A Game of Evolutionary Chess

Saturday, March 24, 2012

Dares and Death

One of the many strange things humans do is dare each other to eat unpleasant things. You can find videos on YouTube of people downing something stinging (e.g. mustard, wasabi), acidic (lemon or lime juice), or spicy (chili powder), all for a small sum of money, or maybe just peer admiration. Sometimes the dare involves a quantity rather than a quality of food (a whole cake, six hot dogs). Of course, the most entertaining involve eating animals–cicadas, cockroaches, worms. The eaters are most often younger males of the Homo sapiens species, and they usually seem to fare OK once the moment of ingestion is over, and digestion has begun.

But sometimes these experiments don’t turn out so well. In 2010, a man in Sydney, Australia nearly died after contracting a rare lungworm from a slug he ate on a dare. And in 1979, an Oregon man did die from eating, while drunk, Taricha granulosa, the rough-skinned newt, which carries in its skin one of the most exquisitely toxic substances known. Tetrodotoxin is a potent neurotoxin that binds to and blocks the sodium ion channels that regulate our heartbeat. Essentially, it prevents the heart from beating.

Evolution certainly is a strange force, to produce both toxic newts and male humans who occasionally choose to eat them.

Newts and Bacteria

It’s pretty clear why the Taricha newts would have evolved to carry this poisonous defense. Newts are small and soft-bodied, and could make an easy meal for a snake or other predator. But the newt only benefits from carrying this toxin if predators know to avoid it before chomping down. And for that purpose, Taricha granulosa has evolved a bright orange belly (orange and red are nature’s standard toxicity warning signs), as well as a back-arching behavior known as “unkenning,” which exposes the bright underside to predators. For some nice close-ups of Taricha granulosa, check out the Backyard Zoologist’s blog post on these critters.

But wait! There’s more to this story. It seems that newts may not produce tetrodotoxin themselves–they may outsource this job to a set of bacteria. Tetrodotoxin is found in widely varying parts of the animal kingdom–sea stars, flatworms, blue-ringer octopi, and pufferfish, among others. It can serve as a predatory toxin as well as a defensive one. It seems likely (to me, at least) that it would have evolved once in bacteria and perhaps been shared through horizontal gene transfer, rather than evolve many times in animals.

The question then becomes, what benefit do the bacteria receive for protecting their diverse hosts? Presumably they get a home and some nourishment, but I’m not sure the precise answer is known. Even the bacterial synthesis of newt tetrodotoxin seems to be uncertain, for now. But if true, it is a beautiful example of symbiosis–of tightly co-evolved species helping each other survive in an uncertain world.

Man and Risk

Meanwhile, the same species that has been puzzling out the secrets of newt toxicity has  also evolved a propensity to engage in risky–and sometimes stupid–behavior, like newt ingestion. And it does seem that males of this species engage in this kind of behavior more readily than females. In a paper that came out in 2008, two evolutionary psychologists found that males were more likely to cut it close when trying to catch a bus, and to take risks when crossing a street–especially if women were watching.

What benefits might accrue from all this risk taking–besides the hope of getting a date? Nowadays, it seems, risk taking is best known for getting us in trouble–refer to the economic collapse of 2008 (caused mostly by men) for more details. But risk taking throughout history has also has its rewards–often spectacular ones. Elizabeth Kolbert, in her profile of Svante Pääbo in the New Yorker last August, explores the idea that it may have been risk taking may have set humans apart from Neanderthals. Neanderthals, it seems, spread until they reached “water or some other significant obstacle.” Humans kept going.

Snakes and Newts

Did the newt get a free pass in the game of chomp, once it teamed up with toxin-producing bacteria? Hardly. There is an animal that can eat the Taricha newt and live to tell the tale–the garter snake Thamnophis sirtalis. Somewhere in evolutionary history, a chance mutation produced a tetrodotoxin-resistant sodium channel, which, as Ed Yong nicely put it, “open[ed] up an exclusive menu of newts unavailable to other predators.” There is a cost though–snakes can be immobilized for up to seven hours while digesting a newt, making them possible prey for another animal.

But again, there’s more. Scientists recently learned that some populations of the garter snake have evolved to become more resistant to tetrodotoxin than others, and don’t need to undergo such a long period of immobility. In fact, these snakes are now so resistant that the researchers believe the newts will not be able to evolve a higher toxicity level to match. But this too comes at a cost–the highly resistant move slower in general, perhaps giving the newts the opportunity to run away more easily.

The scientists publishing these results describe the newt toxin/snake resistance co-evolution as an “arms race,” and posit that the snakes have temporarily escaped from the race. But what if we instead consider the metaphor of a chess game, where each species makes its evolutionary move (newt: form symbiosis with toxic bacteria; snake: develop resistant sodium channels), and waits for its opponent to counter? I like this metaphor, because it suggests a very close co-evolutionary bond, like two competitors sitting across a table. The question then becomes, have the snakes checkmated the newts, or do the newts have another move in store?

Chess and Life

Chess players (who tend to be mostly men) must take risks if they want to win, and sometimes these risks go badly: a sacrificed piece doesn’t pay off; an exposed king comes under attack. But the consequences are limited–a lost game, maybe some lost money or pride. In life, the stakes are higher, and the set of options much larger and harder to evaluate. How many snakes chomped a newt and died before one managed to survive the meal and, belly full, reproduce and pass on its resistant genes? Or, as Kolbert quotes Pääbo, “‘How many people must have sailed out and vanished on the Pacific before you found Easter Island?’”

The garter snake that survived the newt toxin clearly benefited from its risk-taking. We humans have also benefited enormously from ours. We have colonized almost every habitat on earth, our average life spans have more than doubled, and our material living standards have increased immensely. And yet, we are taking larger risks than ever before with the very resources and ecosystems that sustain us. Natural communities depend on ancient co-evolutionary associations that are often finely matched to each other, and we are changing these balances, often drastically–akin to removing pieces from the chessboard, if you will. How long can we continue tampering with the game before we make our planet toxic to all life, including ourselves? Only time will tell.