In certain Lake Tanganyika cichlids breeding in empty snail shells, there are two extreme sizes of males: giants and dwarfs. Researchers from the University of Bern and the University of Graz have analyzed the genomes of these fish and found out how the peculiar sizes of males and females evolved in conjunction with the genetic sex determination mechanism.
Difference in body size (or sexual dimorphism) between males and females is common across the animal kingdom. One of the most extreme examples of sexual dimorphism is found in the cichlid fish species Lamprologous callipterus from Lake Tanganyika in East Africa, where males are 12 times bigger (heavier) than females. The ecological reason for this remarkable size difference is the fact that this species uses empty snail shells found at the bottom of the lake to build nests. Hence males must be large enough to carry shells with their mouths, whereas females need to be small enough to fit inside the snail shells to lay eggs, where they are well protected from predators. Sex-specific differences in body size are important for the biology of this species, as small males would not be able to carry empty snail shells and large females would not be able to enter the shells for breeding.