Sexual Dimorphism and Size Evolution in Animal Systems

Sexual dimorphism, the systematic difference in form between males and females, is a pervasive feature of animal biology. One of its most studied manifestations is sexual size dimorphism (SSD), in which one sex consistently attains a different average body size from the other. The evolution of SSD reflects a balance of selective pressures, including mate competition, fecundity advantages, ecological niche divergence and life-history trade-offs. Male-biased size dimorphism often arises through direct competition for access to mates or through female choice for larger partners, whereas female-biased size dimorphism frequently reflects selection for greater reproductive output or differential energy allocation. Across the animal kingdom, patterns of SSD vary markedly: some lineages conform to allometric scaling rules linking body size and dimorphism, while others deviate owing to ecological constraints or complex mating systems. Recent advances in large-scale datasets and comparative methods have refined our understanding of how directional selection acts on each sex, revealing that macroevolutionary trends can involve increases, decreases or stasis in body size that differ between males and females. These insights have profound implications for conservation biology, pest management and predicting species’ responses to environmental change.

Research from Nature Portfolio

Large-scale analyses of wild mammalian data have challenged long-standing assumptions about male-biased size dimorphism. Using taxonomically balanced sampling of species-rich families and sex-segregated body-mass records, recent work shows that although males tend to be larger where dimorphism exists, they are not larger in the majority of species. This finding prompts a reassessment of sexual-selection paradigms and encourages more nuanced treatment of variance and sampling biases in meta-analyses. Complementary phylogenetic comparative studies across tetrapods have dissected directional size changes in males and females, demonstrating that the mechanisms driving SSD differ among clades. In amphibians, male size increases are the predominant driver of dimorphism, whereas in mammals the decline of male size underpins female-biased patterns. Such lineage-specific pathways highlight the interplay between sexual, ecological and developmental selection in shaping body-size evolution.

Sexual Dimorphism and Size Evolution in Animal Systems publication trend

The graph below shows the total number of articles in sexual dimorphism and size evolution in animal systems across all publications each year (not limited to Nature Index journals).

Technical terms

Sexual dimorphism: Morphological or behavioural differences between males and females of a species.

Sexual size dimorphism (SSD): Systematic difference in average body size between the sexes.

Sexual selection: Evolutionary process by which traits increase mating success through competition or mate choice.

Rensch’s rule: Allometric pattern in which SSD increases with body size when males are larger, and decreases when females are larger.

Phylogenetic comparative methods: Statistical approaches that account for shared ancestry when comparing traits across species.

References

1. New estimates indicate that males are not larger than females in most mammal species. Nature Communications (2024).
2. Evolution of sexual size dimorphism in tetrapods is driven by varying patterns of sex-specific selection on size. Nature Ecology & Evolution (2024).
3. Climate and ecology predict latitudinal trends in sexual selection inferred from avian mating systems.. PLOS Biology (2024).
4. The macroevolutionary dynamics of mammalian sexual size dimorphism. Proceedings of the Royal Society B (2023).
5. Sexual selection explains Rensch's rule of allometry for sexual size dimorphism. Proceedings of the Royal Society B (2007).