Introduction to Snakes

I’m no fan of vertebrates… except for snakes. They’re just cool. I’m not qualified to write much on their biology as a reader requested, so this is more like a collection of trivia.

Systematically, they are diapsid amniotes, belonging in the Lepidosauria (Estes et al., 1988). They clade together with the lizards as the Squamata, defined mainly by the moveability of their skulls. Where they belong in there is still disputed though: they are either sister to Amphisbaenia or to the Anguimorpha, or not to any of those and instead sister to an extinct marine reptile from the Mesozoic.

There are more than 3000 described snake species (Zug et al., 2001), all sharing the same type of body: long body, short tail, no legs. They’ve used their body to conquer all types of habitats: some live underground as burrowers, most live on the surface, while some are tree-dwellers and some are aquatic; one genus, Chrysopelea, can even glide. Interestingly, snakes might have first evolved from burrowing forms, as represented nowadays from the basalmost scolcophidian and annilidan snakes (Wiens et al., 2006), although this is not a certain result, as taking into account the fossil record gives a picture of them having evolved from marine ancestors.

Of course, their leglessness is commonly regarded as a defining trait, even though it convergently evolved in squamates multiple times (Wiens et al., 2006), and boas have vestigial hind limbs. Their development, not only as it related to limblessness, is fascinating (see Cohn & Tickle, 1999), especially from an evo-devo perspective, as we can clearly see the effect of Hox genes in patterning the body plan.

Other characteristics include their elongated body, achieved by an insane number of vertebrae (120-500) and their skull. The latter is especially exciting. It’s very light, and can flex from many points. When opening its mouth, the palate will move forward, the fangs are pushed forward and the jaw joint is expanded to three times its regular size. When it has captured its prey, it will dig its teeth into it, and other skull bones, the maxillae, palatines and pterygoids will be moved backwards, while the snake crawls over its prey (if its a large mammal) to literally stuff the entire prey down its throat.

Egg-eating snakes have an extra step. They swallow the egg, but then use the enlarged hypophyses of their first seven vertebrae to break the shell, which they then vomit.

On their venom: considered potent enough to have been a factor in primate behavioural evolution (Isbell, 2006), it contains peptides, making it a potential source of new drugs (Lewis & Garcia, 2003); for example the venom of the Malayan pit viper is used as a source of anticoagulant. On the flipside, some of the nastier venoms are the ones that disable clotting processes, leading to death by haemorrhage. Most venomous snakes are in the Viperidae family, including vipers, adders and rattlesnakes.

Ecologically, snakes are present in every biome and have their respective adaptations. For example, desert rattlesnakes lose only 0.5% of their body weight in water every day (McDonald, 2003), meaning they can go without drinking for over 2 months.

The snake fossil record begins in the Early Cretaceous, feeding on insects before undergoing a radiation in response to the mammalian radiation in the Tertiary (Rage, 1984).

References:

Cohn MJ & Tickle C. 1999. Developmental basis of limblessness and axial patterning in snakes. Nature 399, 474–479.

Estes R, de Queiroz K & Gauthier J. 1988. Phylogenetic relationships within Squamata. In: Estes R & Pregill G (eds.). Phylogenetic Relationships of the Lizard Families.

Isbell LA. 2006. Snakes as agents of evolutionary change in primate brains. Journal of Human Evolution 51, 1-35.

Lewis RJ & Garcia ML. 2003. Therapeutic potential of venom peptides. Nature Reviews Drug Discovery 2, 790-802.

McDonald GM. 2003. Biogeography: Space, Time and Life.

Rage JC. 1984. Serpentes. Handbuch der Paläoherpetologie 11, 1-80.

Wiens JJ, Brandley MC & Reeder TW. Why does a trait evolve multiple times within a clade? Repeated evolution of snakelike body form in squamate reptiles. Evolution 60, 123-141.

Zug GR, Vitt LJ & Caldwell JP. 2001. Herpetology: an introductory biology of amphibians and reptiles. 2nd ed.

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