Energetic benefits of resting behaviour in humpback whale mother-calf pairs revealed by biologging and UAS-photogrammetry.

Quantifying energy expenditure of cetacean mother-calf pairs is essential for modelling population-level responses to environmental change and anthropogenic disturbance, especially for capital breeders where mothers face the compounding demands of fasting while lactating. Technological advances in biologging and unoccupied aerial system (UAS; drone) photogrammetry help overcome challenges in translating respirometry methods from animals under human care to the field, creating better energy-use estimates in large cetaceans. We quantified energy expenditure of humpback whale (Megaptera novaeangliae) mothers and calves on their winter breeding ground in Hawai'i using 51 biologging suction-cup tag deployments paired with simultaneous UAS-photogrammetric measurements (n = 20 mothers, n = 31 calves) from 2020 to 2025. We used two independent methods, Thrust Power and Breathing Frequency, to calculate per-second energy expenditure, divided deployments into resting dives and other behavioural states and modelled state-specific energy use and daily energy expenditure for mothers and calves. Calves spent more time in resting dives as they aged, and energy expenditure was on average 44% and 35% lower during resting dives for mothers and calves, respectively, compared to other behaviour. Simulated daily energy expenditure for calves ranged from 92 MJ for the smallest and youngest (3.5 m, 1 day) to 452 MJ for the largest and oldest (7.7 m, 110 days), and for mothers from 817 MJ for small females (10 m, 15 000 kg) to 2013 MJ for large females (14 m, 50 000 kg). To illustrate the conservation implications of these energy savings, we simulated the energetic cost of vessel-disturbance-induced reductions in resting behaviour for a representative mother-calf pair. Under a realistic daytime disturbance scenario, daily pair energy expenditure increased by 64 MJ day-1, accumulating to an additional 150 kg of maternal blubber catabolized over 60 days of disturbance. Our findings provide best estimates of humpback whale mother-calf daily and behavioural state-specific energy expenditure, enabling improved quantification of the energetic consequences of disturbance.