• When they are very numerous, it happens that mallard males catch up with a female and force her to land on the water and copulate, according to our partner The Conversation.
  • In a 1912 memo, zoologist Huxley estimated that between 7 and 10% of females drowned in this way, causing significant harm to the population.
  • This analysis was conducted by Loïc Bollache, professor of ecology at the University of Burgundy – UBFC.

Between 1910 and 1912, Balliol College, University of Oxford offered Julian Huxley a teaching position. Born into a family of renowned scientists and writers – his grandfather Thomas Henri Huxley was a biologist close to Darwin, his father Leonard and his brother Aldous recognized writers – the young Huxley had received a scholarship in zoology and ornithology a few years earlier at the same college.

In 1912, just before his departure for Houston, Texas and Rice University, he published a note on the strange sexual habits of mallards (Anas platyrhynchos). Forced relationships between males and females are described as "disharmonies" of nature, according to the expression borrowed from biologist Elie Metchnikov corresponding to adaptations that cause harmful effects for individuals.

The love story of mallards seems similar to that of most monogamous bird species, at least initially. If the male does not participate in brooding or rearing young, he nevertheless shows a considerable attachment to his mate.

A proof of love, you may ask? It is regularly observed at its side not far from the nest and it does not hesitate to follow it over great distances when it flies away in search of food.

Females very "courted"

Outside these periods, the different males, single or in couples, gather in bands to pass the time, but not only. When a female flies away from her nest to feed, she is not only accompanied by her partner, but pursued by other males whose number can reach more than a dozen individuals.

When they are very numerous, they sometimes catch up with the female and force her to land on the water. A first male immediately forces the female to copulate. As she struggles and desperately tries to keep her head above water, the male weighs with his whole body to subdue her.

The scene is extremely violent. As soon as the first male has finished his work, a second takes over, then a third, the female appears exhausted, unable to resist for long the repeated assaults of the different males. The gang rape continues until the males are full and may end up drowning the female.

In his 1912 note, Huxley estimated that between 7 and 10 percent of females drowned, causing significant harm to the population.

A phenomenon influenced by the environment

From the 1970s, observations of forced copulations in Anatidae (family including ducks, geese, dendrocygns, ducks and other related species) accumulated, showing that this phenomenon was more common than biologists had imagined.

In his 1983 paper, Frank McKinney and his collaborators identified thirty-nine species practicing forced copulations. The modus operandi follows a well-established sequence. Each attempt is usually preceded by a chase between the female and the males, whether in flight, on land or on water.

If she cannot escape the aggressors, the female can quickly find herself under a cluster of males struggling among themselves to be able to fertilize her. The local context can also influence the phenomenon. Urban gardens and parks host populations of ducks and geese whose densities and sex ratio (the ratio between the number of males and females) can amplify the frequency of forced copulations.

It happens even in semi-domesticated populations that promiscuity leads to the observation of rapes between different species of ducks.

This raises many questions. Natural selection sorts through every variation, rejecting what is bad and preserving what is good for individuals. This is not a moral process, good and bad refer only to the effects of traits on an individual's ability to survive and reproduce.

"Frustration" is not the only issue

The causes of rape behaviour have been the subject of intense literature. In 1912, Huxley estimated that the sexual instinct of males continued during the incubation period. As long as the female incubates her eggs, this instinct cannot be satisfied; This is why, when a female leaves her nest, she is often pursued by a number of dissatisfied males.

But the frustration of the males alone cannot explain these behaviors. For example, rapist males are not automatically frustrated single males. Males in pairs also participate in collective forced copulations.

While many authors believe that forced copulations are part of a set of male reproductive strategies to increase their reproductive success, other hypotheses point out that these violent behaviors could be a byproduct of natural selection promoting aggression in response to intense competition between males.

It is not aggressiveness towards females as such that would be selected but aggressive behavior during interactions with males during the breeding season, an extreme aggressiveness that would unfortunately become uncontrollable.

More resistant females?

An interesting fact to note concerns the responses of females to the harmful behaviors of males. Their reactions are of two types.

First, a physical resistance to forced copulation attempts that can have various functions. Emma Cunningham of the Department of Zoology at the University of Cambridge analysed the role of female resistance in mallards in 2003.

It appears that, if they resist the attempts of males, it is more to avoid multiple copulations always risky (fatal injuries, increased transmission of diseases) than to promote competition between males. Proof of this is the morphological evolution of the genitals of males and females, which Patricia Brennan and her collaborators carefully analyzed in sixteen species in their 2007 paper.

Females of these species have developed vaginal characteristics capable of antagonizing unwanted males. Meticulous examination of the reproductive organs in these species showed that the longer and more elaborate the sex of the males, the longer and more complex the females had.

Some vaginas thus had spiral shapes limiting the introduction of the male phallus. Others supernumerary pockets, real dead ends to trap spermatozoa.


These morphological characteristics were present only in species known for male violence and high frequency of forced sexual intercourse. An evolving "arms race" for reproductive control. When the male develops a longer and more elaborate phallus to force copulation, the females regain control of their fertilization by developing brakes against the rapist males!

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This article is taken from a passage from the book by Loïc Bollache (University of Burgundy) Quand les animaux font la guerre.

Published on March 8, 2023 by HumenSciences, the book consists of an astonishing exploration, through a wide variety of examples, of situations of conflict, exclusion, but also pacification that cross the animal world.

The above text is taken from the chapter devoted to the war of the sexes, in which the author is interested in the "disharmonic" manners of ducks.

This article is produced by The Conversation and hosted by 20 Minutes.

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