Lower Predation with Increasing Altitude in the Mesquite Lizard Sceloporus grammicus

We evaluated differential predation pressure on the lizard Sceloporus grammicus at 3 sampling sites along a high-mountain gradient, using 3 methods of assessment: attacks on plasticine lizard models, frequency of lizard tail autotomy, and estimation of the richness of potential predators. We placed a total of 720 lizard models at each sampling site (2600, 3100, and 4150 m asl) on 6 d during the reproductive season and on 6 d during the nonreproductive season. Each day we placed 60 models (n = 30 for each sex, with each sex having 10 each of 3 gular color morphotypes [gray, yellow, and orange]) at lizards' previously observed basking sites. Additionally, we conducted 20 mark-recapture sampling visits at each site from 2014 to 2019 to determine the number of individuals with tail autotomy; we also made observations to estimate the richness of potential predators and supplemented our observations with information from the available literature. We expected (1) an inverse effect of altitude on predation pressure due to a possible decrease in the richness of potential predators with altitude, and (2) differential predation pressure according to sex and gular morphotype due to differential behavioral and morphological conspicuity. Our findings support a decrease in predation pressure with altitude according to our 3 methods of assessment. However, we did not find evidence for an effect of sex or color of gular morphotype on predation pressure, neither from the number of attacks on the plasticine models nor from the frequency of tail autotomy. Lower predation with increasing altitude could be due to the following combined effects: lower richness of predators, lower visibility of lizards because of less contrast of their bodies with the substrate, and reduced locomotor, foraging, and social movements due to thermal restriction.