Latitude, digit ratios, and Allen's and Bergmann's rules: a comment on Loehlin, McFadden, Medland, and Martin (2006).

In a recent article in this Journal, Loehlin, McFadden, Medland, and Martin (2006) investigated the relationship between latitude and digit ratio (2D:4D) first commented upon by Manning (2002). Manning suggested that inter-ethnic differences in mean digit ratio were a function of latitude, with populations at intermediate latitudes having the highest digit ratios and those at the equator and at high latitudes having the lowest. This trend was seen in the populations studied in Manning et al. (2000) (see Fig. 1, left panel). Higher digit ratios appeared to be more common at intermediate latitudes. However, the data collected by Loehlin et al. do not appear to fit this trend (Fig. 1, right panel). Latitude has long been thought to influence the evolution of body shape. Modern humans follow Allen’s rule: populations in colder climates have relatively shorter limbs and relatively shorter distal segments within limbs (Roberts, 1978; Ruff, 1994, 2002). Another sexually dimorphic trait showing robust latitudinal variation is height. In accordance with Bergmann’s rule, body size increases as mean annual temperature decreases, resulting in larger mean height at higher latitudes (Katzmarzyk & Leonard, 1995; Roberts, 1978; Ruff, 1994). This suggests that the inter-ethnic variation attributed to latitude may not be due to such factors that vary as a direct consequence of latitude, such as day length (Loehlin et al., 2006; Manning, 2002), but may instead reflect inter-ethnic variation in the morphological traits associated with latitude due to Allen’s and Bergmann’s rules, such as height.