Sexual activity is predicted by digit ratio in rats

The ratio of the second and fourth digits (2D:4D) is a morphological marker reflecting fetal exposure to sex steroid hormones. This ratio exhibits sexual dimorphism, with males typically showing a significantly lower ratio than females, which results from higher androgen exposure during the fetal period. While studies in humans have suggested a relationship between sexual orientation and the 2D:4D ratio, this relationship in rodents remains elusive. Here, we investigated this relationship using rats as an experimental model. We found that male rats exhibited significantly shorter 2D length than females, resulting in a lower 2D:4D ratio in males, similar to humans. Observations of sexual behavior revealed that males that ejaculated during the first mating test exhibited shorter 2D length compared to males those that did not ejaculate. When males were classified into two groups based on 2D length (long-2D and short-2D groups), short-2D males were more sexually active than long-2D males. Additionally, only short-2D males showed a preference for female odors. These findings suggest that, in rats, 2D length is a useful morphological marker reflecting sexual activity and preference. Furthermore, they provide evidence supporting the potential use of the 2D:4D ratio as a tool for studying the relationship between sexual orientation and the 2D:4D ratio in humans.