Amphibian responses in experimental thermal gradients: Concepts and limits for inference

Longer days, larger grays: carryover effects of photoperiod and temperature in gray treefrogs, Hyla versicolor

Environmental conditions like temperature and photoperiod can strongly shape organisms' growth and development. For many ectotherms with complex life cycles, global change will cause their offspring to experience warmer conditions and earlier-season photoperiods, two variables that can induce conflicting responses. We experimentally manipulated photoperiod and temperature during gray treefrog (Hyla versicolor) larval development to examine effects at metamorphosis and during short (10-day) and long (56-day) periods post-metamorphosis. Both early- and late-season photoperiods (April and August) decreased age and size at metamorphosis relative to the average-season (June) photoperiod, while warmer temperatures decreased age but increased size at metamorphosis. Warmer larval temperatures reduced short-term juvenile growth but had no long-term effect. Conversely, photoperiod had no short-term carryover effect, but juveniles from early- and late-season larval photoperiods had lower long-term growth rates than juveniles from the average-season photoperiod. Similar responses to early- and late-season photoperiods may be due to reduced total daylight compared with average-season photoperiods. However, juveniles from late-season photoperiods selected cooler temperatures than early-season juveniles, suggesting that not all effects of photoperiod were due to total light exposure. Our results indicate that despite both temperature and photoperiod affecting metamorphosis, the long-term effects of photoperiod may be much stronger than those of temperature.

Behavioral responses during sickness in amphibians and reptiles: Concepts, experimental design, and implications for field studies

In ectothermic vertebrates, behavioral fever, where an individual actively seeks warmer areas, seems to be a primary response to pathogens. This is considered a broad and evolutionarily conserved response among vertebrates. Recent population declines in amphibians are associated with an increase of infectious disease driven largely by climate change, habitat degradation, and pollution. Immediate action through research is required to better understand and inform conservation efforts. The literature available, does not provide unifying concepts that can guide adequate experimental protocols and interpretation of data, especially when studying animals in the field. The aim of this review is to promote common understanding of terminology and facilitating improved comprehension and application of key concepts about the occurrence of both sickness behavior or behavioral fever in ectothermic vertebrates. We start with a conceptual synthesis of sickness behavior and behavioral fever, with examples in different taxa. Through this discussion we present possible paths to standardize terminology, starting from original use in endothermic tetrapods which was expanded to ectothermic vertebrates, particularly amphibians and reptiles. This conceptual expansion from humans (endothermic vertebrates) and then to ectothermic counterparts, gravitates around the concept of 'normality'. Thus, following this discussion, we highlight caveats with experimental protocols and state the need of a reference value considered normal (RVCN), which is different from experimental control and make recommendations regarding experimental procedures and stress the value of detailed documentation of behavioral responses. We also propose some future directions that could enhance interaction among disciplines, emphasizing relationships at different levels of biological organization. This is crucial given the increasing convergence of fields such as thermal physiology, immunology, and animal behavior due to emerging diseases and other global crises impacting biodiversity.

How seasonality influences the thermal biology of lizards with different thermoregulatory strategies: a meta-analysis

Ectotherms that maintain thermal balance in the face of varying climates should be able to colonise a wide range of habitats. In lizards, thermoregulation usually appears as a variety of behaviours that buffer external influences over physiology. Basking species rely on solar radiation to raise body temperatures and usually show high thermoregulatory precision. By contrast, species that do not bask are often constrained by climatic conditions in their habitats, thus having lower thermoregulatory precision. While much focus has been given to the effects of mean habitat temperatures, relatively less is known about how seasonality affects the thermal biology of lizards on a macroecological scale. Considering the current climate crisis, assessing how lizards cope with temporal variations in environmental temperature is essential to understand better how these organisms will fare under climate change. Activity body temperatures (Tb ) represent the internal temperature of an animal measured in nature during its active period (i.e. realised thermal niche), and preferred body temperatures (Tpref ) are those selected by an animal in a laboratory thermal gradient that lacks thermoregulatory costs (i.e. fundamental thermal niche). Both traits form the bulk of thermal ecology research and are often studied in the context of seasonality. In this study, we used a meta-analysis to test how environmental temperature seasonality influences the seasonal variation in the Tb and Tpref of lizards that differ in thermoregulatory strategy (basking versus non-basking). Based on 333 effect sizes from 137 species, we found that Tb varied over a greater magnitude than Tpref across seasons. Variations in Tb were not influenced by environmental temperature seasonality; however, body size and thermoregulatory strategy mediated Tb responses. Specifically, larger species were subjected to greater seasonal variations in Tb , and basking species endured greater seasonal variations in Tb compared to non-basking species. On the other hand, the seasonal variation in Tpref increased with environmental temperature seasonality regardless of body size. Thermoregulatory strategy also influenced Tpref , suggesting that behaviour has an important role in mediating Tpref responses to seasonal variations in the thermal landscape. After controlling for phylogenetic effects, we showed that Tb and Tpref varied significantly across lizard families. Taken together, our results support the notion that the relationship between thermal biology responses and climatic parameters can be taxon and trait dependent. Our results also showcase the importance of considering ecological and behavioural aspects in macroecological studies. We further highlight current systematic, geographical, and knowledge gaps in thermal ecology research. Our work should benefit those who aim to understand more fully how seasonality shapes thermal biology in lizards, ultimately contributing to the goal of elucidating the evolution of temperature-sensitive traits in ectotherms.

Ecophysiology of neotropical amphibians and reptiles: lessons learned from Colombia

Ecophysiology and herpetology share a close historical relationship, but earlier work at the interface of these disciplines was carried out in temperate regions. Tropical regions like the Neotropics exhibit the highest species richness for amphibians and reptiles, but the pace for ecophysiological research on tropical herpetofauna has been slower relative to temperate counterparts. We are a group of early-career, Latin American researchers interested in the physiological diversity exhibited by neotropical herpetofauna. As such, we have engaged in the organization of the Symposium on the Ecophysiology of Neotropical Amphibians and Reptiles (ECOPHYSHERP) to integrate the scientific community interested on these topics. ECOPHYSHERP has been held three times already within the Colombian Congress of Herpetology, and collectively it has hosted >60 contributions from researchers at 26 institutions and eight countries. Participation has been diverse in terms of gender, age, and career stage, but most participants have been young undergraduate biology students. This generation of early-career researchers is producing excellent research in a broad range of topics, but difficulties to convert this research into scientific publications may exist. Identifying and contributing in order to solve such problems are priorities for this organizing committee, and also our endeavours towards ECOPHYSHERP 4.0 in Santa Marta in 2025.

Response to thermal and hydric regimes point to differential inter- and intraspecific vulnerability of tropical amphibians to climate warming

In Puerto Rico, an island threatened by climate warming, only one of two species of frogs that share part of their distribution has undergone a recent range contraction to higher elevations. We questioned if differences in their physiological response to temperature and dehydration might explain this distributional change. We studied a lowland and a highland population of Eleutherodactylus coqui, a widespread generalist, and E. portoricensis, an endangered species that is currently found only above 600 m. We compared various physiological aspects: operative temperature; temperature selection; critical temperatures; and their response to jumping performance tests at various thermal and hydric regimes. Results revealed that E. portoricensis had the highest CT_min and lowest CT_max and selected a cooler range of temperatures from the experimental gradient. Jumping performance increased with temperature for the three populations until attaining maximum performance. Afterwards, performance dropped drastically until reaching CT_max. Dehydration had a negative effect on performance for both species, particularly on maximum performance. This effect was greatest for E. portoricensis, followed by high-elevation E. coqui. The significantly greater thermo-hydric physiological limitations of E. portoricensis may explain its recent range contraction, potentially, as a response to climate warming. Low-elevation E. coqui had the lowest operative warming tolerance and was the only population to select temperatures like those encountered in their environment, indicating it may be narrowly adapted to local thermal conditions and thus, also vulnerable to climate change. Our results point towards plasticity in the response of E. coqui to varying climatic conditions, and present evidence of different physiological responses between closely related species at the same locality. This work highlights the importance of studying the combined effects of temperature and hydration to understand the response of ectotherms to warming environments and presents further evidence that desiccation may be a limiting factor determining which species may survive.

Staying warm is not always the norm: behavioural differences in thermoregulation of two snake species

Thermal biology research compares field with laboratory data to elucidate the evolution of temperature-sensitive traits in ectotherms. The hidden challenge of many of these studies is discerning whether animals actively thermoregulate, since motivation is not typically assessed. By studying the behaviours involved in thermoregulation, we can better understand the mechanisms underlying body temperature control. Using an integrative approach, we assessed the thermoregulatory and thermotactic behaviours of two sympatric snake species with contrasting life histories: the generalist Eastern Garter Snake (Thamnophis sirtalis sirtalis ( Linnaeus, 1758 )) and the semi-fossorial Northern Red-bellied Snake (Storeria occipitomaculata occipitomaculata ( Storer, 1839 )). We expected that thermoregulatory behaviours would be optimized based on life history, in that T. s. sirtalis would show higher evidence for thermally oriented behaviours than S. o. occipitomaculata due to its active nature. Thamnophis sirtalis sirtalis actively thermoregulated, had higher thermal preferences (29.4 ± 2.5 vs. 25.3 ± 3.6 °C), and was more active than S. o. occipitomaculata, which showed relatively low evidence for thermotaxis. Our results build on the notion that evaluating movement patterns and rostral orientation towards a heat source can help ascertain whether animals make thermally motivated choices. Our data provide insight into the thermoregulatory strategies used by snakes with different life histories and maximize the information provided by behavioural thermoregulation experiments.