Rensch’s Rule is an allometric principle in biology describing a pattern that connects a species’ body size to the size difference between its sexes. Formulated by German evolutionary biologist Bernhard Rensch in 1950, the rule provides a framework for examining how male and female body sizes have evolved across related species, and the interplay between natural selection and sexual selection.
Understanding Sexual Size Dimorphism
Sexual size dimorphism (SSD) refers to the consistent size difference between males and females of the same species, excluding the primary reproductive organs. In many mammal species, such as lions or elephant seals, males are considerably larger than females, while the reverse is true for animals like many insects and spiders. These differences are not limited to body mass but can also include specific features, like the peacock’s elaborate tail feathers compared to the peahen’s. The degree of SSD within a species is linked to its reproductive strategies and the environmental pressures each sex faces.
Defining Rensch’s Rule
Rensch’s Rule describes a predictable relationship between a species’ average body size and the degree of its sexual size dimorphism. First, in species where males are the larger sex, the size difference between males and females becomes more pronounced as the average body size of the species increases. For example, among a group of related primate species, the largest species will show the greatest size gap between males and females.
The second part of the rule addresses species where females are the larger sex. In these groups, the size difference between the sexes tends to decrease as the average body size of the species increases.
Examples of Rensch’s Rule in the Animal Kingdom
Among primates and pinnipeds, such as seals and sea lions, males are larger than females, and larger species consistently exhibit a greater degree of sexual size dimorphism. For example, the massive southern elephant seal has males that can be more than three times heavier than females, a disparity much greater than that seen in smaller seal species, illustrating the rule.
Conversely, the rule is also demonstrated in lineages where females are the larger sex. In many spider species, females are larger to maximize egg production. As the average body size within a related group of spiders increases, the size difference between females and males tends to shrink. A similar pattern has been observed in some groups of insects and fish, where the disparity lessens in larger-bodied species.
Evolutionary Significance of Rensch’s Rule
The evolutionary drivers behind Rensch’s Rule are a combination of sexual selection and natural selection pressures that act differently on males and females. In species with larger males, intense male-male competition for mates is a primary factor. Larger body size provides an advantage in physical contests, so as a species evolves a larger average size, the selection for even larger males becomes disproportionately stronger, accelerating the size difference.
In species where females are larger, the primary selective pressure is fecundity selection—the advantage of a larger body size for producing more or healthier offspring. While sexual selection might still favor larger males to some degree, it is often outweighed by the benefits of large female size.