|The Question: Why would five mallard drakes gang up on a female mallard with chicks trying to drown her?
Submitted by: S., British Columbia, Canada
Warning: I have to begin this answer by warning against applying human moral standards to animals, because it’s very easy to do in this situation. But mallards aren’t thinking about right and wrong when they engage in behaviors. They’re acting in the ways that have proven reproductively successful. They aren’t being mean or immoral.
The Short Answer: The short answer is that the mallard (Anas platyrhynchos) males (drakes) are forcing the female mallard to mate with them. And for the reasons discussed below, forced copulation is far more common in waterfowl (ducks, geese, swans) than in other types of birds. Mallards are most famous for it, probably because they are the most commonly seen duck and can now be found nearly worldwide.
There is a significant skew in the sex ratio of mallards in favor of males, so in any mallard population there are a number of unmated males. By forcing copulation with a female who is mated to another drake, unmated drakes gain a chance to pass on their genes. And if those genes include genes that favor the forced copulation behavior, it will be likely to persist. But it isn’t just the unmated males who take part in the forced copulations. In fact, males who have their own mate are even more likely to force copulations with females other than their mate.
This is simply the way things work in the world of ducks, but for us humans, it can be hard to watch. The drakes you saw were not trying to drown the hen. Mallards mate on water as well as on land and the drakes may just have caught up to this hen on the water, or she may have tried to escape by going into the water. In the vast majority of cases, the hen will eventually escape. They do sometimes get injured or even killed in the course of forced copulations, and that’s probably because they simply get battered and smothered by all the aggressive males. There are records of as many as 39 drakes chasing and repeatedly trying to copulate with a single mallard hen, although typically, only one or two of the males will actually copulate with the female.
This video is one of the milder ones you can find on youtube: http://www.youtube.com/watch?v=Y1X-7FeddeI
More Information – Why ducks? It’s been shown that in most (if not all) birds, females will accept and even solicit copulations with males other than their mates. Even in birds that have one-to-one pairings that last years and years, females will occasionally copulate with males other than their mate, especially if the male is of higher quality or is higher on the pecking order than the female’s mate. The standard explanation for this is that these females are gaining direct benefits such as access to food or territory, or indirect benefits such as “better genes” for their offspring. Even when females are not actively seeking copulations with other males, females of most species do not vigorously resist copulation by males that are not their mates.
What’s odd about mallard hens and hens of many other waterfowl species is that they resist and resist very vigorously, to the point where they risk injury by aggressive males. Why do they do that?
There are several theories:
- Female mallards already have “good genes” at their disposal. The very fact that there are more males than females means that females get the pick of the male crop when they pair up. Most females are already mated to a high quality male and any unmated males are the ones who have been rejected by other females and are therefore probably of lesser quality. Females don’t want to mate with the lesser males so they resist forced copulations. If this were the driving factor, however, you’d expect to see flexible behavior on the part of females. They should resist forced copulations by low ranking males and welcome matings from high ranking males. But what is observed is resistance to all males.
- The presence of a penis. 97% of bird species do not have a penis. Males simply rub their cloaca (combined urinary, intestinal and reproductive opening) against the cloaca of the female, and release sperm. It appears that it is very difficult to rub cloacas if the female resists. Waterfowl on the other hand, have what anatomists call an “intromittent organ,” or what most people would call a penis. And it appears that mallards can forcefully insert this penis before ejaculating. Of course, this immediately raises the question of which came first, the penis or the forced copulation? Do ducks engage in forced copulation because having a penis makes it possible? Or did the reproductive success of forced copulation favor the evolution of a penis to make it even more effective? There is some evolutionary evidence that the first birds had penises, suggesting that the penis came first. So then the question becomes why do waterfowl have penises and other birds don’t? One answer may be that waterfowl often mate on water, where a penis that introduces sperm into a female’s reproductive tract may help prevent sperm from washing away or being damaged by water. And because ducks have a penis, they may be able to force copulations, where the 97% of birds that don’t have a penis are simply unable to force copulation. And the presence of a penis may contribute to the resistance behavior because it’s been shown that venereal diseases, which can take a heavy toll on female birds, are more common in birds with penises. Presumably, because the penis is forcing bacteria and other disease-causing organisms further into the female’s reproductive tract, the risk is greater than in birds that simply rub cloacas. So maybe the resistance of female ducks is an attempt to limit the number of copulations to avoid disease.
- Maintaining pair bonds. There is some evidence that male ducks who witness a mate undergoing forced copulation are more likely to abandon the female and her nest, presumably because the male can no longer be sure the nest contains his offspring. So maybe female ducks resist forced copulation to keep their mate from abandoning them. But this doesn’t explain why female ducks resist more than other birds, since males in many other bird species provide far more help to their offspring in terms of feeding and protection. This would suggest that females in those species should work even harder to maintain the pair bond. Yet they don’t.
- Are females creating competition? In non-waterfowl birds in which females allow and even seek out extra-pair-copulations, females sometimes seem to encourage competition between males. This may be a way of figuring out which males have the “good genes.” It has been suggested that female ducks may be engaging in an extreme version of this. By forcing males to chase them, sometimes for quite long distances, and making multiple males compete for the chance to force copulation, female mallards may be conducting a very strong screening for “good genes.”
- Choosing males who are successful. In some bird species only a few males get most of the matings. For example, in many species of grouse, the males display in a prominent location and females choose which male to mate with. Often, nearly all the females in one location will pick the same male or one of just a couple of males. Females are presumably choosing on the basis of good genes. But one factor of “good genes” is whether the male is attractive to the other females and whether the male’s offspring will share their father’s attractiveness to other females. This favors both the genes of the attractive males and any females who mate with them. In the case of the mallards, this would mean females may “choose” males who are successful at forced copulations because they are likely to pass that trait on to their sons, resulting in numerous grand-offspring for the female who mates with such a male. And the way females “choose” is by resisting all males, thereby ensuring that only the males who are most effective at forced copulation will be able to fertilize their eggs.
As is often the case with a complex system like this, I suspect the answer is some combination of the above factors and maybe others that haven’t yet been identified. Margo Adler, a researcher at the University of New South Wales, believes that females are making the most of a bad deal. Like female birds in other species, female waterfowl want the best genes for their offspring. But they also want to avoid disease. They would probably avoid forced copulation if they could and only mate with the males they choose to mate with. But because male ducks have penises, they are able to force copulations. Given that the females probably can’t avoid forced copulations entirely, they resist vigorously, so that at least if they have to undergo forced copulation, only some males are successful. The males that are successful are likely to pass on the traits that made them successful, which means that the female duck’s male offspring are likely to carry those traits and also be successful at forcing copulations … and that’s how natural selection works.
Why So Many Male Ducks: One question you might be wondering is, “Why are there more male ducks than females?” It turns out that at the time of hatching, the sex ratio of baby ducks is pretty much even – the same number of males and females. But in adult mallard populations, there may be 10% more males than females. Why is that? It appears that female ducks are particularly susceptible to mortality. It has been shown, for example, that female ducks fall prey to foxes and other predators at a higher rate than males, probably because only females sit on the large, difficult to hide, nests. It’s also possible that simply building all those eggs – mallards may lay a dozen eggs and have two or three clutches each year – puts a nutritional strain on females that reduces their survival. For whatever reason, survival of females is lower than that of males, which results in the excess of males, contributing to the forced copulation behavior.
By the way, forced copulation behavior seems to be most strong in parks, where many ducks are crowded together and where the sex ratio skew seems to be strongest. So if you find it hard to watch this behavior, it may be some consolation to know that it is less common “in nature” than it is in the parks where people often witness it.
Sources: Thank you to Margo Adler, PhD, for her help.
Cunningham, E. (2003). Female mate preferences and subsequent resistance to copulation in the mallard. Behavioral Ecology, 14(3), 326-333.
Adler, M. (2010). Sexual conflict in waterfowl: Why do females resist extrapair copulations?. Behavioral Ecology, 21(1), 182-192.
Donald, P F. (2007). Adult sex ratios in wild bird populations. Ibis, 149(4), 671-692.