[Rough-Skinned Newts cultivate a bacteria on their skin that produces a deadly toxin: tetrodotoxin]

Now: when the snakes eat Rough-Skinned Newts, they may sometimes show signs of discomfort. The snake may visibly gag.  It may writhe in obvious unease.  In some cases, it may go into respiratory distress.  Eating the newt looks pretty unpleasant.  Yet the snakes persist.

Okay then — if evolving toxin resistance carries a cost, and if even with resistance eating the newts is unpleasant, then why then do garter snakes insist on eating newts?  They cheerfully eat frogs, fish, and other non-toxic prey items.  Why don’t they just eat those, and leave the poor newts alone?

Turns out there is an answer:  the garter snakes sequester the tetrodotoxin, storing it in their livers.  This makes them toxic to their own predators.  (Of which there are plenty.  They’re not large snakes, so they’re hunted by everything from raccoons to ravens.)  But they don’t harbor the bacteria, so they don’t produce tetrodotoxin of their own. So eventually, the toxin that they’ve ingested breaks down.  And then they need to eat another newt to refresh their defense.

So this explains why the snakes go after the newts particularly, preferring them to less toxic prey.  They want to eat toxic newts.  And it explains why the newts keep evolving to be more toxic: the snake may want to eat newts generally, but if an individual newt packs enough of a wallop, the snake may just retch it up and go after a different one.  Newts with weaker poison?  They get eaten.  Snakes with less resistance?  Have trouble finding newts they can choke down, and don’t get to steal their poison.  So the arms race continues.

FROM:

Doug Muir & Rick Shapiro & Eric Pruitt & Mike Huben

Occasional Paper: The Impossible Predicament of the Death Newts

Source

Just so interesting to see so many layers of evolutionary call and response.