Sexual selection constrained by internal fertilization in the livebearing fish Xenotoca eiseni

Fish of the family Goodeidae have an advanced form of viviparity but males lack a specialized copulatory organ. Goodeids show reduced sexual-size dimorphism compared with the other livebearing families of the order Cyprinodontiformes, I investigated some mechanisms of sexual selection acting on body size in the goodeid fish Xenotoca eiseni. Both males and females strongly prefer mates of their own size. Comparison of mating activities in pairs with various degrees of size difference between males and females showed that matched pairs copulated more successfully. Similar mate preference in the two sexes thus appears to be the 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 determined the hierarchy in a small group, However, unless the size of the female was close to that of the dominant male, assortative mate preferences prevailed and the large-male advantage in competition was offset. Reduced size dimorphism in X. eiseni and in other goodeids may be explained by constraints on the action of sexual selection imposed by the need for effective fertilization. (C) 1997 The Association for the Study of Animal Behaviour.