Image Credit: Connor Long, CC BY-NC-SA 3.0, Image Cropped

Of poisons and parasites—the defensive role of tetrodotoxin against infections in newts (2018) Johnson et al., Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.12816

The Crux

Many organisms in nature produce powerful (and sometimes deadly) toxic substances, often taken as evidence that prey evolved chemical defenses against predators. Interestingly, these chemical defenses are deadly not only to predators, but also to parasites. This complementary defense, in addition to the ubiquity of parasites themselves, indicate that parasites may have had a hand in the evolution of host toxicity.

One particularly potent toxin found in the animal kingdom is tetrodotoxin (TTX). It can cause paralysis, difficulty with breathing, and even death in some cases. Newts in the genus Taricha are notorious for having high concentrations of TTX in their skin and eggs, and this has long been thought to have evolved as a defense against predators. In particular, Taricha newts and garter snakes (Thamnopholis spp.) are a classic example of arms-race dynamics (see Did You Know). Despite this relationship, newt toxicity and snake resistance to the toxin don’t always match up perfectly in nature, suggesting that other factors may influence newt toxicitiy. The goal of today’s study was to study parasitic infection and compare it to variation in toxicity among two newt species, the rough-skinned newt (T. granulosa) and the California newt (T. torosa).

What They Did

The researchers sampled 42 ponds in the Eastern San Francisco Bay area, allowing them to sample habitats in which both newt species occurred. Each collected newt was sampled for a variety of parasites, including the chytrid fungus (Batrachochytrium dendrobatidis). Chytrid causes a potentially lethal skin disease and has decimated amphibian populations the world over. In addition to checking for parasites, each newt was also sampled to determine how much TTX it had. 

Did You Know: The Poisonous Predator Prey Arms Race

Garter snakes consume their prey by swallowing it whole, and as such toxic newts are well-suited to discourage these predators from attempting to make a meal out of them. That being said, some populations of garter snakes have evolved a tolerance to TTX, which leads the newts to become even more toxic. Because the snakes feed on the newts, they must continue to evolve increased tolerance in order to feed on them. Hence the “arms race”, with the prey becoming more and more toxic and the predators evolving increased resistance.

A fun little aside: the snakes that can eat these newts retain TTX in their livers for weeks after consuming a newt, meaning that they become poisonous themselves! So garter snakes can be both venomous (mildly, don’t worry about it if you’re a person, and I hope you are if you’re reading this) AND poisonous for a time.

TTX has long been thought to be the result of an arms race between newts and garter snakes like this, but today’s study has shown the multi-faceted nature of its potential benefits (Image Credit: Wilson44691, Public Domain)

What They Found

The researchers found at least one infection at 95% of the sites, with 79% of all of the newts sampled infected with at least one parasite. Increasing amounts of TTX correlated negatively with parasite species per host, meaning that more toxic newts were infected with a smaller number of parasitic species. Relating to chytrid itself, the more toxic newts were the less likely to be infected with this deadly fungus.

Problems

Of the 42 sites sampled, only 17 had both the rough-skinned newt and the California newt. While not a problem for this study, I would have liked to see more data from sites in which the two newt species co-occurred.  Ecological systems are incredibly complex, with many different organisms interacting with one another in countless ways. Because both newt species are toxic and prey to garter snakes, I am interested in potential differences in their toxicity at sites where snakes may prefer one newt over the other. If garter snakes prefer one species over the other, it may be more toxic, and thus more protected against parasites than the other newt species. 

So What?

Recent work has focused on the role of chemical defenses beyond predator-prey relationships, and this study showed that TTX in newts may serve a dual purpose by protecting hosts against both predators and parasites. Research that bridges the gap between separate disciplines (predator and parasite ecology) to unite theory and evidence (natural enemy ecology) are increasingly important for explaining patterns in the natural world.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, or follow him on Twitter here.