Marcelo Sánchez-Villagra from the University of Zurich and researchers have uncovered the evolutionary history of the mole’s extra “thumb.” As it turns out, this polydactyl animal evolved an elongated wrist bone to serve as a sort of extra finger, widening the paw for more effective tunneling.
The researchers examined embryos of the Iberian mole (Talpa occidentalis) and the closely related—but five-fingered—North American least shrew (Cryptotis parva). They found that the “thumb” didn’t begin to grow until the embryos were 18 days old, after the other fingers had already begun to develop.
The digit, which does not bend but can wiggle, suggests a relationship with the testosterone level of these animals. According to a recent Science Now article, “True to their oddness, many female moles grow not only ovaries but also some testicular tissue, hinting that they have too much of the hormone, Sánchez says. Testosterone is well known for building bones, and some evidence suggests that human polydactyly—people can occasionally develop genuine sixth fingers—coincides with high levels of maternal testosterone.”
Read the original press release “How the mole got its 12 fingers.”