Rensch's Rule

Content of the rule and measurement

Surveying birds, mammals and other groups, Rensch (1950, 1959) reported that among related species, males tend to be relatively larger than females in larger-bodied species. Today the rule is tested by regressing log male size on log female size and asking whether the slope exceeds one. A slope greater than one means male size changes more than female size, producing increasing dimorphism in male-biased clades and decreasing dimorphism in female-biased clades. Abouheif and Fairbairn (1997) assessed this slope systematically with phylogenetically informed comparative analyses.

Explanation by sexual selection

Fairbairn (1997) summarized Rensch's rule as the outcome of correlated evolution of male and female body size in which male size shows greater evolutionary variation. The most widely accepted explanation is sexual selection through competition among males. Székely, Freckleton and Reynolds (2004) in shorebirds and Dale et al. (2007) across many birds reported that Rensch's rule is stronger in lineages with intense male–male competition. Clutton-Brock, Harvey and Rudder (1977) likewise linked polygyny, body size and dimorphism in primates.

Exceptions and links to other ecogeographical rules

Webb and Freckleton (2007) argued that the rule is only half right because many clades with female-biased dimorphism fail to follow it. In groups such as insects, spiders and many reptiles, where fecundity selection makes females larger, slopes near or below one are common. Cox, Butler and John-Alder (2007) and Lindenfors, Gittleman and Jones (2007) showed that the strength of the rule varies among lineages of reptiles and mammals. Blanckenhorn et al. (2006) combined it with Bergmann's rule to ask whether dimorphism changes systematically with latitude but found no consistent trend.

FAQ

What is Rensch's rule in simple terms?

In animal groups where males are bigger, males of larger species are disproportionately bigger than females; in groups where females are bigger, the size difference shrinks in larger species.

Does it apply to all animals?

No. It fits well in male-larger groups such as birds and mammals but often fails in female-larger groups such as many insects and spiders.

Sources reviewed: 2026-10-26

References

1. Rensch, B. (1950). Die Abhängigkeit der relativen Sexualdifferenz von der Körpergrösse. Bonner Zoologische Beiträge, 1, 58–69.
2. Rensch, B. (1959). Evolution Above the Species Level. London: Methuen.
3. Abouheif, E., & Fairbairn, D. J. (1997). A comparative analysis of allometry for sexual size dimorphism: Assessing Rensch's rule. American Naturalist, 149(3), 540–562.
4. Fairbairn, D. J. (1997). Allometry for sexual size dimorphism: Pattern and process in the coevolution of body size in males and females. Annual Review of Ecology and Systematics, 28, 659–687.
5. Dale, J., Dunn, P. O., Figuerola, J., Lislevand, T., Székely, T., & Whittingham, L. A. (2007). Sexual selection explains Rensch's rule of allometry for sexual size dimorphism. Proceedings of the Royal Society B, 274(1628), 2971–2979.
6. Székely, T., Freckleton, R. P., & Reynolds, J. D. (2004). Sexual selection explains Rensch's rule of size dimorphism in shorebirds. PNAS, 101(33), 12224–12227.
7. Webb, T. J., & Freckleton, R. P. (2007). Only half right: Species with female-biased sexual size dimorphism consistently break Rensch's rule. PLoS ONE, 2(9), e897.
8. Blanckenhorn, W. U., Stillwell, R. C., Young, K. A., Fox, C. W., & Ashton, K. G. (2006). When Rensch meets Bergmann: Does sexual size dimorphism change systematically with latitude? Evolution, 60(10), 2004–2011.
9. Fairbairn, D. J., Blanckenhorn, W. U., & Székely, T. (Eds.). (2007). Sex, Size and Gender Roles: Evolutionary Studies of Sexual Size Dimorphism. Oxford: Oxford University Press.
10. Cox, R. M., Butler, M. A., & John-Alder, H. B. (2007). The evolution of sexual size dimorphism in reptiles. In D. J. Fairbairn et al. (Eds.), Sex, Size and Gender Roles (pp. 38–49). Oxford: Oxford University Press.
11. Lindenfors, P., Gittleman, J. L., & Jones, K. E. (2007). Sexual size dimorphism in mammals. In D. J. Fairbairn et al. (Eds.), Sex, Size and Gender Roles (pp. 16–26). Oxford: Oxford University Press.
12. Gidaszewski, N. A., Baylac, M., & Klingenberg, C. P. (2009). Evolution of sexual dimorphism of wing shape in the Drosophila melanogaster subgroup. BMC Evolutionary Biology, 9, 110.
13. Clutton-Brock, T. H., Harvey, P. H., & Rudder, B. (1977). Sexual dimorphism, socionomic sex ratio and body weight in primates. Nature, 269(5631), 797–800.