In recent
years, a quirky area of research has developed in which researchers measure the
length of the second and fourth digits on the hand and foot, calculate the
ratio (2d:4d) and then compare this ratio between the sexes. Surprisingly, in
many species there are consistent differences between males and females. In
mammals, that ratio is smaller for males, whereas in birds, the opposite
occurs. But few studies have looked at the other vertebrate classes.
With this in
mind, Direnzo and Stynoski recently calculated digit ratios for several common Costa Rica
anoles and frogs. The abstract of their paper, published
in Anatomical Record last year, tells the story:
“It is now well
documented that androgen and estrogen signaling during early development
cause a sexual dimorphism in second-to-fourth digit length ratio (2D:4D).
It is also well documented that males of mammalian species have a smaller
2D:4D than females. Although there are discrepancies among 2D:4D studies
in birds, the consensus is that birds exhibit the opposite pattern with
males having a larger 2D:4D than females. The literature
currently lacks substantial information regarding the phylogenetic pattern
of this trait in amphibians and reptiles. In this study, we examined 2D:4D
in two species of frogs (Oophaga pumilio and Craugastor bransfordii) and
two species of lizards (Anolis humilis and Anolis limifrons) to determine
the existence and the pattern of the sexual dimorphism. Male O. pumilio
and C. bransfordii displayed larger 2D:4D than females in at least one of
their two forelimbs. Male A. humilis had larger 2D:4D than females in both
hindlimbs, but smaller 2D:4D than females in both forelimbs. Male A.
limifrons may also have smaller 2D:4D than females in the right forelimb.
Finally, digit ratios were sometimes positively related to body length,
suggesting allometric growth. Overall, our results support the existence
of the 2D:4D sexual dimorphism in amphibians and lizards and add to the
knowledge of 2D:4D trait patterning among tetrapods.
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