The effect of body size on mating system and parental roles in a biparental cichlid fish (Julidochromis transcriptus):: a preliminary laboratory experiment

Julidochromis species, biparental substrate-brooding cichlids in Lake Tanganyika, exhibit a variety of mating systems, including monogamy, polygyny and polyandry. A wide range of body size in both sexes is suggested to be one of the main factors inducing this variation. We studied the effect of body size on the formation of mating system in J. transcriptus in experimental aquaria with an equal sex ratio (three males, three females). In mating trials of six similar-sized fish, monogamous pairs were predominantly formed. In trials with one large male and five similar-sized fish, the large males monopolized two or three females and polygynous harems were formed. In trials with one large female and five similar-sized fish, two of five large females mated polyandrously with two males. These results suggest that body size is likely to affect the formation of mating system in this fish. We also examined parental roles of the mated fish. In monogamous pairs, both parents shared their parental tasks. In contrast, polygynous males and polyandrous females showed less parental care. These results indicate that harem owners may reduce the amount of care regardless of sex. Although the benefit of polyandrous mating remains unclear, harem males fertilized more eggs than monogamous males, indicating that polygyny may be a better option for male mating tactics.