The second-to-fourth finger length ratio (2D:4D) is an established morphological marker that reflects fetal exposure to sex steroid hormones. In humans, this ratio shows a clear sex dimorphism, being lower in men than in women. A study in Experimental Animals shows that the 2D:4D ratio also exhibits sexual dimorphism in rats and suggests that this ratio can predict sexual activity levels in males. Morphological analysis revealed that males exhibited a lower 2D:4D ratio than females. In addition, males with shorter 2D showed a higher sexual activity with faster ejaculation and high ejaculation rates when compared with males with longer 2D. This group also showed a higher preference for bedding with female odor compared to longer 2D male counterparts. These results suggest that 2D length may serve as a non-invasive predictor of sexual activity in rats, potentially reflecting fetal androgen exposure. However, further studies are necessary to understand the relationship between finger ratio and sexual differences in rodents.
Original reference: Hayashi, H. & Sakamoto, H. Exp. Anim. https://doi.org/10.1538/expanim.24-0159 (2025)
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