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Biological warfare and the coevolutionary arms race :

Nastier than nasty newts

Butch discovered that newts are loaded with TTX. In fact, they are SO poisonous that just one newt could kill over 100 people! Humans don't eat newts, but a bird might. One of these newts could kill 200 herons or 2000 kingfishers! The discovery of extreme toxicity in newts explained what killed the hunters — but it also led to a new question: why would a newt evolve to be so toxic? Isn't all that TTX, quite literally, overkill?

great blue heron belted kingfisher
One rough-skinned newt could have enough TTX to kill 200 great blue herons (left) or 2000 belted kingfishers (right).

After all, newts use a lot of energy making TTX — energy that they could otherwise spend on finding food, growing faster, or reproducing. Why would a newt waste all that energy making more TTX than it needed? In evolutionary terms, it doesn't make sense. Natural selection should cause newts to evolve TTX levels just high enough to protect them from predators. It works like this...

  1. Imagine a population of newts in which the amount of TTX produced varies from newt to newt...

    an initial population of newts, with varying levels of TTX production


  2. Because producing chemicals, like TTX, requires energy, newts that produce too much TTX have less energy to invest in offspring and produce fewer offspring. However, newts that produce too little TTX tend to get eaten by predators and fail to reproduce at all. Newts that produce just enough TTX to protect them from predators have the most offspring. If toxicity level is indeed inherited (as it seems to be), high toxicity newts will produce highly toxic offspring and low toxicity newts will produce offspring with low toxicity.

    newts that produce too much TTX have few offspring, newts that produce too little TTX don't survive, and newts that produce just enough TTX have the most offspring.


  3. Because they had more offspring, newts with just the right amount of toxin are more frequent in the next generation.

    newts with just the right amount of TTX are more frequent in the next generation


  4. Over time, this cycle repeats itself and eventually the entire population consists of newts with just enough TTX. Newts that waste energy on too much TTX have died out, and newts without enough TTX defense have died out.

    eventually the entire population consists of newts with just enough TTX


Evolutionary theory predicts that natural selection will weed out newts with too little or excess TTX so that newt populations should be equipped with just enough TTX to kill their predators and no more. But if that's true, then how did newts evolve to be so poisonous?
Great blue heron photo by Gerald and Buff Corsi © California Academy of Sciences; belted kingfisher photo © Joyce Gross

Biological warfare
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