What's love got to do with it? Lots

Mistletoe, office parties, festive spirits in generous measure and a scientist's mind turns naturally to mate selection. But how do we choose our partners, and what makes some potential mates more attractive than others?

For most animal species, the females are choosier than the males. For females, producing eggs or rearing young is a big investment and a poor choice of mate can be expensive. Males are generally less particular about their choice of partner - sperm is cheap.

Some males are more attractive to females than others. Recent experiments with peacocks and mice have shed new light on how animals choose their mates but the latest psychological research says that humans do it differently.

Dr Marion Petrie, an evolutionary biologist from the Department of Zoology at the University of Oxford, studies peacocks. Her research shows that those males with particularly elaborate and spectacular tails are the most desired.

"There is scientific evidence that females prefer exaggerated features," she says. This is true of other species. Female swallows prefer males with long tails, and baboons with the most brightly coloured genitals are particularly attractive to female baboons.

Alternatively, according to Dr Sarah Lenington, a professor at the Institute of Animal Behaviour at Rutgers University in the US, some animals use status, rather than appearance, to select a mate. "Whenever animals live in groups with a hierarchy, such as mice or monkeys, the highest-ranking males are generally favoured," she says.

Selecting the right mate is important. Mating with the preferred male, whether it is for status or appearance, has measurable advantages. Dr Petrie compared the survival of progeny from preferred male peacocks with those from less popular specimens.

Her work, published recently in Nature showed that the offspring of the most attractive birds fared better. They were fitter and survived longer than offspring from less popular cocks.

The results of studies on mice show that mating with the "chiefs" has similar advantages. The offspring are fitter, healthier and have lower mortality, say Professors Gary Beauchamp and Ted Boyes, who conduct research at the Sloan Kettering Institute in the US. Whether partners are chosen for their looks or their social position, the progeny from preferred matings fare better.

Scientists do not yet understand why this should be, but they have a few theories. Some evolutionary biologists believe that the genes for a particularly exaggerated characteristic happen to be next to other genes that provide an enhanced resistance to disease or confer an advantage when escaping predators. The linkage of genes for health with those for beauty could contribute to the evolution of the peacock's elaborate tail.

Dr Lenington is one of many biologists who believe that mate selection also has a genetic component. "In the last five years there has been a profusion of studies suggesting that genes are important in mate selection in species as diverse as ladybirds, beetles, guppies and mice," she says.

In mice, a specific set of genes known as the Major Histocompatibility Complex (MHC), well established for its important role in rejecting tissue or organ transplants, also seems to be important in mate selection. Surprisingly, the genes that are involved in differentiating the animal's own tissue from foreign are also important in differentiating closely related animals from others.

Mice prefer to mate with animals whose MHC is different from their own. According to Dr Lenington, mice can sense different MHC types through urinary odour.

"The differences may be small but they seem to be an important enough factor for mice to use this information when choosing a mate," she says.

The MHC is present in most vertebrates, including humans. "There is some evidence that humans also choose mates with dissimilar MHC region, in that most partners differ in their MHC type," says Dr Lenington. How this happens in humans remains a mystery, particularly since we depend so much less on smell to guide our behaviour.

For humans, mate selection is more complex than a clinical choice of genetically programmed odours or the selection of a prominent piece of anatomy. If humans were to select mates in the same way as peacocks or mice, then prominent physical characteristics or status might explain the obvious popularity of men such as Mick Jagger or David Mellor.

Cole Porter would have us believe that we, like birds, bees and even educated fleas, have a much more romantic approach to choosing a partner. Biologists would probably disagree, arguing that falling in love is primarily a human emotion.

According to Dr Niels Waller of the psychology department at the University of California in Davis, there is almost no genetic component to the way in which we do it. When it comes to love, nurture is more important than nature.

Psychologists have found that people can adopt one of six different love-styles when choosing a mate (although they can also have mixed styles). The six styles, first proposed by Professor J A Lee in 1988, all have Greek names. Those who exhibit the "Agape" style show a spiritual and selfless form of love in which the person concentrates on caring for their partner. "Eros" types favour a passionate and intimate approach whereby the individual is self-confident, falling in love fairly quickly. "Ludus" people are fun-loving and seek the excitement of love but, for them, commitment and thought-sharing are not key components. Those who score high on "Mania" have a desperate and conflicted form of love that can also bring pain and jealously. "Pragma" peopleare practical and reasoned. This approach to love is favoured by people who consider the outcome and relative advantages of a relationship before entering into it. Finally, those who score high on "Storge" value commitment, reliability and affection. For them, love can easily develop out of friendship rather than passion.

Dr Waller studied the responses of 890 pairs of twins and their partners, examining whether identical and non-identical twins adopted the same styles when choosing their mates. He found that "shared experiences, not shared genes, account for similarity in love attitudes".

Genetic factors accounted for virtually none of the individual differences in the approach to love. There must, however, be some influence of genetic make-up on mate selection in humans.

Dr Glen Wilson, a psychologist at the Institute of Psychiatry in London, believes that we have a tendency to choose partners in our own image.

"Height, skin colour and eye colour are characteristics which are strongly correlated between partners. We seem to pick people who are like us, but are not close relatives," he says.

Parents are important role models for their children's choice of mate. Infants imprint on their opposite sex parent, remember their characteristics and frequently recover the images when selecting their partners later. Women choose men who look somewhat like their father did when they were girls of two to three years old, according to Dr Wilson.

For humans, the choice of mate is complex. It can be based on many factors including physical appearance, male status or genetic determinants. The style in which we do it, though, seems to have little to do with genes.

W Somerset Maugham may well have got it right when he said: "Love is only the dirty trick played on us to achieve continuation of the species."