Feeling tired? Don't know if you'll make it to the gym today? Well, maybe you'd better measure your fingers, because a new study is contributing to the scientific understanding of the relationship between finger length and voluntary exercise. Researchers from the Department of Psychology at the University of Alberta in Canada and the Department of Biology at the University of California Riverside studied finger lengths in both mice and humans, and reached some surprising conclusions published this month in the journal PLoSOne. The results will complicate the research on inherited traits and finger length, an area of study that is also of great importance to the debate on the origins of homosexuality.
For years scientists have observed various relationships between the relative lengths of people's index (or pointer) and ring fingers and other qualities. Start with a basic gender difference: As the study authors write, "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." Men's ring fingers tend to be longer than their index fingers; in women, the two fingers are nearly the same, or the ring finger may be slightly shorter. This ratio is also associated with athleticism—the lower, more "masculine" ratio is found in both men and women who are more physically fit or athletic. The current study involved testing not only that conclusion, but the means by which this difference occurs. Why, in other words, do our fingers reflect a quality such as athleticism? For some time, scientists have theorized that prenatal exposure to hormones may be the cause of these complex genetic changes, rather than simply inherited genes from one's parents. In particular, testosterone has long been the suspected culprit.
The current study suggests another possible answer. The researchers at Alberta and Riverside bred a special kind of super-mouse, one that was particularly eager to spend its time running on a wheel. Why mice? Because mice, like humans, have variations in their digit ratios that are associated with secondary characteristics, such as athleticism. And, as with humans, there is reason to believe that hormones in the uterine environment impact these secondary characteristics—and the length of fingers along with them. So, in breeding an eagerly athletic mouse, researchers hoped to demonstrate that intra-uterine hormones shape the relationship between digit length and athleticism. And, as their mice became more athletic, they also developed greater cardiovascular efficiency, more circulating testosterone in their bloodstreams, and increased predatory aggression. In short, they became more masculine.
However, for better or worse, that masculinization did not happen with their little mouse fingers. As opposed to the course of nature in humans, the mouse digits' ratios increased—became more feminine—as the mice became more athletic. And that, in turn, suggests that the cause of digit length ratios may not be testosterone after all, since these mice were exposed to more intra-uterine testosterone without becoming "masculinized" in their "fingers."
The current study poses more questions than it answers. In that regard, it is just another piece of an increasingly complex puzzle of research trying to tease out the relationship of prenatal hormone exposure and genetic changes. A 2005 study found that very short—that is, extremely masculine—ring fingers were associated with high degrees of aggression in men. And a study published in Nature in 2000 found that lesbians were exposed to more prenatal testosterone than were straight women, and had a correspondingly masculine finger ratio. Men, that study found, who had at least one older brother were more likely to be gay, thanks to the mother's womb's ability to "remember" prior sons and prep their environment with testosterone. But measurements of gay men's relative finger lengths were inconclusive, not being statistically different from those of straight men. The impact of prenatal hormones is therefore still unclear.
The Alberta and Riverside researchers suggest that, rather than testosterone, prenatal stress hormones may be responsible for changing relative finger lengths and the other characteristics that seem to track with them. If your mother was stressed during pregnancy, in other words, you may be athletic—or gay. Whether this is an accurate picture, and whether it will help to clarify the origins of same-sex attraction, remains unclear.