Filed under: Uncategorized | Tags: folklore, latin, literature, lizards, reptiles, science, top ten
One of my secret, silly passions in life is the history of biology, and from that rich and varied history, one of my favorite parts is the medieval bestiary. For those of you who have never seen one, a bestiary was a richly illustrated textbook somewhere between Animal Planet and Sunday School. Illustrations of animals would be accompanied by text that described, in greater or lesser detail, their natural history, and what religious lessons could be drawn from these animals and their lives: Pelicans, for example, said to feed their young on their own blood, represented the incredible mercy and self-sacrifice of Christ, while serpents, of any kind, generally represented the devil.
Now, medieval bestiaries were written by monks, who very rarely got into the fresh air, and while they might have been fairly familiar with the habits of common barnyard dogs and cats, they had to take a little license and a lot of faith when describing animals they had never seen; like crocodiles, elephants and giraffes. Understandably, they didn’t always get things right: while we might, today, associate the giraffe with the fabulous camelopard, it’s a far cry from its description today, and no one has yet found a living unicorn or (my personal favorite) bonnacon.
That being said, sometimes natural history has a way of sneaking back on you and asserting itself in the strangest of medieval legends. Take the following passage from Isidore of Seville (my very rough translation follows, and better classicists than me are invited to tell me that I have failed).
Amphisbaena dicta, eo quod duo capita habeat, unum in loco suo, alterum in cauda, currens ex utroque capite, tractu corporis circulato. Haec sola serpentium frigori se committit […] Cuius oculi lucent veluti lucernae.
Etymologies, Book 12
It is said of the Amphisbaena that it is a serpent with two heads, one in its proper place, the other on the tail. It can run in the direction of either head, moving its body in a circular motion. This is the only snake able to live in the cold. His eyes shine like lamps.
Like I said, I’m no classicist, and the translation is a pretty rough one. (I know enough to get the gist of a text, not enough to stake my life on knowing what it says.) But, what’s interesting is that there’s a real-life beast called an amphisbaenian, named after the mythological monster.
And while they don’t look exactly like their medieval counterparts, they are more similar than you might think. We’re going to take a look at one particularly unique member of the group. So, by all means, read on to find out more.
Most amphisbaenians are limbless and earthworm-shaped. They burrow into the sediment using their modified skulls as a combined shovel and jackhammer, smashing through dirt in search of tasty little invertebrates. Most are tiny, and if you weren’t looking closely, you might dismiss them as tiny worms, instead of the reptiles that they truly are. Because of their incredibly weird, derived skull morphology and elongated body shape, they used to be considered a totally separate order of reptiles, only distantly related to snakes and lizards. But, like the snakes, their classification has recently undergone an update, and it is now thought that their closest relatives are the lacertillians, or true lizards (all of this work is based on DNA, and not on their bizarrely derived morphology).
So, what could these beasts, who are almost entirely confined to the tropics have in common with a two-headed serpent of medieval mythology? To illustrate that, I’d like to introduce you to a unique member of the Amphisbaenian group, the ajolote, also known as Bipes. Here’s a close-up of his cute little face … along with a unique anatomical feature that separates him out from the rest of the world’s legless lizards.
Waitaminute. I know, I know. I told you that amphisbaenians were legless, and, yet, that little lizard clearly has two pairs of well-developed, if stubby legs, complete with toes, claws and some pretty hefty musculature. It doesn’t look much like a worm-lizard to you …
That’s because Bipes is the world’s only amphisbaenian with functional legs. Those legs aren’t some evolutionary relic, like the cloacal spurs of pythons or the flaps on pygopodids (remember Lialis?) — they serve a very important function in the amphisbaenian’s day-to-day life. Because, unlike other amphisbaenians, Bipes uses these little legs, not his head, to dig through the soil of his Mexican and Californian habitat. Beyond that, it’s not even the most primitive member of the amphisbaenian group; instead its evolutionary relationships suggest that Amphisbaenians lost their legs on more than one occasion, as a convergent adaptation to their soil-swimming lifestyle. This may make them a very good candidate for examining the developmental processes that lead to limb loss, since most of the developmental work that’s been done on the subject to date involves snakes, which have undergone a remarkable genetic shift that completely eliminates their pectoral girdle and front legs. Understanding amphisbaenian limb reduction is probably a better way to understand the general rules governing limb shortening, digit reduction and limb loss, since their repeated re-evolution of the character provides a good experimental background (unfortunately, the little guys are a pain in the butt to breed in a laboratory).
This unique mode of locomotion, as the only leggy member of an almost entirely legless clade would earn it a place on our remarkable reptiles list, but it has more interesting stories to tell. But, the other reason it’s earned a place on my list is that the medieval legends are true … sort-of. Amphisbaenians don’t have two heads, although their tail and head do look remarkably similar, but they can crawl backwards in their tunnels just as easily as they go forwards. Sometimes, mythology gets things surprisingly right.
The sensory systems of amphisbaenians are also unique. While they can make sounds (a youtube video can be found here) , they have no external ears, but have a remarkably complex internal ear that has much better auditory sensitivity than that found in snakes. While the ear is primarily attuned to low-frequency sounds, it has a pretty good range into higher frequencies. Unfortunately, though, they don’t have luminously shining eyes. In fact, most amphisbaenians are practically blind: their skin-covered eyes can sense dark and light, but probably can’t form images. Nevertheless, when they are exposed to dark or light only for prolonged periods of time, their retinas degenerate even further, and they can be left totally blind (so if you decide to keep one as a pet, make sure it has a regular day/night cycle).
And to tie things back to mythology … apparently in Baja California, there is a legend that you have to be careful where you sit; otherwise an adventurous amphisbaenian might just mistake your unsavory bits for its own tunnel …
Tomorrow we’ll tackle another burrower. But unlike the amphisbaenian, tomorrow’s really does live up to its frightful legend.
Photoreceptor degeneration in the eyes of an amphisbaenian in response to constant light or constant darkness
G.C. Gundy, Journal of Experimental Zoology, August 1977
The Ear and Hearing in Bipes biporus
Ernest Wever and Carl Gans, Proceedings of the National Academy of Sciences, September 1972
Phylogenetic relationships among amphisbaenian reptiles based on complete mitochondrial genomic sequence
J.R. Macey et al., Molecular Phylogenetics and Evolution, October 2004
Appendicular Skeleton in Amphisbaenians (Reptilia: Squamata)
Maureen Kearney, Copeia, August 2002
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