Why the horned lizard has horns: More than a just-so story

Shrikes are basically nature’s version of Vlad the Impaler. While less gory birds feed on nuts and others peck at insects, shrikes impale their prey onto sharp spikes. Once the unfortunate animal is firmly attached and appropriately subdued, shrikes then tear their prey apart. The result is an array of dismantled corpses of lizards, small mammals, and birds dangling from spiny trees or barbed wire – not necessarily a sight you’d be pleased to finding in your backyard.

Loggerhead shrike and prey
Left: A loggerhead shrike eating prey off of barbed wire. Right: A lizard that fell prey to a shrike. Both images from Wikimedia commons (left, right).

But you might be excited if you’re an evolutionary biologist. In the cases of Kevin Young, Edmund Brodie Jr. and Edmund Brodie III, they saw that one specific species of shrike, the loggerhead shrike, could be extremely useful to their research. These researchers were interested in one particular species that the shrikes prey upon, the flat-tailed horned lizard, which is found in the Sonoran desert. This lizard has a distinctive crown of large horns on its head. Biologists had previously assumed these horns served a defensive purpose against predators. The rationale was that it would be too energetically costly to lizards to grow and carry the heavy horns for no reason, so the horns were probably useful somehow; ergo, the horns likely helped the lizards avoid being eaten.

https://www.flickr.com/photos/californiadfg/7421150358
A flat-tailed horned lizard. Photo by California Department of Fish and Wildlife on Flickr.

The problem is that while explanations like this make a great deal of intuitive sense, they often have little or no evidence backing them up – sometimes garnering the uncomplimentary descriptor of ‘just-so’ stories, after Rudyard Kipling’s fanciful children’s tales. Many of these explanations remain no more than just-so stories because they’re extremely difficult to test scientifically. It is often impractical or even impossible to prove that a particular trait has evolved for a specific purpose, no matter how appealing the idea is.

However, Young and colleagues tried to do just that by taking advantage of the shrikes’ hunting style. As shrikes leave distinctive records of their kills–dangling lizard corpses are unique to shrike predation, since no other animal hunts in the same way–the biologists could simply find shrike-killed lizards and measure their horns. Since the horns are bone, they stay intact, even as the rest of the lizard’s body decomposes. The scientists then compared those measurements to ones taken from horned lizards that were still alive to see whether shrike-killed lizards had differently sized horns.

Lizards that hadn’t been eaten by shrikes had horns that were about 10% longer than those of shrike-killed lizards. This was a difference of less than one millimeter for the smaller horns, and about two millimeters for the very largest horns, which doesn’t sound like much, but is a relatively large difference for a small-bodied lizard (and its small-bodied bird predator).

These results show that natural selection favors lizards with longer horns because they’re less likely to be eaten by shrikes than lizards with shorter horns. While these results may seem obvious (of course longer horns will make it harder for a predator to get close enough to attack an animal!), Young and colleagues turned the ‘just-so’ story into empirical data. While it’s still impossible to prove that the horns evolved in the first place to help lizards avoid predation (we have to wait for time machines to be invented for that), these biologists provide compelling evidence to support this idea. Natural selection is occurring: the lizards’ horns are getting longer as a response to the bloodthirsty shrikes. It’s worth noting that this research is relatively old–published in 2004. In today’s genomic era, many biologists identify traits under natural selection using genome sequences, but they are often still interested in accomplishing the same thing that Young and colleagues did: linking that selection back to a cause.

Find Young’s paper here: Young KV, Brodie Jr. ED, Brodie III ED. 2004. How the horned lizard got its horns. Science 304(2): 65.

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