Strong Artifactual Effect of Starting Distance on Flight Initiation Distance in the Actively Foraging Lizard Aspidoscelis exsanguis

Abstract

Escape theory predicts flight initiation distance (FID), distance between predator and prey when escape begins, for factors affecting costs of fleeing and not fleeing. Starting distance (SD), distance between predator and prey when approach begins, appears to affect FID for some species and risk levels, but not others. I predicted that in an active forager under low risk FID would appear to increase as SD increases due to (1) inclusion of SDs shorter than optimal FID and (2) spontaneous movements not motivated by escape. I predicted that FID and SD are unrelated at low risk when SDs shorter than optimal FID and spontaneous movements are excluded. When I approached the lizard Aspidoscelis exsanguis, an active forager, all predictions were confirmed. Effects of short SD and spontaneous movement might account for relationships between FID and SD in some birds and lizards. Nevertheless, as a predator approaches, costs of monitoring may increase, as may assessed risk and risk-averse errors due to shorter available assessment time. Risk-aversion might account for restriction of the relationship between FID and SD to high risk levels in an ambushing lizard species, for which spontaneous movements are far less frequent than for active foragers such as A. exsanguis. Although FID and SD are unrelated in A. exsanguis, SD and the related alert distance can provide valuable insights into risk assessment in other taxa having escape movements distinct from other movements. Present findings emphasize the importance of distinguishing between escape and other movements that may be initiated during approaches. A previously untested prediction of escape theory is that escape begins immediately when a prey detects an approaching predator closer than optimal FID, leading to the prediction of a slope of 1.0 between FID and SD. This prediction was confirmed for SDs less than 1.5 m.