Accordingly, typical masculine traits, such as aggression ( Bailey & Hurd, 2005 Gorka, Norman, Radtke, Carré, & Hariri, 2015), reproductive success, sociosexuality, dominance ( Manning & Fink, 2008), and competitiveness ( Manning & Taylor, 2001), have been shown to be inversely correlated with 2D:4D ratios. In utero testosterone exposure not only decreases the 2D:4D ratio but also “defeminizes” the brain ( Williams et al., 2000). Twin studies, however, have shown that genetic factors are also contributed to this ratio ( Gobrogge, Breedlove, & Klump, 2008 Paul, Kato, Cherkas, Andrew, & Spector, 2006). As a consequence of fetal testosterone exposure, the 2D:4D ratio has a sexually dimorphic feature, with men tending to have smaller values than women ( Manning et al., 2000). Also consistently, lower 2D:4D ratios have been reported in women with congenital adrenal hyperplasia, a condition involving exposure to elevated levels of testosterone in utero ( Okten, Kalyoncu, & Yariş, 2002). Differences in 2D:4D ratios between men and women are consistent with a relationship between prenatal testosterone concentrations and 2D:4D ratios ( Fink, Neave, & Manning, 2003 Malas, Dogan, Evcil, & Desdicioglu, 2006 McFadden & Shubel, 2002). Prenatal androgen exposure and responsiveness have been cited as predictors of 2D:4D ratios ( Lutchmaya, Baron-Cohen, Raggatt, Knickmeyer, & Manning, 2004 Manning, Bundred, Newton, & Flanagan, 2003). In conclusion, 2D/4D appears to be a rather species-and method-specific measure and researchers should be careful when generalising its applicability to study early hormonal effects in vertebrates.The ratio of second-to-fourth digit length (2D:4D ratio) is determined during the fetal period and becomes stable across the lifespan starting at 2 years of age ( Manning, Trivers, Thornhill, & Singh, 2000). Our results also indicate that in this species, 2D/4D is laterally asymmetric, but it may not be either fixed in utero or sexually dimorphic.
Our findings thus suggest that X-rays, due to their greater accuracy, should be preferred over photographs for measuring digit ratios in rodents. We found no evidence for a sex difference in 2D/4D using either of the measurement methods. Adult voles had a lower 2D/4D than juveniles when measured from X-rays, but not when measured from photographs.
According to both measurement methods, the right paw showed higher 2D/4D values than the left paw (lateral asymmetry). Our results show that 2D/4D measurements from photos had a higher measurement error and gave consistently higher 2D/4D than those from X-rays.
We examined the age and sex dependency and lateral asymmetry of 2D/4D in field voles (Microtus agrestis) and compared whether these effects differed between 2D/4D measurements taken from photographs and X-rays. The non-human studies conducted so far have yielded inconsistent results, perhaps due to the variety of different methods employed in measuring 2D/4D. This information would be especially valuable to field biologists to whom measuring steroids in utero is often impossible. Numerous studies mainly on humans have supported this conclusion, but it is yet unclear how well this applies to other vertebrates. The ratio of second-to-fourth digit length (2D/4D) has been suggested to be a useful adult age marker of intrauterine exposure to steroids because it should be sexually dimorphic and fixed already in utero.
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