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, 3 (9), e3216

Selective Breeding for a Behavioral Trait Changes Digit Ratio


Selective Breeding for a Behavioral Trait Changes Digit Ratio

Reginia H Y Yan et al. PLoS One.


The ratio of the length of the second digit (index finger) divided by the fourth digit (ring finger) tends to be lower in men than in women. This 2D:4D digit ratio is often used as a proxy for prenatal androgen exposure in studies of human health and behavior. For example, 2D:4D ratio is lower (i.e. more "masculinized") in both men and women of greater physical fitness and/or sporting ability. Lab mice have also shown variation in 2D:4D as a function of uterine environment, and mouse digit ratios seem also to correlate with behavioral traits, including daily activity levels. Selective breeding for increased rates of voluntary exercise (wheel running) in four lines of mice has caused correlated increases in aerobic exercise capacity, circulating corticosterone level, and predatory aggression. Here, we show that this selection regime has also increased 2D:4D. This apparent "feminization" in mice is opposite to the relationship seen between 2D:4D and physical fitness in human beings. The present results are difficult to reconcile with the notion that 2D:4D is an effective proxy for prenatal androgen exposure; instead, it may more accurately reflect effects of glucocorticoids, or other factors that regulate any of many genes.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.


Figure 1
Figure 1. Measurement of digit lengths.
Right rear palm of male mouse from a line bred for high running, illustrating digit lengths, measured from the mid-point of the basal crease to digit tip. The 2D∶4D ratio was 573.6∶585.0 pixels, yielding a 2D∶4D of 0.9805.
Figure 2
Figure 2. Digit ratios by sex and selection line.
2D∶4D finger length ratios by sex and selection regime for the eight lines of mice. Control are lines 1, 2, 4 and 5 (lab designations), High Runner selected lines are 3, 6, 7 and 8), open bars indicate females, and shaded bars males. Values are least square means and associated standard errors from models (separately for Control and Selected lines) including sex, line, sex-by-line interaction, and generation as fixed effects (SAS Procedure Mixed: family nested within line and the sex-by-family(line) interaction were random effects). See text for P values and for results of analyses comparing the four Control with the four High Runner lines.

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