Physiological plasticity in lizard embryos exposed to high-altitude hypoxia

Responses to Hypoxia and Hyperoxia in Embryonic Tiger Keelbacks (Rhabdophis tigrinus lateralis; Colubridae)

Studies examining the oxygen dependency of embryonic survival, growth, and differentiation have been conducted for decades in a diverse array of animal taxa but including only one oviparous snake, the viperine water snake Natrix maura. Here, we describe a study incubating eggs of the tiger keelback Rhabdophis tigrinus lateralis (Colubridae) under four oxygen conditions, hypoxia (7% and 11% O2), normoxia (21% O2), and hyperoxia (31% O2), for different lengths or at different stages of incubation. The length of hypoxic exposure is important in affecting embryonic development in R. t. lateralis, with prolonged hypoxic exposure retarding embryonic growth and differentiation, increasing embryonic mortality and deformity, reducing hatchling size and mass, and altering hatchling body shape relative to normoxic controls. Embryonic tiger keelbacks are most susceptible to hypoxia late in development, as revealed by the fact that a 5-day exposure of eggs to hypoxia of 7% O2 reduced embryo mass and hatchling mass if it occurred at late stages of incubation. Hyperoxia of 31% O2 did not enhance development of R. t. lateralis embryos, only affecting hatchling head width, which slightly differed between hyperoxic hatchlings and their normoxic siblings. This study demonstrates the importance of avoiding hypoxic exposure at late stages of embryonic development in snakes.

Sex-dependent shifts in body size and condition along replicated elevational gradients in a montane colonising ectotherm, the common wall lizard (Podarcis muralis)

In ectothermic animals, elevational gradients, such as mountainous environments, are often associated with shifts in body size, although patterns differ across taxa and contexts. Mountain landscapes are characterised by relatively rapid shifts in biotic and abiotic conditions along an elevational gradient, commonly referred to as elevational zonation. Such zonation can reduce the geographic scale at which organisms experience the effects of climate change. The upslope range shifts will expose organisms at the colonization front to sub-optimal conditions. We can expect these challenging conditions to influence many life-history traits including growth rates and reproductive output. We tested the hypothesis that body size varies across elevational gradients in a contemporary montane colonizer, the common wall lizard (Podarcis muralis). Further, we assessed active body temperatures and available environmental temperatures in an attempt to discern a potential abiotic factor that might drive such a pattern. We quantified body size in lizards along four replicate transects ranging from 400 to 2400 m above sea level in the Pyrenees. Male body size decreased with increasing elevation. While female body size was invariant, females at higher elevation exhibited lower body condition. These results suggest that the effects of abiotic limitations or selective pressures experienced at the high-elevation colonisation front are sex-specific. Furthermore, lizards from both sexes were able to maintain similar field active body temperatures across elevation, despite reduced ambient temperature. If available temperatures limit activity periods or necessitate higher thermoregulatory investment, as suggested by our results, then further warming may benefit lizards and favour further upslope migration.

Developmental plasticity of the cardiovascular system in oviparous vertebrates: effects of chronic hypoxia and interactive stressors in the context of climate change

Animals at early life stages are generally more sensitive to environmental stress than adults. This is especially true of oviparous vertebrates that develop in variable environments with little or no parental care. These organisms regularly experience environmental fluctuations as part of their natural development, but climate change is increasing the frequency and intensity of these events. The developmental plasticity of oviparous vertebrates will therefore play a critical role in determining their future fitness and survival. In this Review, we discuss and compare the phenotypic consequences of chronic developmental hypoxia on the cardiovascular system of oviparous vertebrates. In particular, we focus on species-specific responses, critical windows, thresholds for responses and the interactive effects of other stressors, such as temperature and hypercapnia. Although important progress has been made, our Review identifies knowledge gaps that need to be addressed if we are to fully understand the impact of climate change on the developmental plasticity of the oviparous vertebrate cardiovascular system.

The integrative biology of the heart: mechanisms enabling cardiac plasticity

Cardiac phenotypic plasticity, the remodelling of heart structure and function, is a response to any sustained (or repeated) stimulus or stressor that results in a change in heart performance. Cardiac plasticity can be either adaptive (beneficial) or maladaptive (pathological), depending on the nature and intensity of the stimulus. Here, we draw on articles published in this Special Issue of Journal of Experimental Biology, and from the broader comparative physiology literature, to highlight the core components that enable cardiac plasticity, including structural remodelling, excitation-contraction coupling remodelling and metabolic rewiring. We discuss when and how these changes occur, with a focus on the underlying molecular mechanisms, from the regulation of gene transcription by epigenetic processes to post-translational modifications of cardiac proteins. Looking to the future, we anticipate that the growing use of -omics technologies in integration with traditional comparative physiology approaches will allow researchers to continue to uncover the vast scope for plasticity in cardiac function across animals.

Embryonic and juvenile snakes (Natrix maura, Linnaeus 1758) compensate for high elevation hypoxia via shifts in cardiovascular physiology and metabolism

The colonization of novel environments requires a favorable response to conditions never, or rarely, encountered in recent evolutionary history. For example, populations colonizing upslope habitats must cope with lower atmospheric pressure at elevation, and thus reduced oxygen availability. The embryo stage in oviparous organisms is particularly susceptible, given its lack of mobility and limited gas exchange via diffusion through the eggshell and membranes. Especially little is known about responses of Lepidosaurian reptiles to reduced oxygen availability. To test the role of physiological plasticity during early development in response to high elevation hypoxia, we performed a transplant experiment with the viperine snake (Natrix maura, Linnaeus 1758). We maintained gravid females originating from low elevation populations (432 m above sea level [ASL]-normoxia) at both the elevation of origin and high elevation (2877 m ASL-extreme high elevation hypoxia; approximately 72% oxygen availability relative to sea level), then incubated egg clutches at both low and high elevation. Regardless of maternal exposure to hypoxia during gestation, embryos incubated at extreme high elevation exhibited altered developmental trajectories of cardiovascular function and metabolism across the incubation period, including a reduction in late-development egg mass. This physiological response may have contributed to the maintenance of similar incubation duration, hatching success, and hatchling body size compared to embryos incubated at low elevation. Nevertheless, after being maintained in hypoxia, juveniles exhibit reduced carbon dioxide production relative to oxygen consumption, suggesting altered energy pathways compared to juveniles maintained in normoxia. These findings highlight the role of physiological plasticity in maintaining rates of survival and fitness-relevant phenotypes in novel environments.

Population divergence in maternal investment and embryo energy use and allocation suggests adaptive responses to cool climates

The thermal sensitivity of early life stages can play a fundamental role in constraining species distributions. For egg-laying ectotherms, cool temperatures often extend development time and exacerbate developmental energy cost. Despite these costs, egg laying is still observed at high latitudes and altitudes. How embryos overcome the developmental constraints posed by cool climates is crucial knowledge for explaining the persistence of oviparous species in such environments and for understanding thermal adaptation more broadly. Here, we studied maternal investment and embryo energy use and allocation in wall lizards spanning altitudinal regions, as potential mechanisms that enable successful development to hatching in cool climates. Specifically, we compared population-level differences in (1) investment from mothers (egg mass, embryo retention and thyroid yolk hormone concentration), (2) embryo energy expenditure during development, and (3) embryo energy allocation from yolk towards tissue. We found evidence that energy expenditure was greater under cool compared with warm incubation temperatures. Females from relatively cool regions did not compensate for this energetic cost of development by producing larger eggs or increasing thyroid hormone concentration in yolk. Instead, embryos from the high-altitude region used less energy to complete development, that is, they developed faster without a concomitant increase in metabolic rate, compared with those from the low-altitude region. Embryos from high altitudes also allocated relatively more energy towards tissue production, hatching with lower residual yolk: tissue ratios than low-altitude region embryos. These results are consistent with local adaptation to cool climate and suggest that this is underpinned by mechanisms that regulate embryonic utilisation of yolk reserves and its allocation towards tissue, rather than shifts in maternal investment of yolk content or composition.

Altitudinal variation in organ mass from three mountain systems: The case of mesquite lizard Sceloporus grammicus

High altitude environments provide a fertile ground for investigating the benefits of phenotypic adjustments at several levels of biological organization. Low oxygen partial pressure and low environmental temperature are the main limiting factors that promote phenotypic variation in different organs, such as the lung and heart. Although high-altitude environments act like natural laboratories, most morphological studies conducted to date lack replication. Here, we evaluated organ mass variation in nine populations of Sceloporus grammicus, throughout three altitudinal gradients (mountains) from the Trans-Mexican volcanic belt. A total of 84 individuals from three different altitudes at three different mountains were collected. Then, we used generalized linear models to analyze the pattern of variation in internal organs mass as a function of altitude and temperature. We observed a striking pattern of altitudinal variation in the size of cardiorespiratory organs: while heart mass increased with altitude and decreased with temperature, the lung showed a significant statistical interaction between mountain transect and temperature. Overall, our results support the hypothesis that cardiorespiratory organs should be bigger in populations occurring at higher altitudes. Moreover, the study of different mountain systems allowed us to observe some differences in one mountain in relation to the other two.

Elevation, oxygen, and the origins of viviparity

Research focused on understanding the evolutionary factors that shape parity mode evolution among vertebrates have long focused on squamate reptiles (snakes and lizards), which contain all but one of the evolutionary transitions from oviparity to viviparity among extant amniotes. While most hypotheses have focused on the role of cool temperatures in favoring viviparity in thermoregulating snakes and lizards, there is a growing appreciation in the biogeographic literature for the importance of lower oxygen concentrations at high elevations for the evolution of parity mode. However, the physiological mechanisms underlying how hypoxia might reduce fitness, and how viviparity can alleviate this fitness decrement, has not been systematically evaluated. We qualitatively evaluated previous research on reproductive and developmental physiology, and found that (1) hypoxia can negatively affect fitness of squamate embryos, (2) oxygen availability in the circulatory system of adult lizards can be similar or greater than an egg, and (3) gravid females can possess adaptive phenotypic plasticity in response to hypoxia. These findings suggest that the impact of hypoxia on the development and physiology of oviparous and viviparous squamates would be a fruitful area of research for understanding the evolution of viviparity. To that end, we propose an integrative research program for studying hypoxia and the evolution of viviparity in squamates.

Effects of hypoxia on the thermal physiology of a high-elevation lizard: implications for upslope-shifting species

Montane reptiles are predicted to move to higher elevations in response to climate warming. However, whether upwards-shifting reptiles will be physiologically constrained by hypoxia at higher elevations remains unknown. We investigated the effects of hypoxic conditions on preferred body temperatures (T_pref) and thermal tolerance capacity of a montane lizard (Phrynocephalus vlangalii) from two populations on the Qinghai-Tibet Plateau. Lizards from 2600 m a.s.l. were exposed to O₂ levels mimicking those at 2600 m (control) and 3600 m (hypoxia treatment). Lizards from 3600 m a.s.l. were exposed to O₂ levels mimicking those at 3600 m (control) and 4600 m (hypoxia treatment). The T_pref did not differ between the control and hypoxia treatments in lizards from 2600 m. However, lizards from 3600 m selected lower body temperatures when exposed to the hypoxia treatment mimicking the O₂ level at 4600 m. Additionally, the hypoxia treatment induced lower critical thermal minimum (CT_min) in lizards from both populations, but did not affect the critical thermal maximum (CT_max) in either population. Our results imply that upwards-shifting reptiles may be constrained by hypoxia if a decrease in T_pref reduces thermally dependent fitness traits, despite no observed effect on their heat tolerance.

A high mountain lizard from Peru: The world’s highest-altitude reptile

Life at high altitudes is particularly challenging for ectothermic animals like reptiles and involves the evolution of specialised adaptations to deal with low temperatures, hypoxia and intense UV radiation. As a result, only very few reptile taxa are able to survive above 5,000 m elevation and herpetological observations from these altitudes are exceedingly rare. We report here an exceptional observation of a lizard population (Liolaemus aff. tacnae; Reptilia, Squamata) from the high Andes of Peru. During an ascent of Chachani mountain (6,054 m, 16°11'S, 71°32'W), we observed and documented photographically this species living between 5,000 and 5,400 m above sea level. Following a review of literature, we show that this is the highest known record of a reptile species.

Phylogenetic patterns and ontogenetic origins of limb length variation in ecologically diverse lacertine lizards

Limb length is intrinsically linked to function and, ultimately, fitness. Thus, it can co-evolve with habitat structure, as exemplified by tropical lizards in highly heterogeneous environments. But does lizard limb length respond in a similar manner during adaptive diversification in temperate zones? Here, we examine variation in habitat preference and limb length in lacertine lizards from the Palaearctic. We tested the following three hypotheses: (1) species of the Lacertini tribe descended from a generalist ancestor and subsequently underwent habitat specialization; (2) specialized ecological roles are associated with relative limb length in extant species; and (3) interspecific differences in limb length emerge in embryonic development. Our comparisons supported an ancestral ‘rocky’ or ‘generalist’ habitat preference, and phenotype–habitat associations were particularly supported when examining size-adjusted forelimb length in 69 species that represented all known Lacertini genera. Moreover, we revealed an elevated interlimb ratio in high-vegetation species, which might be linked to climbing performance in species with relatively longer forelimbs. Furthermore, embryonic limb variation was detected solely against an Eremiadini outgroup species. Instead, hind limb length differences within Lacertini originated in post-hatching ontogeny. The mechanisms that modulate limb growth are likely to be limited in Lacertini, because adaptive morphological change might mirror historical contingency and the ecological context wherein this clade diversified.

High temperatures limit developmental resilience to high-elevation hypoxia in the snake Natrix maura (Squamata: Colubridae)

Climate change is generating range shifts in many organisms, notably along the altitudinal gradient. However, moving up in altitude exposes organisms to lower oxygen availability, which may negatively affect development and fitness, especially at high temperatures. To test this possibility in a potentially upward-colonizing species, we artificially incubated developing embryos of the viperine snake Natrix maura Linnaeus 1758, using a split-clutch design, in conditions of extreme high elevation or low elevation at two ecologically-relevant incubation temperatures (24 and 32 °C). Embryos at low and extreme high elevations incubated at cool temperatures did not differ in development time, hatchling phenotype or locomotor performance. However, at the warmer incubation temperature and at extreme high elevation, hatching success was reduced. Further, embryonic heart rates were lower, incubation duration longer and juveniles born smaller. Nonetheless, snakes in this treatment were faster swimmers than siblings in other treatment groups, suggesting a developmental trade-off between size and performance. Constraints on development may be offset by the maintenance of important performance metrics, thus suggesting that early life-history stages will not prevent the successful colonization of high-elevation habitat even under the dual limitations of reduced oxygen and increased temperature.

High-elevation hypoxia impacts perinatal physiology and performance in a potential montane colonizer

Climate change is generating range shifts in many organisms, notably along the elevational gradient in mountainous environments. However, moving up in elevation exposes organisms to lower oxygen availability, which may reduce the successful reproduction and development of oviparous organisms. To test this possibility in an upward‐colonizing species, we artificially incubated developing embryos of the viperine snake (Natrix maura) using a split‐clutch design, in conditions of extreme high elevation (hypoxia at 2877 m above sea level; 72% sea‐level equivalent O₂ availability) or low elevation (control group; i.e. normoxia at 436 m above sea level). Hatching success did not differ between the two treatments. Embryos developing at extreme high elevation had higher heart rates and hatched earlier, resulting in hatchlings that were smaller in body size and slower swimmers compared to their siblings incubated at lower elevation. Furthermore, post‐hatching reciprocal transplant of juveniles showed that snakes which developed at extreme high elevation, when transferred back to low elevation, did not recover full performance compared to their siblings from the low elevation incubation treatment. These results suggest that incubation at extreme high elevation, including the effects of hypoxia, will not prevent oviparous ectotherms from producing viable young, but may pose significant physiological challenges on developing offspring in ovo. These early‐life performance limitations imposed by extreme high elevation could have negative consequences on adult phenotypes, including on fitness‐related traits.

Excess postexercise oxygen consumption decreases with swimming duration in a labriform fish: Integrating aerobic and anaerobic metabolism across time

Many vertebrate animals employ anaerobic pathways during high-speed exercise, even if it imposes an energetic cost during postexercise recovery, expressed as excess postexercise oxygen consumption (EPOC). In ectotherms such a fish, the initial anaerobic contribution to exercise is often substantial. Even so, fish may recover from anaerobic pathways as swimming exercise ensues and aerobic metabolism stabilizes, thus total energetic costs of exercise could depend on swimming duration and subsequent physiological recovery. To test this hypothesis, we examined EPOC in striped surfperch (Embiotoca lateralis) that swam at high speeds (3.25 L s^-1 ) during randomly ordered 2-, 5-, 10-, and 20-min exercise periods. We found that EPOC was highest after the 2-min period (20.9 mg O₂  kg^-1 ) and lowest after the 20-min period (13.6 mg O₂  kg^-1 ), indicating that recovery from anaerobic pathways improved with exercise duration. Remarkably, EPOC for the 2-min period accounted for 72% of the total O₂ consumption, whereas EPOC for the 20-min period only accounted for 14%. Thus, the data revealed a striking decline in the total cost of transport from 0.772 to 0.226 mg O₂ ·kg^-1 ·m^-1 during 2- and 20-min periods, respectively. Our study is the first to combine anaerobic and aerobic swimming costs to demonstrate an effect of swimming duration on EPOC in fish. Clarifying the dynamic nature of exercise-related costs is relevant to extrapolating laboratory findings to animals in the wild.

Developmental differences between two marine turtle species and potential consequences for their survival at hatching

Anatomical variation is a major source for natural selection. Marine turtles are endangered and survival predictions are of important biological, ecological, social, and political value. Here, we perform a preliminary study illustrating how comparative embryology permits understanding of ontogenetic variation as a contributor for evolutionary fitness. To that end, we studied samples of Chelonia mydas and Caretta caretta embryos relative to a standardized staging system from the literature. We examined external anatomy using discrete characters in order to document interspecific variation during development of these species. We employed the 'Standard Event System to Study Vertebrate Embryos' to examine fitness-relevant structures. These include the limb paddles and elbows of Ch. mydas, which differentiate relatively late in ontogeny. We detected interspecific variation in the timing of trait differentiation - such as the egg tooth, closure of skull vault, carapace formation, and scale covering - and propose that these differences might be functionally and ecologically relevant for marine turtles.

Lizards at the Peak: Physiological Plasticity Does Not Maintain Performance in Lizards Transplanted to High Altitude

Warming climates are facilitating the range expansion of many taxa to habitats that were formerly thermally inhospitable, including to higher latitudes and elevations. The potential for such colonization, however, varies widely among taxa. Because environmental factors may interact to affect colonization potential, an understanding of underlying physiological and behavioral mechanisms is necessary to predict how species will respond to potentially suitable habitats. For example, temperature and oxygen availability will interact to shape physiological and performance traits. Our model species, the wall lizard, Podarcis muralis, is a widely distributed ectotherm that continues to expand its range in Europe despite being limited by cold temperatures at high elevations and latitudes. To test the potential for organisms to expand to warming high-altitude environments, we conducted a transplant experiment to quantify the within-individual effects of high-altitude hypoxia on physiological and performance traits. Transplanted lizards maintained individual differences in physiological traits related to oxygen capacity and metabolism (hemoglobin concentration, hematocrit, and peak postexhaustion metabolic rate), as well as performance traits tied to fitness (sprint speed and running endurance). Although lizards altered blood biochemistry to increase oxygen-carrying capacity, their performance was reduced at high altitude. Furthermore, lizards at high altitude suffered a rapid loss of body condition over the 6-wk experiment, suggesting an energetic cost to hypoxia. Taken together, this demonstrates a limited potential for within-individual plasticity to facilitate colonization of novel high-altitude environments.

Transplanting gravid lizards to high elevation alters maternal and embryonic oxygen physiology, but not reproductive success or hatchling phenotype

Increased global temperatures have opened previously inhospitable habitats, such as at higher elevations. However, the reduction of oxygen partial pressure with increase in elevation represents an important physiological constraint that may limit colonization of such habitats, even if the thermal niche is appropriate. To test the mechanisms underlying the response to ecologically-relevant levels of hypoxia, we performed a translocation experiment with the common wall lizard (Podarcis muralis), a widespread European lizard amenable to establishing populations outside its natural range. We investigated the impacts of hypoxia on the oxygen physiology and reproductive output of gravid common wall lizards and the subsequent development and morphology of their offspring. Lowland females transplanted to high elevations increased their haematocrit and haemoglobin concentration within days and maintained routine metabolism compared to lizards kept at native elevations. However, transplanted lizards suffered from increased reactive oxygen metabolite production near the oviposition date, suggesting a cost of reproduction at high elevation. Transplanted females and females native to different elevations did not differ in reproductive output (clutch size, egg mass, relative clutch mass, or embryonic stage at oviposition) or in post-oviposition body condition. Developing embryos reduced heart rates and prolonged incubation times at high elevations within the native range and at extreme high elevations beyond the current range, but this reduced oxygen availability did not affect metabolic rate, hatching success, or hatchling size. These results suggest that this opportunistic colonizer is capable of successfully responding to novel environmental constraints in these important life-history stages.