Thursday, February 08, 2007

Recycling poison

Sometimes nature is just so remarkably cool.

This time, it’s one of the most remarkable stories I’ve read in a while. Though nature never ceases to amaze me, this story is quite something.

There’s this snake called Rhabdophis tigrinus, common in Japan and other parts of East Asia. Now, some (but not all) snakes of this species carry poisons in little sacks along the top ridge of their neck (called nuchal gland). These snakes, it turns out, are the brave ones. When threatened by hawks or other natural enemies, they arch their necks and expose these poison glands. This usually deters predators from biting or attacking them. What’s funny is that many of these snakes do not have poisons in their nuchal glands, so if predators come by they just run….er….slither away!

So, where does this poison come from, and why do only some snakes (of the same species) have the poisons? Some researchers asked the same question, and hypothesized that this snake could do something that is much more common in non-vertebrates (like sea slugs). Perhaps it acquires the poison from its diet.

These snakes apparently feed on toads (if they can get them). Many toads have poisons, called bufadienolides (which are supposed to protect them from predators). Not only does this snake apparently happily eat the poisonous toads, it recycles the toad’s own poison for personal use.

Some justice!

But the study itself that went on to prove this was very nicely done. In an article in PNAS, the authors first collected snakes from toad free islands in Japan, or islands where toads were very common, or regions where they occurred. They then sampled these snakes for bufadienolides in their glands, and what they saw was that the snakes from islands were toads were common had a lot of bufadienolides in their glands, the snakes from islands where there were no toads had no bufadienolides, and the snakes from islands which had some (but not too many) toads had an intermediate range of poison.

So then they (the scientists) went on to do more thorough research. They acquired hatchlings from snakes which did not have bufadienolides (were fed with fish). When the hatchlings were fed with toads they rapidly accumulated the poison in their glands, showing that the snakes could sequester dietary toxins. The researchers also further studied the type of bufadienolides in the snake glands, and this correlated with the type of toads the snakes were gorging themselves on.

It basically looks like this snake decided not to take the trouble of evolving poison glands. “Thank you ma’am, I get my poisons from my food.”

I never quite figured out why I loved National Geographic or Animal Planet so much. But this story just told me why.

Because it’s cool, that’s why.

(You can read all the technical details at the Proc. Natl Acad. Sci. USA website, doi:10.1073/pnas.0610785104, published online Jan 29)


Chetan Rao said...

"Sometimes nature is just so remarkably cool."

Sometimes? Nature is always remarkably cool. :)

The really weird thing is, most people I know just aren't awed by it. Come to think of it, most people I know aren't awed by anything. Except perhaps food. Sheesh.

Sunil said...

heh......nature is always cool. Sometimes, it's remarkably, fantastically, outrageoulsy cool.

I cannot imagine why someone won't be awed by it. If they aren't, they're missing out on something.

But is good too.

Wavefunction said...

It's fascinating. I think some of the alkaloid poison-bearing frogs also ingest the poison from their environment and then sequester it in their skin, rather than manufacturing it.
Another interesting story I read was about a species of stick insect that stuns its predators with a noxious spray. The reason why the predators have not evolved to resist the spray is because apparently, the stick insect keeps varying the relative proportions of stereoisomers of the active ingredient in the spray.

Sunil said...

the stick insect story is beautiful......and reminded me of a parasite called the African Trypanosome. It lives in the bloodstream, and escapes the immune system by constantly changing the glycoproteins on its own cell surface, with every life cycle (these parasites have over 300 different surface glycoproteins, that they keep changing, so the immune system cannot react to generate antibodies towards all the parasites in the bloodstream).

Nature is magnificient.

Anonymous said...

HIV destroys CD4 cells. Rhinoviruses keep changing their genomes every now and then. There are sporadic significant changes in influenza virus genomes that cause flu outbreaks. So, lots of patterns there.
Btw, why do I see the pattern that riboviruses (all the viruses listed above) have a greater tendency for changing genomes than DNA viruses? Any theories?