Moles’ thumbs

Vertebrates living on land and creatures that descend from terrestrials are stuck with a five-finger and five-toe construction. A whale’s skeleton will reveal that five fingers are hidden by the fin and while dogs may seem to have four toes, the “dewclaw”, located further up the leg, is number five and corresponds to our thumbs. If, with this knowledge in mind, you should study a mole’s leg, you will see that it has five fingers neatly in place. But there’s something else: it has a sickle-shaped thing that looks like a sixth finger. So what is it and how does a mole avoid the limitations of pentadactyly?  An international team of researchers, including Leiden biologists Mike Richardson and Merijn de Bakker, has provided the answers in Biology Letters: moles’ thumbs are not really digits at all, but a growth on the wrist bone that develops later in a baby mole’s development than the fingers. The biologists also describe the genes responsible for this feat in the article.

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