Effects of sex ratio on intra- and inter-sexual behaviour in sand gobies

Reproductive behaviour and reproductive success in a marine fish, the sand goby, Pomatoschistus minutus, were affected by changes in the sex ratio. When the sex ratio was male biased (six males:three females), aggressive male-male interactions were more frequent per individual than in a female-biased situation (three males:six females). Accordingly, in the female-biased treatment females interacted more with each other than in the male-biased treatment. There was no difference between treatments in male interactions towards females, nor in female interactions towards males. Controls, with even sex ratios at two different densities (three or six of each sex), did not differ from each other in intra- or inter-sexual interactions. This shows that the differences in intra-sexual behaviour, found in males as well as females, were caused by the sex ratio and not by density. In the male-biased treatment and in the unbiased controls, nest-building males were larger than non-building males, whereas in the female-biased treatment there was no difference in size between builders and non-builders. Thus, our experiments show that a change in sex ratio changes competition for mates, and that individuals adjust their reproductive behaviour to the current sex ratio. (C) 1995 The Association for the Study of Animal Behaviour