From more than 200,000 historical whaling records, scientists report that longer baleen whale mothers are more likely to have daughters. The team analysed 209,254 sexed fetuses from seven species of rorquals — the baleen whales that include blue, fin, humpback, sei, and minke whales — to test how a mother’s size relates to the sex of her calf.
That finding pushes back on a classic idea from land mammals — the Trivers–Willard hypothesis — which suggests well-provisioned mothers should have more sons because big males often win more mates. In the water, size may not grant the same mating lock-in, and raising calves is extraordinarily costly.
As the authors put it, “Our results likely reflect both the difficulty of excluding small males from competition in aquatic environments and the exceptionally high costs of gestation and lactation in baleen whales.”
To get here, the researchers combed the International Whaling Commission’s individual catch database, which spans 1899–2019 (most entries are from 1910–1985). They first checked for sexing mistakes in very small fetuses and then trimmed the data to sizes where sex ID was at least 99% reliable before modelling the link between maternal length and fetal sex.
Across species, the chance of a fetus being male fell as mothers got longer. In plain terms, the authors estimate “a 90% posterior probability that longer mothers have more female offspring overall, ranging from 77% for humpback whales to 99% for sei whales.” Here, “posterior probability” simply means how confident their statistical analysis is in the direction of the trend — roughly nine times out of ten in favour of more daughters for longer mothers.
They also uncovered a wrinkle in the raw records: in four species, tiny female fetuses were often recorded as males, while Antarctic minke whales showed the opposite bias. Correcting for this measurement issue was key to a fair test.
Why might bigger mothers lean female offspring? In many baleen whales, males can’t easily monopolise mates at sea, and songs or roaming strategies may help smaller males reproduce too. Meanwhile, reproduction is energetically brutal for females; body condition can plummet during lactation, so producing larger daughters — who in turn may be better able to bear those costs — could pay off over generations.
The result also matters for conservation. Industrial whaling removed many of the biggest animals, likely leaving a higher share of smaller females in some populations. As oceans warm, body sizes — especially of females — have been shrinking in a few baleen whale species. If smaller mothers tend to produce more sons, as the model predicts, population recovery could be slower.
Journal Reference: Proceedings of the Royal Society B Biological Sciences. DOI: 10.1098/rspb.2025-1427
