Rensching cats and dogs: feeding ecology and fecundity trends explain variation in the allometry of sexual size dimorphism.

P J Johnson,L A Harrington,C Newman,M J Noonan,D W Macdonald,A C Kitchener

doi:10.1098/rsos.170453

Abstract

The tendency for sexual size dimorphism (SSD) to increase with body mass in taxa where males are larger, and to decrease when females are larger, is known as Rensch's rule. In mammals, where the trend occurs, it is believed to be the result of a competitive advantage for larger males, while female mass is constrained by the energetics of reproduction. Here, we examine the allometry of SSD within the Felidae and Canidae, demonstrating distinctly different patterns: in felids, there is positive allometric scaling, while there is no trend in canids. We hypothesize that feeding ecology, via its effect on female spacing patterns, is responsible for the difference; larger male mass may be advantageous only where females are dispersed such that males can defend access to them. This is supported by the observation that felids are predominately solitary, and all are obligate carnivores. Similarly, carnivorous canids are more sexually dimorphic than insectivores and omnivores, but carnivory does not contribute to a Rensch effect as dietary variation occurs across the mass spectrum. The observed inter-familial differences are also consistent with reduced constraints on female mass in the canids, where litter size increases with body mass, versus no observable allometry in the felids.

Highlights

The tendency for sexual size dimorphism (SSD) to increase with body mass in taxa where males are larger, and to decrease when females are larger, is known as Rensch’s rule
Carnivorous canids are more sexually dimorphic than insectivores and omnivores, but carnivory does not contribute to a Rensch effect as dietary variation occurs across the mass spectrum
The observed interfamilial differences are consistent with reduced constraints on female mass in the canids, where litter size increases with body mass, versus no observable allometry in the felids

Summary

Introduction

The tendency for sexual size dimorphism (SSD) to increase with body mass in taxa where males are larger, and to decrease when females are larger, is known as Rensch’s rule. We hypothesize that feeding ecology, via its effect on female spacing patterns, is responsible for the difference; larger male mass may be advantageous only where females are dispersed such that males can defend access to them This is supported by the observation that felids are predominately solitary, and all are obligate carnivores. The maintenance of male-biased SSD by sexual selection 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 mass (hyperallometry) in taxa where males are larger, and negatively correlated (hypoallometry) where females are larger (Rensch 1950 cited by Abouheif & Fairbairn [11]).

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