A functional trade-off between trophic adaptation and parental care predicts sexual dimorphism in cichlid fish.

Fabrizia Ronco,Marius Roesti,Walter Salzburger

doi:10.1098/rspb.2019.1050

Abstract

Although sexual dimorphism is widespread in nature, its evolutionary causes often remain elusive. Here we report a case where a sex-specific conflicting functional demand related to parental care, but not to sexual selection, explains sexual dimorphism in a primarily trophic structure, the gill rakers of cichlid fishes. More specifically, we examined gill raker length in a representative set of cichlid fish species from Lake Tanganyika featuring three different parental care strategies: (i) uni-parental mouthbrooding, whereby only one parental sex incubates the eggs in the buccal cavity; (ii) bi-parental mouthbrooding, whereby both parents participate in mouthbrooding; and (iii) nest guarding without any mouthbrooding involved. As predicted from these different parental care strategies, we find sexual dimorphism in gill raker length to be present only in uni-parental mouthbrooders, but not in bi-parental mouthbrooders nor in nest guarders. Moreover, variation in the extent of sexual dimorphism among uni-parental mouthbrooders appears to be related to trophic ecology. Overall, we present a previously unrecognized scenario for the evolution of sexual dimorphism that is not related to sexual selection or initial niche divergence between sexes. Instead, sexual dimorphism in gill raker length in uni-parental mouthbrooding cichlid fish appears to be the consequence of a sex-specific functional trade-off between a trophic function present in both sexes and a reproductive function present only in the brooding sex.

Highlights

Sexual dimorphism—that is, the different appearance of males and females within a species—is a prevalent phenomenon in animals [1,2]
We examined gill raker length in a representative set of cichlid fish species from Lake Tanganyika featuring three different parental care strategies: (i) uni-parental mouthbrooding, whereby only one parental sex incubates the eggs in the buccal cavity; (ii) bi-parental mouthbrooding, whereby both parents participate in mouthbrooding; and (iii) nest guarding without any mouthbrooding involved
We addressed the question of whether a conflicting functional demand linked to parental care can explain sexual dimorphism in an otherwise trophic trait

Summary

Introduction

Sexual dimorphism—that is, the different appearance of males and females within a species—is a prevalent phenomenon in animals [1,2]. The gill rakers of cichlid fishes from East African Lake Tanganyika provide a rare opportunity to test, in a comparative framework, for a sex-specific trade-off related to brood care—but not to sexual selection—in an otherwise trophic trait. (iii) In uni-parental mouthbrooders, the functional trade-off between feeding and parental care should only occur in the mouthbrooding sex (females), whereas gill raker morphology in the non-mouthbrooding sex (males) should be selected towards the trophic optimum (figure 1c). We hypothesized that trophic ecology determines the strength of the conflict (i.e. how divergent the two conflicting optima are) as a result of different trait optima in different trophic niches To test these hypotheses, we examined a representative set of cichlid species for sexual dimorphism in gill raker length and tested for an association with breeding mode and trophic ecology. All graphing and statistical analyses were conducted in R [29]

Results

Findings

Discussion