IF YOU find yourself fancying a member of another species, the chances are that you're female, and somewhat in need of company from your own kind. This is the surprising conclusion of a new study of cross- species mating in animals by Peter Wirtz, of the Max Planck Institute in Germany. Wirtz's conclusion is surprising on two counts. First because, if he is right, mating across the species boundary seems to be due to behaviour designed to boost breeding success, and, second, because the individuals responsible for it are female.

Cross-species breeding is not in fact uncommon. It has been a potent evolutionary force in plants, for example, where more than half of all known flowering species have some kind of hybrid ancestry. But it has generally been considered to be rare among animals because differences in behaviour and anatomy add to the barriers that are created by incompatibility between eggs and sperm.

Despite this, accumulating evidence from genetic studies is now pointing to a more extensive role for hybridisation in the origin and evolution of animal species, particularly fish and birds.

Hybridisation can work in two ways. Species can mate reciprocally, so males of species A mate with females of species B, and vice versa. Or they can mate in one direction; males of species A fertilise females of species B but not the other way round. Biologists can tell which has occurred by looking at a special form of DNA known as mitochondrial DNA (because it is found in the organelles - mitochondria - that provide energy to the cell). Mitochondrial DNA has the curious property of being inherited only from the mother, not from both parents like normal (germ- line) DNA.

So if a hybrid contains the mitochondrial DNA of only one of its "parent" species, we can infer that this is the "mother" species and that it has played the maternal role in all its cross-species matings. In many species the so-called Y sex chromosome, which is passed on by the father, tells a similar story from the paternal side.

Reviewing published studies of animal hybrids, Wirtz found that an astonishingly high proportion, almost two-thirds of cases, were unidirectional: one species had consistently provided the mother, the other the father. A few of these may be due to simple mechanical limitations, such as size differences dictating which species can play the female role. A few others may reflect isolated chance encounters. But the high proportion of unidirectional hybrids, and the fact that they spread right across the animal kingdom, suggests that something more fundamental is at work. The most likely explanation, according to Wirtz, is that such hybrid events are the product of female mating preferences.

On the face of it, this seems to make little sense. Basic evolu-tionary reasoning says that females should be much fussier than males in choosing a mate, because their relatively few eggs each represent a greater proportion of their potential lifetime quota of offspring. Wasting one on a duff mating is a serious blunder. If anything, we might expect males, with their myriad, replenishable sperm, to be the driving force behind cross-species matings.

However, the "expensive egg" argument works in two ways. True, females should be choosier, but if they're too choosy, and fail to mate at all, the egg is wasted by default. A simple scenario thus emerges. When two species first come together, a few individuals of one are likely to mingle with a sizeable population of the other. Indiscriminate newcomer males happily court females of the indigenous species, but get nowhere because the indigenous females have plenty of their own males to choose from. Newcomer females, by contrast, have a hard time locating their own kind but are surrounded by plenty of obliging suitors of the local variety.

As the quest for a desirable home-grown hunk becomes ever more desperate, the fussiness of the newcomer females weakens and the temptation to succumb to the overtures of the natives rises. Rarity and frustrated female choice eventually conspire to produce a hybrid.

While this may smack of armchair theorising, studies ranging from fish to fur seals (pictured) provide strong circumstantial evidence for Wirtz's lonely female theory. Certainly other mechanisms could account for the absence of mitochondrial DNA from one of the parent species, but there is little to suggest they have been a force in species evolution. In contrast, exotic fancies born of frustrated female desire may have been a frequent novelty source in the evolution of animal species diversity.

Dr Chris Barnard is a member of the behaviour and ecology research group at the University of Nottingham