Foraging Modes of Some American Lizards: Relationships among Measurement Variables and Discreteness of Modes

• Published in: Herpetologica 2001-03, Vol.57 (1), p.65-76
• Main Authors: Cooper, William E., Vitt, Laurie J., Caldwell, Janalee P., Fox, Stanley F.
• Format: Article
• Language: English
• Subjects: Body temperature; Canyons; Ecological genetics; Evolution; Foraging; Herpetology; Lizards; Phylogeny; Skinks; Taxa
• ISSN: 0018-0831; 1938-5099

We made quantitative field observations of 16 species of American lizards to characterize foraging movements and modes. Consistent with previous qualitative assessments and published data for their families, five teiid and a scincid species were active foragers, whereas eight phrynosomatid, a crotaphytid, and a gekkonid species were ambush foragers. In addition to traditional measures of foraging mode [i.e., number of movements per minute (MPM) and proportion of time spent moving (PTM)], we calculated the proportion of attacks on prey discovered while moving (PAM) as an independent index of the relative importance of ambush and active foraging. Active foragers had significantly higher number of movements per minute (MPM), proportion of time spent moving (PTM), and proportion of attacks on prey discovered while moving (PAM) than did ambushers. For these species, foraging modes are dichotomous, with large gaps separating their PTM and PAM values. MPM values overlapped between modes. Although MPM is less useful than the PTM and PAM for assigning foraging modes, we advocate its continued use for two reasons. It is the best measure for studies of factors affecting foraging activity in ambushers because it measures frequency of movement, which, along with duration and speed, is one of three key ways in which animals vary foraging movements. Comparisons of our data with published figures for related taxa showed that PTM values agreed closely for ambush foragers and were in broad agreement for active foragers where direct comparisons could be made. Factors that may explain discrepancies are differences among species, sites (in food density and temperature), and observational protocols. With data points considered independent, PTM and PAM were almost perfectly correlated. The other pairs of variables were correlated, but less closely. No correlated evolution between variables was found using independent contrasts. PTM and PAM are the best single indicators of foraging mode.