Effects of thermophily-relevant temperature variation and sex on digestive performance in pythons

Different physiological performances are often optimized at slightly varying temperatures, which can lead to ectotherms selecting higher body temperatures during certain physiological efforts (e.g., digestion, reproduction). Such thermophilic responses can lead to temperature-based tradeoffs between two physiological activities with differing optimal temperatures or between optimizing a physiological activity and water balance, as water loss is elevated at higher temperatures. For example, ectotherms will often select a higher body temperature after consuming a meal, but the extent to which body temperature is elevated after eating is affected by its hydric state. Despite this known hydration state-based suppression of thermophily associated with digestion, the impact of this reduced body temperature on digestion performance is unknown. Accordingly, we determined whether small, thermophily-relevant changes in body temperature impact digestive efficiency or passage time and whether sex influenced the extent of the effect. Eighteen (9 female and 9 male) Children's pythons (Antaresia childreni) each consumed a meal at three temperatures (29 °C, 30 °C, and 31 °C), and gut passage time and digestive efficiency were determined. We found that neither metric was affected by temperature over the range tested. However, digestive efficiency was significantly impacted by the interaction between sex and temperature with males having significantly lower digestive efficiency than females at 31 °C, but not 29 °C or 30 °C. Our results provide insight into the effects of temperature on digestive physiology across narrow temperature ranges as well as demonstrate a sex-based difference in digestive physiology.

Hydration state does not affect selected body temperature during gravidity or gravidity duration in pythons (Antaresia childreni)

The embryonic development of many ectothermic species are highly sensitive to temperature and typically have a higher thermal optima than do most other physiological processes. Thus, female ectotherms often maintain a higher and more carefully controlled body temperature when she is supporting developing embryos (early development in oviparous species, throughout development in viviparous species). Considering the positive correlation between body temperature and evaporative water loss, this response could potentially exacerbate female water imbalance in water-limited environments, suggesting that female water balance and egg development may be in conflict. Using Children's pythons (Antaresia childreni), we hypothesized that water deprivation reduces thermophily during gravidity. We split reproductive females into two thermal treatments: those provided with a continuously available thermal gradient of 25-45 °C and those kept at a constant 31 °C. We also had seven non-reproductive females that were provided a thermal gradient. Within each thermal treatment group, we alternatingly assigned females to either have or not have water throughout gravidity. We found that reproduction increased female body temperature, but this increase was not affected by water regime. Reproduction also increased plasma osmolality, and lack of water during gravidity exacerbated this effect. We also found that thermal treatment, but not water regime, significantly influenced gravidity duration, with females given a thermogradient having a shorter gravidity duration, likely as a result of having a higher average body temperature than did the females provided constant heat. Finally, we found that females provided water throughout gravidity had greater clutch masses than did females without water. Further research is needed to improve scientific understanding of the interactions among water balance, body temperature, and various physiological performances.

Dehydrated snakes reduce postprandial thermophily

Transient thermophily in ectothermic animals is a common response during substantial physiological events. For example, ectotherms often elevate body temperature after ingesting a meal. In particular, the increase in metabolism during the postprandial response of pythons - known as specific dynamic action - is supported by a concurrent increase in preferred temperature. The objective of this study was to determine whether hydration state influences digestion-related behavioral thermophily. Sixteen (8 male and 8 female) Children's pythons (Antaresia childreni) with surgically implanted temperature data loggers were housed individually and provided with a thermal gradient of 25-45°C. Body temperature was recorded hourly beginning 6 days prior to feeding and for 18 days post-feeding, thus covering pre-feeding, postprandial and post-absorptive stages. Each snake underwent this 24 day trial twice, once when hydrated and once when dehydrated. Our results revealed a significant interaction between temperature preference, digestive stage and hydration state. Under both hydrated and dehydrated conditions, snakes similarly increased their body temperature shortly after consuming a meal, but during the later days of the postprandial stage, snakes selected significantly lower (∼1.5°C) body temperature when they were dehydrated compared with when they were hydrated. Our results demonstrate a significant effect of hydration state on postprandial thermophily, but the impact of this dehydration-induced temperature reduction on digestive physiology (e.g. passage time, energy assimilation) is unknown and warrants further study.

Sex and Season Affect Cortical Volumes in Free-Living Western Fence Lizards, Sceloporus occidentalis

The hippocampus plays an important role in spatial navigation and spatial learning across a variety of vertebrate species. Sex and seasonal differences in space use and behavior are known to affect hippocampal volume. Similarly, territoriality and differences in home range size are known to affect the volume of the reptile hippocampal homologues, the medial and dorsal cortices (MC, DC). However, studies have almost exclusively investigated males and little is known about sex or seasonal differences in MC and/or DC volumes in lizards. Here, we are the first to simultaneously examine sex and seasonal differences in MC and DC volumes in a wild lizard population. In Sceloporus occidentalis, males display territorial behaviors that are more pronounced during the breeding season. Given this sex difference in behavioral ecology, we expected males to have larger MC and/or DC volumes than females and for this difference to be most pronounced during the breeding season when territorial behavior is increased. Male and female S. occidentalis were captured from the wild during the breeding season and the post-breeding season and were sacrificed within 2 days of capture. Brains were collected and processed for histology. Cresyl-violet-stained sections were used to quantify brain region volumes. In these lizards, breeding females had larger DC volumes than breeding males and nonbreeding females. There was no sex or seasonal difference in MC volumes. Differences in spatial navigation in these lizards may involve aspects of spatial memory related to breeding other than territoriality that affect plasticity of the DC. This study highlights the importance of investigating sex differences and including females in studies of spatial ecology and neuroplasticity.

Reproductive state and water deprivation increase plasma corticosterone in a capital breeder

Plasma corticosterone (CORT) concentrations fluctuate in response to homeostatic demands. CORT is widely recognized as an important hormone related to energy balance. However, far less attention has been given to the potential role of CORT in regulating salt and water balance or responding to osmotic imbalances. We examined the effects of reproductive and hydric states on CORT levels in breeding Children's pythons (Antaresia childreni), a species with substantial energetic and hydric costs associated with egg development. Using a 2 × 2 experimental design, we examined how reproduction and water deprivation, both separately and combined, impact CORT levels and how these changes correlate with hydration (plasma osmolality) and energy levels (blood glucose). We found that reproduction leads to increased CORT levels, as does dehydration induced by water deprivation. The combined impact of reproduction and water deprivation led to the largest increases in CORT levels. Additionally, we found significant positive relationships among CORT levels, plasma osmolality, and blood glucose. Our results provide evidence that both reproductive activity and increased plasma osmolality can lead to increased plasma CORT in an ectotherm, which could be explained by either CORT having a role as a mineralocorticoid or CORT being elevated as part of a stress response to resource imbalances.

Egg desiccation leads to dehydration and enhanced innate immunity in python embryos

The immune system is essential for survival and its performance can vary depending on the physiological state of the organism. Much of the current research into immune function dynamics has examined newborn to adult life stages, despite previous studies documenting physiological responses in embryos to environmental stimuli. While energy balance has been the predominant focus as the driver of changes in immune function, recent research has found a positive relationship between dehydration and innate immune performance in adult reptiles. We expanded the understanding of this relationship by examining trans-generational immune effects of female dehydration as well as the effects of egg desiccation on embryonic hydration state and innate immunity using Children's pythons, Antaresia childreni. We used a 2 × 2 experiment with hydrated or dehydrated mothers and eggs either incubated under continuous optimal conditions or experiencing desiccating conditions for 24 h. Our results demonstrate that, similar to adults, embryos enhance some metrics of innate immunity when they are dehydrated.

Dehydration during egg production alters egg composition and yolk immune function

Parent-offspring conflicts occur when resources are limited for allocation, and, historically, energy has been the primary currency of focus when examining these trade-offs. Water is a fundamental resource that has received far less consideration for parent-offspring conflicts. Previous research suggests that, when water is limited, reproductive females are compromised in favor of developing embryos. However, these studies limited their assessments to standard metrics such as clutch size and mass. We tested the hypothesis that the mother-offspring conflict over limited water resources leads to finer scale morphological and physiological impacts on the eggs in Children's pythons (Antaresia childreni). We predicted that water deprivation during gravidity alters female investment into her eggs, impacting egg water content and shell development. Additionally, we predicted that the yolk in these dehydrated eggs would have enhanced immune performance metrics, as has been documented in dehydrated adults. We found that eggs from water-deprived females were dehydrated as indicated by reduced percent water and greater yolk osmolality compared to eggs from females that received ad libitum water. We also found that eggs from dehydrated mothers had thinner shells and higher water loss rates. The impacts were not entirely negative as dehydrated eggs had higher antimicrobial capabilities. Also, thinner and more permeability eggshells might allow for elevated rates of rehydration from nest substrate. Overall, by examining an array of egg traits, we demonstrated that dehydration of gravid females impacts the eggs, not just the females as previously reported. As a result, the mother-offspring conflicts are indeed two-sided.

Muscles provide an internal water reserve for reproduction

The use of fat to support the energy needs of reproduction (i.e. capital breeding) has been studied in a diversity of taxa. However, despite reproductive output (i.e. young or eggs) being approximately 70% water, little is known about the availability of internal resources to accommodate the hydric demands of reproduction. Recent research suggests that dehydration increases the catabolism of muscle as a means of maintaining water balance. Accordingly, we investigated the interactive effects of reproductive investment and water deprivation on catabolism and reproductive output in female Children's pythons (Antaresia childreni). Both reproductive and non-reproductive females were either provided water ad libitum or were water-deprived for three weeks at the time when reproductive females were gravid. We found that water-deprived reproductive females had, in general, greater body mass loss, epaxial muscle loss, plasma osmolality and plasma uric acid concentrations relative to the other groups. Furthermore, water-deprived females had similar clutch sizes compared with females with access to water, but produced lighter eggs and lower total clutch masses. Our results provide the first evidence that selective protein catabolism can be used to support water demands during reproduction, and, as a result, these findings extend the capital breeding concept to non-energetic resources.

Glucocorticoids in tuatara (Sphenodon punctatus): Some influential factors, and applications in conservation management

Glucocorticoid (GC) hormones promote basic life processes, regulate life-history transitions, and help individuals cope with challenges and stressors, thereby playing an important fitness role. Here, we review recent evidence for several factors that influence plasma concentrations of corticosterone (CORT), the main GC in tuatara (Sphenodon punctatus), and discuss the application of CORT as a physiological tool to monitor conservation efforts. Observational studies show an association between CORT concentrations and seasonal reproductive activity, ambient temperature, and ecological habitat parameters (including presence of rats/seabird abundance, sex-ratio, and genetic diversity), and experimental studies show a positive influence of acute temperature increase on the CORT response. Recently, CORT physiology has been applied as a monitoring tool in tuatara translocation programmes. No signs of chronic stress in CORT profiles were observed during standard short- and long-term translocation and rat eradication procedures, giving confidence that current conservation efforts are supportive in population recovery. These results provide a foundation for comparative understanding of stress physiology in reptiles, and will be critical for managing future population viability of tuatara in a changing environment.

Clutch size influences embryonic stages at oviposition in a lizard with prolonged egg retention

We examined the possible interaction between reproductive effort and embryonic stages at oviposition in oviparous form of the lizard Zootoca vivipara. Our results reveal that the percentage of total embryonic development time (%TEDT) reached at oviposition is negatively correlated to clutch size (adjusted to maternal body size). We found no influence of reproductive burden of female (relative clutch mass, RCM) on %TEDT. The significant effect of fecundity supports the hypothesis that a resource limitation such as oxygen may exist for developing embryos in oviducts. The absence of RCM effect suggests that the available space (abdominal burdening of the mother) does not limit the embryonic stages at oviposition.

Relative role of population density and climatic factors in shaping the body growth rate of Lilford’s Wall Lizard (Podarcis lilfordi)

The body growth rate in small reptiles is modulated by per-capita food resources and recent evidences suggested that this constraint is the mechanism underlying differences between cohorts. Per-capita food resources depend on population size and climatic factors, but their relative role in explaining the variations in growth rate is unclear. We used morphological data collected over 6 years to model the body growth of an insular lizard (Lilford’s Wall Lizard, Podarcis lilfordi (Günther, 1874)). We used nonlinear equations to describe the appropriate length-at-age relationship. For each sex, seasonal growth was observed and the oscillatory von Bertalanffy curve was fitted to the data. Three age classes were recognized, and we investigated the relative role of density, spring rainfall, and temperature in explaining the variation of the growth rate in each sex-by-age class. Results showed that the relative role of density and climatic factors varied according to the age considered. While population size and temperature had a negative effect on the growth rate of juveniles, rainfall had a positive influence on the growth of subadults. Adult growth was near zero and constant over time. The different role of density-dependent and climatic factors in explaining age-dependent growth rate provides an important insight in understanding lizard population dynamics and life-history tactics.

Determinants of anti-predator tactics in hatchling grass snakes (Natrix natrix)

Many organisms exhibit diverse anti-predator tactics, influenced by genetics and prior experience. In ectothermic taxa, offspring phenotypes are often sensitive to developmental temperatures. If the effectiveness of alternative anti-predator responses depends on thermally sensitive traits, then the temperatures experienced during embryonic life should also affect how offspring respond to an approaching predator. We incubated 16 clutches of Swedish grass snakes (Natrix natrix) at a range of developmental temperatures, and scored body size, colour pattern, locomotor performance and anti-predator responses of 213 hatchlings from those clutches. A hatchling snake's size and locomotor abilities were affected by its clutch of origin, its developmental temperature, and by an interaction between these two factors. Anti-predator tactics were strongly linked to locomotor ability, such that slower snakes tended to rely upon aggressive displays rather than flight. Incubation temperatures that generated slow (and thus aggressive) snakes also modified the colour of the snake's nuchal spot. Temperatures in the low to medium range generated mostly cream, white and orange spots, whereas medium to high temperatures generated more yellow spots. Incubation effects, and gene X environment interactions, thus may generate complex correlations between morphology, locomotor ability, and anti-predator tactics.

Influence of temperature on the corticosterone stress-response: an experiment in the Children's python (Antaresia childreni)

To cope with environmental challenges, organisms have to adjust their behaviours and their physiology to the environmental conditions they face (i.e. allostasis). In vertebrates, such adjustments are often mediated through the secretion of glucocorticoids (GCs) that are well-known to activate and/or inhibit specific physiological and behavioural traits. In ectothermic species, most processes are temperature-dependent and according to previous studies, low external temperatures should be associated with low GC concentrations (both baseline and stress-induced concentrations). In this study, we experimentally tested this hypothesis by investigating the short term influence of temperature on the GC stress response in a squamate reptile, the Children's python (Antaresia childreni). Snakes were maintained in contrasting conditions (warm and cold groups), and their corticosterone (CORT) stress response was measured (baseline and stress-induced CORT concentrations), within 48h of treatment. Contrary to our prediction, baseline and stress-induced CORT concentrations were higher in the cold versus the warm treatment. In addition, we found a strong negative relationship between CORT concentrations (baseline and stress-induced) and temperature within the cold treatment. Although it remains unclear how cold temperatures can mechanistically result in increased CORT concentrations, we suggest that, at suboptimal temperature, high CORT concentrations may help the organism to maintain an alert state.

Maternal influences on early development: preferred temperature prior to oviposition hastens embryogenesis and enhances offspring traits in the Children’s python, Antaresia childreni

Embryonic life is particularly sensitive to its surroundings, and the developmental environment can have long-lasting effects on offspring. In oviparous species, the impacts of the developmental environment on offspring traits are mostly examined during development within the egg. However, as more than 25% of the development of squamate reptiles can occur prior to oviposition, we explored the effect of thermal conditions on development prior to oviposition in an oviparous snake species, the Children’s python (Antaresia childreni). We housed gravid female pythons under three thermal cycles: an optimal regime that reflected maternal preference in a non-constrained environment (constant preferred body temperature of gravid females, Tset=31.5°C) and two mildly suboptimal regimes that shared the same mean temperature of 27.7°C, but differed in the duration at Tset. In one of the constraining regimes, females had access to Tset for 4 h daily whereas in the other regime, females never reached Tset (maximal temperature of 29.0°C). Thermal treatments were maintained throughout gravidity in all three groups, but, after oviposition, all eggs were incubated at Tset until hatching. Compared with the optimal regime, the two suboptimal regimes had a longer duration of gravidity, which resulted in delayed hatching. Between the two suboptimal regimes, gravidity was significantly shorter in the treatment that included time at Tset. Furthermore, suboptimal regimes influenced offspring traits at hatching, including body morphology, antipredator behavior, strength and metabolism. However, partial access to maternal Tset significantly enhanced several offspring traits, including performance. Our results demonstrate the importance of time at Tset on early development and suggest an adaptive significance of maternal thermoregulation prior to oviposition.

Parental behavior in pythons is responsive to both the hydric and thermal dynamics of the nest

Parental behavior contributes to the success of a diverse array of taxa, and female-only nest attendance is particularly widespread. Python egg-brooding behavior is an intriguing example of female-only nest attendance because it significantly influences several critical developmental variables, namely embryonic predation, hydration, respiration and temperature. During brooding, females predominately adopt a tightly coiled posture that reduces the exchange of heat, water vapor, O2 and CO2 between the nest and clutch environment, which benefits egg water balance at the cost of respiration. To determine the plasticity of this important behavior, we manipulated nest temperature and humidity while monitoring nest–clutch thermal, hydric and respiratory relationships to test the hypothesis that female Children's pythons (Antaresia childreni) modify their egg-brooding behavior due to an interaction between environmental thermal and hydric conditions. During moderate and high nest humidity treatments (23 and 32 g m−3 H2O, respectively), females spent more time coiling tightly when the nest was cooling than when it was warming, which benefited clutch temperature. However, brooding females in low-humidity nest environments (13 g m−3 H2O) showed a high frequency of tight coiling even when the nest was warming; thus, nest temperature and humidity had an interactive effect on egg-brooding behavior in support of our hypothesis. Our results also suggest that certain egg-brooding behaviors (i.e. postural adjustments) are more energetically costly to females than other behaviors (i.e. tight coiling). In sum, we provide empirical support for the adaptive plasticity of python egg-brooding behavior, which offers insight into the general significance of female-only nest attendance in animals.

Effect of nest temperature on egg-brooding dynamics in Children's pythons

Parental care meets several critical needs of developing offspring. In particular, egg brooding in pythons is an exceptional model for examining environmental influences on specific parental behaviors because brooding behaviors are dynamic yet simple and modulate embryonic temperature, respiration, and water balance. We used captive Children's pythons (Antaresia childreni) to assess their facultative endothermic capability, the influence on egg-brooding behaviors of the gradient between the nest temperature (T(nest)) and clutch temperature (T(clutch)), and the effect of these behaviors on the developmental micro-environment. We monitored maternal egg-brooding behavior, rates of brooding unit (i.e., female and associated clutch) respiratory gas exchange, T(nest), T(clutch), and intra-clutch oxygen tension (PO(2)clutch) during acute changes among four incubation temperature conditions: constant preferred temperature (31.5 degrees C); 'cooling' (T(nest)<T(clutch)); constant cool temperature (25.6 degrees C); and 'warming' (T(nest)>T(clutch)). We demonstrated that A. childreni are not facultatively endothermic because brooding unit temperature coefficient (Q(10)) for V(O2) and V(CO2) was similar to other ectothermic boid snakes (1.9-5.7) and T(clutch) conformed to T(nest) at the constant, cool temperature treatment. Females coiled tightly around eggs more often during cooling than during warming. Further, the amount of time that females spent tightly coiled during warming significantly affected the T(nest)-T(clutch) gradient. Together these results indicate that, although female A. childreni are not facultatively endothermic, they are capable of assessing the T(nest)-T(clutch) gradient and making behavioral adjustments to enhance the thermal micro-environment of their developing offspring.

Alternating egg-brooding behaviors create and modulate a hypoxic developmental micro-environment in Children's pythons (Antaresia childreni)

Parental care is a widespread and ecologically relevant adaptation known to enhance the developmental environment of offspring. Parental behaviors, however, may entail both costs and benefits for developing offspring. In Children's pythons (Antaresia childreni), we monitored both maternal egg-brooding behavior and intra-clutch oxygen partial pressure (PO2) in real-time to assess the effects of various brooding behaviors on PO2 in the clutch micro-environment at three stages of development. Furthermore, at the same developmental stages, we measured O2 consumption rates (VO2) of eggs at varying PO2 to determine their critical oxygen tension (i.e. the minimal PO2 that supports normal respiratory gas exchange) and to predict the impact that naturally brooded intra-clutch PO2 has on embryonic metabolism. At all three stages of development, a tightly coiled brooding posture created an intra-clutch PO2 that was significantly lower than the surrounding nest environment. Maternal postural adjustments alleviated this hypoxia, and the magnitude of such corrections increased with developmental stage. Mean intra-clutch PO2 decreased with stage of development, probably because of increasing egg VO2. Additionally, embryo critical oxygen tension increased with developmental stage. Together, these results suggest that python embryos are unable to maintain normal metabolism under brooded conditions during the final 10% of incubation. These results demonstrate that specific parental behaviors can impose obligatory costs to developing offspring and that balancing these behaviors can mediate deleterious consequences.