Under some concrete rubble in a wasteland in La Paz, towards the tip of the arid peninsula of Baja California, was the site of one of the most memorable and peculiar herpetological encounters of my life.
As the heavy slab was levered up with a grunt, a strange, elongated pink creature was revealed. At first glance, it looked a bit like a thick earthworm, but one that had what appeared to be a pair of hands and black glittering eyes.
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It was the animal I had come a long way to see: Bipes biporus or, as identified in A Field Guide to Western Reptiles and Amphibians, the five-toed worm lizard (also known as the Mexican mole lizard).
There’s nothing else like it in the book or, for that matter, in the world.
Watching this curious beast pushing its vermiform body lazily forwards, I could see it using peristaltic compressions, while simultaneously shovelling dirt with a pair of well-clawed front limbs.
It was moving like a worm and a mole simultaneously. It’s no accident that it shares morphological and behavioural similarities with these two well-known fossorial beasts.
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By the process of convergent evolution, all have solved the challenges of digging and moving through the soil in similar ways – it’s just that this animal uses the lot.
The five-toed worm lizard is a burrowing specialist. It lives, for the most part, in a burrow system just below the surface and underneath objects.
While it rarely slips out of hiding, it regulates its temperature with a sort of ‘cryptic basking’ called thigmothermy, moving towards the surface to warm up first thing in the morning, then heading down to deeper burrows during the daytime to avoid the brutal heat of the Baja Californian desert.
Its body is smooth, shiny, slinky and flexible to aid its passage through the soil and burrow systems. With little to hamper it, the five-toed worm lizard’s forward movement through complex ground is termed terradynamic.
Even the eye-catching front limbs – which really stick out of the overall body plan – can fold flat when not in use shovelling soil and sand, and sit in a slight depression on its flanks.
The rest of the time, the animal moves by peristalsis, with bands of muscular compression simultaneously anchoring and pushing against its burrow walls in sections, while stretching forwards with others.
Up front, a streamlined, bony, well-ossified and large-scaled head with no external ears proves to be an excellent ram to push through the soil and substrate.
Typical of many creatures that rarely see daylight, five-toed worm lizards have little need for disruptive patterns or camouflaged tones to their skin and scales, so they are, in effect, pigmentless, making them appear a pale pink or even white to our eyes.
Given that in this Stygian world there is little ambient light, their eyes are much reduced in size and function too, only standing out to us because of the contrast between the eyes and their pallid skin.
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So, with little in the way of eyesight, the remaining senses of smell and vibration detection are thought to be of primary importance in locating their small invertebrate prey.
The most obviously odd thing about five-toed worm lizards, however, is their partially reduced limb count. It’s as if they are stuck between two forms, legless and full-legged lizard, and this may well be the case.
Having had the surreal experience of placing a dead specimen under a La Paz vet’s X-ray, I can confirm that hidden beneath the surface is a collection of tiny bones, all that remains of a pelvic girdle.
They could be in the process of either losing or gaining limbs (some lizard lineages have done this several times in their evolutionary history), but we’ll probably never know whether their legs are coming or going.
Top image: a five-toed worm lizard. Credit: marlin harms, CC BY 2.0 https://creativecommons.org/licenses/by/2.0, via Wikimedia Commons
