Longer body size means more female calves for baleen whale momsGillian Dohrn UW NewsLong baleen whale mothers are more likely to have female calves than males, according to a new study led by the University of Washington. In 1973, Robert Trivers and Dan Willard proposed that fit female mammals can improve their odds for grandchildren by having males. The results varied some by species: There was a 77% chance that longer female humpbacks have more female calves, and that probability increased to 99% for sei whales. 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. For baleen whale mothers, investing energy in female calves is the best way to ensure generations of grandchildren.