Baleen whale mothers tip the scales in a surprising way

Long baleen whale mothers are more likely to have female calves than males. The skew toward daughters grows with maternal length, hinting that in the open ocean, “investing” in big girls pays off more than producing big boys.

A research team at the University of Washington tested that pattern at unprecedented scale in marine mammals.

The analysis centers on rorquals – the baleen whale family that includes blue, fin, sei, Bryde’s, minke, and humpback whales – and matches maternal length to fetal sex across an archive built during the whaling era. 

The results challenge the popular Trivers-Willard hypothesis, which long held that the fittest female mammals gain more by birthing sons.

Testing an old idea at ocean scale

In 1973, Robert Trivers and Dan Willard proposed that strong, well-conditioned mothers should tilt births toward males. The logic was simple. Large males can outcompete rivals and sire many offspring, amplifying a mother’s genes. 

Evidence for that idea came mostly from land mammals such as deer and elk, often with small samples.

The new study moved offshore and widened the lens to more than 100,000 whales. The researchers compared maternal length – a proxy for condition and reproductive potential – to the sex of fetuses large enough to determine they gender reliably.

If Trivers-Willard held in the sea, longer mothers should have carried more males. The opposite emerged.

A daughter advantage for baleen whales

Across seven rorqual species, the fetal sex ratio grew more female as mothers grew longer. The trend varied by species but told a consistent story. In humpbacks, longer females were 77% more likely to carry a female calf. In sei whales, that probability climbed to 99%. 

The simplest interpretation is that heritable fitness benefits flow more strongly to daughters. Big mothers tend to produce big daughters who, in turn, become long, fecund females.

Raising young is costly in whales. Many breed far from food and must run pregnancy, birth, nursing, and early calf care on stored fat. Under those constraints, size and condition in females may be the key to producing the next generation, while male-male competition might matter less than on land.

“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 in quantitative ecology and resource management.

A treasure trove built in a grim chapter

The data came from historical whaling records. Early in the 1900s, a Norwegian law required crews to log each whale’s length, sex, and pregnancy status, plus the sex and size of any fetus. In the 1930s, those rules became international. 

“When they hunted whales, there were often biologists around who were knee-deep in the carcasses, measuring and collecting samples,” Rand said. Whaling ended under a global moratorium in 1986, but the ledgers remain.

“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.

For this study, the researchers modeled fetal sex against maternal length for fetuses at least about three feet long, the threshold where sex can be identified in the field. The approach allowed comparisons across species and regions, minimizing biases that can plague small, local studies.

Can mothers steer sex at conception?

Experts have long debated whether mammals can, in subtle ways, bias offspring sex soon after conception. Mechanisms remain murky. Hormonal cues, maternal condition, and timing relative to ovulation have all been floated.

“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.”

Whether whales actively bias sex or whether sex-specific survival differs early in gestation, the pattern is clear in the records: longer mothers are more likely to carry daughters.

Baleen whale mothers: Bigger is better

In some whales, males compete for access to mates, but the advantage of being big may be more consistent for females. Small female whales are likely to struggle to conceive, carry, and nurse successfully. 

Large females, by contrast, can store more energy, migrate farther, and invest more in calves. Daughters inherit that body plan, creating a compounding return. 

In an environment where mothers often feed far from breeding grounds and rely on blubber to fuel reproduction, that compounding effect may outweigh any male advantage.

Conservation signals in the stats

Several baleen whale populations show signs of shrinking body size over recent decades, possibly due to warming oceans, prey shifts, or past overharvest. If females trend smaller, their ability to support pregnancies and raise robust calves could erode. 

A daughter-biased payoff makes that prospect more troubling. It also suggests that protecting food supply and foraging habitat – so females can build and keep fat stores – may be especially important.

“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.”

A smart evolutionary bet

The Trivers-Willard hypothesis remains a powerful idea, and it may hold under certain ecologies. But the ocean is different. Life histories stretch out. Migrations are long. Energy budgets hinge on fat.

In that world, the best route to more descendants may run through well-provisioned daughters who become well-provisioned mothers.

The takeaway is not that sons never help fitness. It is that assumptions built on hoofed mammals don’t always translate to whales. In rorquals, bigger moms skew toward daughters – and for these giants, that looks like the smarter evolutionary bet.

The study is published in the journal Proceedings of the Royal Society B.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–