Dear Dr Tatiana,

Every year, after only a little bit of sex, my mate imprisons me in a hole in a tree-trunk. He claims that this is for my safety, but I think he's just insecure. Who is right?

TRAPPED IN ANN ARBOR

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MY DEAR, your husband seems a little behind the times. Even Darwin thought that safety was an improbable explanation for the devoted and careful imprisoning behaviour of the male hornbill. It is much more likely that you are correct, and that walling you up is his way of solving a perennial problem: being sure of paternity.

In these days of DNA tests, perhaps you may be able to persuade your mate to think again. But I doubt he would agree. Genetic analysis of parents and offspring in many species--including people--suggest that females are adept at slipping off for what biologists call an "extra-pair copulation", or what the rest of us call a quickie.

Some of the biggest conflicts caused by quickies concerns the rearing of young. Males do not want to waste precious energy raising someone else's offspring, but they have trouble knowing for certain which young are their own. So natural selection has led to an impressive range of ways by which males prevent females from having quickies, and by which females sneak out to have them anyway. Your mate uses one of the more extreme strategies--but there are many others.

For example, the female chimpanzee and the lioness go for massive obfuscation. Both are so flagrantly promiscuous that all the males in a troupe or a pride must think they might be the father of every offspring, and so are less likely to kill any one of them. A lioness, for instance, requires more than 3,000 copulations to make a cub. As a result, many males pitch in.

When your mate walls you in, at least he is paying attention. Some species conduct their sexual wars without as much as a meeting between the male and female. For instance, the female abalone releases her eggs into the water and then the male comes along to fertilise them. Sperm that can penetrate the covering of an egg more quickly are the most likely to succeed in fertilisation--so any gene that contributes to rapid penetration will be favoured. But if sperm become too quick, an egg will have trouble ensuring that it is fertilised by only one sperm, and will thus cease to be viable. Any gene in the egg that contributes to slowing down the sperm will therefore also be favoured--and so on, through the rumba of time.

You see the pattern? Each sex is constantly evolving to defeat the other's machinations. Give yourself a few epochs, and you may find a way out of your hole.

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