Evolutionary biologists from mount Holyoke College in the US have shown that the length of the male genital organs in animals is determined by intraspecific competition. The researchers showed that the sizes of genitals can change, not only in the course of evolution, but for the life of individuals of such species, such as ducks. Article scientists published in the journal the Auk: Ornithological Advances, and briefly about the study described in Nature.
Specialists conducted a long-term experiment involving two types of ducks — small sea tufted ducks (Aythya affinis) and American white-headed ducks (Oxyura jamaicensis). Birds were divided into two groups. In one males and females were in pairs, the other one female had several males, which increased competition. Within two years, scientists measured the size of the genitals to determine if he could change, depending on external conditions during the life of the individual. In the studied species grow penises before each breeding period, and then degenerates.
It turned out that A. affinis in competition the length of the penis increases and in favorable conditions, the sex organs are normal size. According to scientists, large genitals increase the chances of fertilization of the female.
From O. jamaicensis in the first year only the large males had long penises, and small birds were very small genitals. In second year males-outsiders got normal genitalia, but for a short time — five weeks. At the same time, duck, occupying a dominant position, kept large penises about three months.
These results, the researchers explain the fact that the American white-headed duck, the ratio of penis size and body one of the highest among duck. Their reproductive organs can be longer than most birds. Thus competition among males often leads to the death of the less fit. In these stressful conditions, some birds have reduced ability to produce hormones, influencing the size of the genitals, and the brightness of the color. In turn, inconspicuous males are less likely to engage in a battle with aggressive dogs.