The size of a duck’s penis depends on the company it keeps

The sight of ducks swimming on the placid surface of a picturesque lake is likely to prompt reflections on the harmonious balance of nature and the peaceful disposition of aviankind.

To entertain these ideas, however, while pleasant, is well wide of the mark. Beneath the water’s lapping meniscus – indeed, beneath the ducks themselves – a ruthless battle for genetic dominance is taking place. To the victor go the spoils, and in this case the symbol of triumph is (pardon us) the biggest penis in the pond.

Most species of birds do not have penises, but ducks are a noted exception, possessing substantial members. Indeed, the biggest penis of any animal, relative to body size, belongs to the Argentine duck (Oxyura vittata), which boasts an organ that runs to a frankly eye-watering 43 centimetres long.

While other species have not evolved to wield quite such improbable appendages, all male ducks sport sizeable willies. Not all, however, are equal, with some species having penises a tad – relatively speaking – on the short side.

In a bid to find out why, ornithologist Patricia Brennan of Mount Holyoke College in Massachusetts, US, and colleagues, decided to test the influence of duck society on duck genitalia.

In a study reported in the journal The Auk, Brennan’s team recount setting up captive colonies of two species. The first, called ruddy ducks (Oxyura jamaicensis), are entirely promiscuous and have very long penises. The second, lesser scaups (Aythya affinis), are monogamous during the breeding season and have relatively small ones (for a duck that is; it’s still big enough to make a donkey wince).

Birds from each species were separated into two cohorts for two breeding season cycles: the first cohort was divided into male-female pairs and the second comprised an all-male group.

Brennan and her colleagues found that when kept in an all-male group, the lesser scaups all grew longer organs. This was in line with predictions, with genitalia length conferring a competitive advantage.

For the normally well-hung ruddy ducks, however, the results were more complicated. Some members of the all-male group did not reach sexual maturity until their second year. All of the ducks, when reaching maturity, grew penises faster than average, but many remained in reproductive readiness for shorter than normal periods.

The scientists suggest that by staggering their sexual maturity and readiness, the ducks were able to reduce in-group competition and male-to-male aggression, thereby optimising their abilities to procreate in a sub-optimal situation.

The results indicate that lesser scaups increase penis length in response to competition. The lengthening effect was less pronounced among the ruddy ducks, but that might have been, Brennan’s team suggests, because they already sport one of the biggest in the business and room for extension is therefore limited.

(For more information about duck penises, by the way, we recommend an excellent pop-science book written by Australian palaeontologist John Long. It is called Hung Like An Argentine Duck.)