The foraging mode controversy: both continuous variation and clustering of foraging movements occur

Abstract

Ambush and active foraging are held to be discrete foraging modes having important theoretical consequences and evolutionary correlates, but foraging variables (number of movements per minute (MPM) and per cent time spent moving (PTM)) vary continuously. Whether foraging movements are unimodally or bimodally distributed is controversial. Foraging modes have been crucial for understanding taxonomic, morphological, ecological, and behavioural diversification of lizards. Extensive data for them are used to examine the modality-continuity controversy. MPM and PTM are lognormally distributed, confirming continuity and unimodality. By separating species into groups of ambush and active foragers, bivariate cluster analysis supports the view that lizards fall into active and ambush modes that may reflect adaptive peaks of joint MPM and PTM values. Phylogenetic analysis is needed to test relationships between variables. Although it has greatly advanced understanding of relationships between foraging and other variables, a two-mode paradigm is too simplistic. Modes defined solely by MPM and PTM may obscure differences in speed and search methods. Overlap between clusters might occur in nearly unstudied major taxa that forage differently. On the MPM×PTM plane connections among ambush, active, and saltatory (start-stop) foraging are apparent, suggesting that saltatory foragers may occur in lizards. Controversies about continuity, bimodality, and dichotomy of modes are resolved. Clusters represent common combinations of unimodally distributed MPM and PTM occupying limited regions of foraging space. Some species fall outside traditional modes, but examination of correlations between clusters and other variables remains a viable approach.