Gold climates and the evolution of viviparity in reptiles: cold incubation temperatures produce poor-quality offspring in the lizard, Sceloporus virgatus

Evolutionary origins of viviparity among the squamate reptiles are strongly associated with cold climates, and cold environmental temperatures are thought to be an important selective force behind the transition from egg-laying to live-bearing. In particular, the low nest temperatures associated with cold climate habitats are thought to be detrimental to the developing embryos or hatchlings of oviparous squamates, providing a selective advantage for the retention of developing eggs in utero, where the mother can provide warmer incubation temperatures for her eggs (by actively thermoregulating) than they would experience in a nest. However, it is not entirely clear what detrimental effects cold incubation temperatures may have on eggs and hatchlings, and what role these effects may play in favouring the evolution of viviparity. Previous workers have suggested that viviparity may be favoured in cold climates because cold incubation temperatures slow cmbryogenesis and delay hatching of the eggs, or because cold nest temperatures are lethal to developing eggs and reduce hatching success. However, incubation temperature has also been shown to have other, potentially long-term, effects on hatchling phcnotypcs, suggesting that cold climates may favour viviparity because cold incubation temperatures produce offspring of poor quality or low fitness. We experimentally incubated eggs of the oviparous phrynosomatid lizard, Sceloporus virgatus, at temperatures simulating nests in a warm (low elevation) habitat, as is typical for this species, and nests in a colder (high elevation) habitat, to determine the effects of cold incubation temperatures on embryonic development and hatchling phenotypes. Incubation at cold nest temperatures slowed embryonic development and reduced hatching success, but also affected many aspects of the hatchlings' phenotypes. Overall, the directions of these plastic responses indicated that cold-incubated hatchlings did indeed exhibit poorer quality phenotypes; they were smaller at hatching (in body length) and at 20 days of age (in length and mass), grew more slowly (in length and mass), had lower survival rates, and showed greater fluctuating asymmetry than their conspecifics that were incubated at warmer temperatures. Our findings suggest that cold nest temperatures are detrimental to S. virgatus, by delaying hatching of their eggs, reducing their hatching success, and by producing poorer quality offspring. These negative effects would likely provide a selective advantage for any mechanism through which these lizards could maintain warmer incubation temperatures in cold climates, including the evolution of prolonged egg retention and viviparity.     

Phenotypic responses of hatchlings to constant versus fluctuating incubation temperatures in the multi-banded krait, Bungarus multicintus (Elapidae)

Most studies on egg incubation in reptiles have relied on constant temperature incubation in the laboratory rather than on simulations of thermal regimes in natural nests. The thermal effects on embryos in constant-temperature studies often do not realistically reflect what occurs in nature. Recent studies have increasingly recognized the importance of simulating natural nest temperatures rather than applying constant-temperature regimes. We incubated Bungarus multicintus eggs under three constant and one fluctuating-temperature regimes to evaluate the effects of constant versus fluctuating incubation temperatures on hatching success and hatchling phenotypes. Hatching success did not differ among the four treatments, and incubation temperature did not affect the sexual phenotype of hatchlings. Incubation length decreased as incubation temperature increased, but eggs incubated at fluctuating temperatures did not differ from eggs incubated at constant temperatures with approximately the same mean in incubation length. Of the hatchling phenotypes examined, residual yolk, fat bodies and locomotor performance were more likely affected by incubation temperature. The maximal locomotor speed was fastest in the fluctuating-temperature and 30 degrees C treatments and slowest in the 24 degrees C treatment, with the 27 degrees C treatment in between. The maximal locomotor length was longest in the fluctuating-temperature treatment and shortest in the 24 degrees C and 27 degrees C treatments, with the 30 degrees C treatment in between. Our results show that fluctuating incubation temperatures do not influence hatching success and hatchling size and morphology any differently than constant temperatures with approximately the same mean, but have a positive effect on locomotor performance of hatchlings.     

Communal nesting under climate change: fitness consequences of higher incubation temperatures for a nocturnal lizard

Communal nesting lizards may be vulnerable to climate warming, particularly if air temperatures regulate nest temperatures. In southeastern Australia, velvet geckos Oedura lesueurii lay eggs communally inside rock crevices. We investigated whether increases in air temperatures could elevate nest temperatures, and if so, how this could influence hatching phenotypes, survival, and population dynamics. In natural nests, maximum daily air temperature influenced mean and maximum daily nest temperatures, implying that nest temperatures will increase under climate warming. To determine whether hotter nests influence hatchling phenotypes, we incubated eggs under two fluctuating temperature regimes to mimic current ‘cold’ nests (mean = 23.2 °C, range 10–33 °C) and future ‘hot’ nests (27.0 °C, 14–37 °C). ‘Hot’ incubation temperatures produced smaller hatchlings than did cold temperature incubation. We released individually marked hatchlings into the wild in 2014 and 2015, and monitored their survival over 10 months. In 2014 and 2015, hot‐incubated hatchlings had higher annual mortality (99%, 97%) than cold‐incubated (11%, 58%) or wild‐born hatchlings (78%, 22%). To determine future trajectories of velvet gecko populations under climate warming, we ran population viability analyses in Vortex and varied annual rates of hatchling mortality within the range 78– 96%. Hatchling mortality strongly influenced the probability of extinction and the mean time to extinction. When hatchling mortality was >86%, populations had a higher probability of extinction (PE: range 0.52– 1.0) with mean times to extinction of 18–44 years. Whether future changes in hatchling survival translate into reduced population viability will depend on the ability of females to modify their nest‐site choices. Over the period 1992–2015, females used the same communal nests annually, suggesting that there may be little plasticity in maternal nest‐site selection. The impacts of climate change may therefore be especially severe on communal nesting species, particularly if such species occupy thermally challenging environments.     

Ecological,evolutionary, and conservation implications of incubation temperature‐dependent phenotypes in birds

Incubation is a vital component of reproduction and parental care in birds. Maintaining temperatures within a narrow range is necessary for embryonic development and hatching of young, and exposure to both high and low temperatures can be lethal to embryos. Although it is widely recognized that temperature is important for hatching success, little is known about how variation in incubation temperature influences the post‐hatching phenotypes of avian offspring. However, among reptiles it is well known that incubation temperature affects many phenotypic traits of offspring with implications for their future survival and reproduction. Although most birds, unlike reptiles, physically incubate their eggs, and thus behaviourally control nest temperatures, variation in temperature that influences embryonic development still occurs among nests within a population. Recent research in birds has primarily been limited to populations of megapodes and waterfowl; in each group, incubation temperature has substantial effects on hatchling phenotypic traits important for future development, survival, and reproduction. Such observations suggest that incubation temperature (and incubation behaviours of parents) is an important but underappreciated parental effect in birds and may represent a selective force instrumental in shaping avian reproductive ecology and life‐history traits. However, much more research is needed to understand how pervasive phenotypic effects of incubation temperature are among birds, the sources of variation in incubation temperature, and how effects on phenotype arise. Such insights will not only provide foundational information regarding avian evolution and ecology, but also contribute to avian conservation.     

Reproductive ecology of the jacky dragon (Amphibolurus muricatus): an agamid lizard with temperature‐dependent sex determination

Abstract The jacky dragon, Amphibolurus muricatus (White, ex Shaw 1790) is a medium sized agamid lizard from the southeast of Australia. Laboratory incubation trials show that this species possesses temperature‐dependent sex determination. Both high and low incubation temperatures produced all female offspring, while varying proportions of males hatched at intermediate temperatures. Females may lay several clutches containing from three to nine eggs during the spring and summer. We report the first field nest temperature recordings for a squamate reptile with temperature‐dependent sex determination. Hatchling sex is determined by nest temperatures that are due to the combination of daily and seasonal weather conditions, together with maternal nest site selection. Over the prolonged egg‐laying season, mean nest temperatures steadily increase. This suggests that hatchling sex is best predicted by the date of egg laying, and that sex ratios from field nests will vary over the course of the breeding season. Lizards hatching from eggs laid in the spring (October) experience a longer growing season and should reach a larger body size by the beginning of their first reproductive season, compared to lizards from eggs laid in late summer (February). Adult male A. muricatus attain a greater maximum body size and have relatively larger heads than females, possibly as a consequence of sexual selection due to male‐male competition for territories and mates. If reproductive success in males increases with larger body size, then early hatching males may obtain a greater fitness benefit as adults, compared to males that hatch in late summer. We hypothesize that early season nests should produce male‐biased sex ratios, and that this provides an adaptive explanation for temperature‐dependent sex determination in A. muricatus.     

Phenotypic variation in an oviparous montane lizard (Bassiana duperreyi): the effects of thermal and hydric incubation environments

Recent studies have shown that incubation temperatures can profoundly affect the phenotypes of hatchling lizards, but the effects of hydric incubation environments remain controversial. We examined incubation-induced phenotypic variation in Bassiana duperreyi (Gray, 1938; Sauria: Scincidae), an oviparous montane lizard from south-eastern Australia. We incubated eggs from this species in four laboratory treatments, mimicking cool and moist, cool and dry, warm and moist, and warm and dry natural nest-sites, and assessed several morphological and behavioural traits of lizards after hatching. Incubation temperature influenced a lizard's hatching success, incubation period, tail length and antipredator behaviour, whereas variation in hydric conditions did not engender significant phenotypic variation for most traits. However, moisture affected incubation period slightly differently in males and females, and for a given snout-vent length moisture interacted weakly with temperature to affect lizard body mass. Although incubation conditions can substantially affect phenotypic variation among hatchling lizards, the absence of strong hydric effects suggests that hatchling lizards react less plastically to variation in moisture levels than they do to thermal conditions. Thus, our data do not support the generalization that water availability during embryogenesis is more important than temperature in determining the phenotypes of hatchling reptiles.     

Thermal regimes during incubation do not affect mean selected temperatures of hatchling lizards (Bassiana duperreyi, Scincidae)

Incubation temperatures profoundly affect many phenotypic traits of squamate reptiles, and mean selected body temperatures of such animals also are plastic in response to environmental factors. Plausibly, then, incubation temperatures might affect hatchling thermoregulation, either via adaptation (i.e., populations that historically experience different nest conditions, also will diverge in hatchling thermoregulatory behaviour) or phenotypic plasticity (incubation temperatures directly modify hatchling behaviours). We tested this hypothesis with a montane scincid lizard (Bassiana duperreyi), using thermal-imaging methods to quantify temperatures (of both head and body) selected by hatchling lizards. The young lizards kept their heads cooler than their bodies, but mean selected temperatures did not differ among hatchlings from three populations with differing thermal regimes in natural nests, nor were they affected by thermal conditions during incubation. The conservatism of mean selected temperatures stands in strong contrast to the lability of many other phenotypic traits in response to incubation temperatures in this species.     

Longterm effects of incubation temperatures on the morphology and locomotor performance of hatchling lizards (Bassiana duperreyi, Scincidae)

The phenotypes of hatchling reptiles are known to be affected by the thermal environments they experience during incubation, but the evolutionary and ecological significance of this phenotypic plasticity remains unclear. Crucial issues include: (i) the magnitude of effects elicited by thermal regimes in natural nests (as opposed to constant-temperature incubation); (ii) the persistence of these effects during ontogeny; and (iii) the consistency of these effects across different test conditions (does the thermal regime during embryogenesis simply shift the hatchling's thermal optimum for performance, or actually modify overall performance ability regardless of temperature?). We examined these questions by incubating eggs of scincid lizards (Bassiana duperreyi) from montane southeastern Australia, under two fluctuating-temperature regimes that simulated ‘cold’ and ‘hot’ natural nests. These thermal regimes substantially modified hatchling morphology (mass, body length, tail length, and the relationship between these variables), locomotor performance (running speeds over distances of 25 cm and lm), anti-predator ‘tactics’ and survival rates. The differences in locomotor performance persisted throughout the 20 weeks of our experiment. Lizards that emerged after ‘hot’ incubation were faster runners than their ‘cold’-incubated siblings under all thermal conditions that we tested. Thus, incubation temperatures modified overall locomotor ability, with only a minor effect on the set-point for optimum performance. The magnitude, persistence and consistency of these incubation-induced phenotypic modifications suggest that they may play an important role in evolutionary and ecological processes within lizard populations.     

恒定和波动温度下丽斑麻蜥孵出幼体的表型变异

作者以丽斑麻蜥(Eremias argus)为模型动物研究恒定和波动孵化温度对孵化成功率和孵出幼体表型的影响。卵在四个恒定[24 ,27 ,30 and 33 (±0·3)℃]、一个波动温度下孵化。不同温度处理下的孵化成功率相同,但孵出幼体表型不同。孵化期随孵化温度升高呈指数式缩短;在相同平均温度下,波动温度孵化卵的孵化期比恒温孵化卵长。在所有被检表型特征中,幼体的干重、剩余卵黄干重和运动表现更易受孵化温度影响。总体而言,低温(24℃、27℃)孵出幼体运动表现最佳,高温(33℃)孵出幼体最差、温和温度(30℃和波动温度)孵出幼体居中。本文研究数据显示: (1)丽斑麻蜥卵每日短期暴露于潜在致死的极端温度下对孵化成功率和孵出幼体形态特征无明显的不利效应; (2)温度波动对孵出幼体运动表现无促进作用,对孵化期的影响则不同于平均值相同的恒定温度。     

How does an increase in minimum daily temperatures during incubation influence reproduction in the great tit Parus major?

Temperature variation affects all life stages of organisms, especially early development, and considering global warming, it is urgent to understand precisely its consequences. In egg‐laying species, incubation behaviour can buffer embryo developmental temperature variation and influence offspring development. We experimentally investigated the effect of an increase in minimum daily nest temperature during incubation in the great tit Parus major, by placing a hand warming pad under the nest in the evenings. As compared to controls, the experimental treatment increased nest temperature at night by an average of 4°C, and this increase carried over to the following day. We measured the consequences of this mainly nocturnal temperature increase during incubation on 1) parental behaviour (incubation and nestling feeding), 2) parental health (quantified by body condition, immune status, physiological and oxidative stress) and 3) reproductive success (nestling body condition, growth, i.e. mass gain, hatching and fledging success, and nestling immune status, physiological and oxidative stress). This study yielded three major results. First, we found that heating the nest did not change the duration of incubation as compared to controls. Second, increasing nest temperature during incubation decreased nestling feeding behaviour but did not affect parental health in terms of body condition, immune status, physiological and oxidative stress. Third, nestling mass at hatching was greater but nestling mass gain was slower in heated nests than in control nests, resulting in similar fledging mass. The present study demonstrates that increased environmental temperatures during incubation influenced nestling development in the great tit and especially hatchling mass, which might produce long‐term life history consequences.     

Phenotypic effects of thermal mean and fluctuations on embryonic development and hatchling traits in a lacertid lizard, Takydromus septentrionalis

How fluctuating temperatures influence reptilian embryos and hatchlings has attracted increasing scientific attention, but is poorly known. We conducted an incubation experiment with a factorial design of two mean temperatures (24 vs. 28 degrees C) and three diel thermal fluctuations (0, +/-3, +/-6 degrees C) to determine the effects of diel thermal fluctuations and mean temperature on incubation duration and hatchling phenotypes. Both diel thermal fluctuations and mean temperature significantly affected incubation duration, but not hatching success. Incubation duration increased with increasing temperature fluctuations at a mean temperature of 24 degrees C, but not at a mean temperature of 28 degrees C. The significant interaction between diel thermal fluctuations and mean temperature on hatchling morphology indicated that the effect of thermal fluctuations depended on the mean temperature. Hatchling mass differed significantly between 24+/-6 and 28+/-6 degrees C, but not between the two constant temperatures or the temperatures with +/-3 degrees C fluctuations. At a mean temperature of 24 degrees C, the effect of thermal fluctuations on hatchling size was marginally significant, with relatively large hatchlings at the constant temperature; at a mean temperature of 28 degrees C, thermal fluctuations had no impact on hatchling size. The locomotor performances were significantly affected by mean temperature rather than diel thermal fluctuations. Therefore, diel thermal fluctuations around a given mean temperature do not affect hatchling phenotypes in a relatively large magnitude, but the influence of diel thermal fluctuations may differ with changing mean temperatures.     

Reconstructing an adaptationist scenario: what selective forces favor the evolution of viviparity in montane reptiles?

It is notoriously difficult to test hypotheses about the selective forces responsible for major phylogenetic transitions in life-history traits, but the evolution of viviparity (live bearing) in reptiles offers an ideal model system. Viviparity has arisen in many oviparous reptile lineages that have invaded colder climates. Thermal advantages (eggs retained within the mother's body will be warmer than those laid in the nest) are almost certainly important, but the actual selective pressures remain controversial. For example, the benefit to retention might involve faster development, protection against freezing, predation, or desiccation, or modification of hatchling phenotypes. I experimentally manipulated incubation regimes of a montane scincid lizard (Bassiana duperreyi, Scincidae) to test these ideas. Eggs maintained in cooler "nests" in the laboratory developed more slowly, were more likely to die before hatching, and produced inferior (small, slow) hatchlings. A 2-wk initial period of higher-temperature incubation (simulating uterine retention, an intermediate step toward viviparity) ameliorated these effects. In the field, I placed eggs in artificial nests at the upper elevational limit of natural nests and also extending a further 100 m higher on the mountain. The results mirrored those in the laboratory: retention at maternal body temperatures accelerated hatching, enhanced hatching success, and increased locomotor speeds of hatchlings. This selective advantage of uterine retention was greater at higher elevations and increased with longer retention. The causal process responsible was prolonged low-temperature incubation rather than freezing, desiccation, or predation, and both hatching success and hatchling viability were affected. Field experiments that directly re-create selective regimes may thus provide robust tests of adaptationist hypotheses.     

Eggs in autumn: responses to declining incubation temperatures by the eggs of montane lizards

Facultative hatching in response to environmental cues may increase the viability of offspring, if the cue that stimulates hatching also predicts the negative consequences of delayed emergence. Declining incubation temperatures might provide such a cue for montane lizards, because eggs that fail to hatch before winter will perish in the nest. I tested this idea by incubating eggs of an alpine scincid lizard ( Bassiana duperreyi ) in the laboratory. For the first half of the incubation period the eggs were kept at nest temperatures typical of those experienced in summer in the field (daily cycle of 18 ± 7.5°C). I then transferred eggs at weekly intervals into cooler regimes (either 15 ± 7.5°C; or with daytime temperatures unchanged but dropping to 0°C overnight). Contrary to prediction, the eggs did not hatch early. However, transfer to lower temperatures caused only a relatively short delay in hatching, because of a virtual temperature-independence of developmental rates late (but not early) in incubation. Decreasing incubation temperatures also modified hatchling running speeds and post-hatching growth rates, even if the thermal decrease occurred only shortly before the usual time of hatching. These processes plausibly affect hatchling fitness in cold-climate reptiles, and might be adaptations to montane habitats. Alternatively, they may prove to be widespread in other (warmer-climate) reptile taxa, in which case no adaptive hypothesis need be proposed. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 71–77.     

Effects of seasonally varying hydric conditions on hatchling phenotypes of keelback snakes (Tropidonophis mairii, Colubridae) from the Australian wet-dry tropics

Does water availability during incubation significantly affect the phenotypes of hatchling reptiles in natural nests? Two obstacles to obtaining any general answer to this question are the scarcity of studies on tropical species, and the difficulty of comparing experimental treatments to actual hydric conditions in nature. We used a split‐clutch design to incubate 102 eggs (eight clutches) of a colubrid snake species (the keelback, Tropidonophis mairii), from a floodplain in the Australian wet‐dry tropics. This species breeds over most of the year, and highly seasonal rainfall regimes generate strong shifts in water content of the soil over this period. We measured soil water content in a natural nest, and incubated eggs in both soil and vermiculite (the usual medium for experimental studies) at a range of water contents. These calibration trials let us compare our experimental ‘wet’ and ‘dry’ incubation treatments to conditions in natural nests, in terms of actual water uptake by eggs. Hatchlings from dry incubation were unable to resorb their desiccated yolk and thus were smaller (17% in mass, 12% in body length) than their siblings from moist substrates. Incubation conditions also affected the hatchlings’ muscular strength and locomotor speed: even at the same body length, dry‐incubated hatchlings were weaker and slower than their wet‐incubated siblings. Incubation moisture affected strength differently in males and females. We conclude that seasonal variation in water content of the soil in natural nests can generate strong phenotypic variation in hatchling snakes. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 76 , 339–347.     

The effects of incubation temperature on the morphology and composition of Australian Brush-turkey (<Emphasis Type="Italic">Alectura lathami</Emphasis>) chicks

Environmental heterogeneity during embryonic development generates an important source of variation in offspring phenotypes and can influence the evolution of life histories. The effects of incubation temperature on offspring phenotypes in reptiles has been well documented but remains relatively unexplored in birds as their embryos typically develop over a narrow range of temperatures. Megapode birds (Order Galliformes; Family Megapodiidae) are unique in that their embryos tolerate and develop over a wide range of incubation temperatures, yet little is known of the effect that temperature has on hatchling morphology and composition. Australian Brush-turkey eggs collected on the day of laying were incubated in the laboratory under constant temperatures of 32, 34 and 36°C until hatching in order to determine the influence of temperature on hatchling mass, size and composition. The dry mass of the yolk-free body and residual yolk of hatchlings were temperature dependent, such that higher temperatures produced chicks of lesser yolk-free body mass and greater residual yolk mass than chicks incubated at lower temperatures. However the overall size (linear dimensions) and lipid, protein and ash content of chicks were independent of temperature.     

Microclimate of American crocodile nests in Banco Chinchorro biosphere reserve, Mexico: Effect on incubation length, embryos survival and hatchlings sex

Crocodilians have temperature-dependent sex determination (TSD) in which incubation temperature determines sex of embryo. Global warming is expected to alter hatchling sex ratio, leading to the extinction of small populations. Regional climate influence on crocodile nest microclimate and hatchlings' characteristics is poorly known. Here, microclimate in natural nests of American crocodile (Crocodylus acutus) and its relation with incubation length, hatchling sex and nesting success was studied in Banco Chinchorro Biosphere Reserve (Mexico) from 2007 to 2010. Temperature and relative humidity in different locations within and outside the nests were registered by data loggers. Incident solar radiation above nest was calculated from hemispheric photographs. Incubation length, proportion of hatchling reaching complete development and hatchling sex were determined at hatching. Nest temperatures exhibited a cyclic daily fluctuation due to solar radiation, which is the major heat source for nests. Clutch temperature was relatively stable and its daily amplitude was negatively correlated with clutch depth and size. Rainfall was the major source of clutch temperature decrease. Clutch and metabolic temperatures increased significantly during incubation. A small sample size failed to demonstrate a statistical relationship between length of incubation and mean clutch temperature. Proportion of embryos reaching complete development depended on maximum and minimum clutch temperature, maximum daily amplitude of clutch temperature and maximum decrease in clutch temperature on a period ≤4 day. Results confirmed a Female-Male-Female TSD pattern for C. acutus, with 31 and 32.5 °C as possible pivotal temperatures. Population and hatchling sex ratios were male-biased and fate of crocodiles of Banco Chinchorro could depend on the magnitude of temperature increase in the future.     

Direct and transgenerational effects of an experimental heatwave on early life stages in a freshwater snail

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Hatching success, delay of emergence and hatchling biometry of the spur-thighed tortoise, Testudo graeca, in south-western Spain

Hatching success, egg incubation, emergence and hatchling characteristics were assessed for 44 naturally incubating nests of Testudo graeca in south-western Spain. Nest predation rate was 4.5% and overall hatching success was 82.4%. Incubation periods ranged from 78 to 114 days, and hatchlings delayed emergence from the nest from one to 23 days. Emergences occurred from mid August to late September, and were not correlated with nesting dates, but earlier laid nests had longer incubation times, which was probably owing to lower temperatures experienced by clutches laid at the beginning of the nesting season. Variance of hatchling body size and mass was high and was mainly influenced by the gravid female. Mean straight carapace length was 34.14mm, and mean body mass 10.8g. Hatchlings from clutches laid last in the nesting season had significantly better physical condition. Hatchling mass was positively correlated with egg mass, and both variables were positively correlated with emergence date. Both better physical condition and relatively late emergence may confer advantages to hatchlings in the face of unfavourable environmental conditions in autumn.     

Are thyroid hormones mediators of incubation temperature-induced phenotypes in birds?

Incubation temperature influences a suite of traits in avian offspring. However, the mechanisms underlying expression of these phenotypes are unknown. Given the importance of thyroid hormones in orchestrating developmental processes, we hypothesized that they may act as an upstream mechanism mediating the effects of temperature on hatchling phenotypic traits such as growth and thermoregulation. We found that plasma T₃, but not T₄ concentrations, differed among newly hatched wood ducks (Aix sponsa) from different embryonic incubation temperatures. T₄ at hatching correlated with time spent hatching, and T₃ correlated with hatchling body condition, tarsus length, time spent hatching and incubation period. In addition, the T₃ : T₄ ratio differed among incubation temperatures at hatch. Our findings are consistent with the hypothesis that incubation temperature modulates plasma thyroid hormones which in turn influences multiple aspects of duckling phenotype.     

The effects of thermal and hydric environments on hatching success, embryonic use of energy and hatchling traits in a colubrid snake, Elaphe carinata.

We examined the effects of thermal and hydric environments on hatching success, the embryonic use of energy and hatchling traits in a colubrid snake, Elaphe carinata. The eggs were incubated at four temperatures ranging from 24 to 32 degrees C on substrates with water potentials of 0 and -220 kPa using a 4x2 factorial design. Both thermal and hydric environments affected the water exchange between eggs and their surroundings. Eggs incubated in wetter substrates gained mass throughout the course of incubation, whereas eggs in drier substrates gained mass during the first half of incubation and lost mass thereafter. Hatching success was noticeably higher at 26 and 30 degrees C than at 24 and 32 degrees C, but among treatments, differences in hatching success were not significant. Temperature significantly affected the duration of incubation and most hatchling traits examined. Deformed hatchlings were found in all temperature treatments, with more deformities observed at 32 degrees C. Hatchlings from eggs incubated at different temperatures differed in wet body mass, but the differences stemmed mainly from variation in water contents. Embryos at different temperatures completed development at nearly the same expenditure of energy and catabolized nearly the same amount of lipids, but hatchlings from different temperatures differed in the development condition of carcass at hatching. Hatchlings from eggs incubated at 26 degrees C were larger in SVL than those from other higher or lower incubation temperatures, characteristically having larger carcasses; hatchlings from 32 degrees C eggs were smaller in SVL and had smaller carcasses but larger residual yolks than those from lower incubation temperatures. Hatchlings from eggs incubated at 24 degrees C were shorter in tail length but greater in size (SVL)-specific body wet mass than those from higher incubation temperatures. Within the range from -220 to 0 kPa, the substrate water potential did not affect hatching success, the embryonic use of energy and all hatchling traits examined, and the effects of temperature were independent of the effects of substrate water potential. Therefore, our data add evidence showing that embryonic development in reptiles with pliable-shelled eggs is relatively insensitive to variation in hydric environments during incubation.