Unusual allometry for sexual size dimorphism in a cichlid where males are extremely larger than females

Abstract

When males are the larger sex, a positive allometric relationship between male and female sizes is often found across populations of a single species (i.e. Rensch's rule). This pattern is typically explained by a sexual selection pressure on males. Here, we report that the allometric relationship was negative across populations of a shell-brooding cichlid fish Lamprologus callipterus, although males are extremely larger than females. Male L. callipterus collect and defend empty snail shells in each of which a female breeds. We found that, across six populations, male and female sizes are positively correlated with not only sexual and fecundity selection indices, but also with shell sizes. Given their different reproductive behaviours, these correlations mean that males are required to be more powerful, and thus larger, to transport larger shells, while female bodies are reduced to the shell size to enable them to enter the shells. Among the three size selections (sexual selection, fecundity selection and shell size), shell size explained the allometry, suggesting that females are more strongly subject to size selection associated with shell size availability than males. However, the allometry was violated when considering an additional population where size-selection regimes of males differed from that of other populations. Therefore, sexual size allometry will be violated by body size divergence induced by multiple selection regimes.

Mating and Parental Care in Lake Tanganyika's Cichlids

“…In a site in northern LT (Kalundu), large shells are less abundant, males grow on average somewhat smaller than in Wonzye, and maximum, but not average, harem size is smaller (average of 5.5 and maximum of 14 females). Finally, on the north-east coast (Rumonge), males do not transport shells, which already occur at high density in the habitat, and remain distinctly smaller and hold smaller harems (average of 2.4 and maximum of 7 females) (see also [181]). …”

“…The habitat at this location provides no other refuge than shells, and at the same time, males at this location are not required to transport shells due to their natural distribution. Small male size in this population was suggested to be an adaptation to sheltering in shells made possible by the relief from shell transport [181]. In the populations other than Rumonge [181], both male and female body sizes were positively correlated with shell size, and the observed negative size allometry (the average size differences between males and females decreased with increasing average body size across populations) was explained by the female growth adjustment to shell size availability.…”

“…Small male size in this population was suggested to be an adaptation to sheltering in shells made possible by the relief from shell transport [181]. In the populations other than Rumonge [181], both male and female body sizes were positively correlated with shell size, and the observed negative size allometry (the average size differences between males and females decreased with increasing average body size across populations) was explained by the female growth adjustment to shell size availability. Additionally, male-male competition for nests and fecundity selection on females were shown to affect body sizes [181].…”

“…In the populations other than Rumonge [181], both male and female body sizes were positively correlated with shell size, and the observed negative size allometry (the average size differences between males and females decreased with increasing average body size across populations) was explained by the female growth adjustment to shell size availability. Additionally, male-male competition for nests and fecundity selection on females were shown to affect body sizes [181]. All in all, the extreme sexual size dimorphism in L. callipterus appears to be a consequence of multiple and variable selection regimes, with only an auxiliary role for sexual selection.…”

The Utility of Geometric Morphometrics to Elucidate Pathways of Cichlid Fish Evolution

“…There are several studies dealing with sexual size dimorphism in cichlids [71–74] but only few authors have addressed shape dimorphism by applying GM methods. We found two studies where sexual dimorphism was touched on within another topic [54, 75] and one recent study by Herler et al [76] where sexual dimorphism was investigated through geometric morphometrics and shape differences among sexes were assessed in relation to the differentiation of populations and species in a Lake Tanganyika cichlid genus.…”