Giant males or dwarf females: what determines the extreme sexual size dimorphism in Lamprologus callipterus?

In the Lake Tanganyika cichlid Lamprologus callipterus, males were >12 time s heavier than females, the most extreme sexual size dimorphism in this dir ection among animals. L. callipterus males construct nests of empty snail s hells in which the females breed. If the ancestors of L. callipterus were s mall cichlids, both sexes may have used shells for shelter. If the ancestor s were larger, snail shells could not be used as shelters and would be impo rtant only for reproduction. In the field and the laboratory, females bred only in shells and the largest spawned in the largest shells. In the field, Females chose larger shells than the average available in males' territori es and did not copy the mate choice of other females. They never hid from p redators in snail shells and they occurred commonly in areas without any po tential shell shelters. In laboratory experiments females used shells only for reproduction and hardly for shelter. Therefore, it seems unlikely that L. callipterus descended from small shell-brooding cichlids which used shel ls for shelter, but more likely that the ancestors were of large or interme diate size, and that female size is constrained by the sizes of snail shell s, which appear to be optimal breeding substrata. (C) 2000 The Fisheries So ciety or the British Isles.