Did you know that, on average, you can predict that a girl is going to have a longer pointer finger than ring finger, and a dude is gong to have a longer ring finger than a pointer finger?
Cocktail party tricks aside, the second-to-fourth digit ratio (2D:4D) is a well-studied sexual dimorphism that results from the influence of androgenic hormones on the expression of certain genes during development. Because the trait is the result of exposure to hormones at critical points in development, the presence of extreme digit ratios has been associated with everything from risk taking behavior in stock trading to performance in sports to the likelihood of developing certain diseases such as autism and heart disease. The genes responsible for determining the length of the digits, the Hox genes, are well conserved across animals, so the sexual dimorphism can be seen across many taxa.
As a fun side project, I have collaborated with researchers at a number of institutions to describe the pattern for the first time in a number of amphibians and reptiles. We have found interesting patterns across taxa regarding whether it is the male or the female of a given species that has a higher 2D:4D, but whether the mechanism behind this pattern is related to a species’ chromosomal sex-determination system or phylogenetic history is unresolved. See my contributions to this field here, here, and here.