The roundtail horned lizard (Phrynosoma modestum) is one of the smaller species of horned lizard. Their specific epithet is from the Latin word modestum, meaning modest or calm. They are found in the United States, in western Texas, New Mexico eastern Arizona, southeastern Colorado and eight states in northcentral Mexico where they are referred to as “tapayaxtin“.
Roundtail horned lizards prefer a sandy, semiarid habitat with sparse vegetation, near harvester ant or especially honeypot ant colonies, which is their primary diet.
The color of roundtail horned lizards usually reflects the color of the soil in their native habitat. Most are uniformly grey in color, but they can also be light brown, or even a pale yellow. Often, darker-colored regions occur around the neck and groin, and sometimes striping on the tail.
They are short, flat, round-bodied lizards with short limbs, and small heads, which have a distinctive crest of nearly equal-length horns. Unlike other horned lizards, they lack a fringe of lateral scales and do not seek to flatten themselves to the ground to eliminate shadow; instead, they hunch their bodies into the shape of a rock for camouflage, boldly casting a shadow.
Their maximum size is 7 cm (2.7 in) snout-to-vent length, and 10.5 cm (4.3 in) total length.
Roundtail horned lizards are oviparous, breeding and laying eggs in early summer.