UC Berkeley

Endotherms, unlike ectotherms, generate metabolic heat to meet their thermoregulatory demands. As homeotherms their body temperature stays within a certain range of body temperature, often higher than the environment. This ability to maintain a biochemically optimal body temperature bestows protection against unfavourable thermal conditions for endotherms. The energetic costs associated with thermoregulatory metabolism and the limits for thermoregulatory capacity will, however, still set a boundary upon the range and duration of conditions that are tolerated by an endotherm. Especially within the context of global climate change, the effect of ahistorically high ambient temperatures on endotherms will depend heavily on the cost the new conditions will impose upon them. Despite various statistical and mechanistic evaluations of endotherm energy metabolism and heat exchange, these costs and limits are poorly known on the interspecific level. In the first chapter, we review and summarise current understanding of endotherm thermal physiology, identify the unresolved questions and examine the conceptual and methodological challenges to a consensus.

The field metabolic rate (FMR) of an endothermic animal represents its energy expenditure in a natural environment, or its energy budget, and the field water flux of an animal reflects the animal’s water requirements. In the second chapter, we examined FMR of 97 avian species and field water flux of 76 species for adults from direct field measurements by the doubly-labelled water (DLW) method, and conducted a phylogenetically informed, comprehensive analysis of the relationship between FMR and field wtaer flux, and multiple environmental and biological variables. FMR was strongly associated with body mass with an allometric exponent of 0.6, and seabirds had lower FMR than terrestrial birds. Birds consuming plant matter had lower FMR compared to omnivores, carnivores or nectarivores, and low ambient temperature was associated with higher FMR. Unlike for BMR, there was scarce evidence for phylogenetic covariance in FMR. Life history traits such as fecundity and migration were also not strongly associated with FMR. Field water flux was strongly associated with body mass with an allometric exponent of 0.6, and seabirds, breeding birds and birds from relatively humid habitats had higher field water flux. There was considerable evidence for phylogenetic covariance, but granivory and mean temperature were not associated with field water flux.

Endotherms maintain a stable body temperature at high ambient temperatures by continuous dissipation of metabolic heat to the environment. This requires endotherms to maintain a balance between heat dissipation and water conservation at multiple temperatures. Birds are relatively small, contain a large amount of metabolically expensive tissue, and are mostly diurnal. As a result, they are severely affected by physiological challenges. In the third chapter, we compiled evaporative water loss measurements for birds exposed to different temperatures and examined their relationships with ambient temperature, precipitation, diet and circadian cycles. While evaporative water loss at normothermic conditions was associated with precipitation and water content in diet, ambient temperature in their natural habitat was important when birds were exposed to heat stress. Additionally, the circadian cycle had an effect on water evaporation. Activephase birds that had higher resting metabolic rates lost more water through evaporation than resting-phase birds. The direction and magnitude of relationship between ambient temperature and evaporative water loss at a high temperature also differed between active- and resting-phase birds. These results suggest that birds do not experience a tradeoff between water conservation at lower temperatures and efficient heat dissipation under heat stress, and that the activity level of a bird may affect the ability to dissipate heat through water evaporation.

The upper critical temperature (UCT) of an endotherm represents the temperature that requires it to actively dissipate mettabolic heat through increased energy consumption. Endotherms have relatively low variation in UCT compared to the variation in the lower critical temperature (LCT), but environmental conditions are also linked to intraspecific and interspecific variation in UCT. Difficulty in estimating UCT and differences in data quality complicate the comparative analysis of this variation in UCT. In the fourth chapter, we compiled estimates of UCT for 210 bird species, categorised the information by data quality, and evaluated the relationship between UCT and ambient temperature, precipitation, seasonality, migration and circadian cycle. UCT was positively correlated with maximum temperature in the natural range and negatively correlated with body mass, long-distance migration and summer precipitation. Birds in the resting phase of the circadian cycle had lower UCT compared to birds in the active phase. Including the studies that did not include temperatures well above UCT resulted in slightly reduced coefficient estimates for UCT and thus lower accuracy. However, data quality did not severely impact the precision of estimates.