Study: Peak energy requirements for humpback moms-to-be coincide with migration, fasting
It would be an amazing accomplishment of its own to be able to eat enough for yourself to have enough energy to take a 3,000 mile swim from your summer home to a winter abode, where you then fast for at least 90 days before swimming back, all the while still not eating.
Now, imagine having to do it weighing 35 tons and pregnant.

Proper intake of food is essential for expecting humpback whale moms to pull off the extreme physical feat of annual migration between Alaska and Hawaiʻi.
Researchers with the Marine Mammal Research Program at the University of Hawaiʻi at Mānoa Hawaiʻi Institute of Marine Biology revealed the energetic cost and vulnerabilities of migratory humpback mothers-to-be in a study recently published in The Journal of Physiology.
The Alaska-Hawai’i population of humpbacks feed in polar waters and then fast and migrate up to about 3,100 miles to the tropical waters around the island state, where they breed and give birth.
Humpback whale females spend about 10 months in pregnancy, averaging about 100 days a trimester.
Using a variety of new and historical records of measurement, the University of Hawai’i-led research team found that the size of expecting humpback mothers directly correlated to the size of the fetus.
The larger the mother-to-be, the larger the fetus — and the larger the growth rate.
The team determined that the energy costs of the first two-thirds of the pregnancy were negligible, comprising 0.01% to 1.08% of the energy used.
The majority of energy needs came in the third trimester, when mom-to-be requirements spiked to 98.2%.
“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,” said Martin van Aswegen, a doctoral candidate and lead author of the study. “Females that are late in the pregnancy 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.”
Once the whales leave their Alaska feeding grounds, they have a finite amount of energy available to invest in their offspring during a 3 to 5 month fasting period, with energy requirements even higher after calf birth.
It’s a particularly vulnerable period for pregnant humpback whales.
A 75.6% decline in the number of humpback whale mothers with calves was observed and documented between 2013 and 2018 off Hawaiʻi.
Another recent study showed calf production was about six times less between 2015 and 2019 in Southeast Alaska compared with years prior to 2015, with mid-summer calf mortality increasing tenfold from 2014 to 2019.
Studies also reported significant and prolonged shifts in the distribution of the marine food web, resulting in poor feeding conditions for humpbacks.
“This research underpins future studies on humpback whale energy demands,” said Lars Bejder, a 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.”

The database contains extensive time and location information offering invaluable insight into the effects of large-scale climatic events on the iconic sentinel species.
Bejder said sustaining such long-term, wide-scale studies is crucial for understanding these impacts within the context of natural variability in whale health.
The new research — done in partnership with the Alaska Whale Foundation¯, Pacific Whale Foundation and others — highlights key factors that will help inform future conservation.
“This research underscores the value of collaboration in tackling complex questions about the lives of humpback whales,” said Jens Currie, another co-author of the study and chief scientist at the Pacific Whale Foundation. “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.”