In our series on Big Science Questions, Tim Birkhead looks at research in male-female attraction.
We have all felt that little zing! when we see someone we fancy, and a much greater ZING! when these feelings are reciprocated. What is happening? The question of why we experience "zings" can be answered at two levels - first, by asking what makes someone attractive, and second, by asking why we have evolved to find particular features attractive. The first is a question of immediate causes, the second addresses the evolutionary significance of attraction. Usually, these two types of question are tackled by different sorts of researchers, concentrating respectively on physiological mechanisms and evolutionary factors. But evolutionary biologists interested in sexual attraction have addressed them together.
Those evolutionary biologists who study animal behaviour believe that a particular male and female have ended up copulating because either the female has chosen the male, the male has competed for and "won" the female, or because both partners have come to some mutual agreement about liking each other.
It was Charles Darwin who first put sexual attraction into an evolutionary context. In formulating his ideas about natural selection, he was concerned about traits that did nothing to promote their owner's survival. The extravagant plumage of male birds and the cumbersome antlers of deer made their owners conspicuous and vulnerable to predators, so how could they possibly evolve by natural selection?
Darwin's answer was sexual selection. Extravagant traits probably did reduce their owner's survival by making them more vulnerable to predators, but they more than compensated by making their owners more competitive or irresistibly attractive to members of the opposite sex, so that they left more offspring - more copies of their genes - than less ornamented males.
Sexual selection accounts for many of the differences between males and females and Darwin saw it operating through two processes: competition between members of the same sex - usually males competing for females - and choice by one sex for some members of the other - usually female choice of males. Competition between males for females accounted for the evolution of weapons such as teeth, spurs and antlers, while choice of males by females accounted for otherwise useless ornaments such as plumes, wattles and perfumes.
But when Darwin died in 1882 the idea of female choice all but died with him for nearly 100 years, until an experiment by behavioural ecologist Malte Andersson of the University of Goteborg in Sweden. This involved shortening and elongating (using scissors and superglue) the tail feathers of male long-tailed widow birds, and showed that females preferred the long-tailed males. Further experiments showed female choice was widespread throughout the animal kingdom.
The rediscovery of Darwin by behavioural ecologists in the early 1970s was a paradigm shift, and one that began with some sloppy thinking and much controversy. In 1981, for example, paleontologist Stephen J. Gould and evolutionary geneticist Richard Lewontin accused behavioural ecologists of telling "Just-So" stories and finding evidence to fit them. But the approach eventually yielded rich rewards. For example, as leading behavioural ecologists Martin Daly and Margo Wilson showed in their study of child abuse and step-parenting, the evolutionary approach provided a much greater insight into the nature of relationships between relatives and non-relatives.
A decade or so later came the evolutionary psychologists - the psychological rediscovery of Darwin. Evolutionary psychology is controversial because testing hypotheses about human behaviour is confused by the influence of culture. Some problems might be avoided by studying preliterate societies, but there are few of them left. The alternative, used by many evolutionary psychologists, is to search for what they call "universals" - traits that are perceived in the same way across all human cultures. This is what David Buss has done. What he found, by studying a number of cultures, was that what men find attractive in women is youth and beauty because all the features that make women beautiful, such as clear skin, luxuriant hair, an hour-glass figure and so on, are signs of fertility. In the cold hard light of evolution, what males want (usually unconsciously) are fertilisations and descendants. Women are programmed to do the same, but they achieve it in a different way.
What are women choosing? The answer, say evolutionary psychologists such as Buss and Robert Wright, author of The Moral Animal , is that women choose men less on the basis of looks and more on status, resources and a willingness to share those resources - particularly in their choice of long-term partners. Women need resources in order to rear their babies, and male resources usually come in the same package as status. In preliterate societies such as the Yanomamo Indians from South America, head men have more wives, more extra-marital affairs and more offspring than other men. Of course, the association between high male status and reproductive success may arise partly because of male-male competition rather than female choice, and indeed in circumstances such as this it is often difficult to distinguish between the two.
Social scientists are critical of the evolutionary psychologists' studies because they focus on the differences in what each sex wants from a partner and ignore the broad similarities, which they consider to be much greater. Evolutionary psychologists counter this with the argument that concentrating on the similarities and ignoring the differences is like thinking chimpanzees and bonobos are really human because they share 98 per cent of their DNA with us.
But social scientists do acknowledge that women are attracted to men with status. So how do men acquire status? The answer is: in any way they can. Overall, men compete more often and go to greater extremes than women in everything. Just look at the Guinness Book of Records under the miscellaneous human achievement category. Men vastly outnumber women in their miscellaneous but extreme activities. What are now needed are studies designed to test evolutionary psychologists' ideas that all male human endeavour is motivated by sex, whether they realise it or not. For example, one could measure male reproductive success and see whether it correlates with male status, within peer groups rather than on a global scale. The problem is, it would be necessary to use molecular paternity analyses to catch any extra-pair offspring, and for ethical reasons this would be extremely difficult to carry out.
The classical view of sexual selection, propounded, for example, by Andersson, is that there are a few high-quality men out there that all women should be keen to mate with. If women do manage to mate with them, what do they gain? The first possibility is that more attractive males are genetically superior, and there is much debate about whether this is true even in non-humans. The other, less contentious, possibility is that more attractive males have more resources that females can use to help rear their offspring. While male status and the resources it brings do seem to be universally admired by women, there are also much more subtle things going on. Mate choice is complex, and humans and non-humans use a wide range of information in choosing partners, some of which is gleaned from experiences during development.
If, for example, young zebra finches are reared by Bengalese finch foster parents, on reaching sexual maturity these zebra finches prefer Bengalese finches to individuals of their own species as sexual partners - a phenomenon known as sexual imprinting. Something similar may occur in humans. Why is it adaptive for men to choose a partner who resembles their mother and for women to choose someone who resembles their father? One possible answer, originally proposed by Patrick Bateson in Cambridge on the basis of a study of quail, is that, provided we avoid breeding with close relatives, individuals that breed with those genetically and culturally similar to themselves leave more descendants than those who do not.
Recent studies also suggest that body odour may say something about a male's quality. The major histocompatibility complex (MHC) is a set of genes responsible for the ability to combat infection and has recently been shown to vary enormously between individuals. Male mice advertise their MHC type through an odour in their urine. Given a choice, females choose to mate with males whose MHC differs, that is, complements, their own. A female mouse living in the territory of a male whose MHC is the same as her own will attempt to seek extra-pair matings with a male of another MHC type. Much the same happens in people. When given a choice of male body odours, women prefer those of men whose MHC differs from their own. This research, conducted by Claus Wedekind at the University of Edinburgh, is controversial, but his findings make evolutionary sense. Spontaneous abortion (of very early embryos) is much more common in humans than was previously thought, and is especially common among couples whose MHC's are similar. So there may be genetic advantages in choosing particular males after all.
Tim Birkhead is professor of behavioural ecology at the University of Sheffield.