The Outside Story: Bright, beautiful creatures advertise their secret weapons
In the natural world predation is relentless, and evading predators strongly favors the evolution of camouflage colors in animals. How contradictory then, for small, defenseless creatures — like red efts and monarch butterflies — to be sporting a bright shade of orange. But there is more to their cheerful color than meets the eye. Both the eft and the monarch are poisonous.
Once a predator has tasted one, it soon gets sick, and from that experience learns not to eat another. Thus an individual eft or butterfly may sacrifice itself, but the education of predators benefits the species as a whole. And, in fact, efts and monarchs often survive predator attacks. Toads and snakes that swallow red efts have been observed to vomit up the prey unharmed in about half an hour. Birds that attack monarch butterflies often go for the brightly colored wings, the most toxic part of the insect. One peck may be all it takes to deter the bird.
Zoologists call the use of bright colors to warn predators “aposematism”; generally, the brighter the color, the more toxic the animal. (The brilliant poison dart frog in South America is a great example.)
The orange skin of the red eft contains tarichatoxin, also known as tetrodotoxin; it’s a potent nerve poison. The toxin causes irritation on contact, and if a lot is eaten, paralysis and death by suffocation. In experimental studies, snakes that tried to eat and then spat out a tetrodotoxin-producing newt first rubbed their gaping mouths on the ground, began writhing, and then exhibited partial paralysis that required over an hour for recovery. A brief contact with newt skin was enough to transfer toxin through the lining of the snakes’ throats.
The monarch butterfly’s toxin is called cardenolide, and is derived from a class of plant steroids that is responsible for livestock poisoning and, paradoxically, is used by doctors to treat congestive heart failure. Monarchs acquire their toxin from milkweed, the exclusive food of their caterpillar. (And yes, the caterpillar is toxic, too.)
Many people don’t appreciate that a red eft they see on a moist forest floor is the larval stage of the aquatic Eastern newt. Eastern newts are toxic, too, albeit only one tenth as toxic as efts, but it is enough. Because they are poisonous they can share waters with predatory fish. The aquatic life stages of other Eastern salamanders, all of which lack toxins, are largely restricted to vernal pools where fish can’t live.
Predators remember the experience of eating an eft or monarch for some time.
So effective is this learning that various unpalatable species have evolved to resemble each other. Among butterflies, the viceroy looks strikingly like a smaller version of the monarch. The viceroy tastes bad because its larvae feed on willow, a source of salicylic acid, the active ingredient in aspirin. The salicylic acid in viceroy butterflies makes them bitter and irritates the stomachs of predators. Both viceroys and monarchs are orange with black stripes and are avoided equally by predators.
While there are no mimics of the red eft, there is a Western U.S. salamander, the yellow-eyed ensatina, which closely resembles the dangerously toxic Coast Range newt. Both have orange skin coloration and yellow eye patches.
There’s even evidence of a toxin “arms race.” Out West, some populations of garter snake have evolved a degree of resistance to tetrodotoxin, allowing them to eat toxic newts with few side effects. This has led to the evolution of higher tetrodotoxin levels in the newts that live where the resistant snakes are found.
One wonders why the toxin of newts and efts does not harm the animal itself. That question was answered when researchers discovered a mutation that makes the eft’s nerve cells immune to tetrodotoxin. It comes with a price: sluggish transmission of nerve impulses. But what does it matter if efts are slow-moving? They don’t need speed.
Li Shen is an adjunct professor at the Dartmouth Medical School and the chair of the Thetford, Vt., Conservation Commission. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlandsmagazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: [email protected].
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