Horned lizards (Phrynosoma) live in the western US and central America. It's difficult to resist these little, spiky cuties. Had this one not been so lively, I would have used a whole roll of film on it. But as soon as I got close, he did a runner. Well, a scuttle.
It's thought that the horns on the back of the head are used in defence. (Seems like a silly place to have them in that case.) Kevin Young and a couple of generations of Edmund D. Brodies decided to investigate this. They used the flat-tailed horned lizard (Phrynosoma mccalli) as their study animal and concentrated on populations on the Barry M. Goldwater Air Force Gunnery Range near Yuma, Arizona. They compared the relative horn lengths of live animals with those hung up in shrike larders in creosote trees. The live ones had horns that were significant larger than those that the shrikes were saving for later on. Young et al. attributed the difference to directional selection.
I found this one on the edge of the road in western Arizona, when I was photographing Lake Havasu, the water body formed by damming of the Colorado River on the California-Arizona border. (If I tried to photograph the dam now, no doubt I'd be whisked away by the Department of Homeland Security.)
Of course, the lizard wasn't sitting on the edge of just any old road. This individual had a feel for popular culture.
Young, K.V., Brodie, jr, E.D. & Brodie III, E.D. (2004). How the horned lizard got its horns. Science 304: 65–66.