Age-specific body length, mass, and energetic cost of growth in humpback whales

Body size is a fundamental parameter in modeling the energetic trade-offs related to growth, activity, maintenance, and reproduction. This study integrated photogrammetry from an unoccupied aerial system (UAS; drone) with individual life histories, historical whaling records, and post-mortem tissue samples to estimate body mass and compare somatic growth costs in humpback whales Megaptera novaeangliae . Using 2651 UAS-derived length measurements and demographic information of 1503 individuals, sex-specific von Bertalanffy growth curves were developed from birth to 50 yr. Asymptotic lengths were estimated at 13 m for females and 12.34 m for males, with calves reaching 71-75% of their asymptotic length by weaning (~1 yr). Volume-at-age curves of whales in average body condition were converted to mass using UAS-derived body volume estimates, post-mortem tissue density samples, and published estimates of tissue composition. The cost of growth in calves and adults was modeled using tissue-specific lipid and protein concentrations derived from post-mortem tissue samples and published estimates, respectively. We found that postnatal offspring growth requires ~38 times more energy than prenatal growth, with calves needing 6-8 times more energy per day than mature adults. The cumulative cost of growth averaged 713 GJ for females and 607 GJ for males. We highlight a critical period of energetic demand for lactating females, who must support one-third of their offspring’s lifetime growth costs in just 1 yr. Estimating body mass and the cost of growth in humpback whales offers valuable insights for bioenergetic models assessing the effects of disturbance and a rapidly changing climate.