Sexual dimorphism in digit‐length ratios of laboratory mice

Abstract

The lengths of the index finger (2D) and ring finger (4D) are sexually dimorphic in humans, and men have a smaller 2D:4D ratio compared to women. Prenatal androgens appear to be important in the development of the 2D:4D sex difference, since it has been reported in children as young as 2 years old, and since humans exposed to supernormal prenatal androgen levels display a smaller 2D:4D ratio. We tested whether another mammalian species in which the process of peripheral sexual differentiation is androgen-dependent might also show a sex difference in digit ratios. The 2D:4D ratio of adult outbred mice was calculated for both the left and right rear paws. A sex difference was observed in the right rear paw: female mice had a larger 2D:4D ratio than did males. We also found this difference in prepubescent weanling mice. This sex difference is in the same direction as that observed in humans, and suggests that sexual dimorphism in digit length ratios is a feature common to many, if not all, mammals. The mouse may therefore be a useful animal model for studying the factors that influence finger length patterns, which have recently been correlated with several specific behaviors and disease predispositions in humans. Anat Key words: digit ratios; sex difference; mice; adult; weanling A nineteenth-century anthropological report (Ecker, 1875) noted that when asked whether the index or the ring finger was longer, people tended to look at their own hand, and the answer depended on the sex of the person. Men generally reported that the ring finger was longer, whereas women more often stated that the index finger was longer. For over 50 years there were conflicting reports about whether there is truly a sex difference in the pattern of finger lengths, until George (1930) standardized a method of measuring fingers, gathered a large sample of subjects, and showed that the sex difference was statistically significant. This sex difference has recently been reformulated as the ratio of the length of the index finger (2D) to the length of the ring finger (4D). This ratio is indeed sexually dimorphic in humans: men have a smaller 2D:4D ratio on average than do women. The sex difference in the 2D:4D ratio is evident, and stable, from 2 years of age though adulthood (Manning et al., 1998).Since the 2D:4D ratio sex difference appears to be unaffected by changing sex hormone levels at puberty, and since most other mammalian sex differences are androgen-dependent (Breedlove et al., 1998), prenatal androgen levels may play a role in digit length development. Supporting this supposition is the finding that individuals with congenital adrenal hyperplasia (CAH) display masculinized finger length patterns (Brown et al., 2001). CAH is a disorder that causes the developing fetus to produce excessive adrenal androgens. Hormone levels are usually normalized at birth, so physical and behavioral differences between CAH and non-CAH individuals can primarily be attributed to the effects of supernormal prenatal androgens. This result further...

Sexual dimorphism in metapodial and phalanges length ratios in the wood mouse

“…Taken together, these two pieces of evidence suggest that the two feet may not be differentially susceptible to androgenization/estrogenization during development. This result, however, differs from previously reported larger sexual dimorphism in 2D:4D of the right compared to the left side in humans and laboratory mice (Manning et al, 1998;Williams et al, 2000;Brown et al, 2002b). …”

“…The pattern of sexual dimorphism we observed among length ratios computed on digits (excluding metatarsals) is similar to that observed in several studies of undissected hand digits in humans (Manning et al, 1998;McFadden and Shubel, 2002) and in the studies of undissected digits of hind paws of laboratory mice or fore paws of rats published to date (Brown et al, 2002b;Manning et al, 2003;McMechan et al, 2004), suggesting that previous observations of variation in relative length of undissected digits actually reflected length ratios between bones. However, the single existing study of undissected digit ratios in nonhuman primates (Roney et al, 2004) has shown a pattern of sexual dimorphism in digit ratios different from that observed in humans and rodents, suggesting that sexual differences in digit ratios can vary even between closely related species.…”

“…Two studies of hind paws of laboratory mice and one study of fore paws of laboratory rats have consistently shown that length ratios of undissected digits show sexual differences similar to those reported for humans (Brown et al, 2002b;Manning et al, 2003;McMechan et al, 2004). In Guinea baboons (Papio papio), females have lower hand 2D:4D, 3D:4D, and 4D:5D than males (Roney et al, 2004), contrary to the pattern documented for humans.…”

Environmental levels of oestrogenic and antiandrogenic compounds feminize digit ratios in male rats and their unexposed male progeny

“…Our findings and the results of other studies on digit lengths and ratios in laboratory rodents [9,11,26,27,45] show this endpoint to be a potentially useful indicator that may be associated with gestational testosterone levels, the testosterone to oestrogen ratio, or prenatal hormonal disruption of the uterine microenvironment. It may indicate hormonal disruptions in response to various types of exposure, e.g.…”

Females Have Larger Ratio of Second‐to‐Fourth Digits Than Males in Four Species of Salamandridae, Caudata

“…It has been reported that, in humans, directional asymmetry is grater in males (Williams et al, 2000;Manning, 2002). Evidence for directional asymmetry in the digit ratio has also been shown in mice, where a lower digit ratio in right compared with left limbs was observed (Brown et al, 2002a). In other studies, differences between digit ratios in right and left limbs have been observed, but such evidence is limited (Chang et al, 2008;Rubolini et al, 2006).…”