Most pawed animals have 10 fingers but one of the main exceptions is the little mole: it has an extra ‘thumb’. But how this extra thumb, which it rests upon while digging and thus increases the size of its digging apparatus, develops in mole was unknown, until now.
An international team of researchers have now uncovered the background to the development of the mole’s extra “thumb”: A bone develops in the wrist that stretches along the real thumb, giving the paw a bigger surface area for digging.
Polydactyly – the presence of supernumerary fingers – is a phenomenon that has already been observed in various land animals in Devon and is also fairly common in humans, dogs and cats. In moles, however, polydactyly is the norm, which means the program is constantly activated during embryogenesis.
Researchers headed by Marcelo Sanchez-Villagra, a professor of paleontology at the University of Zurich, studied the molecular-genetic origin and development of the extra thumb in moles.
Unlike the other fingers on the mole’s hand, the extra thumb does not have moving joints. Instead, it consists of a single, sickle-shaped bone. Using molecular markers, the researchers can now show for the first time that it develops later than the real fingers from a transformed sesamoid bone in the wrist. In shrews, however, the mole’s closest relative, the extra thumb is lacking, which confirms the researchers’ discovery.
The researchers see a connection between the species-specific formation of the extra thumb in the mole and the peculiar “male” genital apparatus of female moles. In many mole species, the females have masculinized genitals and so-called “ovotestes”, i.e. gonads with testicular and ovary tissue instead of normal ovaries. Androgenic steroids are known to influence bone growth, transformation and changes, as well as the transformation of tendons in joints.
The study has been published in the journal Biology Letters.