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

Abstract

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.

Competition-driven speciation in cichlid fish

“…However, they could alternatively be an innate behaviour linked to requirements for optimal habitat, and specifically the requirements for self-protection, egg laying and brood care 38 . Small females may be unable to defend shelter on rock habitat because of competition from large growing females, but they may be favoured on shell habitat as they can escape predators and lay eggs deep enough in shells to avoid the numerous egg predators, including freshwater crabs 26 . Similarly, large growing females would be the most successful competitors on rock habitat, and would be favoured by fecundity selection on rock given their higher reproductive potential 26 .…”

“…Small females may be unable to defend shelter on rock habitat because of competition from large growing females, but they may be favoured on shell habitat as they can escape predators and lay eggs deep enough in shells to avoid the numerous egg predators, including freshwater crabs 26 . Similarly, large growing females would be the most successful competitors on rock habitat, and would be favoured by fecundity selection on rock given their higher reproductive potential 26 . However, establishment of mature rock ecomorph females on shell habitat is also likely to be prevented because of the high potential for predation of both adult fish and eggs laid close to shell entrances 26 .…”

“…Similarly, large growing females would be the most successful competitors on rock habitat, and would be favoured by fecundity selection on rock given their higher reproductive potential 26 . However, establishment of mature rock ecomorph females on shell habitat is also likely to be prevented because of the high potential for predation of both adult fish and eggs laid close to shell entrances 26 .…”

“…1a). The two habitats are typically separated by stretches of sand or muddy habitat [24][25][26] . Analyses of fieldcollected samples have shown evidence of reproduction isolation between the T. temporalis ecomorphs 23,24 , and has revealed differences between them in reproductive biology (egg number 23 and size at reproduction 23 ) and morphology (body shape 24 ).…”

Lake level fluctuations and divergence of cichlid fish ecomorphs in Lake Tanganyika

“…In this species substrate use of individuals appears strongly linked to the availability of shelter and predation threat. Adults of both ecomorphs are believed to be highly vulnerable to multiple predators that characterise hard substrate environments of Lake Tanganyika (Takahashi et al 2012), including piscivorous fish (catfishes, mormyrids, cichlids, mastacembelid eels), birds (kingfishers and cormorants), mammals (spotted-neck otters) and reptiles (water cobra) (Konings 1998). It has been found that body size matches available shelter size in T. temporalis, and that in transplant experiments rock ecomorphs are unable to make use of empty shells as shelter against predators (Takahashi et al 2012).…”