I now, officially, love “Ask a Biologist”

22 02 2009
Good tidings and well-wishes!

Regular readers of David Hone’s blog  (also known as ‘your one-stop pterosaur site’) are undoubtedly familiar with the wonderful website “Ask a Biologist” at which he volunteers his services along with dozens of other academics ranging in expertise from cell biology to genetics to botany to zoology and everything in between answer various questions concerning their science. I’ve been aware of this site for a few years, but for some reason, the idea of actually posting a question there never occurred to me.

Recently, however, I’ve been wondering about arboreal (tree-dwelling) reptiles. I’ve noticed that many, such as chameleons and tree boas, are laterally-compressed (flattened from side to side), though I couldn’t for the life of me figure out why. Similarly, I’ve noticed that extinct drepanosauridswere fairly similar in this regard and are also frequently interpreted to have lead a similar lifestyle to modern chameleons, yet I was equally baffled by the presence of this feature in them.

This photograph of a veiled chameleon (Chamaeleo calypatratus) demonstrates the lateral compression which had confused me earlier.

This photograph of a veiled chameleon (Chamaeleo calypatratus) demonstrates the lateral compression which had confused me earlier.


Upon revealing these concerns to the AAB forums, however, none other than the famed Darren Naish delivered a prompt and quite logical response:

“This is presumably because chameleons are specialised for ‘above-branch climbing’: a behaviour (elsewhere seen in some primates and tree frogs) where the animals climb along horizontal supports, their hands and feet being held directly beneath the body. Above-branch climbers tend to be slow movers, and to therefore rely upon crypsis for protection. A deep and narrow body may also help chameleons in avoiding predators, as it means that they can hide behind narrow branches. I would hypothesise that the narrow body evolved in concert with a vertical gait and specialisation for above-branch climbing, and that a role in crypsis came later as an exaptation. It’s hard to test this idea because the early history of chameleons is very poorly known.”

This makes a lot of sense to me, and I can see how this model would also work for arboreal snakes such as the ones below:

Amazon tree boa (Corallus hortulanus)

Amazon tree boa (Corallus hortulanus)

That deep, narrow body would likely help these serpents to likewise hide behind narrow branches.

I heartily recommend this site to anyone even vaguely interested in biology with a few questions on their mind. Keep up the good work, AAB staff!!

May the fossil record (and the rest of the natural world) continue to enchant us all!




One response

18 11 2010
Hannah Bonner

I know this is an older post… but I only just read it and it seems to me that in addition to providing visual protection, once animals have upright legs and are using them to move along a narrow branch, it would also be an advantage, ergonomically and balance-wise, to not have weight spilling out to the sides above this narrow base. Better to keep the center of gravity between the legs as much as possible. Plus the limb girdles would need to be narrow to match the average branch width.

Thanks for the great posts!

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