The Horned Features of Certain Lizards and Snakes: An Inquiry into Their Purpose

Horns are not only found on rhinos and cattle but also on some snake and lizard species, often adorning their heads with spiky, scaly accessories. However, the way these reptiles get their food may dictate if these protrusions are an advantage or a disadvantage.

A large percentage of horned lizards and snake species use an ambush strategy to catch their prey, rather than chasing them, according to a study published on November 22 in Biology Letters. Though horns and other projections may provide camouflage benefits for stationary animals, they might be a disadvantage for more active species, as they could reveal the reptiles to both prey and predators.

Lizards and snakes, collectively known as squamates, have developed horns on different parts of their bodies, including their heads, eyebrows, and snouts. Previous studies have suggested these formations might serve diverse functions, such as courtship, defense, or evasion. However, Federico Banfi from the University of Antwerp and his team wondered if the camouflage benefits of horns are still useful for reptiles that are more mobile when hunting.

The researchers analyzed previously published data that categorised lizards and snakes into two groups: those that wait and attack their prey and those that actively pursue them. There were 1,939 different species, 175 of which had horns—defined as bone or keratin projections on the animal's snout, eyebrows, or head.

Upon mapping the occurrence of horns and the reptiles’ hunting methods onto a previously published evolutionary tree, the researchers found that these protrusions evolved independently around 69 times. As they suspected, horns were more common in ambush predators. Out of the horned reptiles, 94 percent were ambush predators, and only 6 percent were identified as active predators.

Banfi noted that while horns might be beneficial for some species, they could be a drawback for others. Animals that need to move a lot might face disadvantages from having large protrusions over their heads, as these could make them more noticeable to their prey and predators.

Theo Busschau, an evolutionary biologist at New York University Abu Dhabi, agrees. Busschau and another researcher published a study in 2022 indicating a connection between different types of horns in vipers and their habitat choices.

If there’s no disadvantage for ambush predators to have these unusual protrusions, they might persist in a population for a long time. Over the course of evolution, there might be selection for these protrusions to evolve into horns, which could enhance an organism’s survival through improved camouflage, defense or mate selection, according to Busschau.

In the rare cases where prey-chasing species have horns, the benefits might outweigh the potential drawbacks faced by other reptiles that hunt actively, Busschau adds.

According to Banfi, understanding why horns do or don’t develop in the animal kingdom opens up exciting research opportunities. For example, the viper species Cerastes cerastes sometimes produces a mix of horned and hornless offspring, and the reason remains a mystery. Moreover, some amphibians and invertebrates have horn-like formations, suggesting that feeding strategies could play a role in their development.

Busschau suggested future research should aim to test the hypothesized evolutionary trade-offs related to horns. While these potential advantages and disadvantages are currently just hypotheses, thorough testing could help uncover the evolutionary origins of this interesting head accessory.