The larger the cetacean, the more stationary their sleep? Thermoregulatory constraints on the sleep behaviour of captive cetaceans

Abstract Sleeping underwater is challenging for lung‐breathing animals because of their inability to breathe and rapid heat loss caused by high thermal conductivity. However, cetaceans can sleep/rest underwater despite these constraints. It has been hypothesized that sleep behaviour is related to the balance between heat loss and thermogenesis, relative to body size. We verified this hypothesis through inter‐and intraspecific comparisons using observational data on the sleeping/resting behaviour of 10 captive cetacean species. Our results quantitatively demonstrated that as body size decreased, the amount of behavioural sleep while being stationary (stationary sleep) reduced and the amount of behavioural sleep while swimming (swim sleep) increased. Conversely, the opposite occurred when body size increased. To further assess this correlation, the influence of ambient temperature on the choice of sleeping behaviour was investigated through intraspecific comparisons using nine captive bottlenose dolphins ( Tursiops truncatus ). At lower ambient temperatures, dolphins exhibited more swim sleep than stationary sleep. This suggests a potential sleep strategy that adjusts behaviour based on body size and ambient temperature, possibly owing to differences in heat loss. Our findings provide valuable insights into the thermal challenges cetaceans encounter during sleeping/resting underwater and how behavioural thermoregulation helps them overcome these challenges.