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Prey specificity and diet selection in a variable environment: Desert horned lizards (Phrynosoma platyrhinos) and ants.
Newbold, T. A. Scott *,1, MacMahon, James1, 1 Utah State University, Logan, UT
ABSTRACT- Diet selection is influenced by a complex set of factors that include prey availability, prey size, and prey defenses, among others. In conjunction with a larger study assessing horned lizard habitat associations, we examined the relationship between availability and selection of ants by the desert horned lizard, Phrynosoma platyrhinos, to determine: 1) the nature of horned lizard preference for harvester ants (e.g., generalist vs. specialist), and; 2) how spatial and temporal variability of ant abundance affects lizard prey preference. Ant species composition and abundance and lizard prey preference were determined using pitfall sampling and lizard scat dissection on 80 plots established along a 12km transect in western Box Elder Co., Utah, U.S.A. Ant availability varied with location (soil type and shrub community) and, to a lesser extent, season (late spring through late summer). Across the entire site, 14 of 24 ant species were represented in the lizard diet. However, 86% of that diet consisted of just 4 ant species (n=63 scat). Other insects, mostly beetles, comprised less than 5% of scat items. In contrast to previous studies (e.g., Rissing 1981, Collins 1988; but see Suarez et al. 2000), lizards did not show a disproportionate preference for harvester ants. Instead, larger ants (>5mm; including harvester ants) were preferentially selected over medium-sized ants (2-5mm) and smaller ants (<2mm) (73% vs. 26% vs. 1%, respectively). Of the medium-sized ants selected, species that were particularly abundant comprised larger proportions of the diet than less abundant species. Also, while diet composition varied considerably based on spatial location, diet selection on plots monitored through time remained relatively constant. Our results suggest that desert horned lizards in our area should be considered opportunistic foragers rather than harvester ant specialists. Moreover, ant body size and ant abundance largely determine desert horned lizard diet choice.
Key words: ant species composition, prey preference, Phrynosoma platyrhinos, diet selection