Homosexuality in the Ten-Spined Stickleback (Pygosteus Pungitius L.) 1)

The normal reproductive behaviour of the Ten-Spined Stickleback (Pygosteus pungitius L.) is described briefly, and the conditions that are found to produce abnormal courtship behaviour are reported. The males of this species build nests in water-weeds, and the territory around each nest is defended by its owner. At a particular population density only some of a group of males are able to set up territories and build nests, and these individuals become dominant over the nestless males. If there is a scarcity of females, the dominant males behave towards the nestless males as if the latter were females. If there is a normal courtship in progress between a dominant male and a female, then the dominated males are often seen to compete with the female of the pair, and to exhibit the complete female courtship pattern. Such fish have been called pseudofemales. These observations reveal that the nervous mechanisms of the sexual behaviour pattern of the other sex are present, although they are not normally functional. Pseudofemale behaviour is analysed further with model experiments. Its function appears to be the relief of frustration in this case, and it is noted that here where a whole pattern is being thwarted, the result is a displacement pattern. The speed of the inversion of the sexual role seems to exclude any possibility of hormonal change being responsible. It is probable that the phemomenon has ecological significance, as an outcome of its occurrence is often the destruction of eggs. Homosexuality in other species is briefly reviewed. Terminology is discussed, and a classification of types of homosexual behaviour is given.