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Many descriptions of evolutionary adaptations 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 features 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 selection by loggerhead shrikes favors longer horns (fig. S1) in the flat-tailed horned lizard (Phrynosoma mcalli).
Predation is difficult to document in the wild. Some predators, however, leave behind explicit records of individual predation events that can be exploited to assay natural selection. Loggerhead shrikes (Lanius ludovicianus) often impale their prey onto thorns, twigs, and even barbed wire as a means of subduing their quarry (2). When shrikes attack homed lizards, they typically spear the lizard through the neck and pull off the soft tissue. What remains is a record of the successful shrike predation attempts marked by desiccated skulls of horned lizards hanging in trees and bushes (Fig. 1A).
We quantified selection (3, 4) on relative horn lengths of flat-tailed horned lizards by comparing the...