GIRLS in the bedroom playing with their dolls. Boys in the back alley chasing one another. Mothers nattering on a park bench. Lads at the pub. Women round the photocopier gossiping about their colleagues鈥� indiscretions. Men in the boardroom discussing the performance of their competitors.
These are blatant sexual stereotypes but, like most stereotypes, they contain at least a grain of truth. We humans do often choose to hang out with friends of the same sex. What鈥檚 more, this behaviour, far from being aberrant, is common throughout the animal world. The experts call it 鈥渟exual segregation鈥� and it is practised by all sorts of species, from chimps and polar bears to albatrosses and dogfish. Until recently, it had been studied primarily in ungulates, but now we are starting to get the bigger picture. And, as it becomes more apparent why many animals separate into different sex groups we are starting to see how our human behaviour fits into the jigsaw.
That male and female animals should band together in separate groups within a herd is puzzling at first glance. But there are several possible explanations. The first focuses on habitat and the notion that males and females choose to congregate in different areas that reflect the requirements of their differing lifestyles. Lactating mothers, for example, might need to eat higher-quality forage and choose their habitat accordingly. The second idea is that segregation stems from the two sexes being subject to different predation pressures. Nursing females might be more at risk than males, for instance. A third idea links segregation with energy requirements and activity levels. This 鈥渁ctivity budget hypothesis鈥� assumes that in some species males and females spend unequal amounts of time active or inactive. The more the sexes differ in their activity levels, the more difficult it is for them to stay together, which is why males and females segregate.
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In an attempt to tease apart these ideas, Kathreen Ruckstuhl and Peter Neuhaus from the University of Cambridge looked at 30 species of ungulates that form single-sex groups within the herd. They found that varying activity levels between males and females could largely explain segregation, with predation risk and foraging selection playing smaller parts. A follow-up study indicated that, in theory at least, activity budget differences alone can create sexual segregation.
Teaming up with Hanna Kokko from the University of Jyv盲skyl盲 in Finland, Ruckstuhl created a computer simulation in which males and females were distinguished only by their activity levels and their patterns of switching from being active to inactive, with males being more ready to get up and more reluctant to lie down than females. All the individuals were social: programmed to congregate with others they encountered. 鈥淎s sexual differences in activity budgets increase, the degree of sexual segregation significantly increased,鈥� says Ruckstuhl. 鈥淪exual segregation reached a peak of around 74 per cent when sexual differences in activity budgets were greatest鈥� (Animal Behaviour, vol 64, p 909).
Segregation by size
Ruckstuhl concludes that in ungulates, at least, different levels of activity between the sexes largely explains segregation. But not all ungulates divide neatly along sexual lines. She wondered whether she could predict which species would do so and which would not. The obvious starting pointed was body size dimorphism, because sexual segregation tends to be most common in species where one sex is larger than the other. And there is a huge range of variation between ungulate species, from Alpine ibex, where males are around twice the size of females, to animals such as oryx and zebras, where both sexes are the same size. When Ruckstuhl and Neuhaus looked at data for 30 species of ungulates they found that the watershed was 20 per cent 鈥� species with at least this difference in size between the sexes tended to form single-sex groups.
Ruckstuhl鈥檚 findings have gone a long way towards unravelling the mystery of sexual segregation in ungulates. But what of other animals? At a meeting organised by Ruckstuhl in September 2002, biologists studying this odd behaviour in a variety of species had a rare opportunity to compare notes. One thing that became apparent was that sexual segregation is not to be undertaken lightly. It can have fatal consequences for albatrosses and giant petrels, for example, both of which segregate because males and females choose different hunting grounds. While males stay close to shore, females go on long foraging trips out to the open ocean. Studies by Jose Xavier from the University of Cambridge, Jacob Gonzalez-Solis from the University of Barcelona, Spain, and their colleagues from the British Antarctic Survey found that the female hunting areas overlap with long-line fishing activity and that birds getting caught up in the lines is the main reason for the highly skewed sex bias in the mortality of these species. The researchers believe it is also the most likely explanation for their dramatic decline in recent years.
As well as driving home the message that segregation can be a conservation issue, the meeting also served to highlight just how common the practice is, and the wide variety of underlying causes. Sexual segregation occurs in several solitary species. It sounds like a contradiction in terms, but in practice, this simply means that males and females hang out in different environments. Dogfish do it, and David Sims of the Marine Biological Association in Plymouth, UK, is trying to find out why. Grizzly bears segregate so that females can avoid contact with infanticidal males. And even among ungulates, the activity budget hypothesis doesn鈥檛 explain all instances of segregation. Johan du Toit from the University of Pretoria in South Africa has found that male and female giraffes browse on the same trees but at different heights to avoid competition. And male and female kudus, an African antelope, segregate primarily to avoid being eaten. Leopards are more of a problem for females, and lions for males, so each sex tends to occupy areas that have lower densities of their respective predator.
Bats do it for their babies
Bats also segregate by sex for a whole range of reasons. Robert Barclay from the University of Calgary, Canada, has found that male brown bats (Myotis lucifugus) and long-eared bats (Myotis evotis), which live in the Canadian Rockies, tend to forage further up the mountainside than females. Males in these species spend the cold nights in a sort of temporary hibernation known as torpor, whereas females cannot go into torpor when they are pregnant because it would endanger their developing fetuses.
John Altringham and Paula Senior from the University of Leeds, UK, found a similar pattern when studying Daubenton鈥檚 bats (Myotis daubentonii). In this species some males forage for food higher upstream than the females. They suggested that the energy demands of pregnancy and lactation might keep females from living at higher elevations or in areas where density of prey is unpredictable. Males can save energy by entering torpor if foraging conditions are bad, but pregnant females do not have that option. Ruckstuhl believes that bats would be a good group in which to search for general patterns underlying sexual segregation, as there are well over 1000 species, many of which engage in the behaviour.
With so many unanswered questions, the field is certainly ripe for further research. African ground squirrels, for example, segregate strictly by sex but no one is sure why. Mysteriously, mice seem to start their sexual segregation in the uterus, with males aligning on one side and females on the other. Nobody knows whether invertebrates segregate by sex. Information is very limited, although likely suspects include the horned beetle, where there is a clear size difference between males and females. But size dimorphism is not always an indicator. Studies of whales by Robert Michaud from Dalhousie University in Halifax, Canada, reveal that there is usually sex segregation in species where males are much larger than females, with one striking exception 鈥� male and female orcas or killer whales, which live in family units called pods. The reason remains unclear.
And what about primates? Sexual segregation is uncommon among our closest living relatives even where males are much larger than females, according to David Watts from Yale University. In some species there are temporary same-sex associations, which tend to be linked with mating behaviour. Among chimps, for example, all-male bands roam their territories to fight off intruders or to check on oestrous females. Spider monkeys also segregate by sex. Could such behaviour provide any clues about our sexual segregation?
We undoubtedly show an extraordinarily high level of sexual segregation for a primate. And, unusually, it starts early in life. Anthony Pellegrini from the University of Minnesota, Twin Cities, has spent many hours observing and analysing children at play. He has found that boys tend to engage in rough-and-tumble play with lots of running around, whereas girls are less energetic. In other words, boys and girls have different activity levels, and this tends to split them into same-sex groups 鈥� supporting the activity budget hypothesis.
But could this simply be a product of upbringing and cultural background? Melissa Hines from City University, London, has found biochemical reasons underlying a tendency for children to seek out playmates of the same sex. Her studies show that tomboyish behaviour is associated with female fetuses being exposed to higher than normal levels of androgens (male hormones) in the womb. But Hines cautions that the reasons for segregation are not simply biochemical. 鈥淏oys discourage other boys from playing with girls,鈥� she says. And single sex associations are both positively and negatively reinforced by children and adults, with boys being more strongly socialised to be boyish.
The reasons for sexual segregation among adults are equally tricky to pin down. They include common interests, a shared outlook on life, and differences in status. But perhaps the best place to look for their biological roots is in traditional societies where strict sexual segregation is the rule, with men hunting and women gathering food. Kristen Hawkes from the University of Utah, Salt Lake City, has spent many years studying modern forager ecology and is convinced that sexual selection underpins this segregation. She points out that men could provide more calories by helping gather roots and berries. But hunting is a way of showing off, and men who are good at it tend to have more sexual partners and enjoy higher status.
If Hawkes is correct, sexual segregation among human adults has its origins in mating behaviour. So it looks as if we are not so very different from other primates after all. And children seem to play in single-sex groups because of their different energy levels 鈥� segregating according to activity budgets, just as many animals do. But it is still a mystery why we humans are so much fonder of consorting with our own sex than other apes are. 鈥淲hat I find fascinating is that wherever you go, you always get men and women forming separate groups, even at conferences,鈥� says Ruckstuhl. None of us is immune, not even the people researching sexual segregation.