Behavioral plasticity during acute heat stress: heat hardening increases the expression of boldness.

Climate change is creating novel thermal environments via rising temperatures and increased frequency of severe weather events. Short-term phenotypic adjustments, i.e., phenotypic plasticity, may facilitate species persistence during adverse environmental conditions. A plastic response that increases thermal tolerance is heat hardening, which buffers organisms from extreme heat and may enhance short term survival. However, heat hardening responses may incur a cost with concomitant decreases in thermal preference and physiological performance. Thus, phenotypic shifts accompanying a hardening response may be maladaptive in warming climates. Understanding how heat hardening influences other traits associated with fitness and survival will clarify its potential as an adaptive response to altered thermal niches. Here, we studied the effects of heat hardening on boldness behavior in the color polymorphic tree lizard, Urosaurus ornatus. Boldness in lizards influences traits such as territory maintenance, mating success, and survivorship and is repeatable in U. ornatus. We found that when lizards underwent a heat hardening response, boldness expression significantly increased. This trend was driven by males. Bolder individuals also exhibited lower field active body temperatures. This behavioral response to heat hardening may increase resource holding potential and territoriality in stressful environments but may also increase predation risk. This study highlights the need to detail associated phenotypic shifts with stress responses to fully understand their adaptive potential in rapidly changing environments.

Thermal ecology of Northern Water Snakes (Nerodia sipedon) in a northern wetland in central Michigan.

Semi-aquatic north-temperate reptiles experience challenging environmental conditions for thermoregulation on both seasonal and daily bases. We studied the thermal biology of north-temperate Northern Water Snakes (Nerodia sipedon) in central Michigan by remote radiotelemetry monitoring of snake body temperature (Tb) using surgically implanted thermally-sensitive radio-transmitters and by measuring operative temperatures (Te) across a range of microhabitats using biophysical models. Our goals included evaluation of thermoregulatory capabilities at a locality that we viewed to be a fine-grained thermal environment, seasonal variations in patterns of thermoregulation, and the effects of sex on thermoregulation. During summer, snakes showed diel Tb cycling apparently using the open Sphagnum mat for late morning warming and shuttling among different microhabitats until early evening when a monotonic decline in Tb ensued and continued through the early morning hours. Snakes attained Tb within their laboratory-determined preferred body temperature range (Tset = 28-33 °C) mostly during late afternoon and with average percentages of Tb values for individual snakes within Tset range when permitted by operative temperatures (Ex) between 68 and 70% of the time depending on method of measurement. Relatively high investment in thermoregulation when thermal conditions were poor occurred only during September but declined thereafter as snakes prepared to overwinter. We did not detect differences in thermoregulation among reproductive females, non-reproductive females, and males. Relative to a population of N. sipedon at a higher latitude in Ontario our snakes showed a relatively high Tset range and thermoregulated more effectively, particularly during the daylight hours. It remains unclear how much inter-population variation in thermoregulation is due to potential adaptations to latitude, to habitat differences, or variations in methodologies.

Territorial status is explained by covariation between boldness, exploration, and thermal preference in a colour polymorphic lizard.

Colour polymorphic species often exhibit variation in morphology, physiology, and behaviour among morphs. In particular, dominance status may be signalled by the interaction between behaviour and colour morph. Behavioural traits associated with dominance include boldness, exploration, and aggression, which influence access to preferred habitat, territorial defence, and mate acquisition. In ectotherms, the social structure associated with morphs may result in the exploitation of structural niches differing in thermal quality. Hence, social interactions among morphs may generate concordant variation in thermal preference and environmental temperature. However, few studies have assessed thermal preference variation in colour polymorphic species and its covariation with behaviour. Doing so can provide insight into niche specialization and the maintenance of colour polymorphism in populations. Here, we investigated the patterns of covariation in boldness behaviour, exploratory behaviour, and thermal preference in the tree lizard, Urosaurus ornatus. We assessed trait variation between territorial and non-territorial male morphs and between orange and yellow female morphs. Boldness and exploratory behaviour were repeatable in male U. ornatus and bolder individuals were significantly more likely to incur tail loss, a potential consequence of bold behaviour. Territorial male morphs were significantly bolder and more exploratory and preferred higher body temperatures with a narrower T set than non-territorial morphs. Female morphs did not vary in behavioural or thermal traits. This study highlights behavioural mechanisms that underly ecological niche segregation and variable habitat use between morphs in a colour polymorphic species.