ANIMAL mating can be a cruel and unusual process. Male bedbugs inseminate females by piercing their bellies and depositing sperm inside their paramours’ body cavities. Male chimpanzees and lions kill the suckling infants of females before mating with them, as this brings those females more rapidly into oestrus. Male dolphins routinely engage in rape. Nor are aggressive mating practices perpetrated solely by males against females. In many species of insects and spiders, females eat their partners after sex.

Such cannibalism clearly brings advantage to the female, who gets an easy snack. But the benefits (if any) for the male are less obvious. That there might sometimes be such benefits, though, is an idea that intrigues zoologists—and so, from time to time, some of them look into the matter.

The latest to do so are Steven Schwartz of Gonzaga University, in the American state of Washington, and Eileen Hebets of the University of Nebraska-Lincoln. Dr Schwartz and Dr Hebets note that, after mating, the males of one species of arachnid, the dark fishing spider, spontaneously die and thus ensure that they get eaten. This is in contradistinction to the behaviour of most male spiders, who usually attempt at least some sort of a getaway, even if it is futile. And, as the two researchers report in a forthcoming paper in Current Biology, there is, indeed, method in the male fishing spider’s suicidal madness.

Dr Schwartz and Dr Hebets came to this conclusion by collecting male and female dark fishing spiders and subjecting them to an experiment. In one group of the animals, females were allowed, as per normal, to eat their deceased partners after mating. In a second, the males’ bodies were removed and the females ate nothing. And, in a third, the males’ bodies were substituted by a cricket of about the same weight as a male spider.

Not surprisingly, the offspring of females in the first group—those allowed to cannibalise their partners—were bigger, more numerous and longer-lived than those of females in the second. But they were also bigger, more numerous and longer-lived than those of females in the third, cricket-fed group. In fact, the offspring of the third group did no better than those whose mothers had received no extra nutrients at all. Evidently, something in male fishing-spider flesh is particularly advantageous for the production and development of young.

Exactly what this something is, Dr Schwartz and Dr Hebets cannot yet say. But they do have a theory about what is going on. The fact that the male spider dies after mating, and thus makes sure his body is available as a feast for his mate, suggests the mysterious extra nutritional value of that body has evolved specifically for the purpose of nurturing the eggs that will turn into his offspring. Possibly, in the past, females have been so good at catching males that few survived to father a second brood anyway. In that case, any adaptation which enhanced the number and fitness of a male’s firstborn clutch, even at the expense of his life, would be favoured by natural selection. Whatever the truth, though, the fate of the poor male dark fishing spider is surely the cruellest and most unusual one-night stand of all.