Ants don't make very good meals. They're tiny, not all that nutritious and many have evolved an array of morphological, chemical and behavioral adaptations to make predators pay. Horned lizards (Phrynosoma spp.) are unique in that their diet consists almost entirely of the nastiest ants imaginable; Harvester ants (Pogonomyrmex spp.). Native to the American southwest, these ants have powerful mandibles, a stinger tipped with potent venom, and a propensity to swarm all over attacking predators. However, horned lizards suck down these ants whole by the hundreds. How can these lizards gulp down intact (and almost certainly very agitated) venomous ants without incurring painful bites and stings all over the insides of their mouths? Wade Sherbrooke and Kurt Schwenk from the Univeristy of Connecticut recently addressed this question in The Journal of Experimental Zoology using a combination of high-speed videography, anatomy, and the natural historian's classic tool, stomach content analysis.
First, the team brought horned lizards into the lab and filmed them gulping down their prey. Usually, lizards will snag prey with their tongues, crush them with their teeth, then slowly move the prey through the mouth to the back of the throat before swallowing. Each eating phase has clear time boundaries in most lizards. Horned lizards have evolved a different feeding strategy. They grab prey items with their tongue, then in one extremely fast motion (∼30 ms) roll the prey directly into their esophagus. This kinematic pattern is unique among lizards studied to date.
Next, Sherbrooke and Schwenk wanted to see what prey items look like after being snapped up by horned lizards. To do this, the team examined ants in the stomachs of freshly road killed animals. They found each individual ant to be wrapped up in a compact ball of mucus; the ants' mandibles, stingers and limbs were totally immobilized. This observation suggests that horned lizards actually roll their prey up in mucus as they suck them down.
Finally, the team went looking for mucus-producing cells in the mouths of captured horned lizards. They found `thick carpets' of highly folded mucus-producing structures lining the floor of the animals' mouths. Furthermore, the authors could find no evidence that other lizards have the same degree of surface area devoted to mucus secretion. Compared with other lizard species, horned lizards clearly have a huge advantage when it comes to mucus production.
Sherbrooke and Schwenk provide several lines of evidence for a unique use of mucus by a predator. The team provides strong circumstantial evidence for a mucus prey-binding system that no other vertebrate has. We don't usually think of mucus as useful, but the work of Sherbrooke and Schwenk reminds us that it's actually pretty handy stuff. In fact, it can be totally crucial in an animal's life. More importantly, this work should serve to remind us that animals can evolve to use almost anything as a weapon – even something as innocuous as mucus.
- © The Company of Biologists Limited 2008