New publication: Maternal investment, body condition and calf growth in humpback whales

Written by Martin van Aswegen

We are pleased to share a new publication in The Journal of Physiology, entitled: Maternal investment, body condition and calf growth in humpback whales. This publication is accompanied by another study estimating the energetic cost of gestation in humpback whales.

Authors: Martin van Aswegen, Andy Szabo, Jens Currie, Stephanie Stack, Lewis Evans, Jan Straley, Janet Neilson, Christine Gabriele, Kelly Cates, Debbie Steel, and Lars Bejder.

Figure 1. Graphical abstract summarizing key methods and findings from the study.

Abstract:

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 Hawaiʻi (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 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. 

Figure 2. Comparison images of the same mother-calf pair measured off Maui, Hawaiʻi. Through repeat measurements, we estimated this female lost ~4.6 tonnes or >10,000 lbs of blubber mass over 42 days. Over the same period, the length and body volume of her calf increased by 25 % and 116 %, respectively. Credit: Martin van Aswegen (Marine Mammal Research Program; NMFS permit 21476).

Study overview:

This study used drone-based photogrammetry to quantify the body size and condition (BC) of humpback whales on their Hawaiian breeding and Southeast Alaskan feeding grounds (Figure 1). A total of 2,410 measurements were taken from 1,659 individuals, with 405 repeat measurements from 137 lactating females used to track changes in maternal body volume over migration.

We found humpback whale mothers in Hawai‘i lost an average of 96.84 kg or 214 lbs of blubber per day, equivalent to 2,641 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. On average, mother whales in Hawai‘i lost 20% of their body volume over the first 60 days of lactation (Figure 2), with the corresponding energy loss surpassing the total energetic cost of pregnancy estimated for humpback whales of similar length.

Maternal investment in calf growth correlated with both maternal length and body condition, with larger females producing larger, faster-growing calves. In Hawai‘i, the ratio between maternal body volume lost and calf body volume gained (conversion efficiency; 33.58%) was relatively low compared to other mammals, suggesting only a third of energy lost by mothers each day translated into calf growth.

Lactating females in Southeast Alaska exhibited the slowest rates of growth in body width and condition over a 150-day period compared to non-lactating females, with pregnant and resting females gaining body condition at six and two times the rate of lactating females, respectively.

Eight lactating females and nine calves were repeatedly measured across Hawai‘i and Southeast Alaska (3,000 miles apart) within the same year (Figure 3), over a mean period of 176 days (SD = 38.67, range = 108 – 223). Over this period, lactating females decreased in body volume by an average 17.27%. Calf body volume increase averaged 394.9% (range = 108.1 – 685.6). Calf length increased by an average of 59.53% (range = 22.68 – 88.69).

Figure 3. Comparison images of the same mother-calf pair measured in both Hawaiʻi and Southeast Alaska within the same year. Differences in body volume (BV) and total length (TL) estimates are provided. Credit: Martin van Aswegen (Marine Mammal Research Program and Alaska Whale Foundation; NMFS permit numbers 21476 and 19703).

In Hawai‘i, humpback whales are important cultural, economic, educational, and environmental pillars. Between 2013 and 2019, studies documented a 76.5% decline in mother-calf encounter rates on the Hawaiian breeding ground between 2013-2018 with crude birth rates declining by ≈80% from 2015 to 2016. On the Southeast Alaskan feeding ground, there was total reproductive failure in 2018 with calf survival decreasing tenfold from 2014 to 2019. These observations coincided with the longest lasting marine heatwave globally, which was responsible for abrupt shifts in food webs and declines in prey availability throughout the North Pacific. It is believed humpback whales were unable to acquire sufficient energy on their feeding grounds, resulting in nutritional stress and marked declines in reproduction and abundance in Hawai‘i.

This study not only informs on the energetic cost required by humpback whales to produce offspring but also highlights the importance of Hawai‘i as a critical habitat during vulnerable phases of reproduction (early calf growth and lactation). This kind of information is key in modeling the effects of human activities and highlighting the importance of key habitats like Hawai‘i, ultimately informing effective conservation management strategies. 

Funding:

Hawaiʻi fieldwork was funded through the University of Hawaiʻi at Manoa, DoD's Defense University Research Instrumentation Program, ‘Our Oceans’, Netflix, Wildspace Productions and Freeborne Media, Office of Naval Research, Omidyar Ohana Foundation, the National Marine Sanctuary Foundation and PacWhale Eco‐Adventures as well as members and donors of Pacific Whale Foundation. Southeast Alaska research was funded through awards from the National Geographic Society, the Lindblad Expeditions‐National Geographic (LEX‐NG) Funds and North Pacific Research Board. Graduate Assistantships for Martin van Aswegen were funded by a Denise B. Evans Oceanography Fellowship, North Pacific Research Board grant and the Dolphin Quest General Science and Conservation Fund.

Acknowledgements:

We acknowledge the cultural significance of humpback whales (koholā) in Hawaiian culture as 'aumakua and a manifestation of Kanaloa. Our work involves a rigorous permitting process and the use of minimally invasive techniques to interact with whales. We thank the numerous research assistants who helped with fieldwork and data processing across Hawaiʻi and Southeast Alaska. We are grateful to Dana Bloch, Sonja Feinberg, Annie Bartlett, Jessie Hoffman, Abigail Machernis, Grace Olson, Florence Sullivan and Elizabeth Beato for their long‐term dedication and support with data collection and processing. We also thank PacWhale Eco‐Adventures, Chrissy Lovitt and Emma Nelson (Maui Ocean Adventures), Lee James, Ed Lyman, Marc Lammers, Jason Jones and the Guth family for their support. We thank C. Scott Baker for providing long-term sighting and sex identification data. We are grateful to Ted Cheeseman for his assistance with Happywhale and the following Happywhale data contributors: Suzanne Yin (Hawaiʻi Marine Mammal Consortium), Rich Dolan, Emily Cramer, Mindy Huston, Celia Ackerman and Annette G. E. Smith. We are grateful to Zoltan Nemeth (cetek‐art.com) for providing illustrations for the abstract figure. Finally, we thank the Applied Research Laboratory (University of Hawaiʻi) and Eva Leunissen for their technical support. 

Full citation:

van Aswegen, M., Szabo, A., Currie, J. J., Stack, S. H., Evans, L., Straley, J. M., Neilson, J. L., Gabriele, C. M., Cates, K., Steel, D., & Bejder, L. (2024). Maternal investment, body condition and calf growth in humpback whales. The Journal of Physiology. doi: 10.1113/JP287379.