Abstract
Rensch's rule states that sexual size dimorphism (SSD) increases with body size in taxa where males are larger, and decreases when females are larger. The dominant explanation for the trend is currently that competitive advantage for males is greater in larger individuals, whereas female size is constrained by the energetics of rearing offspring. This rule holds for a variety of vertebrate taxa, and opposing trends are rare. We examine the allometry of SSD within the Musteloidea and demonstrate a hypo‐allometry contrary to Rensch's rule, with lower SSD associated with larger body size. We provide evidence that feeding ecology is involved. Where diet promotes group‐living, the optimal strategy for the males of larger species is often not to attempt to defend access to multiple females, obviating any competitive advantage of relatively greater size. We conclude that the effect of feeding ecology on mating systems may be a hitherto neglected factor explaining variation in SSD.
Highlights
Dimorphism in secondary sexual characteristics—those not directly involved with the reproductive process—of sexually reproducing species has long attracted the attention of biologists. Darwin (1871) was among the first to survey the diversity of sexual dimorphism across the animal kingdom and to speculate on its causes
In those taxa for which a positive Rensch effect has been reported, it is invariably associated with polygynous mating systems and assumed to be the product of sexual selection, operating through body- size advantages
There is no consensus on the mechanisms involved in determining sexual size dimorphism (SSD) in those taxa that counter the Rensch allometry
Summary
Dimorphism in secondary sexual characteristics—those not directly involved with the reproductive process—of sexually reproducing species has long attracted the attention of biologists. Darwin (1871) was among the first to survey the diversity of sexual dimorphism across the animal kingdom and to speculate on its causes. The importance of sexual selection in maintaining male-biased SSD has been implicated in explaining an allometric pattern known as “Rensch’s rule.” This rule states that, within a lineage, SSD is positively correlated with mean body size (hyperallometry) in taxa where males are larger, and negatively correlated (hypoallometry) where females are larger (Rensch 1950 cited by Abouheif & Fairbairn, 1997). The analysis of Lukas and Clutton-Brock (2013) provided some support for this; transition from the ancestral state to social monogamy was associated with lower population density of individuals (adjusting for body size) They speculated this occurred where species came to rely on resources with high nutritional value, but low abundance. This spatial arrangement results in polygynous mating systems, and males must compete for access to females, conferring a selective advantage to larger males
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