|Hens eject sperm; roosters are clueless|
Edward R. Winstead
June 16, 2000
ize matters, but not as much as social status. Roosters are big enough to have their way with hens, but hens, it turns out, have the ability to eject sperm following a sexual attack. Victimized hens squirt semen back on the assailant before he even pulls away; the male assumes everything went according to plan and has no reason to retaliate. With this trick of evolutionary quality control, mother hens are the architects of the species.
A pair of British researchers discovered this behavior in a group of two dozen free-ranging chickens in Sweden. The fowl, descendents of a primitive strain of domesticated chickens, are essentially wild but tolerate close observation by humans. During the two-year study, the hens consistently retained the sperm of the one or two dominant roosters in the group and ejected all other.
"We found that although females can be coerced into copulating and have limited choice in their male partners, they have evolved a mechanism to control paternity," says Tommaso Pizzari, of the University of Sheffield. "This is the first time that females have been shown to be able to bias the utilization of sperm toward the type of male they seem to prefer."
A gregarious rooster has sex appeal, and for good reason. By making fathers out of the dominant roosters, mother hens are giving their chicks the best possible start in life. Studies have shown that dominance has a genetic component: the offspring of top roosters are more likely to grow up to be leaders than are the offspring of low-ranking males.
Since the 1970s animal scientists have described species in which the sexes compete over how sperm is used. Female fowl appear to be unique in controlling the entire process and in selecting sperm based on appearance: she will accept or reject sperm after sizing up the male.
"Female choice in sperm competition occurs at different levels, and the process in chickens works at a behavioral level," says T.R. Birkhead, the Honorable Curator of Alfred Denny Museum at the University of Sheffield. Birkhead is author of several books that describe different approaches to sperm competition in animal species. In some varieties of birds, for example, males physically manipulate the female before or after copulation to increase the odds of fatherhood. More impressive are females who can store and manipulate the sperm of many males in their reproductive organs.
Discovering the mechanisms of sperm competition is not easy, and most studies have focused on males. "What males do is often crude and obvious," says Birkhead. "Females can be incredibly subtle. Only recently have we begun to discover the sophisticated mechanisms that females have evolved to counter male sexual behavior." The results of this study appear in the current issue of Nature.
The low-ranking males in the study were not subtle, and hens nearly always tried to resist the one-second copulation, says Pizzari. For a sexually harassed female, sperm ejection was the last resort. A hen would first try to run away. If caught, she would then issue a call for help to the dominant male, who sometimes would chase away the aggressor. Even in that single second, Pizzari notes, the female always seemed to have a clear idea of what was on top of her.
Whether controlling paternity directly benefits the mother is not clear, but life appears to be easier for hens favored by dominant males. For instance, dominant males are vigilant. Birkhead estimates that some roosters spend 70 percent of their time with their heads up, on watch for predators. Dominant roosters are also good providers of high-quality food (insects and worms) for expectant mothers.
By contrast, low-ranking males are "opportunists" that harass hens when the top roosters aren't around, says Birkhead. They have nothing to offer females. Even so, non-dominant males probably succeed in fertilizing some eggs. "It seems likely that this system isn't 100 percent safe," says Birkhead. "If none of the sperm of subordinate males ever made it through, it wouldn't be in the interest of these roosters to copulate."
Chicken chastity belts
Birkhead and Pizzari knew their study had one potential weakness. They could see for themselves whether a hen ejected semen or not. But they also needed to show that the dominant roosters were actually getting the job done. Even at close range, the researchers couldn't see semen enter the female. Moreover, since dominant males mated more often than other males, it was entirely possible that their reserves were occasionally depleted. Perhaps the lesser roosters were the real fathers after all.
"A crucial part of the study was to check that the dominant male was ejaculating normally," says Birkhead. "So we fashioned a kind of chastity belt for the hens." A swatch of film caught the male ejaculate. After working out the average volume of ejaculate for the group, the researchers were surprised to learn that dominant males seemed to be producing more semen than other males.
Both Pizzari and Birkhead say their findings have no bearing on human behavior. Pizzari has begun preparations for follow-up studies that will attempt to get a better sense of how many offspring are fathered by sub-dominant roosters. The challenge will be to do so without breaking the trust that allows him to observe chickens copulating from distances as close as a meter.
"The chickens are extremely tame in their own environment but seem to behave like wild birds," says Pizzari, adding that the group is a "gold mine" for researchers. They function as a single social unit and allow him to record detailed information about their behavior. "They have a roost where they go at night," he says.
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