Insights into Humpback Whale Maternal Energy Demands Revealed by Drone Technology

A new study published in The Journal of Physiology provides valuable insights into the energy demands of lactating humpback whales and their calves during migration. Led by biologists at the University of Hawaii at Manoa's Marine Mammal Research Program, in collaboration with several partners, the research highlights the urgent need to understand the challenges faced by mother-calf pairs, especially given recent declines in the reproductive success and survival of calves in the North Pacific.

The team utilized drone imagery to measure the growth of whale calves and the condition of their mothers shortly after calf birth in Hawaii. These measurements were compared with data from humpback females in the feeding grounds of Alaska, assessing both pregnant and lactating females. The study revealed how maternal energy expenditure during lactation impacts both the mothers and their calves.

The research found that larger female humpback whales produced larger, faster-growing calves. Over six months, lactating females lost about 17% of their body volume, while calves saw an incredible 395% increase in body volume and a nearly 60% increase in length. In Hawaii, the mothers lost an average of 214 pounds of blubber per day, equating to a loss of about 50 tons of krill or 25 tons of Pacific herring over 60 days. This energy expenditure was greater than the total cost of their year-long pregnancy.

In Southeast Alaska, lactating mothers gained weight at the slowest rate compared to non-lactating females, with weight gains about six times slower than pregnant females and 32 pounds per day slower than non-pregnant females. One of the most surprising aspects of the study was the ability to track individual mothers and calves across vast distances. The team was able to take repeat measurements from 137 lactating females and follow eight mother-calf pairs over 3,000 miles between Hawaii and Alaska, offering unique insights into their migratory journey.

The findings of this study are particularly timely, as the North Pacific has experienced significant disruptions to marine ecosystems. Research has shown that from 2013 to 2018, the rate of mother-calf encounters in Hawaii dropped by 76.5%, and birth rates fell by 80% between 2015 and 2016. Additionally, in Southeast Alaska, reproductive failure was reported in 2018, with calf survival dropping tenfold from 2014 to 2019. These declines coincided with a prolonged global marine heatwave, which altered food webs and reduced prey availability, leading to nutritional stress in humpback whales.

This study sheds light on the critical role of Hawaii as a breeding ground and refines our understanding of the energy requirements for humpback whales during calf production. It also emphasizes the vulnerability of whales during early calf growth and lactation stages, underscoring the importance of managing these habitats to protect both the whales and their calves, ensuring the survival of the species.