"If females must compete, evolution will furnish them with weapons to do so"
Mar 4th 2010 | From The Economist print edition
WHEN a species evolves traits that seem to have little to do with individual survival—bright colours, say, or oversize horns, it is typically the male alone who sports these excesses. Observing this, Charles Darwin proposed the idea of “selection in relation to sex” as a follow-up to his theory of natural selection. He defined it as the struggle between members of one sex, “generally male”, to possess the other. The plumage of peacocks attracts peahens. The stag’s antlers are there to fight off other stags. And so on.
But females, it turns out, have some tricks of their own. Nicola Watson and Leigh Simmons of the University of Western Australia have published a paper this week in the Proceedings of the Royal Society about Onthophagus sagittarius, a species of dung beetle in which not only do both sexes sport horns, but those of the females are larger than those of the males. They set out to discover whether female competition accounted for these impressive armaments, and whether there was a trade-off between horns and fecundity.
There are around 2,000 species of dung beetle. All, though, live their lives around faeces. In the case of O. sagittarius, each female constructs a tunnel after she has mated and then packs it with the stuff in the form of a brooding ball, on which she lays her eggs. Her mate guards the entrance, fighting other males to stop them entering the tunnel and cuckolding him. Tunnels are often so close together, however, that other females may break in to their neighbours’ underground, to try to steal dung. Females, therefore, are constantly in conflict with other females, which is why they need horns.
In their laboratory in Perth, Ms Watson and Dr Simmons divided their female beetles into three groups, according to body size. Some of each group were allowed to mate with fertile male beetles. The others mated with beetles rendered sterile by irradiation. This ensured that all female beetles would become pregnant, but those who mated with irradiated males were impregnated by damaged sperm, and would not lay viable eggs. The researchers could thus put three females into a burrow and allow them to compete yet, by ensuring that only one of those females had mated with a fertile male, they could be sure that all the grubs in a burrow were hers.
By comparing all possible combinations of females in this way (mating two of the three sizes with sterile males), and also looking at the success of females who were able to lay their eggs without competition, Ms Watson and Dr Simmons showed that the bigger a female is, the more reproductively successful she is. No surprises there.
The next stage, though, was to do the same experiment, but match females who were the same size except for their horns, in order to see if a bigger horn results in more offspring. The reason to ponder that it might not—and the presumed reason why females of most species do not go in for sexually selected accoutrements—is that such things are costly to grow and maintain. The resources a female spends doing so are therefore unavailable for turning into eggs.
In fact, Ms Watson and Dr Simmons found, horn size was even more important than body size for determining reproductive success. Fending off females who have designs on your dung-ball is evidently more important than laying extra eggs.
If the evolutionary circumstances demand it, then, females can be just as aggressive as males. But they are being aggressive to a different end. This is no struggle to possess the opposite sex, so does it qualify as sexual selection?
That is a matter of definition, but it does go to the heart of the difference between the sexes. Males compete because the more females they inseminate, the more genes they will leave behind. Females mainly let the males get on with this, and pick the winners. They increase their genetic contribution not by promiscuity but by nurturing. If that requires violence, so be it. As to whether there are any human parallels, Ms Watson herself would not be drawn. She did, however, observe that “somebody suggested stilettos.”
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