How did the horned lizard get its horns?

IN THEIR BREVIA “HOW THE HORNED LIZARD got its horns,” K. V. Young et al. present an important example of natural selection in the wild, suggesting that loggerhead shrike predation drove the evolution of elongated horns in the flat-tailed horned lizard (2 Apr., p. 65). Although the authors acknowledge that selective forces other than shrike predation may also be involved, they make no mention of the possibility that one of these potential forces could have been the first instigator of the directional selection for horn elongation. Under this hypothetical scenario, the horns would have then only subsequently served to reduce shrike predation. Other likely cases of preadaptation [or exaptation (1)] have been described in vertebrates (2–5), some of which involve important transitions in evolutionary history. Perhaps the role of preadaptation in evolution is of great importance and is deserving of more widespread appreciation. Given the possibility of a preadaptation scenario in the evolution of crown horns in horned lizards, I find it ironic that Young et al. commented on the weakness of “just-so stories” (6) and also chose a title that reads remarkably like the titles of Kipling’s stories. Until presented with evidence suggesting that the horns were mere nubs until the onset of shrike predation, I will remain convinced that “How the horned lizard got its horns” is a poor choice for what is presumably meant to be an informative title. WILLIAM R. FOUTS Department of Biology, Nevada State College, 1125 Nevada State Drive, Henderson, NV 89015, USA.