Males can be distinguished from Andean duck (Oxyura ferruginea), a similar species, by being smaller and having a flatter head.

[2] The lake duck holds the Guinness World Record for having the largest avian reproductive organ, from a specimen in Córdoba, Argentina that had a penis measuring 42.5 centimetres (16.7 in).

[3] It is theorized that the size of their spiny penises with bristled tips may have evolved in response to competitive pressure in these highly promiscuous birds, removing sperm from previous matings in the manner of a bottle brush.

[8] Little is known about the lake duck's diet, but it is believed to consist of small invertebrates, seeds, and plant remains.

[2] It is very widespread, naturally occurring in Argentina, Brazil, Chile, Paraguay, and Uruguay, and having been introduced to the Falkland Islands.