Some mammals have a dangerous-looking penis because their glans is covered in spines. In a few cases, we know why they're there (typically, to induce ovulation during copulation). But in many other species, the purpose of the spiky bits is something of a mystery.
Bats are one group of mammals in the still-a-mystery camp. Not all bats have spiny penises. But the ones that have them (like the bat penis pictured above) grow their sharp bits in an amazingly diverse number of shapes. In an article at BBC Earth, Lesley Evens Ogden asks Teri Orr, a biologist at the University of Massachusetts, about that diversity and about the mating behaviours that may drive their evolution. Orr explained one possibility:
One idea is that penis spines allow male and female hoary bats to stay coupled for mating in the air. This is called the "in-flight locking hypothesis".
In tree-roosting bats, which include hoary bats and eastern red bats, it's possible their remarkably long penis spines evolved to facilitate this locking. "They're huge spines, and these are small bats," says Orr.
Like car park spikes, the spines on these bats' penises orient backwards. That means it is easy for the penis to go in, but difficult for it to go out.
But that's just one hypothesis: others suggest that penile spines act like a bottle brush to remove rival males' sperm from the vagina, or play some part in stimulating the female to use his sperm over his rivals'. Orr's study is still trying to tease out the details.
Image by Teri Orr, used with permission