with temperature compared to the other traits, and there was no genetic variance in the shape of the reaction norm. Consequently, age at maturation and somatic growth rate were strongly negatively correlated. Both traits showed a strong genotype–environment interaction, and the estimated T_m was 14 °C for both age at maturation and growth rate. This value of T_m corresponds well with mean summer temperatures experienced by the population and suggests that the population has evolved under stabilizing selection in temperatures that fluctuate around this mean temperature. These results suggest local adaptation to temperature in the studied population and allow predicting evolutionary trajectories of thermal reaction norms under changing thermal regimes.     

Immune Profile Predicts Survival and Reflects Senescence in a Small,Long-Lived Mammal,the Greater Sac-Winged Bat (Saccopteryx bilineata)

The immune system imposes costs that may have to be traded against investment of resources in other costly life-history traits. Yet, it is unknown if a trade-off between immunity and longevity occurs in free-ranging mammals. Here, we tested if age and survival, two aspects associated with longevity, are linked to immune parameters in an 8 g bat species. Using a combination of cross-sectional and longitudinal data, we assessed whether total white blood cell (WBC) counts, bacterial killing ability of the plasma (BKA) and immunoglobulin G (IgG) concentration change with age. Furthermore, we asked if these immune parameters impose costs resulting in decreased survival probabilities. We found that WBC counts decreased with age both within and among individuals. IgG concentrations were higher in older individuals, but did not change with age within individuals. Furthermore, individuals with above average WBC counts or IgG concentration had lower probabilities to survive the next six months. High WBC counts and IgG concentrations may reflect infections with parasites and pathogens, however, individuals that were infected with trypanosomes or nematodes showed neither higher WBC counts or IgG concentrations, nor was infection connected with survival rates. BKA was higher in infected compared with uninfected bats, but not related to age or survival. In conclusion, cellular (WBC) and humoral (IgG) parts of the immune system were both connected to age and survival, but not to parasite infections, which supports the hypothesis that energetically costly immunological defences are traded against other costly life-history traits, leading to a reduced lifespan in this free-ranging mammal.     

Reptilian endothermy: a field study of thermoregulation by brooding diamond pythons

Miniature temperature-sensitive radiotransmitters were surgically implanted into free-ranging adult diamond pythons ( Morelia s. spilota ), which are medium-sized boid snakes of south-eastern Australia. Four female pythons oviposited during the study, and constructed incubation mounds. These apparently provided excellent insulation, and the snakes maintained high (approx. 31 C) and relatively constant body temperatures throughout the two-month incubation period. They apparently maintained these temperatures primarily by endogenous heat production (shivering thermogenesis), but also basked briefly on most mornings.
Brooding females maintained a body temperature differential above ambient of about 9C, occasionally up to 13C; their temperatures were significantly higher amd less variable than those of non-brooding females or males. The energetic cost of brooding must be high, but these costs may be outweighed by the benefits of rapid embryonic development and high embryonic survivorship.     

GENETIC VARIATION FOR PHENOTYPIC PLASTICITY IN THE LARVAL LIFE HISTORY OF SPADEFOOT TOADS (SCAPHIOPUS COUCHII)

Phenotypic plasticity in life-history traits is common. The relationship between phenotype and environment, or reaction norm, associated with life-history plasticity can evolve by natural selection if there is genetic variation within a population for the reaction norm and if the traits involved affect fitness. As with other traits, selection on plasticity in a particular trait or in response to a particular environmental factor may be constrained by trade-offs with other traits that affect fitness. In this paper, I experimentally evaluated broad-sense genetic variation in the reaction norms of age and size at metamorphosis in response to two environmental factors, food level and temperature. Differences among full-sib families in one or both traits were evident in all treatments. However, variation among families in their responses to each treatment (genotype-environment interaction) resulted in variation among treatments in estimated heritabilities and genetic correlations. Age at metamorphosis was equally sensitive to temperature in all families, but size at metamorphosis was more sensitive to temperature in some families than in others. Size at metamorphosis was equally sensitive to food level in all families, but age at metamorphosis was sensitive to food in some families but not in others. At high temperature or low food, the genetic correlation between age and size at metamorphosis was positive, generating a potential trade-off between metamorphosing early to attain higher larval survival and metamorphosing later to achieve larger size. This trade-off extends across treatments: families with the largest average size at metamorphosis achieved larger size with the longest average and greatest plasticity in age at metamorphosis. Other families achieved shorter average larval periods by exhibiting greater plasticity in size at metamorphosis but had the smallest average size at metamorphosis. This trade-off may reflect an underlying functional constraint on the ability to respond optimally to all environments, resulting in persistent genetic variation in reaction norms.     

Effects of food supplementation on the physiological ecology of female Western diamond-backed rattlesnakes (<Emphasis Type="Italic">Crotalus atrox</Emphasis>)

Food availability is an important factor in the life histories of organisms because it is often limiting and thus can affect growth, mass change, reproduction, and behaviors such as thermoregulation, locomotion, and mating. Experimental studies in natural settings allow researchers to examine the effects of food on these parameters while animals are free to behave naturally. The wide variation among organisms in energy demands and among environmental food resources suggest that responses to changes in food availability may vary among organisms. Since most supplemental feeding field experiments have been conducted on species with high energy demands, we conducted a supplemental feeding study on free-ranging, female Western diamond-backed rattlesnakes (Crotalus atrox), a species with low energy demands and infrequent reproductive investment. Snakes were offered thawed rodents 1–4 times per week. Over two active seasons, we collected data on surface activity, home range size, growth, mass change, and reproduction of supplementally fed and control snakes. Fed and control snakes did not differ in surface activity levels (proportion of time encountered above versus below ground) or home range size. Fed snakes grew and gained mass faster, and had a dramatically higher occurrence of reproduction than control snakes. Also, fed snakes were in better body condition following reproduction than snakes that were not fed. However, litter characteristics such as offspring number and size were not increased by feeding, suggesting that these characteristics may be fixed. These data experimentally demonstrate that food availability can directly impact some life history traits (i.e., growth and reproduction for C. atrox), but not others (i.e., surface activity and home range size for C. atrox). The relationship between food availability and life history traits is affected in a complex way by ecological traits and physiological constraints, and thus interspecific variation in this relationship is likely to be high.     

The adaptive significance of sexually dimorphic scale rugosity in sea snakes

In terrestrial snakes, rugose scales are uncommon and (if they occur) generally are found on both sexes. In contrast, rugose scales are seen in most sea snakes, especially in males. Why has marine life favored this sex-specific elaboration of scale rugosity? We pose and test alternative hypotheses about the function of rugose scales in males of the turtle-headed sea snake (Emydocephalus annulatus) and conclude that multiple selective forces have been involved. First, rugosities may aid male positioning during courtship, because histology shows that tubercles are more highly innervated than adjacent flat areas of each scale and hence are presumably more sensitive to tactile cues, and because biomechanical tests show that rugosities enhance friction between the bodies of males and females. Second, the occurrence of rugosities over the entire body of males and (albeit less well developed) in females as well suggests that rugosities also play a hydrodynamic role by modifying water flow across the snake's surface. Flow tank tests show that rugosities reduce the thickness of the boundary layer by almost 50% and create turbulent flow that should massively enhance rates of cutaneous oxygen uptake and hence prolong maximal courtship duration by males.     

Patterns of fluctuating asymmetry in sexual ornaments predict female choice

Extravagant secondary sexual characters are assumed to have arisen and be maintained by sexual selection. While traits like horns, antlers and spurs can be ascribed to intrasexual competition, other traits such as extravagant feather ornaments, displays and pheromones have to be ascribed to mate choice. A number of studies have tested whether females exert selection on the size of male ornaments, but only some of these have recorded female preferences for the most extravagantly ornamented males. Here I demonstrate that female choice can be directly predicted from the relationship between the degree of fluctuating asymmetry and the size of a secondary sexual character. Fluctuating asymmetry is an epigenetic measure of the ability of individuals to cope with stress, and it occurs when an individual is unable to undergo identical development of an otherwise bilaterally symmetric trait on both sides of its body. There is a negative relationship between the degree of fluctuating asymmetry and the absolute size of an ornament in those bird species with a female preference for the largest male sex trait, while there is a flat or U-shaped relationship among species without a female preference. These results suggest that females prefer exaggerated secondary sexual characters if they reliably demonstrate the ability of males to cope with genetic and environmental stress. Some species may demonstrate a flat or U-shaped relationship between the degree of fluctuating asymmetry and the absolute size of an ornament because (i) the genetic variance in viability signalled by the secondary sex trait has been depleted; (ii) the secondary sex trait is not particularly costly and therefore does not demonstrate condition dependence; or because (iii) the sex traits can be considered arbitrary traits rather than characters reflecting good genes.     

Developmental constraints on an adaptive plasticity: reaction norms of pigmentation in adult segments of Drosophila melanogaster

SUMMARY Variation of dark pigmentation according to developmental temperature was investigated in two geographic populations (France and India) with the isofemale line technique (20 lines for each population). The response curves called the reaction norms, were established in females for seven different segments: the mesothorax and abdomen segments 2–7 (Abd 2–7). In all cases the response curves were non-linear and had to be described either by a quadratic convex polynomial for thorax and Abd 2–5, or by a cubic polynomial for Abd 6 and 7. Among abdomen segments, increasing antero-posterior gradients were observed for several traits, including average pigmentation, overall phenotypic plasticity, the temperature of minimum pigmentation, and the curvature parameter of quadratic norms. Genetic correlations between abdomen segments were high when adjacent segments were considered, but became nil when more distant segments were correlated, suggesting that different pigmentation genes are expressed in the anterior and the posterior part of the abdomen. Characteristic values of reaction norms provided information either on trait value (i.e., the extension of pigmentation) or on plasticity. Correlations between plasticity and pigmentation were generally low and non-significant, suggesting their genetic independence. The overall darker pigmentation which is observed at low temperatures is assumed to be an adaptive plasticity. However, the differences which are evidenced among segments reveal strong interactions with developmental genes. These interactions are less likely to be a consequence of natural selection and are better interpreted as developmental constraints. The reaction norms analysis reveals the complexity of these interactions and should help, in the future, in the identification of the responsible thermosensitive genes.     

Sexual dimorphism in life history plasticity in the gypsy moth (Lymantria dispar L.)

Sexual differences in reaction norms of life history traits (larval development time--LDT, pupal weight--PW and adult longevity--L) were investigated in the gypsy moth reared on young or old oak leaves during the first larval instar. Sexual dimorphism was revealed for genetic variation in reaction norms that was expressed only for LDT in males, and PW and L in females. Higher mean plasticity of longevity was found in males compared to females indicating that the sexes are exposed to divergent selective pressures. Greater dependence of males on energy resources (carbohydrates and lipids) may account for the observed differences.     

How a thermal dichotomy in nesting environments influences offspring of the world's most northerly oviparous snake,Natrix natrix (Colubridae)

Temperature has a major influence on the rate of embryonic development in ectothermic organisms. While incubation experiments unambiguously show that constant high temperature accelerates development and shortens embryonic life, studies on the effect of fluctuating temperatures have generated contradictory results. Grass snakes (Natrix natrix) occur at latitudes and altitudes that are unusually cool for an oviparous reptile. In these cool climates females typically lay their eggs in heat‐generating anthropogenic microhabitats that provide either a highly fluctuating (compost piles) or a relatively constant (manure heaps) thermal nesting environment. A laboratory experiment with fluctuating and constant incubation temperatures mimicking those recorded in such nests in the field showed that this nest‐site dichotomy influences the development of the embryos, and the morphology and locomotor performance of the hatchlings. The incubation period increased at fluctuating temperatures and the fact that the rate of embryonic development showed a decelerating pattern with temperature suggests that periods of low temperature had a relatively larger influence on average development than periods of high temperature. Our study demonstrates how a dichotomy in the nesting environments available to female grass snakes in cool climates can affect variation in the duration of the incubation period and offspring phenotypes in ways that may have consequences for fitness. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ??, ??–??.     

Effects of nest temperature and moisture on phenotypic traits of hatchling snakes (Tropidonophis mairii, Colubridae) from tropical Australia

Previous research on developmentally plastic responses by reptile embryos has paid relatively little attention to tropical species, or to possible interactions between the effects of thermal and hydric regimes. In the present study, eggs of keelback snakes ( Tropidonophis mairii ), from a tropical area with strong temporal and spatial variation in soil temperatures and moisture levels, were incubated. The phenotypic traits of hatchling snakes (body size, shape, muscular strength) were affected by moisture content of the incubation medium (vermiculite plus 100% vs. 50% water by mass), by mean incubation temperatures (25.7 vs. 27.9 °C) and by diel thermal variation (diel range 6.0 vs. 8.4 °C). Interactions between these factors were negligible. Cooler, more thermostable, moister conditions resulted in larger offspring, a trait under strong selection in this population. Thermal and hydric conditions covary in potential nest-sites (e.g. deeper nests are more thermostable as well as moister). This covariation may influence the evolution of reaction norms for embryogenesis. For example, if moister nests enhance offspring fitness and are cooler, then selection will favour the ability to develop in cool as well as moist conditions. Thus, the evolution of optimal incubation conditions with respect to one variable (e.g. temperature) may be driven by patterns of association with another variable (e.g. soil moisture) among natural nest-sites. Perhaps for this reason, the thermal optimum for incubation is surprisingly low in this tropical species.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 159–168.     

Prenatal sex ratios and expression of sexually dimorphic traits in three snake species

Variation in intrauterine exposure to hormones associated with variation in the sex of litter mates has well-established and far-reaching effects on sexual development in some mammals. Research on this phenomenon in reptiles is scant, but suggests that lizards may follow the mammalian model whereas snakes may be affected differently. We examined sex-specific expression of four sexually dimorphic traits (tail length, head length, ventral scale count, swimming speed) in three species of snakes (Nerodia sipedon, Thamnophis sirtalis, T. sauritus) relative to litter sex ratios. We found little evidence that traits in either sex were masculinized or feminized in response to variation in litter sex ratio. The one significant result appeared best explained as a statistical artifact attributable to a single litter. Our results indicate that snakes are different from the one lizard studied to date. Unlike previous suggestions that prenatal hormonal mechanisms operate differently in snakes and lizards, however, the difference appears to be that development of sexually dimorphic traits in lizards is affected by litter sex ratios whereas in snakes it is not.