Golden Moles & Marsupial Moles

Convergent Evolution

We start our final ‘Mole Month’ article in an unusual place: looking at the world’s largest nocturnal primate, the aye-aye. This peculiar lemur almost seems to have been assembled using the body parts of various other animals: the big, leathery ears of a bat, the gnawing teeth of a beaver, the long, bushy tail of a squirrel.

But its most curious feature must surely be the grotesquely long, thin middle finger that it has on each hand. The aye-aye uses these elongated fingers to tap on the bark of a tree, after which it listens closely. Its excellent hearing enables it to detect tiny changes in resonance that indicate the presence of hollow cavities beneath the bark, which are often home to wood-boring insect grubs. Using its sharp, chisel-like front teeth, the aye-aye chews a hole in the wood and then, with extreme delicacy and precision, inserts one of those long, double-jointed middle fingers inside to try and pry the larva out.

In most parts of the world, woodpeckers are so adapted to finding and eating wood-boring larvae that few other animals can compete with them. But Madagascar, where the aye-aye lives, is devoid of woodpeckers – they never reached this huge, ancient island. This means there was an ecological niche to fill, with a largely untapped source of food. And so the aye-aye, by evolving long, flexible fingers, became a specialist at extracting insects from inside wood.

The island of New Guinea is also woodpecker-free so, in their absence, a marsupial called the striped possum has filled the same niche there. It has the same diet as the aye-aye and much the same equipment needed to obtain its food. It possesses very similar chisel-like teeth and a sharply hooked, elongated finger on each hand – though this time, the fourth one along rather than the third.

When organisms that are not closely related to one another independently evolve similar traits as a result of having to adapt to similar environments or ecological niches, it is called ‘convergent evolution’. In other words, the same solution to the same problem is reached by completely different animals, via their own evolutionary routes. The aye-aye and the striped possum showcase just one example of this fascinating process, but there are many other instances throughout the world.

And with that thought, we finally return to moles. Well… not quite.

You see, not all creatures that we call moles are technically moles. Some are very different, taxonomically speaking, and belong to completely separate groups. The golden moles of Africa, though they may look superficially similar to true moles, are more closely related to a group of mammals endemic to Madagascar called the tenrecs. Like the tenrecs (but unlike true moles), golden moles possess a cloaca – a single orifice through which they pass both urine and faeces, a physiological characteristic seen in reptiles, amphibians and birds, but rarely in mammals. This is because golden moles have developed such efficient kidneys that most of them don’t need to drink at all, thus negating the need for a dedicated urinary tract.

Of course, it’s fairly easy to see why these creatures were called moles when they were first discovered. They have powerful claws for digging, they have the same burrowing habits, they spend most of their lives underground, and their eyes are non-functional and covered in skin.

One species, Grant’s golden mole (seen in the image at the top of the article), lives in the sandy deserts of southern Africa. It has a small, leathery nose, silky fur, strong shoulders and front legs, and a wedge-shaped snout. Its curved front claws are used as tiny shovels, while its hind feet are webbed for scooping sand backwards. However, it cannot construct permanent burrows because sand is too small and loose and simply falls back into place after the animal has passed. This species therefore simply ‘swims’ through the sand as it searches for its invertebrate prey. At night, when the desert is much cooler, Grant’s golden mole may emerge to forage on the surface rather than wasting energy shifting sand.

In Australia, meanwhile, there lives another small, mole-like creature that lives in the desert and ‘swims’ through the sand. It is the marsupial mole, Australia’s only specialised burrowing mammal.

The marsupial mole has fine, sandy-coloured silky fur, spade-like claws, and a hard nose shield. Its eyes are vestigial and it lacks external ears altogether. Meanwhile, its neck vertebrae are fused to give its head extra rigidity when pushing through sand in search of food. Being a marsupial, the female has a pouch, although it faces backwards so that it doesn’t fill up with sand as she digs.

Golden moles and marsupial moles are so similar in both lifestyle and physical appearance that for a long time, the marsupial/placental divide notwithstanding, they were thought to be closely related to one another. We now know, however, that their strikingly similar body plans are due to an amazing example of convergent evolution. After all, what works well in a particular environment or ecological niche in one part of the world can work equally as well in another part. In this case, this is clearly the best mammalian body if you live buried in the desert sand.

We hope that you’ve enjoyed our little wander through the world of moles (and not-quite-moles) this month. If you missed any of our other mole-related articles, here are links to our posts on the European mole, the star-nosed mole, and the desmans.