Rough-skinned newts (newts of the genus Taricha granulosa) are one of the most poisonous animals in the world. In certain populations, the poison in a single newt can kill up to 20 humans. However, the newt’s predator, the common garter snake (Thamnophis sirtalis), has developed resistance to the newt’s toxin over time as well. Why is the rough-skinned newt so poisonous and who will triumph in this competition between predator and prey?
The rough-skinned newt, which is found along the west coast of Northern America, is the amphibious equivalent of the deadly puffer fish that is eaten by the Japanese as a delicacy. This is because it secretes from its skin tetrodotoxin (TTX), the same poison that causes the tingling sensation on the lips and tongue when eating a fugu meal and which could lead to possible death if ingested in larger quantities.
"Ounce for ounce, some of these populations are the most toxic amphibians on the planet,"
so says Charles Hanifin, a postdoctoral scholar at Stanford's Hopkins Marine Station who recently conducted a study on the rough-skinned newt and the common garter snake. However, rough-skinned newts are mostly harmless to human beings unless eaten as the poison cannot be absorbed through the skin.
Evolution Toxic Newts
Video with 2 of the co-authors in the study
Scientists have theorized that the reason for this level of toxicity found in the rough-skinned newt is that it is in response to evolutionary pressure in the form of its predator, the common garter snake. The common garter snake, similarly, has developed resistance to TTX in response to the TTX found in its prey, the rough-skinned newt. Newts develop greater toxicity over time as snakes become more resistant to TTX and snakes become more resistant to TTX over time as newts become more toxic.
The study by Charles Hanifin uncovers a few interesting findings:
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Prey toxicity and predator resistance to toxicity closely correlate with each other; in regions where newts have high levels of toxicity, the garter snakes have correspondingly high levels of resistance. In regions where there are no newts or where the newts are non-toxic, the garter snakes have no resistance to tetrodotoxin.
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In 30% of the regions studied, the populations of garter snakes have developed such great resistance to TTX that snakes with the least resistance are able to eat newts with the greatest toxicity.
The first finding is further evidence of coevolution, where two species exert selective pressures on each other such that certain traits develop over time. In the case of garter snakes and newts, this results in an evolutionary arms race as the two species compete to develop traits that result in greater fitness for itself at the expense of the opposing species.
The second finding shows that the garter snakes appear to have won the evolutionary arms race with the newts. The newts have been unable to keep up in increasing its toxicity in response to the increasing resistance to TTX found in their predators.
There are two reasons why the newts have lost. Only a single mutation in the garter snakes’ genes is responsible for its resistance to tetrodotoxin, making it faster and easier for successive generations of snakes to evolve greater resistance to tetrodotoxin. A number of genes are involved in increasing the toxicity in newts however, and the increases in toxicity are more gradual than the increase in resistance to TTX in the snakes. The newt also faces limits in increasing its toxicity as the poison is only produced in the skin. Some snakes have already developed such a great resistance to tetrodotoxin that it would be biologically impossible for the newt to produce in its skin the amount of TTX needed to have an effect on the snakes.
"It is pretty much biologically impossible for the newts to ever catch up,"
Hanifin says. However, there is hope yet for the newts. There appears to be a cost to the super-resistance found in certain populations of garter snakes. These snakes crawl slower than other snakes with lower levels of resistance as can be seen in the experiment in the video above. It is thus harder for the snakes to catch the newts and the snakes are more suceptible to its own predators like birds as well because of its slower speed. Hence, the population of super-resistant garter snakes is controlled and rough-skinned newts are still found in areas with super-resistant snakes.
References:
Hanifin CT, Brodie ED Jr & Brodie ED III, 2008. Phenotypic mismatches reveal escape from arms-race coevolution. PLoS Biol 6(3): e60. doi:10.1371/journal.pbio. 0060060
“Snakes vault past toxic newts in evolutionary arms race,” by Shelby Martin. Stanford News Service, 11 March 2008