Traveling up to 5,000 kilometers every year is no mean feat. Every year, humpback whales make this journey without fail: they feed on krill and small fish in the polar regions to build up their fat reserves, then embark on their annual migration to the tropics where some will mate and others will give birth. 

For the pregnant females undertaking this journey, the demands of pregnancy substantially add to the physical challenges they face. A new study published in The Journal of Physiology looks into the energetic burdens they encounter.

It focuses on how pregnant humpback whales use energy for reproduction and how such energy use can impact the survival of the offspring, and subsequently their population. 

“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,” said lead author Martin van Aswegen, a PhD candidate at the Marine Mammal Research Program (MMRP). 

Findings of the study

The researchers looked at the following: the body-lengths of mother and fetus in historical whaling records, body-lengths of mother and calf using drone photogrammetry and tissue samples from deceased whales.

They found that fetal growth rates and birth size increased with maternal length, and that fetal length, volume and mass increased exponentially throughout the pregnancy. 

In addition, during the first two-thirds of the journey, the energetic cost comprised only 0.01 to 1.08 percent of the energy used throughout the pregnancy. Then, in the final one-third of the pregnancy, the energetic burden shot up to 98.2 percent, signifying a huge draw on the mother's energy reserves during the final stage of the migration. 

“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 three- to five-month fasting period, with energy requirements being even higher after calf birth,” said van Aswegen. 

Importance of inter-agency collaborations

“This research underpins future studies on humpback whale energy demands,” said co-author Lars Bejder, MMRP Director. “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.”

Collaborations among researchers from various organizations have been instrumental to the study's success. These partnerships have yielded crucial insights into the lives of whales and the challenges they encounter, thereby enhancing the development of informed conservation strategies.

“Together, we can address large-scale ecological challenges that no single institution could achieve alone,” said co-author Jens Currie, MMRP PhD candidate, Chief Scientist at Pacific Whale Foundation.