Dear colleagues, My co-authors and I are pleased to share our recent open-access publication in Marine Ecology Progress Series: van Aswegen, M., Szabo, A., Currie, J., Stack, S., Straley, J., Neilson, J., Gabriele, C., Cates, K., Baker, C., Steel, D., Pack, A., West, K., Hofmann, N., Hofmann, N., Toonen, R., Toonen, R., Loockerman, C., Sullivan, F., Cheeseman, T., Sharpe, F., Bejder, L. (2025). Age-specific body length, mass, and energetic cost of growth in humpback whales. *Mar Ecol Prog Ser*, 770, 171-194. doi: 10.3354/meps14937 Abstract: 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. Please feel free to email me if you have any questions. Best, Martin *Martin van Aswegen* Postdoctoral Researcher *|* Marine Mammal Research Program Hawaiʻi Institute of Marine Biology *|* University of Hawaiʻi at Mānoa _______________________________________________ *E: *mvanas@hawaii.edu *W: *www.mmrphawaii.org *A: *46-007 Lilipuna Rd, Box 1346, HI 96744