Dear colleagues, On behalf of my co-authors, we are pleased to share our recent publication published in The Journal of Physiology: van Aswegen, M, Szabo, A., Currie, J.J., Stack, S.H., West, K.L., Hofmann, N., Christiansen, F., & Bejder, L. (2024). Energetic cost of gestation and prenatal growth in humpback whales. *The Journal of Physiology*. doi: 10.1113/JP287304 <https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP287304>. Abstract: Improving our understanding of energy allocation in reproduction is key for accurately parameterizing bioenergetic models to assess population responses to environmental perturbations and anthropogenic disturbance. We quantified the energetic cost of gestation in humpback whales (*Megaptera novaeangliae*) using historical whaling records, non-invasive unoccupied aerial system (UAS) photogrammetry, and post-mortem tissue samples. First, we estimated relative birth size using body length measurements of 678 mother-fetus pairs from historical whaling records and 987 mother-calf pairs measured *in-situ* using UAS-photogrammetry. The total energetic cost of gestation includes fetal growth, heat increment of gestation, and placental tissue development. Fetal growth was modeled from conception to birth, with fetal volume and mass estimated using the volume-to-length relationship of perinatal calves and published humpback whale tissue composition estimates. Tissue-specific energy content was quantified using bone, muscle, viscera, and blubber samples from a post-mortem neonatal humpback whale. Placental tissue development was estimated using humpback whale placental tissue and published equations. Relative birth length was found to be 33.75% (95% CI: 32.10-34.61) of maternal length. Fetal growth rates and absolute birth size increased with maternal length, with exponential growth in fetal length, volume, and mass resulting in minimal energetic costs over the first two quadmesters (0.01-1.08%) before increasing significantly in the final quadmester (98.92%). Gestational heat constituted the greatest energetic cost (90.42-94.95%), followed by fetal (4.58-7.76%) and placental tissue growth (0.37-1.83%). Our findings highlight the energetic costs endured by capital breeding females preceding parturition, with the most substantial energetic costs of gestation coinciding with migration and fasting. Please feel free to email me for a pdf copy or if you have any questions. Best, Martin van Aswegen PhD Candidate | Marine Mammal Research Program Hawaiʻi Institute of Marine Biology |University of Hawaiʻi at Mānoa E: mvanas@hawaii.edu W: www.mmrphawaii.org