Underground! From rabbit warrens to nuclear fallout shelters, it’s a great place to hide out. If you’re a terrestrial vertebrate, the safest place you could conceivably be is underground, where you’re protected by a temperature-stable bunker, an ocean of dirt, rocks, and roots. But it’s one thing to dig a burrow, and another to spend all your time underground, swimming through the soil. If you’re a full-time tunneler, there are really just two body types you can evolve to fit: the “mole” model, and the “earthworm” model.
Consider the difficulties of underground travel. You don’t want to be too large, or digging would be exhausting. You don’t want large eyes, which would be useless and become full of grit. You’ll need a keen sense of smell and touch, as you’re likely to be finding food by chemical and tactile signals rather than visual ones. This star-nosed mole from North America is a great example of the mole archetype: small, compact, wedge-shaped, with sealed-off eyes and ears, powerful front claws, and 22 fleshy appendages that are among the most sensitive touch receptors in the animal kingdom. Star-nosed moles are true swimmers; they breaststroke through soil, but are also quite adept at catching prey in the water. Moles are insectivores, related to that most ancient of mammals, the shrew. But thanks to the awesome power of convergent evolution, you don’t have to be related to the moles to become a mole.
The Marsupial Mole of Australia. Same design, but even more drastically evolved: the marsupial mole’s eyes have been entirely lost, and the cervical vertebrae have been fused to turn its head into a sturdy prow. Its pouch faces backwards so its young don’t get a faceful of sand.
The Golden Mole of Africa. Neither a member of Insectivora nor Marsupialia, this blind sand-swimmer is a member of the fantastic sub-order Afrotheria, a relative of elephants, manatees, and aardvarks.
The Pink Fairy Armadillo of South America. Yes, even armadillos can evolve into moles. Its armor serves not just to protect it from predators, but from being rubbed raw by the sand it swims through. Delightfully, they also have a round shield of armor on their posteriors to protect them from getting nipped in the butt when they escape.
Another African, the Naked Mole Rat. This rodent’s incisors grow outside its lips, so it can chisel the dirt without swallowing it. The Naked Mole Rat has taken underground existence to the extreme: it is one of the only known mammals to have a eusocial organization of life, like ants and termites. A queen mole rat resides over her offspring, sterile female worker drones, and a small entourage of breeding males who live in her harem. Their system of interconnected chambers resemble those of ant colonies, and they have taken to resemble the insects so much that they have entirely lost the ability to regulate their own body temperatures. It is the only ectothermic, or “cold-blooded,” mammal.
And what of the true ectotherms? Once they go subterranean, they tend to take up the “earthworm” style of burrowing. Instead of digging, they creep through the loose soil and sand. They are slimy enough to slip through dirt, but have circular folds and scales — annuli, you may remember — which give them enough friction to pump their round bodies through the strata.
Earthworm, right? Nope: amphibian. It’s a caecilian, a little-known and distant relative of the salamander. Once thought to be blind (“caecus”), we now know that their primitive eyes can at least detect light. All except one species possess lungs and a skeleton and every other feature you might expect an amphibian to have, except limbs. This worm-sized caecilian might eat termites, while a snake-sized caecilian might eat worms as well.
Speaking of snakes, surely they must have been quick to adapt to an earthworm’s lifestyle, right? Right. There are 203 species of “blind snake,” tiny reptiles with small or no eyes and, in most, a hard shovel-like horn or scale at either end. One blind snake which could be under my feet right now is the Western Threadsnake, a native of New Mexico. A curious fact about this snake: like a scorpion, it glows fluorescent under a blacklight. The reason is a mystery.
Snakes aren’t the only reptiles trying to get into the earthworm game. Witness the Ajolote, or Mexican Mole Lizard, a native of Baja California which manages to combine the “mole” and “earthworm” models: a thin, round, pink, ringed body with a wedge-shaped head and two powerful front claws for digging. There are several legless lizards, but alone among the reptiles known as amphisbaenans — or worm lizards — the ajolote managed to find enough use for the front two limbs to preserve them while evolution was rolling it out like Play-Doh in its hands. The ajolote has the best of both underworlds.
I often rhapsodize about the forest canopy and the deep ocean, two zoological continents we have barely begun to explore. There is a third frontier, that tideless ocean beneath us. We know relatively little about the animals that dwell beneath the surface of the Earth, swimming blindly through the foundation on which we tread. But we can be certain of one thing: despite the dazzling diversity of species that have chosen to dwell in the basement of the world, once they’re down there, they begin to display an equally dazzling similarity.