For years, scientists have been puzzled by a strange feature of mole anatomy. It appears to be an extra thumb, and it makes moles super-powered burrowers. But is the mole's extra thumb all that it seems?
When shaking hands within the animal kingdom, you generally have to deal with a maximum of five fingers. Some animals have three digits per appendage. Others have four. Humans have five, including a thumb. No animal consistently seems to manage more than that. The mole was, for a long time, considered an exception to this rule. Moles, which spend most of their day digging through the dirt, benefit from having as many scooping digits as possible. Scientists noticed that their hand came with six digits; four fingers, a thumb, and an 'extra thumb' with limited movement.
Recently, though, scientists took a closer look at that extra thumb. Specifically, they looked at the way it wiggled, but couldn't be bent by the mole. It widened the paw of the mole, but was useless in manipulating objects the way a regular thumb would. Having satisfied themselves that the thumb didn't act the way thumbs should, scientists looked at whether it developed the way they should. A certain gene switches on bone growth. In embryonic moles, the gene switched on early for five of the fingers, but didn't kick in on the extra thumb until it was dying down in the rest of the digits.
The thumb isn't a thumb. It's a wrist bone. Over time, as the mole was evolving, the wrist bone slowly extended, possibly making a 'snkt' sound, until it jutted out of the hand and up beside the fingers. These massive wrist bones are only prominent in the digging moles. The ones that stay above ground most of the time have only nubs.
And what's the culprit responsible for these extended wrist claws? Scientists say it's testosterone. Moles have an excess of it, so much so that female moles develop the beginnings of testes. Testosterone promotes bone growth and gave the mole's wrist bones the outward kick in the first place.
Mole Image Credit: C. Mitgutsch et al., Bio. Lett. (2011)