Evolution of sexual size dimorphism in mammals: Sexual or natural selection?

Abstract

Sexual size dimorphism may have evolved through two processes: sexual or natural selection. The sexual selection theory states that males compete for mate monopolization and larger males can sire more offspring than smaller ones—factors that resulted in the evolution of sexual size dimorphism. An alternative hypothesis suggests that there was a change in ecological conditions (e.g., from close to open habitats that increased predation risk or a climatic change that increased thermoregulation requirements) that favoured an increase in body size that was more significant in males than in females. In the present study, phylogenetic confirmatory path analyses were conducted to test the hypotheses on the causal relationships between five characters: breeding system, sexual size dimorphism, body weight, daily activity (representing the initial change in habitat that induced female grouping), and mating system (monogamy and polygyny) or variance in genetic paternity (measures of intensity of sexual selection). The best-fit models in the path analyses assumed that dimorphism evolved through natural selection following the evolution of large body size, which in turn influenced the evolution of breeding groups and polygyny.