Humpbacks are found in oceans throughout the world and are among the largest aquatic mammals. They feed in polar waters and then must fast and migrate up to 5,000 km to the tropical waters, where they breed and give birth. Proper intake of food is essential for humpbacks to pull off the extreme physical feat of annual migration, and the added demands of pregnancy make that energetic balance all the more critical if a healthy humpback calf is to enter the world.
“We know a calf’s survival is linked to maternal characteristics like body size and condition and that the calf’s size and energy stores determine its resilience and likelihood of survival,” explains Martin van Aswegen, MMRP Ph.D. candidate and lead author of the study. “It is therefore important to understand how female humpbacks use energy for reproduction and how such energy use can impact survival of their offspring, and ultimately their population.”
Using a combination of body length measurements of mother-fetus pairs from historical whaling records, length estimates of mother-calf pairs using drones, and tissue samples from deceased whales retrieved at sea, the study yielded a few key takeaways. Researchers confirmed that fetal growth rates and birth size increased with maternal length and made the new discovery that fetal length, volume, and mass increased exponentially throughout the pregnancy. The research team determined that the energy cost of the first two-thirds of the pregnancy was negligible, comprising only .01–1.08% of the energy used throughout the pregnancy. The majority of the energetic needs came in the final third of the pregnancy when energetic requirements ticked up to a whopping 98.2%. This means that the most demanding period of a humpback pregnancy occurs right when the females must fast and migrate thousands of miles across the ocean.
“It was surprising to see how the peak of energy requirements coincided with the onset of fasting in pregnant females, ultimately highlighting how crucial those final 100 days of pregnancy are for this migratory species,” notes van Aswegen. “Late-pregnant females are therefore particularly vulnerable to disruptions in energy balance, given periods of greatest energetic stress coincide with fasting and migration to sub-tropical breeding grounds,” explains van Aswegen. “Our study highlights a particularly vulnerable period for pregnant humpback whales. This is important because once these whales leave their high-latitude feeding grounds, they have a finite amount of energy available to invest in their offspring over a 3–5 month fasting period, with energy requirements being even higher after calf birth.”
Humpback whales in Hawai’i have been widely celebrated as a success story following decades of population growth and subsequent delisting from endangered species status. However, marked declines in abundance reproductive output, and survival in recent years have caused concern. A 75.6% decline in mother-calf encounter rates was documented in Hawai’i between 2013-2018. In Southeast Alaska, a principal foraging ground for Hawaiian humpback whales, a recent study shows calf production was approximately six times lower between 2015–2019 compared to pre-2015 years, with mid-summer calf mortality increasing tenfold from 2014–2019. These observations coincided with extended periods of intense warming anomalies across the Gulf of Alaska, reflecting the convergence of the 2014–2016 Pacific Marine Heatwave, a strong El Niño phase, and a positive Pacific Decadal Oscillation. Studies have subsequently reported significant and prolonged shifts in the distribution of the marine food web, resulting in poor feeding conditions for humpback whales. With the intensity and frequency of extreme climate events anticipated to increase, improving our understanding of the energetic cost of reproduction is a critical step in quantifying the effects of natural and anthropogenic perturbations on breeding humpback whales.
“This research underpins future studies on humpback whale energy demands,” explains Lars Bejder, co-author of the study and Director of the Marine Mammal Research Program. “Our drone-collected whale health database, developed in partnership with the Alaska Whale Foundation, includes over 11,000 measurements from 8,500 individual North Pacific whales. Its extensive temporal and spatial scale offers invaluable insights into the effects of large-scale climatic events on this iconic sentinel species. Sustaining such long-term, wide-scale studies is crucial for understanding these impacts within the context of natural variability in whale health.”
The success of this study, and many to follow, hinges on close partnerships among researchers.
“This research underscores the value of collaboration in tackling complex questions about the lives of humpback whales,” emphasizes Jens Currie, MMRP Ph.D. candidate, Chief Scientist at Pacific Whale Foundation, and co-author of the study. “Through large-scale collaborations, we’re able to gain critical insights into the challenges migratory whales face during pregnancy to better inform conservation strategies. Together, we can address large-scale ecological challenges that no single institution could achieve alone.”
Cetaceans throughout the globe face a slough of threats that include habitat degradation, climate change, fisheries, and chemical and noise pollution. One-quarter of the 92 known cetacean species are at risk of extinction, and there is a clear and urgent need to implement effective conservation strategies on their behalf. The animals’ ability to reproduce successfully is key to their survival, and carefully tracking and understanding nuances in this essential behavior makes resource managers better poised to adeptly monitor and conserve humpback feeding and breeding grounds.
A number of generous donors made this work possible. Hawaiʻi fieldwork was funded through the University of Hawaiʻi at Mānoa; 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; 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 (NGS), Lindblad Expeditions-National Geographic (LEX-NG) Funds, and the 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. Stranding response, necropsy, and tissue processing of the humpback whale calf were supported by the NOAA John H. Prescott Marine Mammal Rescue Assistance Grant Program.
