Keratin barbs on the tongue and glans of the domestic housecat.
You may have heard that cats have “spiked penises”, which is true, to an extent. The barbs on the glans of the male consist of the same backward-facing keratin barb structures that are present on the tongue, though clearly they’re not used for the same purposes.
While many male mammals have similar spines on their genitals, the purpose of them is not universally known. In felines, however, it’s believed that the barbs raking the inside of the vagina induces ovulation in the queen (female cat). [read more on why cats don’t menstruate but humans do]
It’s less understood why our ape bretheren, who have cyclical ovulation similar to humans, also have keratin spikes on their penises. It may be an evolutionary holdover from a common ancestor with prosimians, who have much more complex penis barbs, and who appear to have similarly triggered ovulation as felines.
Somewhere along the line, the genes that create these keratin spikes were lost in humans, but in some people, the non-barbed keratin (“pearly”) papules around the glans or shaft are a benign throwback to an ancient ancestor. Luckily, even though we have the genes to create the papules (generally not activated), we’ve literally lost the genes that create the spiked barbs on top of them.
[Penile spines at Wikimedia Commons]
P.S. When a cat is neutered, its penis loses its barbs. One of the ways to detect if a stray cat who appears to have been fixed actually has one or more retained testes (where the testicle does not descend into the scrotum) is to check the glans of its penis for barbs.