Buffalo have horns, sure, and so do rhinos. But reptiles? Some types of lizards and snakes do have spiky structures on their heads. How they catch dinner could determine whether those horns are a help or a hindrance.
Most horned lizards and snakes don’t chase down animals for food. Instead, they lie in wait to ambush their prey. When standing still, horns can help animals blend into their environment. But they could make more active reptiles stand out. Any showy headgear could reveal their presence to prey and predators alike, scientists now conclude. They reported their new findings November 22 in Biology Letters.
A number of reptiles have evolved horns on their heads, eyebrows or snouts. Prior studies suggested the animals may use these ornaments in courtship, for defense or sometimes for avoiding detection. But what about when the animals search for prey, wondered Federico Banfi. He’s a herpetologist at the University of Antwerp in Belgium.
Banfi was part of a team that decided to study whether the camouflage (KAM-oh-flaaj) benefits of horns helped reptiles that move a lot while hunting. If not, they reasoned, the active species might be less likely to evolve such spiky bumps.
Costs vs. benefits
The team pulled together published data that sorted lizards and snakes as either sit-and-wait predators or active pursuers. They found info on 1,939 different species. Of those, 175 had horns on the snout, eyebrows or head. Those horns were made of either bone or keratin (CAER-uh-tin).
The team then mapped two features — hunting style and presence (or absence) of horns — onto a previously published lizard-snake evolutionary tree. Horns evolved independently about 69 times, they found.
As expected, horns were far more common in sit-and-wait predators. Among horned species, 164 (94 percent) were relatively still, ambush-type predators. Just 11 (6 percent) were active hunters.
Horns might help some species, but burden others, Banfi says. “Animals that need to move a lot may be disadvantaged by possessing large appendages over their heads.” While moving, those spikes may make them more visible to prey, he says.
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The idea makes sense, says Theo Busschau. He’s an evolutionary biologist at New York University’s campus in Abu Dhabi, the capital of the United Arab Emirates. Busschau and a colleague published an earlier study that linked preferred habitat with different horn types in vipers.
Such spiky growths might persist in a population of sit-and-wait predators for a long time. Why? There isn’t a costly downside, he says.
“Over evolutionary time, there may be selection for these projections to form horns that could increase an organism’s fitness by enhanced camouflage, defense or mate selection,” Busschau says.
But he says it’s important to consider the costs as well as benefits when studying the evolution of a certain trait. “There are trade-offs that might depend on an organism’s unique lifestyle.” Still, some prey-chasing species do have horns. In those, Busschau says, “the benefits of having horns may simply outweigh the potential costs experienced by other active-foraging reptiles.”
There are plenty of options in the animal kingdom to study why horns do or don’t occur, Banfi thinks. For one, eggs of the viper Cerastes cerastes sometimes yield a mix of horned and hornless offspring. It’s not clear why. Some amphibians and invertebrates also have hornlike structures. Researchers could test whether feeding strategies might be a factor there, too.
As for Busschau, he would like to see tests of the proposed evolutionary trade-offs that animals make with their horns.
“So far, the potential advantages and costs of horns in reptiles are only hypotheses,” Busschau says. Testing all those ideas won’t be easy, he notes. But it might help researchers unearth the evolutionary roots of this showy headgear.