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Nightjars represent a model taxon for investigating physiological limits of heat tolerance because of their habit of roosting and nesting in sunlit sites during the heat of the day. We investigated the physiological responses of Rufous-cheeked nightjars (Caprimulgus rufigena) and Freckled nightjars (Caprimulgus tristigma) to high air temperatures (T a ) by measuring body temperature (T b ), resting metabolic rate (RMR) and total evaporative water loss (TEWL) at T a ranging from 10 to 56 °C. Both species became hyperthermic at T a  > T b . Lower critical limits of thermoneutrality occurred at T a between 35 and 37 °C, whereas we detected no clear upper critical limits of thermoneutrality. Between T a  ≈ 37.0 and 39.9 °C, rates of TEWL increased rapidly with T a . At T a  ≥ 40 °C, fractional increases in mass-specific TEWL rates were 78-106% of allometric predictions. Increasing evaporative heat dissipation incurred only small metabolic costs, with the RMR of neither species ever increasing by more than 20% above thermoneutral values. Consequently, both species displayed extremely efficient evaporative cooling; maximum evaporative heat dissipation was equivalent to 515% of metabolic heat production (MHP) at T a  ≈ 56 °C in C. rufigena and 452% of MHP at T a  ≈ 52 °C in C. tristigma. Our data reiterate that caprimulgids have evolved an efficient mechanism of evaporative cooling via gular fluttering, which minimizes metabolic heat production at high T a and reduces total heat loads. This likely aids in reducing TEWL rates and helps nightjars cope with some of the most thermally challenging conditions experienced by any bird.

In mammals, reproduction, especially for females is energetically demanding. Therefore, during the reproductive period females could potentially adjust patterns of thermoregulation and foraging in concert to minimise the energetic constraints associated with pregnancy and lactation. We assessed the influence of pregnancy, lactation, and post-lactation on torpor use and foraging behaviour by female little brown bats, Myotis lucifugus. We measured thermoregulation by recording skin temperature and foraging by tracking bats which carried temperature-sensitive radio-tags. We found that individuals, regardless of reproductive condition, used torpor, but the patterns of torpor use varied significantly between reproductive (pregnant and lactating) females and post-lactating females. As we predicted, reproductive females entered torpor for shorter bouts than post-lactating females. Although all females used torpor frequently, pregnant females spent less time in torpor, and maintained higher skin temperatures than either lactating or post-lactating females. This result suggests that delayed offspring development which has been associated with torpor use during pregnancy, may pose a higher risk to an individual's reproductive success than reduced milk production during lactation. Conversely, foraging behaviour of radio-tagged bats did not vary with reproductive condition, suggesting that even short, shallow bouts of torpor produce substantial energy savings, likely obviating the need to spend more time foraging. Our data clearly show that torpor use and reproduction are not mutually exclusive and that torpor use (no matter how short or shallow) is an important means of balancing the costs of reproduction for M. lucifugus.

Most studies of hibernation physiology sample individuals from populations within a single geographic area, yet some species have large ranges meaning populations likely experience area-specific levels of energetic challenges. As well, few studies have assessed within-season variation. Since physiological adjustments often are influenced by environmental factors, and the types of environments vary with geography, we expected variance in hibernation patterns among geographically separated populations. Our specific goal was to measure intraspecific variation in torpid metabolic rate (TMR) and body temperature (T (b)) as a function of ambient temperature (T (a)) for a non-migratory and migratory species to determine whether there is a continuum in physiological responses based on latitude. We chose big brown (Eptesicus fuscus) and eastern red bats (Lasiurus borealis) as model species and sampled individuals from populations throughout each species' winter range. In both species, individuals from southern populations maintained higher TMR at cooler T (a)s and lower TMR at warmer T (a)s than those from northern populations. Big brown bats from southern populations regulated T (b) during torpor at higher levels and there was no significant difference in T (b) between populations of eastern red bats. Although metabolic responses were similar across the gradient between species, the effect was more dramatic in big brown bats. Our data demonstrate a continuum in thermoregulatory response, ranging from classic hibernation in northern populations to a pattern more akin to daily torpor in southern populations. Our research highlights the potential usefulness of bats as model organisms to address questions about within-species physiological variation in wild populations.

Altricial mammals and birds become endothermic at about half the size of adults and presumably would benefit energetically from entering torpor at that time. Because little is known about torpor during development in endotherms, we investigated whether after the establishment of endothermic thermoregulation (i.e. the ability to maintain a high body temperature during cold exposure), Sminthopsis macroura, a small (approximately 25 g) insectivorous marsupial, is capable of entering torpor and whether torpor patterns change with growth. Endothermic thermoregulation was established when the nest young reached a body mass of approximately 10 g, and they were capable of entering torpor early during development at approximately 10-12 g, lending some support to the view that torpor is a phylogenetically old mammalian trait. Torpor bout length shortened significantly and the minimum metabolic rate during torpor increased as juveniles approached adult size, and consequently total daily energy expenditure increased steeply with age. Relationships between total daily energy expenditure and body mass during development of S. macroura (slope approximately 1.3) differed substantially from the relationship between basal metabolism and body mass in adult endotherms (slope approximately 0.75) suggesting that the energy expenditure-size relationship during the development differs substantially from that in adults under thermo-neutral conditions. Our study shows that while torpor can substantially reduce energy expenditure during development of endotherms and hence is likely important for survival during energy bottlenecks, it also may enhance somatic growth when food is limited. We therefore hypothesize that torpor during the development in endotherms is far more widespread than is currently appreciated.