Humans are descended from anthropoid primates and they in turn from mouse-like, tree-dwelling mammals. The males of some rodent species, for example the squirrel, leave behind special mating plugs. These are nothing more than a sticky, tacky secretion that prevents other males from gaining vaginal access after the first male’s ejaculation. Evolution has equipped the penis of the male rodent with a special protrusion with which it can make a hole in the mating plug. Equality for all.
Male hookworms have a gland close to their sperm production centre that secretes a sticky substance. Hookworms also seal off the female’s sexual orifice after successful mating. The unusual feature is that their sperm cells stay alive for a very long time, so that the female can lay the eggs fertilized by a donor even after his death. Male hookworms also use the adhesive substance to attack male rivals by sealing their genitals, so that for a while they are incapable of emitting any more sperm.
During mating the glands of Cowper in male moles swell to a tenth of their body weight, and they literally fill the female’s vagina to the brim with their ejaculate. Female readers may be interested to know that in these creatures the clitoris is the same size as the penis.
The typical stickiness of human sperm is undoubtedly a remnant of the above-mentioned adhesive quality: when it dries it clings to the hair and skin. In What is Sex? by American biologist Lynn Margulis and journalist Dorion Sagan we are told that drones, after mating with the queen bee, forfeit their lives, leaving behind not only their genitalia but also a slimy substance. One may compare this sealing off with the frantic way in which females are guarded in some frog species. These are the ultimate sperm competition avoiders. Even if the ova have been amply fertilized by the sperm cells the males still do not let go, but cling to their partner for months on end. The writers of the above-mentioned book assume that the tough, syrupy texture of human sperm is the result of our distant mammalian origins: from mammals which had sperm that ‘went hard’ and acted as a kind of natural chastity belt, making access for any later would-be fertilizers difficult or even completely impossible.