The Garter Snake, the Rough-Skin Newt, and the Caddisfly
What is a "biological arms race"? Well, the term refers to the coevolution of two sets of organisms. Imagine a population of orange striped butterflies that is preyed on by small red birds with orange crests and black wings. Initially, the butterflies had no defense against their flying predators. Their predator was thus free to attack any butterfly that was unfortunate enough to enter their line of sight.
That is until the day one butterfly was born with a mutation which fatally poisoned any bird that tried to eat it. This mutation allowed that butterfly to escape predation and increased its chances of contributing offspring to the subsequent generation. It is at this point where the beauty of natural selection comes into play. The mutation, clearly an advantageous trait, would be selected for against the less toxic variation. In so doing the number of butterflies in the population with the mutation grew until these were the most common butterflies in the population.
So, wait, if the population of butterflies consists mostly of butterflies with a defense to protect against predation by their orange-crested predator, what happens to their predator? Surely they must eat, right? I’m glad you asked that question because it is at this point that something interesting happens. The predator evolves a mechanism to counteract the defense of the butterflies.
Well, at first, one bird does; that bird and subsequent birds that carry the trait are selected for in the population until they are the most common birds in the population. This then places selective pressure on the butterflies. Any butterfly that has a stronger defense is favored and, well, you know how the story goes. This process continues on and on, each time the butterflies evolve a defense that is more effective than previous iterations and each time the birds evolve a counter defense that counteracts it.
The Case of Three Mysteriously Dead Hunters
In the state of Oregon, there is a story about three dead hunters who were found mysteriously dead by their campsite in the 1950s. Nothing was stolen and their bodies bore no signs of physical violence. The most unusual thing found on the scene was a roughskin newt in the hunter’s coffee pot, which was apparently boiled to death. Investigators had no way of explaining the deaths of the hunters.
It seemed like the perfect mystery that is until in the 1960s when an undergraduate student named Edmund “Butch” Brody Jr decided to test a theory of his. The newt, he believed, was the key to this mystery. Roughskin newts have brown backs, which allow them to blend in with their environment. Their undersides, however, have a distinct orange color. When threatened, roughskin newts arch their head and tails upward to display their brightly colored underside.
Butch knew that bright colors are associated with poisonous and venomous animals such as coral snakes and monarch butterflies. In these species they act as a signal, warning potential predators of the animal’s toxicity. Butch deduced that the brightly colored undersides of the newt meant that they were poisonous and that the death of the hunters was due to the ingestion of that poison along with their coffee.
He proceeded to prove this theory by conducting a series of experiments. He grounded up the skin of roughskin newts and then, with it, created mixtures of varying concentrations. These he then injected into potential predators and depending on the concentration the effect on the injected animal was one of or a combination of four symptoms: wobbly movement, immobility, uncontrollable vomiting or worst yet instant death.
The Poison That Packs a Punch
Researchers would later discover that the poison was a neurotoxin called tetrodotoxin, the same toxin found in pufferfish, which is 10, 000 times more potent than cyanide!! Tetrodotoxin works by binding to the sodium channels on the surface of neurons. By doing this it prevents the passage of sodium ions into the cell. Neurons can no longer fire and the nervous system breaks down.
With no signals to tell muscles to contract, paralysis occurs. Breathing comes to a halt, the heart stops beating, and death follows. But that is only if the dose is high enough, if not tetrodotoxin causes numbness, muscle spasms, loss of speech, dizziness, and paralysis. What makes this a terrifying experience is the fact that the brain is impervious to tetrodotoxins so victims remain conscious and aware of all that is happening, but they are unable to communicate their distress (sheesh reminds me of night terrors).
So why would a newt need such a powerful toxin? Butch would find a clue to this troubling question when one day he found a garter snake making a quick meal of a newt in one of his traps, and to his surprise, the snake survived.
The Game of Catch-Up: Garter Snakes and Roughskin Newts
When Butch stumbled upon a garter snake devouring a newt he took his first steps towards discovering a tale that goes as far back as prehistoric times. You see, what he was unaware of was that rough skin newts and garter snakes are locked in a biological arms race that began millions of years ago. Fuelled by curiosity he began collecting garter snakes, which he then fed newts. What he observed was that the snakes suffered no ill effects from doses of the toxin that would have killed animals one hundred times its size. How could this be possible? How did the snakes avoid death or displaying even the milder symptoms of tetrodotoxin poisoning?
The answer to these questions would come in 2005 when Butch discovered that garter snakes have oddly shaped sodium channels. The odd shape of their sodium channels prevents tetrodotoxin from binding to their surface effectively rendering the snakes immune to its effects. The mutation, however, makes the snakes slower than other species of snakes which lack the mutation. He hypothesized that through time the newt became more and more toxic to avoid predation and in response, the garter snakes evolved resistances in order to keep eating the newts. Selective pressure on one group drove the evolution of a stronger defense. This, in turn, placed selective pressure on the other group which resulted in the evolution of a counter defense.
Butch and his son Edmund Brodie III began studying the toxicity of newts and the resistance of snakes along the west coast of North America. They found that the resistance of the snakes mirrored the toxicity of the newts in the area in which they were found. Where there were mildly toxic newts they were accompanied by mildly resistant snakes. Where there were extremely toxic newts they were accompanied by extremely resistant snakes, which is what you would expect to find when two groups experience localized coevolution.
The Gift That Keeps on Giving
The newts having evolved the nearly perfect defense against predation didn’t stop at just protecting themselves. To increase the number of offspring and genes they contribute to the following generation, the newts incorporate tetrodotoxin into their eggs. This protects the eggs from being eaten by predators.
To determine whether or not incorporating tetrodotoxin into their eggs protects the eggs from predation Butch, his son, and their students went to some ponds in central Oregon to study them. They gathered predators, which were known to eat the eggs of other species of animals, from the pond and placed them in buckets which contained newt eggs and pond muck. Almost all the predators failed to eat the eggs, all except one. It turned out that caddisfly larvae were the only predator that dared eat the eggs. Not only did they eat the eggs, but it was found that caddisfly larvae that were fed newt eggs actually grew larger than those that fed on pond muck alone.
Much like the garter snake, it seems the caddisfly larvae had evolved a defense against tetrodotoxin. The Brodies also discovered that the ingested tetrodotoxin remained in the tissues of the caddisfly larvae weeks after ingesting it. Could it be that the caddisflies are ingesting the poison as a means of avoiding predation? Whether or not sequestering the poison protects the caddisfly from predation is still unknown but it opens up the possibility of further research. All we know for certain is that caddisflies are the only known predator of roughskin newt eggs.