Sex-specific association between sleep and basal metabolic rate in great tits

Differences in animal behaviour can arise from individual variation in energy resource allocation decisions. Because energy is essential to fuel all processes that permit behaviour, it is necessary to consider metabolism for a more complete understanding of behavioural ecology. Although many studies have explored interspecific relationships between metabolic rate and behaviour, few studies have evaluated within-species relationships between metabolism and sleep. We investigated the relationship between basal metabolic rate (BMR) and components of sleep behaviour measured in wild great tits, Parus major, during winter and the prebreeding period. Individuals with higher metabolic rates may partially offset their costs by using sleep as an energy conservation strategy, where individuals with higher BMR may sleep more. On the other hand, the energetic savings of longer sleep may not be worth the lost foraging opportunities and therefore higher BMR individuals may sleep less. Our results suggest that the relationship between BMR and sleep behaviours may depend on sex. Female great tits displayed a positive relationship between metabolic rate and sleep duration consistent with energy conservation, or protection, while male great tits displayed a negative relationship. Differences in sleep duration came about largely due to a sex-specific interaction between BMR and sleep onset time; we found no relationship between BMR and time of awakening in either sex. Nor did it appear that individuals compensated for changes in duration of sleep with changes to quality of sleep, measured as frequency of night-time awakenings. This suggests that male and female great tits use different sleep strategies based on their metabolic requirements which may contribute to variation in sleep behaviour within a species. Sex-specific differences in metabolic physiology, or fitness-enhancing behaviours throughout the circannual cycle may contribute to variation in energy-balancing strategies within and between the sexes.