Maxwell, Stephen J., Rymer, Tasmin L., and Congdon, Bradley C. (2021) A theoretical composite model for population sex-specific shell size dynamics in Strombidae (Gastropoda, Neostromboidae). Journal of Natural History, 55 (41-42). pp. 2661-2672.

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Abstract

Modelling sexual dimorphism has important implications for understanding the evolution of size relationships among and within organisms. We present a composite model for the regulation and evolution of sex-specific inter-population shell size in a family of herbivorous marine molluscs, the Strombidae. In particular, this model postulates that gene flow acts as a size regulator that limits inter-population divergence by not allowing the mean size of each sex to deviate significantly between disparate populations. The mean shell size of individuals within populations is affected by the ecological setting in which the animal is growing. The model considers niche divergence as inconsequential in the regulation of sexual dimorphism, whereas environmental regulation for the mean size of a population is considered a significant driving factor. Sexual competition, where males may dislodge other males during copulation, is also considered a non-significant driver of male body size because of the open mating system. In such a system, smaller males can simply mate before larger males have matured. Selection for earlier mating in males so as to maximise lifetime fecundity favours male maturation at small body sizes. Conversely, greater fecundity at larger body size in females argues for larger females. The postulated composite model presented here fills a knowledge gap by graphically illustrating the factors affecting inter-and intra-specific variation. These differences include the modelling of mean inter-population shell size, as well as the regulation of intra-population sex size ratios. The components of this new model can be applied to a wider range of sexually dimorphic organisms to explain the evolutionary factors involved in regulating the observed sexual dimorphism.

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