Sexual behavior and sex preference in animals and humans are shaped by complex interactions between the brain and hormones during early development. One critical factor is exposure to androgens (male sex hormones) during fetal development, which influences the sexual differentiation of the brain. This early hormonal exposure is linked to a wide range of adult behaviors, like sexual behavior and cognitive traits, but finding a reliable, non-invasive biological marker for the same has remained a challenge.
Now, a team of researchers led by Professor Hirotaka Sakamoto, along with Dr. Himeka Hayashi from Okayama University, Japan, reveal that a simple anatomical feature—the ratio of the lengths of second to fourth digits (2D:4D)—can predict the sexual activity and preference in rats. Their findings, published online in the Experimental Animals on May 14, 2025, discuss how the 2D:4D digit ratio reflects the impact of fetal hormone exposure on the brain’s sexual development.
The 2D:4D ratio has long been studied in humans, where it tends to be lower in males than in females due to prenatal androgen exposure. This low ratio arises due to a shorter index finger (2D) and is linked to a range of sex-typical traits like personality, cognition, and even sexual orientation. However, there were very limited studies that investigated whether the same relation existed in animals—which led the researchers to study rats.
“We found that the 2D:4D ratio serves as a reliable biological marker for predicting sexual activity in rats,” says Prof. Sakamoto. “Rats with shorter second digits were not only more sexually active but also showed a clear preference for female scent.”
The team started by confirming if male rats had shorter second digits than females, which would mirror the lower 2D:4D ratios found in human males. They then ran a series of mating tests. They observed that males who ejaculated during their first trial had shorter second digits in comparison to those who didn’t. Additionally, these male rats with shorter second digits also displayed a higher sexual vigor indicated by frequent ejaculations, shorter latency to ejaculation, and better erectile function.
To explore sexual preference, the researchers conducted a bedding preference test, offering male rats a choice between bedding soiled by females or males. It was observed that only the males with shorter 2D showed a preference for female-scented bedding and spent more time investigating it. Notably, all the test males initially explored the male-scented bedding, but only those with shorter 2D sustained interest in the female scent—highlighting the distinction between initial curiosity and true preference. “While most studies have only linked the digit ratio to morphological sex differences, we noticed a surprising lack of research connecting digit ratio to the biological basis of sex itself—especially sexual activity and preference,” notes Dr. Hayashi, “This curiosity marked the start of our study.”
Apart from identifying sexual behavior, the digit ratios also have broader implications— they serve as a powerful predictor of how fetal androgen exposure influences brain sexual differentiation and adult behavioral traits. They also reinforce the idea that biology, shaped before birth, can subtly influence an individual’s actions, even in complex behaviors like sexuality. Additionally, insights from this study could have real-world implications in understanding sex differences in conditions like autism, depression, or attachment disorders, which show varying prevalence between males and females.
“Our results suggest a profound link between the body and mind, with significant scientific and clinical implications,” says Prof. Sakamoto, “Simply examining our fingers could one day help identify aspects of our behavioral tendencies.”
The study also offers a valuable animal model to investigate questions that are ethically and practically inaccessible in human research. The 2D:4D ratio is also linked to cognitive abilities, personality, and psychiatric conditions, and could therefore contribute to identifying early developmental markers for a better understanding of diversity in gender, sexual orientation, and mental health.
About Okayama University, Japan
As one of the leading universities in Japan, Okayama University aims to create and establish a new paradigm for the sustainable development of the world. Okayama University offers a wide range of academic fields, which become the basis of the integrated graduate schools. This not only allows us to conduct the most advanced and up-to-date research, but also provides an enriching educational experience.
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About Professor Hirotaka Sakamoto from Okayama University, Japan
Dr. Hirotaka Sakamoto is a behavioral neuroendocrinologist and Professor in the Faculty of Environmental, Life, Natural Science and Technology, Okayama University, Japan. He earned his Ph.D. in Neuroscience from Hiroshima University, Japan, in 2002. He specializes in research on hormonal regulation of behavior in vertebrates, with a specific focus on the effects of stress or sex steroid hormones on behaviors, or both, including neural networks underlying sexual function. He has more than 180 publications to his credit, earning him numerous awards, including the JSPS, Japan Neuroscience Society, and Minister of Education Award.
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