Desert Regal Horned Lizard (Phrynosoma solare)
Main sources: Stebbins, Robert, 2003, A Field Guide to Western Reptiles and Amphibians, Houghton Mifflin; Behler, John & Wayne King, 1979, The Audubon Society Field Guide to North American Reptiles and Amphibians, NY: Alfred Knopf. See also Sherbrooke, W.C., 1981, Horned Lizards: Unique Reptiles of Western North America, Globe, AZ: Southwest Parks and Monuments Association, and Sherbrooke & Schwenk, 2008, "Horned lizards (Phrynosoma) incapacitate dangerous ant prey with mucus", Journal of Experimental Zoology, Part A, Vol 309A, Issue 8, pp 447-459.
Horned Lizards (Iguanid lizards of the genus Phrynosoma) are particularly abundant in our Southwestern deserts, and unlike most lizards they specialize in consuming one type of food: ants. They are also somewhat bizarre in appearance, possessing a squat, tank-like body that is dorsoventrally compressed, short limbs, a short tail, a head with blunted snout and rear-directed cranial horns, and relatively large stomachs. (Sherbrooke & Schwenk, cited above p. 449)
All the variously colored Desert Horned Lizards shown in our images here (above & below) -- light gray, beige to reddish -- belong to the Regal species since they have four large horns at the back of the head with the bases of these horns in contact with one another. This is the main Horned Lizard residing in our immediate area around Cascabel, Arizona. The one shown below left from March 2005 is quite small (the length of the bar code on the pen is just under an inch (2.5cm); the one below right was fat from the ant-succulent summer of 2001 (click on each image to enlarge it.).
these two Regals below show, quite clearly, the whitish pineal (light-sensitive) gland at the back of the top of the head which is a characteristic of these lizards. Ignore the flash of light color above the eye on the figure at the right, which is distinct from the sunlight reflections further forward on the head. (Click on each image to enlarge it.)
The two shown below was one animal, lurking among the rocks in open sun atop a Cascabel Pasture ridge in the intense heat of a July day in 2000, and quietly "panting". Its tongue appeared considerably redder to the naked eye than appears in the photograph. This one was unusual in our experience because they usually blend more fully into their backgrounds. When encountered at any distance, Horned Lizards usually freeze, depending on their camouflage to remain unseen. This one froze at the approach of a large animal, but stood out visually like a sore thumb, having been caught on these reddish brown rocks. (Click on each image to enlarge it.)
Horned Lizards are often found near anthills, since ants are their main food source and they need to eat lots of them. Remarkably, these lizards are able to consume large quantities of one of the most aggressive and venomous stinging insects known, the Harvester Ants residing here in southeastern Arizona (see that link for a description of these ants). Sherbrooke & Schwenk (cited above) and other researchers have now shown that most of the unusual features of these lizards derive from the way they have evolved to prey specifically on these defensively-dangerous ants. Since ants are a small and nutritionally poor food source, the lizard must eat a great many while avoiding arousal of the ant colony (which would lead to being stung) and also avoiding movement that might attract their own predators. So they search out columns of the ants, whereupon they adopt an immobile stance and capture ants "with exceptional speed by using precisely targeted tongue protrusion and rapid prey handling" (ibid., p.449). Before the ant column begins to respond with ant-typical "mobbing" behavior, the lizards cease that particular situation and move on to find another column.
Other lizards do eat these kinds of ants occasionally, but when they do so they rapidly bite and chew the prey before swallowing (a time-consuming process for such low-nutrition prey), whereas the Horned Lizards swallow their ants quickly and whole and can collect large numbers at one sitting. The question arises: how do they do this without incurring severe stings in their mouths, throats, and even guts? Sherbrooke & Schwenk show that when Horned Lizards swallow each ant, the prey is moved rapidly past the teeth and deep into the throat, a probably-unique ingestion procedure and one which simultaneously curls the prey into a ventrally-compact ball completely encased with strands of mucus that immobilize all body parts including stingers. Horned lizards produce this mucus in distinctly specialized cells distributed from their tongue down through the throat.
Since x-ray videography shows that these ants remain alive while being swallowed and presumably also survive for some time in the gut, this mucus immobilization process is probably crucial for Horned Lizard survival under this adaptation, and the researchers suggest that Horned Lizards and Harvester Ants have probably undergone a long process of predator-prey co-evolution here in the Southwest.