Energetic Costs of Extreme Heat: High Temperatures Elevate Daytime Activity and Suppress Nighttime Foraging in...

Extreme heat weather events can lead to energy deficits by increasing thermoregulatory energy costs while reducing energy intake because of limits to dissipating the metabolic heat generated by foraging activity. Understanding the consequences of exposure to extreme heat is increasingly important under ongoing climate warming. We measured acceleration forces using collar-mounted loggers to gain indices of activity and energy expenditure in grey-headed flying-foxes (Pteropus poliocephalus) during mild and hot summer conditions. Overall dynamic body acceleration (ODBA) is positively correlated with energy expenditure in a wide range of animals. During the daytime rest phase, air temperatures above 30°C decreased low-level activity indicative of rest, whereas during the nocturnal active phase, temperatures above 21°C reduced high-level activity indicative of flight. On hot days (maximum T _a : 40.6°C ± 3.1°C), ODBA increased threefold during the daytime and decreased by one-third during the first 2 h of the night, compared to milder summer conditions. These patterns indicate that extreme heat simultaneously elevates energy expenditure and constrains foraging activity. Our data support the hypothesis that heat-induced energetic deficits are an important component of the negative effects of extreme heat exposure. Such energy deficits could reduce survival and population growth, especially as heat events become more frequent and severe under climate change.