Our monogamous lifestyle may explain why humans, unlike many other mammals, lack a penis bone.

The bone, called a baculum, rests at the end of the penis and is thought to provide structural support and prolong copulation. Matilda Brindle and Christopher Opie at University College London analysed the size of bacula in nearly 2,000 species of mammal, including primates and carnivores. They found that species that copulate for longer tend to have longer bacula. The same is true of animals that have more than one mate or have seasonal-breeding patterns, which lead to intense competition between sperm from different males after mating.

The results show that the baculum first evolved 145–95 million years ago, in the common ancestor of primates and carnivores. It disappeared from the human lineage after our split with chimpanzees, and this may have coincided with the switch towards a more monogamous lifestyle, the authors say.

Proc . R. Soc. B 283, 20161736 (2016)