How the Horned Lizard Got Its Horns

Many descriptions of evolutionary adapta-tions are criticized as “just-so stories ” (1) that are based more on intuition than on direct tests of adaptive hypotheses. The elaborate crowns of horns possessed by many species of horned lizards (genus Phrynosoma) are classic examples of intuitively adaptive fea-tures that lack direct tests of function. The bony horns that give horned lizards their name are presumed to function as a defense against predators (Fig. 1B). Here we present data from the wild showing that natural se-lection by loggerhead shrikes favors longer horns (fig. S1) in the flat-tailed horned lizard (Phrynosoma mcalli).