Latent effects of egg incubation temperature on growth in the lizard Anolis carolinensis

Varied egg incubation temperatures can result in immediate effects on the phenotype of reptiles, and also latent effects that can augment or contradict effects evident at egg hatching. I examined the effects of incubation temperature on embryonic development, hatching morphology, and subsequent growth in multiple populations of the lizard Anolis carolinensis. Eggs from wild-caught females in four populations were incubated at up to three temperatures, 23.5, 27, and 30 degrees C. Measures of body size were collected immediately after hatching and weekly thereafter, while juveniles were maintained in a common laboratory environment for 8 weeks. Cooler incubation temperatures resulted in longer incubation periods but did not affect conversion of egg mass to hatchling mass. Incubation temperature did not affect hatchling mass or snout vent length (SVL), but did affect subsequent growth in both mass and SVL, which varied by population. Cooler incubation temperatures generally resulted in greater overall growth over 8 weeks of housing all juveniles in a common environment. In A. carolinensis, egg incubation temperature had latent effects on juvenile growth despite the absence of any detected immediate effects on hatchling phenotype. Therefore, the total impact and evolutionary importance of developmental environment should not be assessed or assumed based solely on the phenotype of reptiles at birth or hatching.     

Sex determination and optimal development in the Moorish gecko,Tarentola mauritanica

Under temperature sex determination (TSD), sex is determined by temperature during embryonic development. Depending on ecological and physiological traits and plasticity, TSD species may face demographic collapse due to climate change. In this context, asymmetry in bilateral organisms can be used as a proxy for developmental instability and, therefore, deviations from optimal incubation conditions. Using Tarentola mauritanica gecko as a model, this study aimed first to confirm TSD, its pattern and pivotal temperature, and second to assess the local adaptation of TSD and variation of asymmetry patterns across four populations under different thermal regimes. Eggs were incubated at different temperatures, and hatchlings were sexed and measured. The number of lamellae was counted in adults and hatchlings. Results were compatible with a TSD pattern with males generated at low and females at high incubation temperatures. Estimated pivotal temperature coincided with the temperature producing lower embryonic mortality, evidencing selection towards balanced sex ratios. The temperature of oviposition was conservatively selected by gravid females. Asymmetry patterns found were likely related to nest temperature fluctuations. Overall, the rigidity of TSD may compromise reproductive success, and demographic stability in this species in case thermal nest choice becomes constrained by climate change.     

Shift in Thermal Preferences of Female Oviparous Common Lizards During Egg Retention: Insights into the Evolution of Reptilian Viviparity

Pregnant female Zootoca vivipara select lower body temperatures than males or nonpregnant females, and this shift in the thermal preferendum is believed to be related to optimising the conditions for embryogenesis. Thus, subjecting embryos to the higher temperature selected by males and non-gravid females might have detrimental effects on embryonic development and on hatchling fitness, according to predictions of the “maternal manipulation” hypothesis on the evolution of viviparity. To test the role of gestation environment on embryonic development in oviparous Z. vivipara, we kept a number of gravid females at the temperature selected by non-gravid females in a laboratory thermal gradient, whereas control females were allowed to regulate their body temperature without restrictions. Developmental stage at oviposition was more advanced for embryos of the experimental clutches, which were heavier than those of the control group. Forced gestation temperature also affected hatching success (58.62% in the experimental treatment vs. 97.37% in the control group). In addition, hatchlings from females subjected to high temperatures during pregnancy were smaller, had shorter head length and performed worse in running trials. Our results fulfil the prediction of the “maternal manipulation” hypothesis, and suggest that the shift in female body temperature during pregnancy optimizes embryogenesis and hatchling phenotype by avoiding the negative effects of the high incubation temperatures preferred by non-gravid females.     

Influence of incubation temperature on morphology, locomotor performance, and early growth of hatchling wall lizards (Podarcis muralis)

Eggs of wall lizards (Podarcis muralis) were incubated at three temperatures approaching the upper limit of viability for embryonic development in this species (26, 29, and 32 degrees C) to assess the influence of temperature on various aspects of hatchling phenotype likely affecting fitness. The thermal environment affected size and several morphometric characteristics of hatchling lizards. Hatchlings from eggs incubated at 32 degrees C were smaller (snout-vent length, SVL) than those from 26 and 29 degrees C and had smaller mass residuals (from the regression on SVL) as well as shorter tail, head, and femur relative to SVL. Variation in the level of fluctuating asymmetry in meristic and morphometric traits associated with incubation temperatures was quite high but not clearly consistent with the prediction that environmental stress associated with the highest incubation temperatures might produce the highest level of asymmetry. When tested for locomotor capacity in trials developed at body temperatures of 32 and 35 degrees C, hatchlings from the 32 degrees C incubation treatment exhibited the worst performance in any aspect considered (burst speed, maximal length, and number of stops in the complete run). Repeated measures ANCOVAs (with initial egg mass as covariate) of snout-vent length and mass of lizards at days 0 and 20 revealed significant effects of incubation temperature only for mass, being again the hatchlings from eggs incubated at 32 degrees C those exhibiting the smallest final size. All together, our results evidenced a pervasive effect of thermal regime during incubation (and hence of nest site selection) on hatchling phenotypes. However, incubation temperature does not affect hatchling phenotypes in a continuous way; for most of the analysed traits a critical threshold seems to exist between 29 and 32 degrees C, so that hatchlings incubated at 32 degrees C exhibited major detrimental effects. J. Exp. Zool. 286:422-433, 2000.     

Altitudinal divergence in maternal thermoregulatory behaviour may be driven by differences in selection on offspring survival in a viviparous lizard

Plastic responses to temperature during embryonic development are common in ectotherms, but their evolutionary relevance is poorly understood. Using a combination of field and laboratory approaches, we demonstrate altitudinal divergence in the strength of effects of maternal thermal opportunity on offspring birth date and body mass in a live-bearing lizard (Niveoscincus ocellatus). Poor thermal opportunity decreased birth weight at low altitudes where selection on body mass was negligible. In contrast, there was no effect of maternal thermal opportunity on body mass at high altitudes where natural selection favored heavy offspring. The weaker effect of poor maternal thermal opportunity on offspring development at high altitude was accompanied by a more active thermoregulation and higher body temperature in highland females. This may suggest that passive effects of temperature on embryonic development have resulted in evolution of adaptive behavioral compensation for poor thermal opportunity at high altitudes, but that direct effects of maternal thermal environment are maintained at low altitudes because they are not selected against. More generally, we suggest that phenotypic effects of maternal thermal opportunity or incubation temperature in reptiles will most commonly reflect weak selection for canalization or selection on maternal strategies rather than adaptive plasticity to match postnatal environments.     

Detrimental influence on performance of high temperature incubation in a tropical reptile: is cooler better in the tropics?

Global temperatures have risen over the last century, and are forecast to continue rising. Ectotherms may be particularly sensitive to changes in thermal regimes, and tropical ectotherms are more likely than temperate species to be influenced by changes in environmental temperature, because they may have evolved narrow thermal tolerances. Keelback snakes (Tropidonophis mairii) are tropical, oviparous reptiles. To quantify the effects of temperature on the morphology and physiology of hatchling keelbacks, clutches laid by wild-caught females were split and incubated at three temperatures, reflecting the average minimum, overall average and average maximum temperatures recorded at our study site. Upon hatching, the performance of neonates was examined at all three incubation temperatures in a randomized order over consecutive days. Hatchlings from the ‘hot’ treatment had slower burst swim speeds and swam fewer laps than hatchlings from the cooler incubation temperatures in all three test temperatures, indicating a low thermal optimum for incubation of this tropical species. There were no significant interactions between test temperature and incubation temperature across performance variables, suggesting phenotypic differences caused by incubation temperature did not acclimate this species to post-hatching conditions. Thus, keelback embryos appear evolutionarily adapted to development at cooler temperatures (relative to what is available in their habitat). The considerable reduction in hatchling viability and performance associated with a 3.5 °C increase in incubation temperature, suggests climate change may have significant population-level effects on this species. However, the offspring of three mothers exposed to the hottest incubation temperature were apparently resilient to high temperature, suggesting that this species may respond to selection imposed by thermal regime.     

Warm developmental temperatures induce non-adaptive plasticity in the intrasexually selected colouration of a dragonfly

1. When the breeding environment fluctuates across generations, reproductive traits may evolve plasticity that optimises the balance between survival and mating success for the prevailing environment. 2. For sexually selected colouration, this balance can depend on environmental temperatures. Accordingly, breeding colouration often co-varies with temperature through space and time. However, whether such traits exhibit plasticity in response to environmental temperatures is poorly understood. 3. In the present study, a dragonfly (Pachydiplax longipennis) was reared under ambient or experimentally warmed conditions and tested for plasticity in its intrasexually selected wing colouration. Although wing colouration improves male territorial success, these advantages are smaller under warmer conditions than cooler conditions. It was therefore predicted that males reared under the ambient thermal conditions of the study site (Cleveland, Ohio) would develop more wing colouration than those reared under experimentally warmed conditions. 4. Contrary to this prediction, males reared in warm larval temperatures produced more wing colouration. Thus, although the secondary sexual colouration of this species displays some thermal plasticity, it does not appear to be adaptive relative to the known thermal variation of intrasexual selection in this population. 5. Given that the environment often determines the strength and direction of sexual selection, future studies should consider the potential for non-adaptive, and even maladaptive, developmental plasticity in the sexually selected traits of insects.     

Influence of temperature and photoperiod on embryonic development in the dragonfly Sympetrum striolatum (Odonata: Libellulidae)

Temperature and photoperiod play major roles in insect ecology. Many insect species have fixed degree‐days for embryogenesis, with minimum and maximum temperature thresholds for egg and larval development and hatching. Often, photoperiodic changes trigger the transfer into the next life‐cycle stadium. However, it is not known whether this distinct pattern also exist in a species with a high level of phenotypic plasticity in life‐history traits. In the present study, eggs of the dragonfly Sympetrum striolatum Charpentier (Odonata: Libellulidae) are reared under different constant and fluctuating temperatures and photoperiodic conditions in several laboratory and field experiments. In general, and as expected, higher temperatures cause faster egg development. However, no general temperature or light‐days for eyespot development and hatching are found. The minimum temperature thresholds are distinguished for survival (2 °C), embryogenesis (6 °C) and larval hatching (above 6 °C). Low winter temperatures synchronize hatching. Above 36 °C, no eyespots are visible and no larvae hatch. In laboratory experiments, light is neither necessary for eyespot development, nor for hatching. By contrast to the laboratory experiments, the field experiment show that naturally changing temperature and photoperiod play a significant role in the seasonal regulation of embryonic development. The post‐eyespot development is more variable and influenced by temperature and photoperiod than the pre‐eyespot development. This developmental plasticity at the end of the embryogenesis might be a general pattern in the Libellulidae, helping them to cope with variation in environmental conditions.     

Embryonic development of the domestic guinea fowl (Numida meleagris)

Inside the viable range 36–39°C, the development of the domestic guinea fowl embryo was studied during artificial incubation of about 5000 eggs. Equations giving internal and external pipping and hatching time as a function of incubation temperature were developed. Whatever temperature was used, internal and external pipping occurred at 89% and 95% of hatching time, respectively. The chronological development in ovo of the guinea fowl is illustrated by growth curves. Guinea fowl grew in ovo at the same rate as the chicken embryo. The embryonic mortality is significantly affected by incubation temperature. The thermal tolerance of the embryo follows a parabolic curve: temperatures below or above 37.2°C increase the rate of mortality. During the course of the incubation, the mortality frequency was mainly distributed around two peaks (each one third of the total), which occurred during the first days of incubation, that is, within the first 12% of the incubation time, and during the last days, that is, after 85% of the incubation time. At the optimal 37.2°C temperature, these two peaks occurred on days 1–3 and 23–27, respectively.     

Constraints on the evolution of thermal sensitivity of foraging in Trichogramma: genetic trade-offs and plasticity in maternal selection

Negative genetic correlation between performance at different temperatures or temperature-dependent mutations may promote evolution of thermal specialization in ectotherms. The first hypothesis implies that a selective change in performance at one temperature simultaneously results in change in performance at others, while the second implies a delay before observing such indirect responses. Comparison of the direction of evolution among Trichogramma lines selected for improvement of parasitization capacity at low, medium, or high temperatures indicated that a change in performance at one temperature concurrently resulted in opposite changes at distant temperatures. Unexpectedly, selection at high temperatures resulted in a decrease in adult fitness components, while adult performance expressed at cold temperatures simultaneously increased. The relationship between maternal fecundity and offspring fitness components varied across the thermal range. No correlation between these traits was present at cold or medium temperatures, but negative relationships appeared at high temperatures. We show that maternal selection resulting from a conflict between adult and offspring fitness components may have resulted in reversed evolution of the adult traits at the high end of the thermal range. Thus, genetic trade-offs in performance at different temperatures and phenotypic plasticity in maternal selection may constrain evolution of the thermal niche in Trichogramma.     

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.     

Hatching of Daphnia sexual eggs. II. The effect of age and a second stimulus

• 1 We examined the effect of age on the hatching response of Daphnia magna sexual eggs of specific families. For old eggs (>2 years), hatching characteristics were compared at two storage temperatures (4°C and 20°C). Also, the hatching response after a second dark incubation and subsequent incubation under conditions favourable for hatching was compared with that after the first stimulus.
• 2 Daphnia sexual eggs were found to remain viable for several (at least 4.5) years. The effect of age on the hatching rate was family dependent. At least in some families, hatching rate was higher for old (>2 years) than for young (<5 months) eggs. Low temperature (4°C) during dark incubation resulted in a higher hatching rate compared with incubation at 20°C.
• 3 The application of a second hatching stimulus resulted in a renewed hatching response. The overall hatching rate after the second stimulus was, however, lower than that of the first stimulus.
• 4 More than 80% of the hatchlings of young eggs appeared on Day 3 or 4, with minor between-family differences in time distribution of hatching. The timing of the response to hatching stimuli was more variable in old than in young eggs, with the average time at hatching being 6.4 instead of 4.0 days. The response to the application of hatching stimuli was also slower after the second stimulus compared with the first stimulus.

     

Survival of Coregonus albula (L.) (Teleostei) embryos incubated at different thermal conditions

Eggs of Coregonus albula were incubated at constant temperatures: 1.1, 2.0, 2.9, 4.9, 6.6, 8.4, and 9.9 °C, and the percentage of normal hatch was 20.6, 11.8, 30.4, 61.0, 51.7, 32.6, and 14.6%, respectively. The lower and upper median tolerance limit (TL 50) defined as the interpolated temperature at which embryos survival to hatch was 50% of the highest response (61% at 4.9 °C) were 2.9 and 8.5 °C, respectively. The optimum temperature range delimited by lower and upper TL 75 was encompassed by 4.0 and 7.2 °C.Eggs of C. albula incubated at variable temperature in a commercial hatchery showed a very high survival (up to 76%). Similarly low survival observed during hatching of embryos at constant temperatures of 1.1 and 2.0 °C could be hightened (to about 90%) by raising the temperature in the beginning of hatching period. This phenomenon was utilized in the technique of delaying C. albula embryos' mass hatching for the purpose of synchronization in time of stocking the lakes with the time of appearence of good thermal and food conditions for C. albula larvae.The conception of the optimal thermal conditions for Coregoninae embryogenesis was developed as the course of incubation temperature, securing the highest survival rate during embryogenesis and also during the larval period.     

Plasticity in incubation behavior and shading by king rails Rallus elegans in response to temperature

King rails experience a wide range of temperatures during the course of the breeding season throughout their rapidly contracting geographic range. Incubating parent birds are adapted to keep their eggs within a temperature range appropriate for embryo development, but king rail clutches are at risk of exceeding lethal temperatures in the latter half of the nesting season. We investigated whether behavioral plasticity during incubation enables parents to maintain clutch temperature within tolerable limits for embryo development. Video revealed that king rail parents interrupted incubation to stand above and shade their eggs. We tested the hypothesis that the onset of shading was a direct response to ambient temperature (adaptive plasticity). We monitored clutch temperature directly by experimentally adding into clutches a model egg embedded with a programmable iButton. We measured ambient temperature at the nest site simultaneously. Parents spent proportionately more time shading and less time incubating their eggs at higher ambient temperatures. Shading may primarily function in cooling the parent. The frequency and duration of shading bouts were significantly greater at higher ambient temperatures. Parents also took more frequent but shorter recesses in hotter conditions. Diurnal recesses exposed eggs to direct sunlight, and the highest clutch temperatures were recorded under these conditions. Complete hatching failure in at least one nest was attributable to high clutch temperature for an extended period. Because mean ambient temperature increases throughout the breeding season, we investigated seasonal patterns in onset of incubation and its effect on hatching rate. Later in the season, parents tended to initiate incubation earlier, and hatching asynchrony increased significantly. Together these results suggest that breeding king rails may be constrained in their ability to cope with sustained high temperatures should seasonal averages continue to rise as predicted.     

Do temperature fluctuations during incubation always play an important role in shaping the phenotype of hatchling reptiles?

Fluctuating temperatures (FTs) influence hatchling phenotypes differently from constant temperatures (CTs) in some reptiles, but not in others. This inconsistency raises a question of whether thermal fluctuations during incubation always play an important role in shaping the phenotype of hatchlings. To answer this question, we incubated eggs of Naja atra under one CT (28 °C, CT), two temperature-shift [cold first (CF) and hot first (HF) in which eggs were first incubated at 24 or 32 °C and then at the other, each for 20 days, and finally at 28 °C until hatching], and one FT thermal regimes. Female hatchlings were larger in snout–vent length but smaller in tail length, head size than male hatchlings from the same-sized egg; female hatchlings had more ventral scales than did male hatchlings. The FT and HF treatments resulted in shorter incubation lengths. Tail length was greatest in the CT treatment and smallest in the FT treatment, with the CF and HF treatments in between; head width was greater in the CT treatment than in the other three treatments. Other examined hatchling traits did not differ among treatments. The observed morphological modifications cannot be attributed to the effect of thermal fluctuations but to the effect of temperatures close to the upper and lower viable limits for the species. Our results therefore support the hypothesis that hatchling phenotype is not altered by thermal fluctuation in species with no phenotypic response to incubation temperature within some thresholds.     

Effect of incubation temperature on green sturgeon embryos, <Emphasis Type="Italic">Acipenser medirostris</Emphasis>

Regulation of river flow and the amount of winter rainfall are the major factors affecting the water temperature of the spawning grounds, for green sturgeon in the Klamath River. During the primary spawning period of green sturgeon, mid-April to June, the water temperature may vary from 8 to 21°C. To estimate the potential implications of this modified thermal regime, we examined the survival and development in three progeny groups of green sturgeon embryos from zygote to hatch, at constant incubation temperatures (11–26°C). Temperatures 23–26°C affected cleavage and gastrulation and all died before hatch. Temperatures 17.5–22°C were suboptimal as an increasing number of embryos developed abnormally and hatching success decreased at 20.5–22°C, although the tolerance to these temperatures varied between progenies. The lower temperature limit was not evident from this study, although hatching rate decreased at 11°C and hatched embryos were shorter, compared to 14°C. The mean total length of hatched embryos decreased with increasing temperature, although their wet and dry weight remained relatively constant. We concluded that temperatures 17–18°C may be the upper limit of the thermal optima for green sturgeon embryos, and that the river thermal regime during dry years may affect green sturgeon reproduction.     

Egg temperature and initial brood patch area determine hatching asynchrony in Magellanic penguin Spheniscus magellanicus

In birds, the adaptive significance of hatching asynchrony has been under debate for many years and the parental effects on hatching asynchrony have been largely assumed but not often tested. Some authors suggest that hatching asynchrony depends on the incubation onset and many factors have been shown to influence hatching asynchrony in different species. Our objective was to analyze the exact timing of the onset of incubation and if this affects hatching asynchrony; and, in addition, which other factors (brood patch development, incubation position, adult body condition, intra‐clutch egg dimorphism, laying date and year) affect hatching asynchrony in Magellanic penguins Spheniscus magellanicus. We first estimated the eggshell temperature at which embryo development starts, with a non‐destructive and novel method. We then recorded individual egg temperatures in 61 nests during incubation, and related them, and other breeding parameters, to hatching asynchrony. We also observed incubation positions in 307 nests. We found a significant positive relationship between hatching asynchrony and the temperature that the first‐laid egg experienced during egg laying and between hatching asynchrony and the initial brood patch area. We also found a negative relationship between hatching asynchrony and the difference in temperature between second and first‐laid eggs within a clutch, measured after the egg‐laying period was finished. We ruled out position of the eggs during incubation, adult body condition, egg volume, laying date, and study year as factors influencing hatching asynchrony. The egg temperature during laying and the difference in temperature between eggs of a clutch are determinants of hatching asynchrony in Magellanic penguins.     

Thermal adaptation and phenotypic plasticity in a warming world: Insights from common garden experiments on Alaskan sockeye salmon

An important unresolved question is how populations of coldwater‐dependent fishes will respond to rapidly warming water temperatures. For example, the culturally and economically important group, Pacific salmon (Oncorhynchus spp.), experience site‐specific thermal regimes during early development that could be disrupted by warming. To test for thermal local adaptation and heritable phenotypic plasticity in Pacific salmon embryos, we measured the developmental rate, survival, and body size at hatching in two populations of sockeye salmon (Oncorhynchus nerka) that overlap in timing of spawning but incubate in contrasting natural thermal regimes. Using a split half‐sibling design, we exposed embryos of 10 families from each of two populations to variable and constant thermal regimes. These represented both experienced temperatures by each population, and predicted temperatures under plausible future conditions based on a warming scenario from the downscaled global climate model (MIROC A1B scenario). We did not find evidence of thermal local adaptation during the embryonic stage for developmental rate or survival. Within treatments, populations hatched within 1 day of each other, on average, and among treatments, did not differ in survival in response to temperature. We did detect plasticity to temperature; embryos developed 2.5 times longer (189 days) in the coolest regime compared to the warmest regime (74 days). We also detected variation in developmental rates among families within and among temperature regimes, indicating heritable plasticity. Families exhibited a strong positive relationship between thermal variability and phenotypic variability in developmental rate but body length and mass at hatching were largely insensitive to temperature. Overall, our results indicated a lack of thermal local adaptation, but a presence of plasticity in populations experiencing contrasting conditions, as well as family‐specific heritable plasticity that could facilitate adaptive change.     

Thermal constraints on embryonic development as a proximate cause for elevational range limits in two Mediterranean lacertid lizards

Local adaptation and range restrictions in alpine environments are central topics in biogeographic research with important implications for predicting impacts of global climate change on organisms. Temperature is strongly coupled to elevation and greatly affects life history traits of oviparous reptiles in mountain environments. Thus, species may encounter barriers for expanding their ranges if they are unable to adapt to the changing thermal conditions encountered along elevational gradients. We sought to determine whether thermal requirements for embryonic development provide a plausible explanation for elevational range limits of two species of lacertid lizards that have complementary elevational ranges in a Mediterranean mountain range (Psammodromus algirus is found at elevations below 1600 m and Iberolacerta cyreni is found at elevations above 1600 m). We combined experimental incubation of eggs in the laboratory with modelled estimates of nest temperature in the field. In both species, increasing temperature accelerated development and produced earlier hatching dates. The species associated with warmer environments (P. algirus) experienced an excessive hatching delay under the lowest incubation temperature. Moreover, newborns from eggs incubated at low temperatures showed poor body condition and very slow rates of postnatal growth. In contrast, eggs of the strictly alpine species I. cyreni exhibited shorter incubation periods than P. algirus that allowed hatching before the end of the active season even under low incubation temperatures. This was countered by lower reproductive success at higher temperatures, due to lower hatching rates and higher incidence of abnormal phenotypes. Elevational range limits of both species coincided well with threshold temperatures for deleterious effects on embryonic development. We suggest that incubation temperature is a major ecophysiological factor determining the elevational range limits of these oviparous lizards with predictable consequences for mountain distributions under future warmer climates.     

Keeping your options open: Maintenance of thermal plasticity during adaptation to a stable environment

Phenotypic plasticity may allow species to cope with environmental variation. The study of thermal plasticity and its evolution helps understanding how populations respond to variation in temperature. In the context of climate change, it is essential to realize the impact of historical differences in the ability of populations to exhibit a plastic response to thermal variation and how it evolves during colonization of new environments. We have analyzed the real‐time evolution of thermal reaction norms of adult and juvenile traits in Drosophila subobscura populations from three locations of Europe in the laboratory. These populations were kept at a constant temperature of 18ºC, and were periodically assayed at three experimental temperatures (13ºC, 18ºC, and 23ºC). We found initial differentiation between populations in thermal plasticity as well as evolutionary convergence in the shape of reaction norms for some adult traits, but not for any of the juvenile traits. Contrary to theoretical expectations, an overall better performance of high latitude populations across temperatures in early generations was observed. Our study shows that the evolution of thermal plasticity is trait specific, and that a new stable environment did not limit the ability of populations to cope with environmental challenges.