Allometry of sexual size dimorphism in domestic dog

Abstract

Background: The tendency for male-larger sexual size dimorphism (SSD) to scale with body size - a pattern termed Rensch's rule - has been empirically supported in many animal lineages. Nevertheless, its theoretical elucidation is a subject of debate. Here, we exploited the extreme morphological variability of domestic dog (Canis familiaris) to gain insights into evolutionary causes of this rule.

Methodology/principal findings: We studied SSD and its allometry among 74 breeds ranging in height from less than 19 cm in Chihuahua to about 84 cm in Irish wolfhound. In total, the dataset included 6,221 individuals. We demonstrate that most dog breeds are male-larger, and SSD in large breeds is comparable to SSD of their wolf ancestor. Among breeds, SSD becomes smaller with decreasing body size. The smallest breeds are nearly monomorphic.

Conclusions/significance: SSD among dog breeds follows the pattern consistent with Rensch's rule. The variability of body size and corresponding changes in SSD among breeds of a domestic animal shaped by artificial selection can help to better understand processes leading to emergence of Rensch's rule.

Figure 1. SSD increases with body size among dog breeds.

Lovich-Gibbons ratios expressing SSD in shoulder height correlate with female shoulder height among 74 breeds of the domestic dog (r = 0.47; p<.0001).

Figure 2. Relationship between mean male and female shoulder heights among 74 domestic dog breeds.

Each point represents means computed from at least 10 males and 10 females. The data are naturally log-transformed. The slope of the fitted line is significantly less than 1, thus conforming to Rensch's rule. The broken line illustrates a 1:1 relationship.