Of the four extant species, only female spotted hyenas have a pseudopenis or fake penis – and in fact, they are even more unique in being the only female mammals to lack an external vaginal opening! We still aren’t sure why the females have a pseudopenis, but it clearly has its perks, and there are two main ideas explaining its evolution.
Pseudopenis benefits
Firstly, it means they have absolute control over who they mate with, and their full cooperation is required. Males do not provide any offspring care, which means females have to bear the burden, so it makes sense for them to be choosy about which males sire their young: they want to maximise the survival of their cubs.
Secondly, because female hyenas are more likely to suffer aggression from other females, it’s possible that the pseudopenis is a sort of masculine ‘camouflage’ to deter attacks – this could be particularly useful for female cubs at risk of infanticide.