Imagine being a humpback whale mom-to-be. You have been pregnant for 10 months. Your 2600-pound calf is due in just four to six weeks. Over the summer and fall, you have consumed as much food as possible, preparing for the journey ahead. Soon, you will embark on a 3000-mile migration south to the Hawaiian breeding grounds. Once you leave your Alaskan feeding grounds, food will become scarce until you return in four months. Therefore, you will be fasting from here on in. Before and after birth, all the energy your calf needs to grow and develop will come from you. Its survival depends entirely on your ability to provide that energy while fasting over the next four months.
But how much energy does a pregnant humpback whale require, and how are these costs distributed throughout a year-long pregnancy? After giving birth, how much energy do mothers in Hawai’i use daily and how is this investment reflected in the growth of their calves? What are the consequences when mothers cannot accumulate enough energy to fuel these energetic costs? Let’s take a deep dive into the costly humpback whale pregnancies, by highlighting the results of two papers, just published in The Journal of Physiology.
Why Study Maternal Energy Costs?
These are critical questions to address, particularly in the face of rapidly changing marine environments. For example, the 2014-16 Pacific Marine Heatwave was the largest marine heatwave event in recorded history, resulting in weakened coastal winds, warmer water, and abrupt ecosystem shifts throughout the North Pacific. Consequently, Hawaiian humpback whale abundance declined by more than one third. Mother-calf encounters between 2013 and 2018 declined by ~75%, and birth rates plummeted by 80%. Alarmingly, there was also a tenfold increase in the number of calves that did not survive their first summer in Alaska. With the frequency, intensity, and duration of marine heatwaves already increasing, there is a pressing need to better understand the factors that amplify or mitigate the impacts of climate and other human-related stressors.
The Price of Pregnancy: Growing a One-Ton Baby
The first study estimated the energy required by pregnant females to produce a calf from conception to birth. Researchers analyzed 678 body length measurements of mother-fetus pairs from historical whaling records. They also included 987 length estimates of live mother-calf pairs obtained using drones, along with samples of calf and placental tissue.
The fetus growth rate and birth size increased with the length of the mom. Additionally, the length, volume, and mass of the fetus increased exponentially over gestation. This exponential growth created huge energy demands later in the pregnancy. For example, energy costs over the first two-thirds of pregnancy are negligible. However, they increase dramatically during the final 100 days. To sustain an average-sized fetus just before birth – growing 97 pounds per day – a female must invest the energetic equivalent of 1,500 Big Mac burgers daily!
In total, an average-sized female requires 22.5 million calories for pregnancy alone. Late-pregnant females are particularly vulnerable to disruptions in their energy balance, as the most energetically demanding stage coincides with fasting and migration to the breeding ground. Bigger females use almost twice as much energy as smaller females, but their larger energy stores help them handle this extra demand more easily. This means smaller females might struggle more to cope with challenges that disrupt their energy balance.
Lactation: The True Marathon
Congratulations – you have now given birth to a 15 ft, 2600-pound calf! Now the real work begins. You now have to produce and transfer high-fat, energy-rich milk to your calf, so that it can grow quickly and be healthy. Lactation is widely recognized as the most energetically demanding phase of reproduction. For North Pacific humpback whales, it coincides with an extended fasting period in Hawaiian waters that lasts for several weeks.
In the second study we are covering, researchers used drone-based measurements to estimate the body size and condition of humpback whales across Hawai’i (their breeding ground) and Southeast Alaska (their feeding grounds). A whale’s body condition represents the amount of energy reserves available to the individual, primarily stored as blubber (the large amount of fat under the whales’ skin). Between 2018 and 2022, the researchers collected over 2000 measurements of 1659 individuals, including some repeat measurements of 275 of the 1659 whales.
On average, lactating mothers in Hawai’i lost 214 pounds of blubber per day. That is the energetic equivalent of 1200 Big Mac burgers or 1830 pounds of krill! Over 60 days, this cost equates to over 110,000 pounds of krill, surpassing the total energy cost of pregnancy. The calves showed remarkable growth rates on the breeding ground. Their length increased by an average of one-inch per day, and their body mass increased by approximately 60 pounds per day. Importantly, larger mothers with better body condition produced larger, faster-growing calves.
In Southeast Alaska, where this population of humpback whales feeds, lactating mothers gained only 30 pounds per day, while pregnant and non-reproductive females built up fat reserves up to six times faster. This dramatic contrast shows the immense energy drain of lactation, which slows mothers’ recovery and affects their ability to rebuild body condition for up to a year (the time it takes to wean the calves).
Looking to the Future
A year has now passed since you gave birth to your calf. You still weigh approximately 20% less than you did when you gave birth, despite having had a full feeding season to accumulate energy. Your calf, however, is about twelve times heavier than it was a year ago and has more than doubled in length.
Many studies have shown that larger offspring have better chances of survival. These larger and fatter calves show better resilience to predation, starvation, and extreme environmental conditions. As we navigate an unprecedented era of rapid climate change, and associated environmental pressures such as marine heat waves, these two studies provide valuable insights into the energetic costs of whale reproduction. They highlight critical factors that may help or hurt these magnificent creatures as they face mounting environmental challenges.
Sources and further reading
The two studies covered in this post:
- van Aswegen, Martin, et al. “Energetic cost of gestation and prenatal growth in humpback whales.” The Journal of Physiology (2025).
- van Aswegen, Martin, et al. “Maternal investment, body condition and calf growth in humpback whales.” The Journal of Physiology (2025).
We would like to warmly thank Zoltan Nemeth for their illustrations of the different stages of pregnancy/calf growth. Please make sure to check Zoltan’s amazing work (Cetek Art) here: https://www.cetek-art.com/
Further reading:
- Adamczak, Stephanie K., et al. “Growth in marine mammals: a review of growth patterns, composition and energy investment.” Conservation Physiology 11.1 (2023)
- Lockyer, C. “Growth and energy budgets of large baleen whales from the Southern Hemisphere.” Food and Agriculture Organization 3 (1981): 379-487.
- Ronget, Victor, et al. “Causes and consequences of variation in offspring body mass: Meta‐analyses in birds and mammals.” Biological Reviews 93.1 (2018): 1-27.
Did you like learning about humpback whale pregnancies? Find out more about humpbacks on Whale Scientists:
Martin van Aswegen
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