ONCE upon a time, sex in the animal kingdom seemed pretty simple. Flamboyant male met coy female, male courted female, male deposited spermatozoa in the vicinity of an ovary, then headed out to do it all again elsewhere.

Then biologists began to look more closely, at what really happens. They found that being the biggest and brashest male doesn't always win you mating rights. Among weaver fish, for example, it is good fathers, the ones who will take care of the fry, who get the girl. Females don't always conform to type either. The female bean weevil, for instance, would rather drink her mate's ejaculate than use it to fertilise her eggs. Reproduction, it turns out, is a complex affair.

Just how complex has been emphasised anew with a slew of studies that highlight the staggering diversity of sexual practice in the animal kingdom. Intercourse is a bizarre and often dangerous pursuit, where sexually transmitted infections can be desirable, living in a male harem inside your mate can make sense, and headless lovers give you extra. Relations between the sexes are also surprisingly convoluted. Biologists have charted virgin births, spontaneous sex changes and, perhaps weirdest of all, males who father their brother's offspring. Human sexual exuberance is tame compared with some of the things that animals get up to in the name of reproduction.

Take the male preying mantis, the poster boy of risky sex. In an ideal world, he will jump onto a female's back, establish a rigid grip, copulate and jump away again, safe to repeat the process with some other female. Much of the time, however, that grip will slip. If it does, the male slides within reach of the female's mandibles and he stands a very good chance of having his head bitten off.

Being eaten by your partner during copulation is clearly not desirable. William Brown at the State University of New York at Fredonia thinks the males tread a delicate line. His research reveals that they approach females with trepidation: the drive to reproduce and the drive to survive are at loggerheads (The American Naturalist, vol 167, p 263). "Our work suggests that males actively assess the level of risk posed by an individual female and alter their behaviour to reduce the risk of sexual cannibalism," Brown says. "We expect that the level of acceptable risk to the male will depend upon features such as the availability of safer mating opportunities, the age of the male - and thus his expectation of future reproduction - and perhaps even the quality of the female."

From the female's point of view, cannibalistic sex looks like a winner on several fronts. Clearly, it provides a nutritious meal, making it particularly popular among females who have not eaten for a while. But there may also be another benefit. In some mantid species, losing your head means that you have also lost the system of nerves that tells you to stop copulating. Meanwhile, the nerves that keep copulation going, which are in your abdomen, remain intact. So following decapitation, the female gets everything the male has to offer, as it were. There's just one downside. "Hungrier, more cannibalistic females attract fewer males," Brown says.

Another species in which females keep males firmly in their place is the green spoonworm, Bonellia viridis. Found in the warm waters of the Mediterranean Sea, B. viridis begins life as free-floating flake-like larvae. When they settle on the sea floor, they mature over a period of years into 10-centimetre-long females. Many, however, do not make it this far. If a larva should settle on top of a female instead, she produces a chemical called bonellin that turns the larva into a tiny male. This male then creeps up her body and into her mouth, from where it migrates down to her uterus. "Once inside the female, males assume a parasitic existence: they depend on the female for their nourishment," says Patrick Schembri of the University of Malta. But there is mutual benefit. With up to 20 males safely holed up inside her genital sac, the female can get her eggs fertilised without expending any effort on finding a mate.

While B. viridis females keep their males captive, aphids prefer a more detached relationship. In fact, many species only copulate once a year and it's not even sperm the females are after.

A female aphid can reproduce without sex. In terms of her genetic legacy, it makes perfect sense to do this because she can produce many clones that carry all her genes down through the generations. So why do aphids make time for an annual bout of sex? This question puzzled Nancy Moran and Helen Dunbar at the University of Tucson in Arizona. Their surprising discovery is that aphids have sex to acquire sexually transmitted infections (Proceedings of the National Academy of Sciences, vol 103, p 12803).

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