Ankylosaurus (Image: Mariana Ruiz Villarreal LadyofHats/Wikimedia Commons)

Humans tend to rely on crafted tools in order to harm one another, but most other species have evolved weapons right on their bodies. Normally, these bashers, spikes, and other instruments of attack appear on heads or limbs. But only rarely do they end up on tails—and scientists want to know why

The tail might seem like a less risky place to evolve a weapon than the head, but only a few animals actually end up using their tails as weapon. Only one living lizard seems to have bony spikes on its tail, for example—the rest of those weapon-wielders are extinct. A pair of researchers in the U.S. and Canada have analyzed some living and extinct examples, and think they’ve found out why that is.

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Essentially, it seems that a bony tail comes along with large body size, having body armor, and being a herbivore, according to the paper published today in the Proceedings of the Royal Society B. “We suggest that the evolution of tail weaponry is rare because large, armored herbivores are uncommon in extant terrestrial faunas,” and have been rare throughout evolutionary history, study authors Victoria Arbour and Lindsay Zanno from the North Carolina Museum of Natural Sciences write.

Today’s giant girdled lizard has bony spikes on its tail—the rest of the species with bony tail weapons included in this research are extinct, like the club-wielding ankylosaurus, spiky-tailed stegosaurus, and some glyptodonts (which look like a cross between an armadillo and a tortoise).

Their conclusion, that bony tail weapons are usually associated with large, armored herbivores, came from analyzing lots of data on animals and their traits. Highly evolved tail weapons may have started as a defense mechanism to ward off predators, but then became more elaborate as the animals used them to fight one another. Think survival of the fittest, but in this case, “fittest” means having the deadliest tail weapon.

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Today’s tail fighters sometimes have armor made from keratin that match the rest of their bodies, like porcupines and pangolins. But others, like some lizards, simply whip around whatever tail they’ve got in order to defend themselves, special machinery or not. These animals frequently have more effective ways to avoid predators, like camouflage or speed, and use tail lashing as a last resort. Evolution might be less likely to favor behavior that puts the animals into direct contact with a predator, the authors write.

Eric Gorscak, postdoctoral researcher at the Field Museum in Chicago, told Gizmodo that tail weaponry is difficult to research. “Their one-of-a-kind study provides a fresh perspective and broad evolutionary context for these often puzzling traits that have fascinated paleontologists and the public for so long

The authors acknowledge that these weapons might also have been used for sexual selection, but that wouldn’t really change their conclusions. Additionally, these are correlations set up by data analysis—correlations do not immediately imply that one thing caused another.

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So if you ever find yourself being smacked in the face by the tail of some animal, just know: It would probably rather not be there either.

[Proceedings of the Royal Society B]

Update 3:55 PM EST: This post has been updated with a quote from Eric Gorscak.

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