AMONG spiders, the female of the species really is more deadly than the male. Lady arachnids have a well-deserved reputation for polishing off their suitors, post copula, in a manner that Hannibal Lecter might have admired. But it has never been clear why this happens. Some biologists believe it is simply a mixture of female hunger and the availability of a meal that is in no position to run away. Others suspect that the male is actually sacrificing his life for the good of his genes. In other words, his becoming a meal for his paramour somehow helps the offspring of their union.
Peng Yu, of Hubei University in China, and his colleagues, decided to try to settle the question. The results of their investigation are published this week in Behavioral Ecology and Sociobiology.
Dr Peng and his team collected almost 400 young wolf spiders, of both sexes, from local fields. They then raised the animals separately (in order to avoid premature cannibalistic accidents) until they were sexually mature.
Then, one at a time, they introduced the females to a male and watched until one of three things happened: the male got eaten; the male mated with the female and successfully avoided being eaten; or the male survived for half an hour, but did not mate with the female within that time. Sometimes the researchers paired virgin males with virgin females; sometimes they paired virgin males with females that had recently bred. And in one crucial set of tests they paired virgin males with virgin females that had been taken off their regular diet of fruit flies a fortnight beforehand, and were thus presumed to be feeling more than a little peckish.
After doing all this, the team chose 16 females that had mated and then eaten their partners, and ten that had mated but not done so, and followed their reproductive success. When these females laid their egg sacs, the researchers picked ten sacs at random from each group and monitored those until the eggs hatched. At that point they selected 20 spiderlings from each group for further study.
Their first pertinent observation was that, while female wolf spiders did indeed sometimes eat males before breeding with them, that happened only 10% of the time, and did not seem to be more frequent if the female had been starved. Their second observation was that if a male was deemed suitable to mate with, he was never eaten in copula—even though copulation could last as long as an hour and a half. Their third was that, 28% of the time, a male that had mated was indeed eaten afterwards. Successful suitors, then, succumbed more often than unsuccessful ones.
The crucial finding, however—which makes sense of all the others—was the success of the spiderlings. Young born of females that had eaten their partners had a 48% chance of making it through their first month of life. Those born of females who had let their partners live, had only a 12% chance of surviving that long.
That is a staggering difference—and certainly, in evolutionary terms, enough to drive self-sacrificial behaviour by males, since a male would have to mate successfully another three times to match the benefit he gains by this one suicidal act. The reason a male is almost three times as likely to be eaten if he has mated with the female in question than if he has not is thus probably that he wants to be eaten, for the good of his posterity.
Just what it is that a male meal gives, via the female’s digestive system, to the hatchling spiders, remains to be determined. Perhaps spider bodies contain some crucial nutrients which are scarcer in other forms of prey. Whatever the details, though, the general answer to the biologists’ question is now clear. In the case of spiders, fathers really do lay down their lives for their children.
This article appeared in the Science and technology section of the print edition under the headline "Having a mate for dinner"