The mystery of sexual segregation

Posted on March 23, 2015

In many animal species, male and female individuals behave differently in terms of their use of space and habitat, Not surprisingly, this has been described as sexual segregation. However, in a number of cases, the observed variations in behaviour have been better explained by reference to male/female body size differences than to fundamental sexual differences. One way to assess the relative importance of sex in spatial behaviour is to look for sexual segregation among species where males and females are outwardly identical. Minnows (Phoxinus phoxinus) are good species for such studies because outside the breeding season the two sexes are indistinguishable. In a recent experiment , minnows were collected from tributaries of the River Tay in Scotland in the breeding season and transported to the laboratory where they were kept in large circular tanks in flowing river water. The sex of each fish was noted and individuals were marked using internal PIT (Passive Integrated Transponder) tags. After the breeding season, 80 fish were tracked continuously in a large indoor stream channel over a period of 14 weeks. Male minnows spent significantly more time with other males than with females, but female fish showed no sexual preferences. Therefore, true sexual segregation can occur in monomorphic species. However, the reasons for this behaviour are currently a mystery, and more research is needed. One possibility is that in the reproductive season the physiological drain on females is greater than on males, which leads to sexual differences in recovery strategies and patterns of behaviour.

Reference: Griffiths, S.W. et al. 2014. Sexual segregation in monomorphic minnows. Animal Behaviour 88, 7-12.