Penises and sperm

It used to be thought that males have evolved to make love and females to make babies. But now biologists have discovered the truth: namely that, from stick insects to chimpanzees, females are hardly ever faithful. In fact, in species after species, rampant promiscuity is beneficial. Which, says Dr Tatiana, goes a long way to explaining why penises and sperm come in so many different shapes and sizes.

More than just sperm delivery
Bushbabies and many other primates have monstrous penises – many of them look like medieval torture instruments. They have spikes and knobs and bristles and are often twisted into weird and sinister shapes. By comparison, the human penis is dull, notable only for its girth.

Penises are for more than just sperm delivery, you see. If females mate with a number of males, each subsequent suitor will sire a larger proportion of her children if his sperm are the ones that do the trick. A male who can stimulate his mate to take up more of his sperm, or who can somehow get rid of the sperm of his rivals, will spread more of his genes than less artful fellows. Thus, the first consequence of female promiscuity is that males are under greater pressure to outdo one another in all aspects of love. For this task, the penis is an important tool.

Scourers and tickling sticks
Consider damselflies. These insects, close relations of dragonflies, look sweet and innocent as they flit along the riverbanks on a sultry summer day. But they have evolved some of the fanciest penises around. A typical damselfly penis has a balloon – an inflatable bulb – and two horns at the tip, plus long bristles down the sides. In the black-winged damselfly, Calopteryx maculate, the male uses this device to scour sperm from inside a female before depositing his own. But in the related Calopteryx haemorrhoidalis asturica, he uses his penis as an instrument of persuasion: by stimulating her in the proper manner, he can induce her to eject sperm from previous lovers.

Meanwhile, the moth Olceclostera seraphica has genitals that resemble a musical instrument: the male rubs one part of his privates against another, producing vibrations with which to thrill his mate.

Unadorned and teeny weenies
In contrast, among termites the female typically mates with only one male – and male termites have plain, unadorned genitalia that do not differ much from one species to the next.

Among primates as among insects, it is a rule of thumb that in species where females consort with one male at a time, penises are small and uninteresting. Take the gorilla – a huge guy with a little teeny weenie. A male gorilla can weigh 210kg (460lbs), but his penis is a measly 5cm (2in) long and entirely devoid of knobs and spikes. The Argentine lake duck puts him to shame. The duck is small, but his penis, which rivals that of the ostrich, is 20cm (8in) long – and it has spines. But then a male gorilla generally presides over a small group and does not often have to worry about other fellows’ sperm.

Frisbees, corkskrews and bearded sperm
The evolution of sperm size and shape is something we know little about. All we can say for sure is that sperm tend to be smaller and simpler in species where the eggs are fertilized outside the female’s body.

Consider sperm shape first. A conventional sperm looks like a tadpole, with a big head and undulating tail. But in many species, sperm deviate from this model. A popular invention is the tandem sperm – sperm that always swim in teams of two. They’ve evolved in American opossums, water beetles, millipedes, fire-brats, and some seafaring snails. Hooks are also fashionable. Koalas, rodents and crickets all have hooked sperm. The protura, tiny critters related to insects, were the first to play Ultimate Frisbee: their sperm are flat discs.

Crayfishes have sperm that resemble Catherine wheels. Some land snails make corkscrews. Some termites make bearded sperm – sperm with 100 tails. Roundworms make amoeboid sperm: these don’t swim, they crawl. And all this before you begin to look at spermatophores, the sperm packets that many creatures deliver. After a lengthy lovemaking session, the giant octopus, for example, hands over a spermatophore that is a huge bomb. Over 1m (3ft) long, it contains more than 10 billion sperm and explodes inside the female reproductive tract.

Since these various shapes have evolved independently in different groups, they must be repeatedly beneficial in some way. Hooks, for example, might help sperm grapple their way along the reproductive tract – but to my knowledge, this has never been demonstrated. The possible advantages of other shapes? Your guess is as good as mine. But as far as we can tell, sperm shape has nothing to do with female promiscuity.

Super sperm
Sperm size, however, probably does. In roundworms, big sperm are more successful at fertilizing eggs, apparently because they crawl faster than small sperm and are less likely to be pushed aside by rivals. Similarly, in the bulb mite, an agricultural pest, males who boast big sperm fertilize more eggs than males whose sperm are puny. Indeed, as a general rule, where females are promiscuous, males don’t just make more sperm, they make bigger sperm. The Giant Sperm Hall of Fame contains a diverse scattering of animals, including featherwing beetles, back-swimming beetles, ostracods (small shrimp that look like kidney beans on legs), ticks, the Australian land snail Hedleyella falconeri, the painter’s frog, but the reigning champion is the fruit fly, Drosophila bifurca, whose solitary sperm takes three weeks to make and has a tail 20 times longer than the fly’s whole body.