How the Horned Lizard got its Horns- supporting online material

The general rugose appearance and horny margin of the body and head of horned lizards are thought to increase crypsis by disrupting the outline of the body form against the background (S1, S2). Published accounts also assert that horns are used to repel or prevent ingestion by predators including other lizards, snakes, mammals and birds (S3-S5). Anecdotal observations suggest that predators occasionally die after eating horned lizards when the horns lodge in the throat (S6, S7), but such occurrences do not offer a fitness advantage to the prey, since they too die. A quantitative relationship (measurable as a covariance (S8, S9) between variation in horn length and the probability of survival in interaction with predators is necessary to establish selection on horn length. Moreover, an understanding of how the horns function to generate such a relationship is fundamental to determining the cause of selection and therefore adaptation (S9-S11). We quantified selection on relative horn lengths of flat-tailed horn lizards, by comparing measurements of live lizards with those from skulls of lizards killed by shrikes and collected from their repository on bushes (primarily creosote). Collections were made on the Barry M. Goldwater Air Force Gunnery Range outside Yuma, Arizona, in 1996 and 1997. Skulls (n = 29