Maternal investment, body condition and calf growth in humpback whales.

Given recent declines in North Pacific humpback whale (Megaptera novaeangliae) reproductive output and calf survival, there is additional urgency to better understand how mother-calf pairs allocate energy resources across their migratory cycle. Here, unoccupied aerial system (UAS; or drone) photogrammetry was used to quantify the body size and condition (BC) of humpback whales on their Hawai'i (HI) breeding and Southeast Alaska (SEAK) feeding grounds. Between 2018 and 2022, we collected 2410 measurements of 1659 individuals. Rates of change in body volume (BV) and length (BL) were quantified using 803 repeat measurements of 275 individuals. On average, HI mothers lost 0.106 m³ or 96.84 kg day^-1 while fasting, equivalent to 2641 MJ day^-1 or 830 kg of krill and 424 kg of Pacific herring daily. HI calf BV and BL increased by 0.035 m³ and 2.6 cm day^-1, respectively. In SEAK, maternal BV increased by 0.015 m³ or 14.54 kg day^-1 (367 MJ day^-1), while calf BV and BL increased by 0.039 m³ and 0.93 cm day^-1, respectively. Maternal investment in calf growth correlated with both female BL and BC, with larger females producing larger, faster-growing calves. Finally, using 330 measurements from 156 females, we quantified differences in BC increase over four feeding seasons. Lactating females exhibited an average BC increase of 6.10%, half that of unclassified females (13.51%) and six times lower than pregnant females (37%). These findings represent novel insights into the life history of humpback whales across their migratory cycle, providing key baseline data for bioenergetic models elucidating the effects of anthropogenic disturbance and rapidly changing ocean ecosystems. KEY POINTS: On average, Hawai'i (HI) mothers lost 0.106 m³ or 96.84 kg day^-1, equivalent to 2641 MJ day^-1. Over a 60 day period, this corresponded to an estimated mean energetic cost of 158 GJ, or ≈50 tons of krill or ≈25 tons of Pacific herring, surpassing the total energetic cost of gestation estimated for humpback whales of similar length. In Southeast Alaska (SEAK), maternal body volume (BV) increased by just 0.015 m³ or 14.54 kg day^-1 (367 MJ day^-1). Further, SEAK lactating females showed the slowest rates of growth in body width and condition over a 150 day period compared to non-lactating females. Maternal investment in calf growth correlated with both maternal length and body condition, with larger females producing larger, faster-growing calves. In HI, however, the ratio between maternal BV lost and calf BV gained (conversion efficiency) was relatively low compared to other mammals.