The central idea of sexual selection theory is pretty simple: Females, who invest relatively more in making and raising offspring, have an incentive to be choosy about mating. Males, on the other hand, may be able to get away with no more investment that a squirt of semen—so they have an incentive to mate with any female who’ll have them. How widely that model applies in the animal kingdom is very much an open question, but it does make some specific predictions that can be tested in an evolutionary context.
One of those predictions is that, when relatively more resources are at stake in the process of making babies, sexual selection should be stronger. Austin Hughes, a biologist at the University of South Carolina, recently set out to test for that pattern in waterfowl [$a].
Ducks and their relatives already look like a good fit for classic sexual selection. In many duck species mating is coercive, so females have evolved maze-like reproductive tracts to slow down unwelcome penises—and males have, in turn, evolved corkscrewing penises to navigate those mazes. And in many species, the sexes have strikingly different coloration—generally thought to mean that males are vying for female attention with brightly colored plumage, while females are more concerned with staying hidden while sitting on a nest.
However, there are also plenty of waterfowl species where males and females are almost indistinguishable—think of swans or geese, especially. If sexual plumage differences are related to the strength of sexual selection, maybe that reflects differences in the sexual “stakes” at play in each species. Hughes tested this hypothesis by comparing closely related pairs of waterfowl species or subspecies.
As an index of the reproductive effort made by females of each species, he used the mass of the average clutch of eggs laid, as a fraction of the mass of the average female. He then tested whether the species in each pair whose females made the larger “investment” in reproducing was also the species in the pair with more pronounced sexual differences in plumage coloration. And this was, indeed, what he found.
So that looks like a neat confirmation for one predicted effect of sexual selection. A worthwhile follow-up might be to add male parental care—which may be, admittedly, harder to measure—into the mix. If males help feed and protect the brood (which is often the case for waterfowl), that should offset the cost of reproduction from a female’s perspective, which might also reduce the strength of sexual selection.◼
Hughes, A. (2012). Female reproductive effort and sexual selection on males of waterfowl. Evolutionary Biology DOI: 10.1007/s11692-012-9188-1