The hidden struggles of pregnant humpback whales

Proper food intake is essential for pregnant humpback whales to complete their extraordinary annual migration between Hawaii and Alaska. 

Researchers with the Marine Mammal Research Program (MMRP) at the Mānoa Hawaiʻi Institute of Marine Biology (HIMB) have revealed the energetic demands and vulnerabilities of these migratory mothers-to-be. 

The findings, published in The Journal of Physiology, show that while the initial stages of pregnancy are relatively low in energy cost, the final trimester dramatically increases the mother’s energetic needs – just when she must fast and cover great distances without feeding.

Humpback whales’ astonishing migrations

Humpback whales feed in polar waters and then embark on migrations reaching up to 5,000 kilometers to the tropical regions where they mate and give birth. 

During pregnancy, which lasts about 10 months, these females must travel and fast, relying solely on the energy reserves built up in feeding grounds. 

By examining a combination of recent and historical data, the researchers found a direct correlation between maternal size and fetal growth: larger mothers carry larger fetuses that grow at faster rates.

Energy demands of pregnant whales

These findings allowed the team to estimate the energy demands for different stages of pregnancy. The first two-thirds of the pregnancy required only minimal extra energy, accounting for just 0.01–1.08% of the mother’s energy expenditure. 

The most significant strain came in the final 100 days, when energy needs soared to 98.2% of the pregnancy’s total metabolic cost.

“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 lead author Martin van Aswegen, a PhD student at MMRP.

“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.” 

Vulnerable period for pregnant whales

According to van Aswegen, the 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 depend on abundant feeding during summer months in high-latitude regions to gather the energy reserves necessary for migration and reproduction. 

The new data highlights how critical it is for these whales to have access to sufficient prey before beginning their long journeys southward to deliver calves in warmer, predator-scarce waters.

Declining calf production raises concerns

Recent observations have shown a worrying trend: there has been a 75.6% decline in the number of humpback whale mothers with calves off Hawaiʻi between 2013 and 2018. 

In Southeast Alaska, a recent study documented that calf production between 2015 and 2019 was about six times lower than in pre-2015 years, while midsummer calf mortality increased tenfold from 2014 to 2019. 

These declines coincided with significant, extended changes in the marine food web, leading to poor feeding conditions for humpback whales.

Complex lives of humpback whales

“This research underpins future studies on humpback whale energy demands,” said Lars Bejder, co-author of the study and director of MMRP. 

“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.” 

Bejder noted that sustaining such long-term, wide-scale studies is crucial for understanding these impacts within the context of natural variability in whale health.”

“This research underscores the value of collaboration in tackling complex questions about the lives of humpback whales,” added Jens Currie, co-author and chief scientist at 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.”

The pressure faced by pregnant whales

By analyzing the whales’ energy use and nutritional demands, this study sheds light on key factors influencing whale reproductive success. 

Understanding the energetic pressures faced by pregnant females can guide future conservation efforts aimed at safeguarding feeding grounds and migration routes. 

In a warming ocean where prey availability and distribution are changing, such science-based insights will prove essential for ensuring that humpback whales continue to complete their life cycle and thrive for generations to come.

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