SEXUAL SELECTION CONSTRAINED BY INTERNAL FERTILIZATION IN THE LIVEBEARING FISH XENOTOCA-EISENI

Fish of the family Goodeidae have an advanced form of viviparity but m ales lack a specialized copulatory organ. Goodeids show reduced sexual -size dimorphism compared with the other livebearing families of the o rder Cyprinodontiformes, I investigated some mechanisms of sexual sele ction acting on body size in the goodeid fish Xenotoca eiseni. Both ma les and females strongly prefer mates of their own size. Comparison of mating activities in pairs with various degrees of size difference be tween males and females showed that matched pairs copulated more succe ssfully. Similar mate preference in the two sexes thus appears to be t he consequence of the primitive method of internal fertilization that requires the partners to be exactly synchronized during sperm transfer . Competition for access to a female was intense and body size determi ned the hierarchy in a small group, However, unless the size of the fe male was close to that of the dominant male, assortative mate preferen ces prevailed and the large-male advantage in competition was offset. Reduced size dimorphism in X. eiseni and in other goodeids may be expl ained by constraints on the action of sexual selection imposed by the need for effective fertilization. (C) 1997 The Association for the Stu dy of Animal Behaviour.