Longer body size means more female calves for baleen whale moms, contrary to popular hypothesis

Long baleen whale mothers are more likely to have female calves than males, according to a new study led by the University of Washington. The findings contradict a popular evolutionary hypothesis postulating that strong mammals benefit more from birthing males.

In 1973, Robert Trivers and Dan Willard proposed that fit female mammals can improve their odds for grandchildren by having males. Large, strong mothers will raise large, strong calves that can outcompete other males for mates. But, according to the hypothesis, female fitness is less consequential. The studies backing this argument focused on land mammals, such as deer and elk, and often included just tens or hundreds of animals.

UW researchers tested the hypothesis in marine mammals by comparing maternal length and fetal sex in more than 100,000 baleen whales. They found that the fetal sex ratio skews female for longer—and thus more fit—rorqual whales, the predominant baleen whale family that includes humpbacks and blue whales. The findings suggest that female calves benefit more from heritable fitness than males do.

Carrying and caring for young is exhausting, and whales often breed far from food sources. They must rely on stored fat to sustain themselves and their young during and after pregnancy. The results are published in Proceedings of the Royal Society B.

“The question we wanted to answer was if you are in good condition, if you’re big and fat and you’re going to have a big fat calf that will survive and reproduce—do you want that calf to be a male or a female?” said Zoe Rand, a UW doctoral student of quantitative ecology and resource management.

To answer this question, the researchers turned to historical whaling data.

Back in the early 1900s, when people hunted whales, a group from Norway began collecting data on their catch. The practice was codified into a law that required all Norwegian hunters to record the whale’s length, sex and pregnancy status, as well as the sex and size of a fetus. In the 1930s, the Norwegian regulation became international law.

“When they hunted whales, there were often biologists around who were knee-deep in the carcasses, measuring and collecting samples,” Rand said. The International Whaling Commission banned whaling in 1986 to protect dwindling populations from further decimation. The IWC data, however, is a treasure trove for researchers.

“We have this enormous data set with hundreds of thousands of data points that doesn’t exist for almost any other wild population,” said Trevor Branch, a UW professor in the School of Aquatic and Fisheries Sciences. In 2023, Branch created an interactive map depicting whale distribution from the data.

The data also gave Rand an opportunity to investigate fetal sex ratios in marine mammals. Experts argue that some animals can influence the sex of their offspring just after conception. No one knows exactly how this works for mammals, but adapting sex ratios based on physical or environmental conditions is considered advantageous.

“I think for our mammal brains, it is a little bit confusing,” Rand said, “But insects, like bees and ants, have a lot of control over the sex of their offspring, so it’s not entirely surprising that mammals might have a little bit of control.”

In this study, the researchers modeled maternal length against sex for fetuses measuring three feet and longer—the size at which sex becomes evident. They included seven whale species in the rorqual family, totaling more than 100,000 whales.

If the Trivers-Willard hypothesis were correct, researchers would have seen a slight increase in the number of male fetuses as maternal length increased. Instead, they observed a downward trend, indicating that fewer males were born to larger mothers. The results varied some by species: Longer female humpbacks were 77% more likely to have a female calf, and that probability increased to 99% for sei whales.

There are several possible explanations for why these findings flip the Trivers-Willard hypothesis, and the trends observed in land mammals. Some male whales compete for mates, but competition might not be as significant a pressure as female size because small female whales will likely struggle to reproduce and raise healthy young. Big whales, on the other hand, will have big female calves that will grow into long mothers with strong reproductive potential.

For baleen whale mothers, investing energy in female calves is the best way to ensure generations of grandchildren.

Research also suggests that many whale species are getting smaller, which could spell trouble for future generations if females are unable to support offspring. The findings could have implications for conservation, but Rand said that this will require further research to confirm.

“Previously it was assumed that if you have male-male competition for mates, bigger mothers will have males,” Rand said. “Our paper shows that you can’t make that assumption because there’s also an advantage to being big as a female.”