The penis bone, or baculum, rests at the end of the penis and provides structural support during copulation in many mammals, although not in humans. Matilde Brindle and Christopher Opie of University College London in the UK have analysed the baculums of nearly 2000 mammal species including primates and carnivores, finding that species that copulate for longer periods have longer bacula, as do those with more than one mate or with seasonal breeding patterns. The baculum first evolved 145–195 million years ago in the common ancestor of carnivores and primates, and disappeared in humans when we split from chimpanzees. This may have coincided with the change towards a more monogamous lifestyle, concludes the team.