Escape and alerting responses by Balearic lizards (Podarcis lilfordi) to movement and turning direction by nearby predators

Escape theory predicts that prey monitoring an approaching predator delay escape until predation risk outweighs costs of fleeing. However, if a predator is not detected until it is closer than the optimal flight initiation distance (FID = distance between predator and prey when escape begins), escape should begin immediately. Similarly, if a change in a nearby predator’s behavior indicates increased risk, the optimal FID increases, sometimes inducing immediate escape. If a predator that has been standing immobile near a prey suddenly turns toward the prey, greater risk is implied than if the predator turns away. If the immobile predator suddenly moves its foot without turning, it might be launching an attack. Therefore, we predicted that frequency of fleeing and preparation to flee are greater when a predator turns toward than away from prey and that frequency of fleeing when a predator suddenly moves decreases as distance between predator and prey increases. We verified these predictions in the Balearic lizard Podarcis lilfordi in field experiments in which an investigator simulated the predator. Lizards fled and performed alerting responses indicating readiness to flee more frequently when the predator turned toward than away from them, and fled more frequently the nearer the predator.     

Effect of Risk on Aspects of Escape Behavior by a Lizard, Holbrookia propinqua, in Relation to Optimal Escape Theory

Prey must balance gains from activities such as foraging and social behavior with predation risk. Optimal escape theory has been successful in predicting escape behavior of prey under a range of risk and cost factors. The optimal approach distance, the distance from the predator at which prey should begin to flee, occurs when risk equals cost. Optimal escape theory predicts that for a fixed cost, the approach distance increases as risk increases. It makes no predictions about approach distance for prey in refuges that provide only partial protection or about escape variables other than approach distance, such as the likelihood of stopping before entering refuge and escape speed. By experimentally simulating a predator approaching keeled earless lizards, Holbrookia propinqua, the predictions of optimal escape theory for two risk factors, predator approach speed and directness of approach were tested. In addition, predictions that the likelihood of fleeing into refuge without stopping and the speed of escape runs increase with risk, in this case predator approach speed, and that lizards in incompletely protective refuges permit closer approach than lizards not in refuges were also tested. Approach distance increased with predator approach speed and directness of approach, confirming predictions of optimal escape theory. Lizards were more likely to enter refuge and ran faster when approached rapidly, verifying that predation risk affects escape decisions by the lizards for escape variables not included in optimal escape theory. They allowed closer approach when in incompletely protective refuges than when in the open, confirming the prediction that risk affects escape decisions while in refuge. Optimal escape theory has been highly successful, but testing it has led to relative neglect of important aspects of escape other than approach distance.     

Optimal escape theory predicts escape behaviors beyond flight initiation distance： risk assessment and escape by striped plateau lizards Sceloporus virgatus

Escape theory predicts that flight initiation distance (FID=distance between predator and prey when escape begins) is longer when risk is greater and shorter when escape is more costly. A few tests suggest that escape theory applies to distance fled. Escape models have not addressed stochastic variables, such as probability of fleeing and of entering refuge, but their economic logic might be applicable. Experiments on several risk factors in the lizard Sceloporus virgatus confirmed all predictions for the above escape variables. FID was greater when approach was faster and more direct, for lizards on ground than on trees, for lizards rarely exposed to humans, for the second of two approaches, and when the predator turned toward lizards rather than away. Lizards fled further during rapid and second consecutive approaches. They were more likely to flee when approached directly, when a predator turned toward them, and during second approaches. They were more likely to enter refuge when approached rapidly. A novel finding is that perch height in trees was unrelated to FID because lizards escaped by moving out of sight, then moving up or down unpredictably. These findings add to a growing body of evidence supporting predictions of escape theory for FID and distance fled. They show that two probabilistic aspects of escape are predictable based on relative predation risk levels. Because individuals differ in boldness, the assessed optimal FID and threshold risks for fleeing and entering refuge are exceeded for an increasing proportion of individuals as risk increases[Current Zoology 55(2):123-131,2009].     

Complex Relationships amongst Parasite Load and Escape Behaviour in an Insular Lizard

Economic escape models predict escape decisions of prey which are approached by predators. Flight initiation distance (FID, predator–prey distance when prey begins to flee) and distance fled (DF) are major variables used to characterize escape responses. In optimal escape theory, FID increases as cost of not fleeing also increases. Moreover, FID decreases as cost of fleeing increases, due to lost opportunities to perform activities that may increase fitness. Finally, FID further increases as the prey's fitness increases. Some factors, including parasitism, may affect more than one of these predictors of FID. Initially, parasitized prey may have lower fitness as well as impaired locomotor ability, which would avoid predation and/or reduce their foraging ability, further decreasing the opportunity of fleeing. For example, if parasites decrease body condition, prey fitness is reduced and escape ability may be impaired. Hence, the overall influence of parasitism on FID is difficult to predict. We examined relationships between escape decisions and different traits: parasite load, body size and body condition in the Balearic lizard, Podarcis lilfordi. Lizards that showed higher haemogregarines load had longer FID and shorter DF. Although results did not confirm our initial predictions made on the basis of optimal escape theory, our findings suggest that parasites can alter several aspects of escape behaviour in a complex way.     

Costs of Refuge Use Affect Escape Decisions of Iberian Rock Lizards Lacerta monticola

Theoretical models of anti-predator escape behaviour suggest that prey may adjust their escape response such that the optimal flight distance is the point at which the costs of staying exceed the costs of fleeing. Anti-predatory decisions should be made based also on consequences for long-term expected fitness, such as the costs of refuge use. For example, in lizards, the maintenance of an optimal body temperature is essential to maximize physiological processes. However, if unfavourable thermal conditions of refuges can decrease the body temperature of lizards, their escape decision should be influenced by refuge conditions. Analyses of the variation in flight distances and emergence latency from a refuge for the lizard Lacerta monticola under two different predation risk levels, and their relationship with the thermal environment, supported these predictions. When risk increased, lizards had longer emergence latencies, and thus costs of refuge use increased (a greater loss of time and body temperature). In the low-risk situation, lizards that were farther from the refuge had longer flight distances, whereas thermal conditions were less important. When risk increased, lizards had longer flight distances when refuges were farther off, but also when the external heating rate and the refuge cooling rate were lower. The results suggest that, in addition to the risk of predation, expected long-term fitness costs of refuges can also affect escape decisions.     

Plesiomorphic Escape Decisions in Cryptic Horned Lizards (Phrynosoma) Having Highly Derived Antipredatory Defenses

Escape theory predicts that the probability of fleeing and flight initiation distance (predator–prey distance when escape begins) increase as predation risk increases and decrease as escape cost increases. These factors may apply even to highly cryptic species that sometimes must flee. Horned lizards (Phrynosoma) rely on crypsis because of coloration, flattened body form, and lateral fringe scales that reduce detectability. At close range they sometimes squirt blood‐containing noxious substances and defend themselves with cranial spines. These antipredatory traits are highly derived, but little is known about the escape behavior of horned lizards. Of particular interest is whether their escape decisions bear the same relationships to predation risk and opportunity costs of escaping as in typical prey lacking such derived defenses. We investigated the effects of repeated attack and direction of predator turning on P. cornutum and of opportunity cost of fleeing during a social encounter in P. modestum. Flight initiation distance was greater for the second of two successive approaches and probability of fleeing decreased as distance between the turning predator and prey increased, but was greater when the predator turned toward than away from a lizard. Flight initiation distance was shorter during social encounters than when lizards were solitary. For all variables studied, risk assessment by horned lizards conforms to the predictions of escape theory and is similar to that in other prey despite their specialized defenses. Our findings show that these specialized, derived defenses coexist with a taxonomically widespread, plesiomorphic method of making escape decisions. They suggest that escape theory based on costs and benefits, as intended, applies very generally, even to highly cryptic prey that have specialized defense mechanisms.     

Flush early and avoid the rush? It may depend on where you stand

Optimal escape theory predicts that animals should flee at an optimal distance from the approaching predator (flight initiation distance, FID). However, FID usually increases with increasing alert distance (AD) or starting distance (SD). As an explanation for this pattern, the “flush early and avoid the rush” (FEAR) hypothesis states that prey should escape soon after detecting an approaching predator due to the cost of continuously monitoring risk. However, the positive relationship observed may also result from a mathematical artefact. Meanwhile, it is unknown whether animals would consistently follow this rule in different environmental contexts. We explored escape behaviours in light-vented bulbuls (Pycnonotus sinensis) perched at different heights. FID generally increased with increasing AD and decreasing perch height. The positive relationships between AD and FID were outside the 95% confidence levels of simulated slopes from Monte Carlo simulations, suggesting that the relationships observed reflect biological effects rather than merely a mathematical artefact. Increasing perch height was also associated with longer buffer distance (defined as FID minus AD or SD), suggesting that the birds tend to delay their flush after detecting an approaching predator when perched high. The effects of environmental contexts (and the associated predation risk) on the AD-FID relationship should be considered when performing inter-specific comparisons or meta-analyses.     

Influence of Some Potential Predation Risk Factors and Interaction between Predation Risk and Cost of Fleeing on Escape by the Lizard Sceloporus virgatus

Escape theory predicts that flight initiation distance (predator–prey distance when escape begins) increases as predation risk increases and decreases as cost of fleeing increases. Scant information is available about the effects of some putative predation risk factors and about interaction between simultaneously operating risk and cost of fleeing factors on flight initiation distance and distance fled. By simulating an approaching predator, I studied the effects of body temperature (BT), distance to nearest refuge, and eye contact with a predator, as well as simultaneous effects of predator approach speed and female presence/absence on escape behavior by a small ectothermic vertebrate, the lizard Sceloporus virgatus. Flight initiation distance decreased as BT increased, presumably because running speed increases as BT increases, facilitating escape. Distance to nearest refuge was unrelated to BT or flight initiation distance. Substrate temperature was only marginally related, and air temperature was not related to flight initiation distance. Eye contact did not affect flight initiation during indirect approaches that bypassed lizards by a minimum of 1 m, but an effect of eye contact found in other studies during direct approach might occur. Predator approach speed and presence of a female interactively affected flight initiation distance, which increased as speed increased and decreased when a female was present. In the presence of a female, flight initiation distance was far shorter than when no female was present. The high cost of forgoing a mating opportunity accounts for the interaction because the difference between female presence and absence is greater when risk is greater.     

Flight initiation distance and escape behavior in the black redstart (Phoenicurus ochruros)

There are many anti‐predatory escape strategies in animals. A well‐established method to assess escape behavior is the flight initiation distance (FID), which is the distance between prey and predator at which an animal flees. Previous studies in various species throughout the animal kingdom have shown that group size, urbanization, and distance to refuge and body mass affect FID. In most species, FID increases if body mass, group size or distance to refuge decreases. However, how age and sexual dimorphism affect FID is rather unknown. Here, we assess the escape behavior and FID of the black redstart (Phoenicurus ochruros), a small turdid passerine. When approached by a human, males initiated flights later, that is allowing a closer approach than females. Males of this species are more conspicuous, and therefore, may exhibit aposematism to deter potential predators or are less fearful than females. Additionally, juveniles fled at shorter distances and fled to lower heights than adults. Lastly, concerning escape strategy, black redstarts, unless other passerine birds, fled less often into cover, but rather onto open or elevated spots. Black redstarts are especially prone to predation by ambushing predators that might hide in cover. Hence, this species most likely has a higher chance of escaping by fleeing to an open spot rather than to a potentially risky cover.     

Ecological factors affecting flight initiation distance in Daurian ground squirrels (Spermophilus dauricus)

More often than not, animals forage under predation risk. Foragers, therefore, face a challenge to balance between two conflicting tasks, namely energy intake and safety. Flight initiation distance (FID, defined as the distance between a prey and a predator when the prey starts to flee) has been widely measured in many taxa to study such economic trade‐offs. However, FID may also be affected by limitations on the prey's ability to detect predators, especially when there is visual obstruction caused by surrounding vegetation. Although both vegetation cover and vegetation height may contribute to such obstruction, the effect of vegetation height on FID has not been well studied. In this study, we explored the effects of vegetation height, vegetation cover and distance to refuge on FID in free‐living Daurian ground squirrels (Spermophilus dauricus) inhabiting a grassland in Inner Mongolia, China. Multiple linear regressions suggested that both vegetation height and distance to refuge significantly affected FID in S. dauricus. Ground squirrels fled earlier when vegetation was low or when foraging farther away from a refuge. No significant effect of vegetation cover on FID was detected. Our results have implications for ecologically based pest control, and FID may be used as an effective and easy‐to‐use behavioral indicator in wildlife management.     

Dynamic Risk Assessment: Prey Rapidly Adjust Flight Initiation Distance to Changes in Predator Approach Speed

The pre‐eminent model of flight initiation distance assumes that the function relating predation risk to distance between predator and prey is constant. However, the risk–distance function can change dramatically during approaches by predators. Changes in predator behavior during approach and in availability of benefits (e.g. food or potential mates) may alter risks and/or costs during encounters. Thus, prey should be able to respond appropriately to changes in cues to risk, such as predator approach speed. Under the assumption that prey assess risk in real time, it was predicted that flight initiation distance (distance between predator and prey when escape begins) decreases when approach speed increases and increases when approach speed decreases during an encounter. Effects of single, abrupt changes from slower to faster approach or the reverse were studied in a lizard, Anolis lineatopus. Flight initiation distances were determined solely by final approach speed, being nearly identical for: (1) continuously fast approaches and approaches initially at the slower and finally at the faster speed and (2) for continuously slower approaches and approaches initially at faster and finally at slower speed. Escape should be adjusted to match changes in risk and cost caused by changes in predator behavior, ability to escape, and costs of escape as attacks unfold. A recent model by Broom and Ruxton [Behavioural Ecology (2004) vol. 16, pp. 534—540] predicts that cryptic prey should stay motionless until detected, then flee immediately. Our results suggest that current escape models can be applied to prey escape strategies when cues to risk change, by assuming that prey base decisions on the current relationship between risk and distance. Empirical studies are needed to test predictions concerning continuous risk assessment.     

Strategic Escape Direction: Orientation,Turning, and Escape Trajectories of Zebra‐Tailed Lizards (Callisaurus draconoides)

A prey's body orientation relative to a predator's approach path may affect risk of fleeing straight ahead. Consequently, prey often turn before fleeing. Relationships among orientation, turn, and escape angles and between these angles and predation risk have not been studied in terrestrial vertebrates and have rarely been studied in the field. Escape angles are expected to lead away from predators and be highly variable to avoid being predictable by predators. Using approach speed as a risk factor, we studied these issues in the zebra‐tailed lizard, Callisaurus draconoides. Lizards fled away from human simulated predators, but most did not flee straight away. Escape angles were variable, as expected under the unpredictability hypothesis, and had modes at nearly straight away (i.e., 0°) and nearly perpendicular to the predator's approach path (90°). The straight away mode suggests maximal distancing from the predator; the other mode suggests maintaining ability to monitor the predator or possibly an influence of habitat features such as obstacles and refuges that differ among directions. Turn angles were larger when orientation was more toward the predator, and escape angles were closer to straight away when turn angles were larger. Turning serves to reach a favorable fleeing direction. When orientation angle was more toward the predator, escape angle was unaffected, suggesting that turn angle compensates completely for increased risk of orientation toward the predator. When approached more rapidly, lizards fled more nearly straight away, as expected under greater predation risk. Turn angles were unrelated to approach speed.     

Predation Risk and Opportunity Cost of Fleeing While Foraging on Plants Influence Escape Decisions of an Insular Lizard

Cost‐benefit models of escape behaviour predict how close a prey allows a predator to approach [flight initiation distance (FID)] based on cost of not fleeing (predation risk) and cost of fleeing (loss of opportunities). Models for FID have been used with some success to predict distance fled (DF). We studied effects of foraging opportunity cost of fleeing and examined differences between age‐sex groups in the omnivorous Balearic Lizard, Podarcis lilfordi. Balearic lizards forage on the ground for invertebrate prey and climb the thistle Carlina corymbosa to forage on its inflorescences. We studied escape behaviour in three experimental groups, with human beings as simulated predators: lizard foraging above ground on C. corymbosa, foraging on the ground away from thistles and on the ground with cut inflorescences. Flight initiation distance was shorter for lizards with cut inflorescences than for (1) lizards above ground due to the greater risk above ground due to conspicuousness of black lizards on yellow flowers; and (2) lizards on ground away from flowers due to the cost of leaving while feeding. The only age‐sex difference was slightly greater FID for adult males than subadults, presumably because larger adult males are more likely to be attacked by predators. Other potential factors affecting this difference are discussed. Experimental group and age‐sex group did not interact for FID or DF. Because lizards foraging on inflorescences above ground fled to the base of the plants to refuge provided by spiny thistle leaves, their DF was shorter than in the other groups, which fled across the ground, usually without entering refuge. DF did not differ between groups on the ground or among age‐sex groups. The predicted shorter DF for lizards with cut inflorescences than on ground without inflorescences did not occur. We hypothesize that the opportunity cost was small due to the abundance of blooming thistles and that DF may be less sensitive to opportunity cost than FID.     

Does Locomotor Ability Influence Flight Initiation Distance in Yellow‐Bellied Marmots?

Flight initiation distance (FID) is the distance between a potential threat and the point at which a potential prey flees. Animals may modify their FID to compensate for increased risk generated by external/extrinsic factors such as habitat type, visibility, group size, time of year, predator‐approach velocity, and distance to burrow, as well as internal/intrinsic factors such as physical condition, body temperature, crypsis, and morphological antipredator defenses. The intrinsic speed at which an animal can escape a predator is a factor that should influence FID. We studied the relationship between an individual's intrinsic escape speed and FID in yellow‐bellied marmots (Marmota flaviventris) to determine whether marmots compensated for slower escape speeds by fleeing at greater distances. We found no evidence of risk compensation. Rather, we found that slower marmots tolerated closer approaches. This behavioral syndrome may be explained by a coevolution of FID and escape speed in determining an individual's antipredator behavior, an idea upon which we expand.     

Risks Associated with Predator Immobility,Movement Direction,and Turn Direction Similarly Affect Pursuit‐Deterrent Signaling and Escape by Zebra‐Tailed Lizards (Callisaurus draconoides)

Some prey may signal to deter pursuit by predators. Because deterrence is not needed when risk is low or useful when capture is imminent, most signaling should occur at intermediate risk. Probability of fleeing increases with risk for various risk factors. At low–intermediate risk, more frequent signaling should occur as assessed risk associated with risk factors increases. I examined the effects of three risk factors related to immobility and movement by a predator: standing distance (distance from prey to immobile predator), directions of walking, and turning by the predator. Risk is greater when the predator stands nearer, walks toward prey vs. retreating, and turns toward prey vs. away. In the lizard Callisaurus draconoides, which signals by elevating and waving its tail, signaling was more frequent before fleeing when I stood immobile at the shorter of two distances. All the lizards fled when I walked toward them, regardless of standing distance. Fewer fled when I moved away and only at the shorter standing distance. At the shorter standing distance, signal probability was high and did not differ between movement directions. At the longer standing distance, fewer lizards signaled and only when I moved toward them. Patterns of response of signaling and escape to combinations of standing distance and turn direction were qualitatively identical. When I turned away from lizards, none displayed or fled at the longer standing distance. At the shorter standing distance, probabilities of displaying and fleeing were higher when I turned toward than away from lizards. Standing distance affected signaling interactively with directions of movement and turning in manners readily interpretable from risk. Signaling was affected by risk associated with all factors, being absent or infrequent at both high‐ and low‐risk levels but frequent at intermediate risk, strengthening evidence for pursuit‐deterrent signaling.     

Influence of gaze and directness of approach on the escape responses of the Indian rock lizard, Psammophilus dorsalis (Gray, 1831)

Animals often evaluate the degree of risk posed by a predator and respond accordingly. Since many predators orient their eyes towards prey while attacking, predator gaze and directness of approach could serve as conspicuous indicators of risk to prey. The ability to perceive these cues and discriminate between high and low predation risk should benefit prey species through both higher survival and decreased energy expenditure. We experimentally examined whether Indian rock lizards (Psammophilus dorsalis) can perceive these two indicators of predation risk by measuring the variation in their fleeing behaviour in response to type of gaze and approach by a human predator. Overall, we found that the gaze and approach of the predator influenced flight initiation distance, which also varied with attributes of the prey (i.e. size/sex and tail-raise behaviour). Flight initiation distance (FID) was 43% longer during direct approaches with direct gaze compared with tangential approaches with averted gaze. In further, exploratory, analyses, we found that FID was 23% shorter for adult male lizards than for female or young male (FYM) lizards. In addition, FYM lizards that showed a tail-raise display during approach had a 71% longer FID than those that did not. Our results suggest that multiple factors influence the decision to flee in animals. Further studies are needed to test the generality of these factors and to investigate the proximate mechanisms underlying flight decisions.     

Effects of risk assessment, predator behavior, and habitat on escape behavior in Columbian black-tailed deer

The relationship between preflight risk assessment by prey andthe escape behaviors they perform while fleeing from predatorsis relatively unexplored. To examine this relationship, a humanobserver approached groups of Columbian black-tailed deer (Odocoileushemionus columbianus), varying his behavior to simulate moreor less threatening behavior. We measured the focal deer's angleof escape, distance moved during flight, duration of trottingand stotting behavior, and change in elevation during flight.Analyses revealed positive relationships between the distancemoved during flight and the distance at which they fled. Whenflight was initiated when the approacher was close, deer fledrelatively shorter distances and took flight paths at more acuteangles, a property that would force a real predator to changedirection suddenly. Our results indicate that deer do not compensatefor allowing the observer to approach more closely by fleeinggreater distances. Rather, distance moved and flight initiationdistance are linked by level of reactivity and habituation:more reactive or less habituated deer both flee at a greaterdistance and move away to a greater distance during flight.More threatening behavior by the approacher led to longer durationsof rapid flight behavior (e.g., trotting and stotting), anddeer tended to flee uphill and into taller vegetation, usingthese landscape features as refuge from danger. Finally, weprovide the first evidence for Pitcher's untested "antiambush"hypothesis for the function of stotting and discuss its significance.In general, both preflight predator behavior and habitat featuresinfluence both duration and direction of escape.     

Universal Optimization of Flight Initiation Distance and Habitat-Driven Variation in Escape Tactics in a Namibian Lizard Assemblage

Some aspects of escape predicted by theoretical models are intended to apply universally. For example, flight initiation distance (distance between an approaching predator and prey when escape begins) is predicted from predation risk and the costs of escaping. Escape tactics and refuge selection are not currently predicted by theoretical models, but are expected to vary with structural features of the habitat. One way of studying such variation is to compare aspects of antipredatory behavior among sympatric species that differ in habitat or microhabitat use. In an assemblage of lizards in northwestern Namibia, we conducted experiments to test predictions of escape theory for three risk factors in representatives of three families and observed escape tactics in additional species. As predicted by escape theory, flight initiation distance increased with directness of a predator's approach and predator speed in Agama planiceps, Mabuya acutilabris, and Rhotropus boultoni, and with distance from refuge in M. acutilabris. As predicted by theory, the probability of entering refuge increased with risk in R. boultoni. All available data indicate that flight initiation distance and refuge entry by lizards conform to theoretical predictions. Escape tactics varied greatly as a function of habitat type: (1) arboreal species fled up and around trees and sometimes entered tree holes; (2) saxicolous species used rock crevices as refuges, but differed in tactics prior to entering refuges; and (3) terrestrial species fled into bushes or other vegetation, often to the far sides of them. Some M. acutilabris entered small animal burrows or buried themselves in sand beneath bushes. Escape tactics varied even among congeners in Mabuya, highlighting the important effect of habitat structure on them. Although habitat partitioning has traditionally been viewed as favoring species coexistence, an interesting by‐product appears to be structuring of escape tactics in lizard communities.     

You can run--or you can hide: optimal strategies for cryptic prey against pursuit predators

We consider the optimal behavior of a cryptic prey individualas it is approached by a predator searching for prey. Althoughthe predator has not yet discovered the prey, it has an increasinglikelihood of doing so as it gets closer to the prey. Further,the closer the predator is to the prey when it discovers it,the more likely the predator will be to capture the prey. Thesearguments suggest that the prey should flee before the predatordiscovers it. However, the act of fleeing will alert the predatorto the presence of the prey and trigger an attack that mightnot have occurred otherwise. We capture these conflicting outcomesin a mathematical model, which we then use to predict the optimalbehavior of the prey and predator. We argue that the optimalstrategy for the prey is either to run as soon as they detecta predator approaching or to only flee in response to havingbeen detected by the predator. Running as soon as the predatoris detected is associated with low predator search speeds, alow nonpredation cost to running, a large advantage to the preyin initiating chases rather than reacting, limited ability tospot the predator at distance, a high ability to spot prey bythe predator, and a high probability that chases will be successful.The optimal strategy for the predator depends on whether itscurrent trajectory is taking it closer to or further from theprey. In the latter case, the predator should attack immediatelyon discovering the prey; in the former case, it should delayits attack until it reaches the point on its current trajectorywhere distance to the prey is minimized.     

Island tameness: living on islands reduces flight initiation distance

One of Darwin''s most widely known conjectures is that prey are tame on remote islands, where mammalian predators are absent. Many species appear to permit close approach on such islands, but no comparative studies have demonstrated reduced wariness quantified as flight initiation distance (FID; i.e. predator–prey distance when the prey begins to flee) in comparison with mainland relatives. We used the phylogenetic comparative method to assess influence of distance from the mainland and island area on FID of 66 lizard species. Because body size and predator approach speed affect predation risk, we included these as independent variables. Multiple regression showed that FID decreases as distance from mainland increases and is shorter in island than mainland populations. Although FID increased as area increased in some models, collinearity made it difficult to separate effects of area from distance and island occupancy. FID increases as SVL increases and approach speed increases; these effects are statistically independent of effects of distance to mainland and island occupancy. Ordinary least-squares models fit the data better than phylogenetic regressions, indicating little or no phylogenetic signal in residual FID after accounting for the independent variables. Our results demonstrate that island tameness is a real phenomenon in lizards.