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A few columns back I touched on the chemical arms race between newts and garter snakes: the newts loaded with a fugu-like toxin to which the snakes have evolved resistance. Well, there are complexities to that story that I wasn’t aware of, some of which are described in a 2004 Journal of Chemical Ecology article entitled “A Resistant Predator and its Toxic Prey: Persistence of Newt Toxin Leads to Poisonous (Not Venomous) Snakes.” The lead author, Becky Williams, is a UC Berkeley graduate student; she collaborated with Edmund Brodie, Jr. of Utah State University and Edmund Brodie III, now at the University of Virginia.
For one thing, it isn’t just any old newt or any old garter snake. The only species known to be resistant to the newt toxin (tetrodotoxin, TTX for short) is the common garter snake (Thamnophis sirtalis), in respect to the rough-skinned newt (Taricha granulosa). And there seems to be a great deal of variation in patterns of toxicity and resistance. Some rough-skinned newt populations pack higher doses of TTX than others.
It’s unclear whether this reflects the newts’ diet or has a genetic basis; other toxic amphibians, the arrow-poison frogs, have been shown to acquire their toxin from the insects they eat. Research appears to have ruled out symbiotic bacteria as a TTX source. In any case, garter snake populations that prey on supertoxic newts have evolved higher levels of resistance.
Oregon’s Benton Couny in the Willamette Valley, where Williams and the Brodies got their specimens, is one of those coevolutionary hot spots. I’m not sure what the picture is in the Bay Area, which has its own populations of common garter snakes (of which the beautiful San Francisco garter snake is a subspecies) and rough-skinned newts, as well as other garter snake and newt species.
The hypothesis Williams and her co-authors were interested in had to do with whether the Willamette snakes made any use of the newt toxin themselves. There was a precedent: a Japanese snake species that feeds on toads, stores the toad toxin in glands on the back of its neck, and displays the glands when threatened by a predatory bird. Was something similar happening with the garter snakes?
After feeding newts to snakes, then sacrificing the snakes and assaying their organs, the biologists concluded that TTX stayed in the garter snakes’ livers for at least seven weeks. Three weeks after a newt meal, the average dose in a snake’s liver was 42 micrograms.
Consuming more newts would crank up the toxicity. Even the one-newt toxin load would be enough to kill typical avian predators of garter snakes, like crows (which are particularly fond of snake livers), northern harriers, red-tailed hawks, and American bitterns. Predatory mammals seem less susceptible.
But here’s a paradox: a defense that does in the attacker has no evolutionary advantage for the prey species. Death short-circuits the learning experience. Toxic defenses only make sense if a predator’s reaction is sublethal: it feels terrible and avoids such prey in the future. Williams and the Brodies say TTX acts quickly enough to cause an emetic response—so a crow might well survive a bite of toxic snake liver, sadder but wiser.
They also speculate that the garter snakes’ coloration may aid that process. Most populations of the species are boldly patterned in red and black; like the monarch butterfly’s orange and black, this could function as a warning to predators with color vision, notably birds. The snakes of Benton County, which would have atypically high TTX levels, also have brighter red coloration. (The newts’ vivid orange underbellies serve to warn their own predators, and are mimicked by nontoxic salamanders like the ensatina).
And the snakes appear to accentuate the visual signals with defensive displays that highlight their red lateral markings. The foul-smelling musk they emit when threatened may contain chemical cues to their unpalatability.
As a sidebar, it seems that resistance to newt toxin involves tradeoffs: resistant snakes can’t crawl as fast as nonresistant ones. Why this should be is unclear—maybe one of those genetic linkage deals. But it would give resistant snakes more of a window of vulnerability to predation.
It’s a complicated world out there, and the ancient dance of predator and prey has infinite variations. The Brodies are still working on the newt-snake interaction, but Becky Williams has moved on to other toxic creatures and is now studying the notoriously venomous Australian blue-ringed octopus. Let’s wish her luck.
Joe Eaton’s column runs every other Tuesday, alternating with Ron Sullivan’s “Green Neighbors,” a column on East Bay trees.