Wide-ranging, year-round breaching behaviour of basking sharks revealed by long-term biologging

Marine megafauna, including various species of cetaceans and fishes, exhibit energetically costly breaching behaviour during which individuals rapidly propel themselves out of the water. The function of breaching remains enigmatic, largely due to short-duration observations of limited spatial extent. Here, we used animal-borne biologgers to record breaching events of plankton-feeding basking sharks (Cetorhinus maximus) over full annual cycles at high temporal resolution (5 s) across broad spatial scales (35° latitude). Recording 577 breaches across four individuals over 366 days (mean 144 yr⁻¹ shark⁻¹), breaching appears to be a routine behaviour in both sexes, peaking in autumn, but occurring across all seasons and habitats, including deep oceanic areas. Notably, we identified thermal constraints on breaching rates in colder water, with a critical minimum observed at 5°C. Beyond demonstrating that breaching is more common than previously recognized, our study suggests it serves several functions, some potentially independent of spatial and temporal contexts (e.g. seasonal courtship), such as ectoparasite removal. The ubiquity of breaching across space and time highlights the need for comprehensive 'behaviour maps' for ocean giants to elucidate the physiological constraints, ecological significance and management implications of breaching in a changing ocean.