Why do some lizards and snakes have horns?

Horns aren’t just for rhinos and cattle. A fair number of snake and lizard species have heads adorned with spiky, scaly accessories. But how the reptiles snag food may determine whether having horns is an asset or a liability. 

The overwhelming majority of horned lizard and snake species lie in wait and ambush their prey rather than chase it down, researchers report November 22 in Biology Letters. Horns and other protuberances potentially provide camouflage to largely static animals but could be costly to more active reptiles, possibly revealing the bearer’s presence to prey and predators alike, the scientists say. 

Squamates — lizards and snakes — have repeatedly evolved horns atop their heads, on their eyebrows and jutting out from their snouts (SN: 6/9/20). Prior studies suggested these ornaments may have different functions, such as being used in courtship, defense or breaking up the body outline to evade detection. But Federico Banfi, a herpetologist at the University of Antwerp in Belgium, and his colleagues wondered whether horns’ camouflage benefits still help animals that move around a lot when hunting. If not, or if horns hinder the animal’s movements, this might discourage the protuberances from evolving in more active species.

The team compiled previously published datasets that classified lizards and snakes as either sit-and-wait predators or active pursuers, yielding 1,939 different species with 175 that had horns — defined as projections of bone or keratin on the animal’s snout, eyebrows or head. 

The team mapped the presence or absence of horns and the reptiles’ hunting style onto a previously published squamate evolutionary tree, finding the projections evolved independently about 69 times. Sure enough, horns were much more common in sit-and-wait predators than in their more active counterparts. Of horned squamates, 164 — 94 percent — were also relatively stationary ambush hunters, with just 11 — 6 percent — categorized as active predators. 

Horns might be a boon for some species, but a burden in others, Banfi says. “Animals that need to move a lot may be disadvantaged by possessing large appendages over their heads. These might make them more conspicuous to prey and predators because a structure that enlarges their head and silhouette may render them more visible while moving.”

The idea makes sense, says Theo Busschau, an evolutionary biologist at New York University Abu Dhabi. Busschau and a colleague published findings in 2022 that linked habitat preferences with different horn types in vipers. 

If weird protuberances aren’t a cost to sit-and-wait predators, they might passively persist in a population for a long time, 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. It’s important to consider the costs as well as the benefits when studying the evolution of a certain trait, and that there are trade-offs that might depend on an organism’s unique lifestyle.” 

For example, in the few exceptions where prey-chasing species have horns, “the benefits of having horns may simply outweigh the potential costs experienced by other active foraging reptiles,” Busschau says. 

Banfi thinks there are plenty of opportunities to delve into why horns do or don’t appear in the animal kingdom. For instance, the viper Cerastes cerastes lays eggs that sometimes hatch a mix of horned and hornless offspring, and it’s not clear why. And some amphibians and invertebrates have hornlike structures, so researchers could test whether feeding strategies could be a factor there, too.

The work Busschau would like to see done is directly testing the hypothetical evolutionary trade-offs animals make with their horns. “So far, the potential advantages and costs of horns in reptiles are only hypotheses,” Busschau says. It won’t be easy testing all these ideas, he says, but it could help researchers find the evolutionary roots of this wild headgear.