Female chickens are among the most promiscuous members of the animal kingdom, with wild and domestic fowl alike tending to mate with way more males than necessary to fertilize their eggs.
But just because they get around doesn't mean these hens aren't selective about who they choose to reproduce with. In fact, recent research shows that female chickens actually practice a pretty reliable method of birth control — but how they do it is as bizarre as it is graphic.
Chickens have long been known to, at times, eject sperm after doing the deed. What wasn't well established was the underlying reason for what's technically known as "seminal evacuation." But in a recently published paper, a team led by Oxford researcher Rebecca Dean explains that this behavior is, in fact, far from random, and that the tendency for females to jettison sperm is actually a finely tuned mechanism of postcopulatory sexual selection.
The researchers were looking at two specific behaviors in the chickens they studied. The first was the probability that a male chicken's ejaculate would be ejected, or "risk." The second was the proportion of semen ejected, or "intensity."
You've heard of a "pecking order," right? Well, the term first came about when people observed the tendency for a social hierarchy to arise within a group of chickens. By comparing the risk and intensity of contraceptive behavior exhibited by female chickens to the relative levels of social status of the roosters with whom they mated, the researchers found that females were not only more likely to reject the sperm of socially subordinate males, but that they were actually wont to do so with greater intensity.
According to Dean, "these results show that promiscuous females can actively bias sperm utilization to exert a strong and predictable influence on the struggle for fertilization." And in doing so, they manage to "retain control of paternity even in species such as fowl where males can force mating."
In other words, even if a female chicken is unable to withstand a rooster's sexual advances, the sperm of a socially subordinate rooster is significantly more likely to get rejected. Rejected hard.
Top image via