Body Armor is Not Always for Protection

When we see an animal covered in scales and plates, we assume that it has evolved this armor to protect itself from predators. It seems obvious, which is probably why scientists had not really tested it... until now. And they found that it is not necessarily true.

Cordylid lizards are a family of small South African lizards that vary considerably from species to species with respect to their degree of body armor. Cordylid body armor is typically a combination of osteoderms (deep-skin scales or plates), epidermal armor (scales or plates made up of the top layer of the skin), and keratin spines. The spines help prevent predators from pulling the lizards out of the rock crevices they hide in. However, the function of the osteoderms and the epidermal armor had not been tested.

Researchers Chris Broeckhoven, Genevieve Diedericks and P. le Fras N. Mouton, at Stellenbosch University in South Africa set out to test whether this skin body armor evolved in these small lizards to protect them from predation from some of their most common predators: mongooses. They looked at sections of skins from five different cordylid lizards under the microscope to compare the thickness of the osteoderm armor and the thickness of the epidermal armor that each lizard species has. One of the five lizard species, the armadillo girdled lizard, is known to have the most body armor, so the researchers measured the skin of individuals of this species from three different locations. The researchers found that indeed, the armadillo girdled lizard had thicker osteoderm armor and epidermal armor compared to the other four lizard species. However, among the three populations of armadillo girdled lizards, one of them (from Namaqua National Park) had body armor that was similar to the other thinner-skinned lizards, and the other two (from Cederberg and Lambert's Bay) had much thicker armor, mostly due to extra-thick osteoderm plates.

The researchers also estimated the skin toughness of these lizards and the bite force of four different predatory mongoose species that live in these areas. The researchers did this by mounting mongoose skulls onto a force transducer, a device that measures how much force is being applied. They placed the upper jaws of the skulls on the lizard skins and gradually increased the pressure with a lever press until the teeth penetrated the skin. They used the amount of pressure needed for the mongoose teeth to penetrate the lizard skin as an estimate of that lizard's skin toughness.

The researchers found that whereas most of the lizards had skin that was equally tough, the armadillo girdled lizard had more skin toughness than all the other lizard species. Among the armadillo girdled lizards, those from Lambert's Bay had the toughest skin, those from Cederberg had medium-tough skin and those from Namaqua National Park had the least tough skin. When the researchers compared the lizard skin toughness to the mongoose bite forces, they found that none of the lizards had skin that was tough enough to withstand the bite of a large grey mongoose; only the armadillo girdled lizards from Lambert's Bay could withstand the bite of a meerkat; only the armadillo girdled lizards from Lambert's Bay and Cederberg could withstand the bite of a yellow mongoose; and only the armadillo girdled lizards from all three sites (and none of the other lizard species) could withstand the bite of a small grey mongoose.

The researchers concluded that the body armor does, in fact, help protect the armadillo girdled lizards from predatory mongooses. However, this does not appear to be true for any of the other lizard species studied. This raises the question: what do these other lizards have body armor for if it is not for protection from mongooses?

Having body armor can have its drawbacks. For example, thicker body armor significantly slows down cordylid lizards. This means that thick body armor could actually increase the animal's risk of getting eaten. Cordylid lizards generally sit at entrances of rock crevices waiting for tasty bugs to come by, allowing them to quickly escape into the rock crevices when predators come by. Because of this, the chance of successfully outrunning a predator depends on how far the lizard is from its retreat site. Most cordylid lizards stay very close to their retreat sites, but armadillo girdled lizards are adventurers that sometimes visit termite nests that are far away from their retreat sites. For armadillo girdled lizards to benefit from body armor, the armor needs to be thick enough to withstand the bites of the predators that are likely to catch them. Armadillo girdled lizards have also developed a strategy of curling up into an impenetrable ball, which forces predators to open their mouth wider to bite them. The wider the mouth is, the weaker the bite force, and the more likely the lizard is to get away unharmed.

Why did some armadillo girdled lizards have more body armor than others? Of the three sites where armadillo girdled lizards were measured, Namaqua National Park has the least vegetation and is the most exposed to aerial predators (as opposed to mongooses). Therefore, this group may benefit from having less body armor to allow them to run faster.

Because body armor has its costs, it is unlikely that the body armor in these other lizard species does not have some important function. It could, for example, protect lizards from aerial predators or venomous snakes, which this study did not test. It could also potentially play a role in regulating body temperature. These ideas all still need to be tested... Maybe you'll be the one to do it!

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Further reading:

Broeckhoven, C., Diedericks, G. and le Fras Mouton, P. What doesn't kill you might make you stronger: functional basis for variation in body armour, Journal of Animal Ecology, 84, 1213-1221 (2015). DOI: 10.1111/1365-2656.12414.

Image Credits:

Ouroborus cataphractus03.jpg by Handré Basson at Wikimedia Commons.

Photo of the force transducer and lever press set-up on the mongoose skull (secured with lots of foam) on a lizard skin. Photo from Broeckhoven et al., 2015.

Ouroborus cataphractus02.jpg by Handré Basson at Wikimedia Commons.