Sexual Dimorphism in Lamprologus Callipterus

Lamprologus callipterus is one of over 300 species of cichlid (a freshwater fish) found only in Lake Tanganyika in Africa. Most African cichlids, including L.callipterus , exhibit sexual size dimorphism-the males are larger than the females. What is unusual about L. callipterus is the magnitude of the size difference. The key to understanding the sexual dimorphism of L.callipterus lies in the reproductive behavior of the species, particularly spawning (release of eggs by females and release of sperm by males) and brooding (protection of eggs). Female L.callipterus spawn and brood their eggs in empty snail shells on the lake bottom. In all other shell-brooding cichlids, both sexes enter the shell to spawn and thus both must be small enough to fit into the shells available on the lake bottom. In L. callipterus , however, only the females enter the shell to spawn and brood their eggs. The males remain outside, releasing their sperm only at the entrance to the shell. Consequently, female L. callipterus are small enough to fit into the shells, but the males, unconstrained by the need to enter shells, are much larger.

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Spawning and brooding inside shells puts females in an evolutionary dilemma with respect to their body size. In cichlids, as in most fishes, the number of eggs spawned per batch (the fecundity) increases with female body size, and this favors females that grow large. In L. callipterus the correlation between female length and number of eggs spawned is as high as 0.95, which means that more than 90 percent of the variation in fecundity among females can be explained by differences in body length. Further, because fecundity is more a function of body volume than length, it increases disproportionally as body length increases. In one study, for example, an increase in female length from 3 centimeters to 4 centimeters was associated with an increase in fecundity from 59 to 134 eggs. In addition to this huge fecundity boost, larger females are able to exclude smaller females from access to the larger, preferred shells and so gain in the competition for spawning sites. All this suggests that female L. callipterus should be large, but this is where the dilemma comes in. No matter how beneficial large size might be, the scarcity of large shells ultimately limits the size that females attain in a given population. In laboratory experiments females actually adjust their growth rates so that their body size at maturity matches that of the available shells, and in the wild the correlation between shell size and female size explains more than 98 percent of the variation in female size among populations. The ultimate size of female L. callipterus is thus a compromise between growing as large as possible to maximize fecundity and remaining small enough to be able to find suitable shells for spawning and brooding their young.

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Finding shells large enough for brooding is so critical for female L. callipterus that, when they are ready to spawn, they generally pay more attention to choosing shells than to choosing mates. Males have adopted a unique strategy to exploit this female choosiness. Rather than trying to attract females from afar with flamboyant displays, male L. callipterus entice females to mate with them by assembling piles of empty snail shells. They set up small territories, less than a meter in diameter, and each territory is centered on a patch of shells. In most areas of the lake these patches of shells are separated by stretches of sandy or rocky bottom devoid of shells. The males create this patchy distribution by picking up and carrying shells from as far away as 20 meters. In addition to clearing the surrounding area of abandoned shells, territorial males frequently steal shells from the shell piles of other males, sometimes with females and offspring inside. When this happens the female and her offspring usually disappear, and the empty shell becomes available for a new female in the territory of the male relocating the shell. The effect of this remarkable shell transporting is to create artificial clumps of shells surrounded by unsuitable stretches of lake bottom. When females are ready to spawn, they have to visit the shell piles of territorial males to find suitable shells, thus creating spawning opportunities for males.

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?Lamprologus callipterus is one of over 300 species of cichlid (a freshwater fish) found only in Lake Tanganyika in Africa. Most African cichlids, including L.callipterus , exhibit sexual size dimorphism-the males are larger than the females. What is unusual about L. callipterus is the magnitude of the size difference. The key to understanding the sexual dimorphism of L.callipterus lies in the reproductive behavior of the species, particularly spawning (release of eggs by females and release of sperm by males) and brooding (protection of eggs). Female L.callipterus spawn and brood their eggs in empty snail shells on the lake bottom. In all other shell-brooding cichlids, both sexes enter the shell to spawn and thus both must be small enough to fit into the shells available on the lake bottom. In L. callipterus , however, only the females enter the shell to spawn and brood their eggs. The males remain outside, releasing their sperm only at the entrance to the shell. Consequently, female L. callipterus are small enough to fit into the shells, but the males, unconstrained by the need to enter shells, are much larger.

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