Prey Preferences in the Desert Horned Lizard: Influence of Prey Foraging Method and Aggressive Behavior

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Abstract
Desert horned lizards Phrynosoma platyrhinos consumed the solitary—foraging seed—harvester ant, Pogonomyrmex califonicus, 10 to 100 times more often than expected based upon its relative abundance in the environment. The group—foraging seed—harvester ants, Pogonomyrmex rugosus and Veromessor pergandei, were correspondingly underrepresented in the horned lizard's natural diet. In a series of laboratory prey choice tests, where prey availabilities were held constant and equal, these preferences were confirmed: P.californicus was eaten more often than P. rugosus which was eaten more often than V. pergandei. These preferences correspond to differences in ant aggressive behavior which, in turn, correspond to differences in ant foraging—group size and foraging method. Colonies of solitary—foraging ants have small foraging groups and are generally incapable of mobbing a lizard, but they are also difficult to find. Colonies of group—foraging ants have large to massive foraging groups and vigorously mob lizards. Such aggressiveness is probably effective by increasing the risk of exposure of a lizard to predators of its own; the indirect costs of preying upon these ant species, normally assumed constant over all prey types for most predators, are variable here. Differences in experience with ant aggressive behavior probably account for differences in prey preferences found among adult lizards captured from the same area. Ant—species—specific predation by Phrynosoma may be an important ecological force structuring the Mohave Desert seed—harvesting ant community.