Some evidence for different ecological pressures that constrain male and female body size

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The dwarf morph of the Lake Tanganyika cichlid Telmatochromis temporalis uses empty snail shells as shelters and breeding sites in shell beds, in which many empty shells exist. Here, we assessed selection forces regulating body size in this fish. Field observations showed that large males tended to have a greater number of females in their territories, suggesting that sexual selection favours large males. Nonetheless, a transplant experiment suggested that male body size was limited by the ability to hide in empty shells from large piscivorous fish. In females, the number of ovarian eggs increased with body size, suggesting that fecundity selection favours large females. However, females are smaller than males. Females spawned eggs close to the apices inside the shells. The small space there would decrease the risk of egg predation by egg predators, and small body size of females may be a result of adaptation to spawn eggs in the small, safe spaces. This study provides support for the idea that male and female body sizes have been limited by different ecological pressures (predation on adult fish in males, predation on eggs in females), which has not been reported previously in any animal.