I studied effects of three risk factors, predator proximity, persistence in attacking, and speed, on latency to emerge after entering a refuge in the scincid lizard Eumeces laticeps. I simulated a predator by directly approaching a lizard until it took refuge, usually a tree hole. Costs per unit time of lost opportunities to forage or engage in other activities enhancing fitness did not vary with risk factors. Skinks remained in refuges longer when I stayed near the refuges than when I withdrew to a greater distance. At both distances, skinks monitored my presence visually from positions at or near refuge entrances. Skinks remained in refuges longer after the second of two successive similar approaches at the same speed than after the first, suggesting that they perceived increased risk due to persistence by an individual predator, but the assessment might have been based on attack rate without individual recognition. Latency to emergence increased with predator approach speed, giving another indication of response to higher risk. If lizards were approached twice in succession, latency to emergence was much greater when predator speed was faster during the second approach. When the second approach was slower, there was no significant difference in latency between trials. Risks associated with speed and persistence thus simultaneously affect risk assessment. A recent model (Sih, 1992) of emergence from refuge supposes that decisions affecting latency to emerge are based on a balance of costs of remaining in refuge (often predominantly costs of lost opportunities for feeding or mating) against risk of predation. All data are consistent with a generalized view of Sih's (1992) model, but studies are needed to assess effects of cost and limited information about predator presence.
Assessed risk for prey should depend not only on distance from a predator, but also on probability that it has been detected. If an indirectly approaching predator close enough to pose a threat suddenly turns toward prey, risk increases because the joint probability that the predator has detected and will attack is raised abruptly. I conducted field experiments on the effect of a predator turning toward prey on escape behaviour by simulating a predator, slowly approaching adult keeled earless lizards, Holbrookia propinqua, on linear paths that did not lead directly toward them, but bypassed them by various distances. At the minimum bypass distance for a path, I stopped and turned directly toward a lizard or directly away. The effect of turning varied dramatically with bypass distance. Regardless of tum direction, almost all lizards fled for very close approaches and no lizards fled for sufficiently large bypass distances. At intermediate distances a greater proportion of prey fled when I turned toward them than away from them. Possible cues used by lizards to assess risk are direction of turning per se and increased visibility of eye and facial features or other morphological correlates of orientation. Ydenberg & Dill's (1986) optimality model of escape behaviour can account for the results if prey can continually assess rapidly changing risk. By turning toward prey, an indirectly approaching predator may suddenly shift from outside to within the break-even point for risk and cost of escape, thereby eliciting flight.
Escape behaviour often differs between sexes, reproductive states and ages. Escape theory predicts that flight initiation distance (FID = predator–prey distance when escape begins) increases as predation risk and fitness increase, and decreases as cost of escaping increases. Similar predictions hold for distance fled and refuge entry, suggesting that age and sex differences in escape behaviour may occur when risk, fitness, and opportunity costs differ. I studied such differences in two lizard species and reviewed relevant literature on escape by lizards. In Sceloporus virgatus no difference occurred between sexes or female reproductive states in FID, distance fled, distance from refuge, or probability of entering refuge. In S. jarrovii juveniles had shorter FID and distance fled than adults; juveniles were closer than females to refuge, but this did not affect FID or distance fled. Juveniles were more likely than adults to be on rocks and use them as refuges. The literature review showed that sexual dimorphism in FID occurs in about 1/5 of species (male FID usually > female FID), but distance fled differed between sexes in only 1 of 21 species. Juveniles had shorter FID than adults in all of five species; the relationship between age and distance fled was highly variable. Reasons for patterns of age/sex differences are discussed. Because age and sex differences in these factors and escape strategy can alter multiple components affecting optimality, sometimes in opposite ways, these factors and escape strategy must be known to predict effects of age, sex and reproductive state on escape.
Lizard foraging activity is assessed quantitatively by measuring various aspects of activity including number of movements per minute (MPM), proportion of time spent moving (PTM), average speed over an observation period (AS), and speed while moving (MS). With some exceptions, single estimates are given without considering possible effects of environmental variables such as air temperature and time of day. Effects of distance between lizard and observer during observation, the standing distance, have not been studied previously. I measured foraging movements in a phrynosomatid ambush forager, Sceloporus virgatus, early in the breeding season to examine possible effects of sex, air temperature, time of day, and standing distance. Males had higher values of all four foraging movement variables than females. The major cause of this difference is presumably greater socially motivated movement by males, which have much larger home ranges than females. Decreased female foraging probably contributed to the difference because gravid and possibly late vitellogenic females reduce feeding and might become less mobile to reduce the likelihood of being detected by predators when a heavy clutch reduces running speed. Time of day and standing distance did not affect foraging movements. MPM, PTM, and AS decreased as temperature increased, but these effects were small. All pairs of MPM, PTM, and AS were positively correlated, but MS was correlated only with AS, and weakly. I discuss reasons for intraspecific and interspecific correlations among movement variables.