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

Maternal egg care enhances hatching success and offspring quality in an oviparous skink

Maternal egg-caring behavior can often be observed in oviparous scincid lizards. The expression of such behavior is predictably affected by the trade-off between its resultant costs and benefits for mothers and/or offspring, which has been investigated in only a few scincid species. Here, post-ovipositional Plestiodon chinensis females were treated to care for their egg clutches without interference, under simulated predation pressure, or to be care-deprived. Potential maternal costs and offspring benefits associated with egg-caring behavior were then evaluated by measuring changes in maternal body condition, egg mortality, and hatchling performance. Egg-caring behavior caused post-ovipositional females to participate less in outside-nest activity, eat less food, and show relatively poorer body conditions at egg hatching. By contrast, compared with care-deprived females, egg-caring females gained mass slightly faster, and achieved a similar body condition within a few months of hatching. Eggs that were cared for by their mothers were more likely to hatch and produced larger, faster-running and better-growing hatchlings with higher survival than uncared-for eggs. Simulated exposure to a potential predator had no distinct impact on maternal egg-caring behavior expression and offspring performance. These results indicated that marked benefits of offspring viability but minor maternal energy costs might play a decisive role in the occurrence of maternal egg-caring behavior in P. chinensis.

The evolution of mechanisms involved in vertebrate endothermy

Endothermy, i.e. the endogenous production of metabolic heat, has evolved multiple times among vertebrates, and several strategies of heat production have been studied extensively by physiologists over the course of the twentieth century. The independent acquisition of endothermy by mammals and birds has been the subject of many hypotheses regarding their origin and associated evolutionary constraints. Many groups of vertebrates, however, are thought to possess other mechanisms of heat production, and alternative ways to regulate thermogenesis that are not always considered in the palaeontological literature. Here, we perform a review of the mechanisms involved in heat production, with a focus on cellular and molecular mechanisms, in a phylogenetic context encompassing the entire vertebrate diversity. We show that endothermy in mammals and birds is not as well defined as commonly assumed by evolutionary biologists and consists of a vast array of physiological strategies, many of which are currently unknown. We also describe strategies found in other vertebrates, which may not always be considered endothermy, but nonetheless correspond to a process of active thermogenesis. We conclude that endothermy is a highly plastic character in vertebrates and provides a guideline on terminology and occurrences of the different types of heat production in vertebrate evolution. This article is part of the theme issue 'Vertebrate palaeophysiology'.

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.

Modeling the costs and benefits associated with the evolution of endothermy using a robotic python

Endothermy provides considerable benefits to an organism but requires large energy investment. To understand potential driving forces that would lead to the evolution of endothermy, it is important to understand the energy costs and potential benefits of intermediate steps between ectothermy and homeothermic endothermy as well as the influences of environmental conditions on energetic costs. However, efforts to examine intermediate conditions are greatly limited by the predominant natural dichotomy between ectothermy and endothermy. Facultative endothermy by brooding pythons provides a fortunate study system where endothermy is beneficial but not essential. As one cannot control the extent of energy investment in heat production by a female python, we created an artificial snake with controllable heating capability. This enabled us to determine the energetic costs of maintaining a clutch at a preferred temperature, and to determine the relative thermal benefit of limited energy-producing capability (i.e. 50% of the required energy to maintain the preferred developmental temperature). We manipulated the pseudoserpent's clutch size (5, 10, 15 eggs), diel ambient temperature cycle (2, 4, 6°C) and insulation (with and without) at each of these power levels: unlimited power, half required power and no power. We found no significant effect of clutch size on either power requirements or developmental temperature. Energy requirements increased with the amplitude of the diel cycle and decreased with the addition of insulation, while the quality of the thermal environment decreased with the amplitude of the diel cycle. Interestingly, the quality of the thermal environment also decreased with the addition of insulation. We discuss these results within the context of the reproductive model of the evolution of endothermy.

Facultative thermogenesis during brooding is not the norm among pythons

Facultative thermogenesis is often attributed to pythons in general despite limited comparative data available for the family. While all species within Pythonidae brood their eggs, only two species are known to produce heat to enhance embryonic thermal regulation. By contrast, a few python species have been reported to have insignificant thermogenic capabilities. To provide insight into potential phylogenetic, morphological, and ecological factors influencing thermogenic capability among pythons, we measured metabolic rates and clutch-environment temperature differentials at two environmental temperatures-python preferred brooding temperature (31.5 °C) and a sub-optimal temperature (25.5 °C)-in six species of pythons, including members of two major phylogenetic branches currently devoid of data on the subject. We found no evidence of facultative thermogenesis in five species: Aspidites melanocephalus, A. ramsayi, Morelia viridis, M. spilota cheynei, and Python regius. However, we found that Bothrochilus boa had a thermal metabolic sensitivity indicative of facultative thermogenesis (i.e., a higher metabolic rate at the lower temperature). However, its metabolic rate was quite low and technical challenges prevented us from measuring temperature differential to make conclusions about facultative endothermy in this species. Regardless, our data combined with existing literature demonstrate that facultative thermogenesis is not as widespread among pythons as previously thought.

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.