Mechanisms and Functions of Sexual Segregation in Habitat Use

In many animals males and females may live apart for some or most of their lives termed sexual segregation. Sex segregation may occur when the sexes use different parts of their environment. Ancient Greek mythology tells of a nation of female warriors known as the Amazons who lived on an island. The Amazons only met with men to trade and reproduce and kept all daughters on the island. Whilst the degree of truth in this ancient myth is open to debate, in animals sex differences in habitat use are common and understanding why and how this behaviour occurs is essential if we are going to conserve and protect species and habitats. For example, if sex segregation occurs it may be necessary to protect different habitats for males and females. However, we have to understand why and how this behaviour occurs to be able to do this. For some time scientists have tried to understand why the sexes segregate in habitat use. In the 1900's Charles Darwin pondered over why red deer live apart for much of the year. In recent years many explanations have been proposed, of which three are probably important. The first idea hinges on the fact that in many species males and females are attacked at different rates by predators. Males may be larger and be equipped with weapons such as the antlers, which may allow them to use habitats with increased predation risk. The second idea suggests that males and females may differ in their nutritional requirements, for example due to pregnancy, which may lead to the sexes selecting different habitats to meet these nutritional needs. Finally, harassment of females by males is common in the animal kingdom and frequently causes injury to females or their offspring. The final idea suggests that females occupy habitats with no males to reduce this harassment. Previous work on sex segregation has generally focused on hoofed animals such as deer and sheep, but because of their large size and the amount of space they need to roam, scientists have generally been able to do little more than describe patterns of segregation. In contrast the proposed study will experimentally test these hypotheses using a small species of freshwater fish 'guppies' on the island of Trinidad in the Carribean as a model system. Guppies form relatively small populations in different streams that vary in the number of predators they face. Furthermore, guppies can easily be kept in the laboratory, making them particularly suitable for this investigation. We know that guppies living under high predation risk segregate by sex, with male guppies (which are more vulnerable to predation) occupying areas of shallow water, which are thought to be safer from predators, suggesting that predation risk is important in driving sexual segregation. However, the benefits that they gain from segregation remain unknown. For example, why do females accept a greater risk of predation? Is it to reduce sexual harassment or because there is less competition for food? Or is it a combination of the two? We also do not know how patterns of sex segregation arise, or why some males and females live in the same parts of the river. Does it dependent on environmental conditions? How is it affected by differences in the number of males and females and the density of the population? I will compare the degree of sex segregation between 10 populations of wild guppies living under different risks of predation in the rivers of Trinidad. In each river I will assess the levels of sex segregation and compare it to the risk of predation, population structure and habitat type (e.g. water depth) testing predictions from the main ideas thought to drive sex segregation. In addition to observing patterns of behaviour in the wild this study will observe the behaviour of guppies in the laboratory where it is possible to measure the level of sex segregation in a standard environment and test the factors thought to drive sex segregation in isolation.