This small duck (which typically weighs around 640 g) was previously reported to have a 20-cm penis1,2, which is about half of the drake's total body length and is comparable to the penis of an ostrich (Struthio camelus)3. A new voucher specimen from Argentina's Córdoba province (Fig. 1) indicates, however, that the penis size of this species was underestimated. We measured the length of the unwound, everted penis of an aroused O. vittata drake and found that it was 32.5 cm long when hanging under the force of gravity, but that it could stretch when fully unwound to 42.5 cm.

Figure 1
figure 1

Photograph of the 42.5-cm penis of a male Argentine lake duck (Oxyura vittata) taken near Arroyo Chucul, Córdoba, Argentina on 30 April 2001.

Like other stifftail ducks, lake ducks are promiscuous and boisterous in their sexual activity4. One explanation for the evolution of such a long penis might be selection for its competitive advantage — for instance, groups of drakes might display their everted penises to attract females. Success in sperm competition5 may also be a factor. The base of the lake-duck penis is covered with spines, yet the tip is soft and brush-like. Before ejaculation, drakes probably use their penises like bottle-brushes to remove sperm stored in the oviduct by the female's previous consort. The larger the bottle-brush, the more effective it will be, clearing the way for the drake to father the ducklings.

Many questions remain unanswered. How much of his penis does the drake actually insert, and does the anatomy of the female's oviducts make them unusually difficult to inseminate? The Argentine lake duck offers a sizeable opportunity to study sexual selection and sperm competition in birds.