Risk and cost of immobility in the presence of an immobile predator

Comparing the antipredator behaviour of two sympatric, but not syntopic, Liolaemus lizards

The microhabitat preferences of prey animals can modulate how they perceive predation risk, and therefore, their antipredator behaviour. We tested under standardized conditions how microhabitat preferences of two Liolaemus lizards affected their responses when confronted with two types of ambush predators (raptor vs. snake), under two levels of predation risk (low vs. high). These lizard species are sympatric, but not syntopic; L. chiliensis basks on bushes, a complex microhabitat that may provide protection against visual predators, while L. nitidus prefers open microhabitats, basking on the top of large bare rocks, highly exposed to visual predators. If microhabitat complexity modulates the antipredator response, L. chiliensis may perceive lower predation risk, exhibiting lower intensity of antipredator responses than L. nitidus. Both species reduced their activity after being exposed to both predators, but lizards differed in the assessment of predation risk; L. nitidus reduced its activity independently of the predation risk experienced, while L. chiliensis only reduced its activity in the high-risk condition. The microhabitat preferences shaped during the evolution of these species seem to modulate their perception of predation risk, which may cause interspecific differences in the associated costs of their antipredator responses.

Predator-prey distance and latency to flee from an immobile predator: functional relationship and importance

When an immobile prey has detected an immobile predator nearby, predation risk is greater when the predator is closer. Consequently, prey flee with shorter latency as standing distance (predator–prey distance when both are still) decreases. Since it was first reported in 2009, this relationship has been confirmed in the few species studied. However, little is known about the functional relationship between standing distance and latency to flee (LF). We hypothesized that LF increases as standing distance increases at short distances, but reaches a plateau at longer distances where prey can escape reliably if attacked. We simulated immobile predators by moving slowly into positions near striped plateau lizards Sceloporus virgatus, stopping and then remaining immobile, and recording LF from the stopping time. LF increased from shorter to longer standing distances in a decelerating manner. The relationship was concave downward, and LF was indistinguishable among the longer standing distance groups. Latency to flee appears to reach a plateau or approach an asymptotic value as standing distance increases. The effect size of standing distance was large, indicating that S. virgatus sensitively adjusts LF to the level of risk associated with standing distance. Relationships between risk assessment and theoretical zones associated with risk, its assessment by prey, and escape decisions are discussed. Effect sizes of standing distance were substantial to large in all studies to date, indicating that standing distance is an important predation risk factor when both predator and prey are immobile.

Directional escape strategy by the striped plateau lizard (Sceloporus virgatus): turning to direct escape away from predators at variable escape angles

A prey’s orientation to a predator’s approach path affects risk of fleeing straight ahead. By turning to flee closer to straight away from the predator before fleeing, prey can reduce risk. Laboratory studies suggest that escape angles should lead away from predators and be unpredictable. I studied orientation, turn, and escape angles and in a study of striped plateau lizards,Sceloporus virgatus. Lizards fled away from a predator, but often not straight away. Escape angles were variable and bimodally distributed: one mode was straight away for distancing prey from predator and one was near 90°, which maintains ability to monitor the predator or requires turning by the predator. Turn angles increased as orientation shifted toward the predator. Escape angle was closer to straight away when turn angle was larger, but turning did not fully compensate for degree of orientation toward the predator. Directional escape strategies of diverse prey are compared.

Averted predator gaze reduces latency to flee by zebra-tailed lizards (Callisaurus draconoides)

Directness of a predator’s gaze affects several aspects of antipredatory behavior, including flight initiation distance (FID = predator-prey distance when fleeing begins), which is longer under direct than averted gaze. However, the effect of gaze directness on probability of fleeing or latency to flee (LF) in a short interval when a prey is immobile near an immobile predator is unknown. We predicted that probability of fleeing is greater and LF is shorter if the predator’s gaze is averted than direct. If the predator has not detected the prey, it will detect it immediately and attack when the prey moves while under direct gaze. Therefore, the prey is less likely to flee than if the predator is looking elsewhere. Under direct gaze the cost of fleeing decreases over time because the predator is increasingly likely to detect the prey. Under averted gaze, risk of being detected is constant for immobile prey and increases if the prey flees. Cost of fleeing under averted gaze is therefore constant and lower if the prey flees than under direct gaze at short latency, predicting shorter LF and greater likelihood of fleeing within a short fixed interval. We test this novel prediction of a model of the effect of gaze directness using the zebra-tailed lizard, Callisaurus draconoides. The proportion of lizards that fled was three times greater for averted than direct gaze, suggesting that prey under scrutiny reduce movement to avoid being detected. We discuss effects of gaze on FID, LF and probability of fleeing.

FEAR and DREAD: starting distance, escape decisions and time hiding in refuge

Flight initiation distance (FID = predator–prey distance when escape begins) increases as starting distance (predator–prey distance when approach begins) increases. The flush early and avoid the rush (FEAR) hypothesis proposes that this relationship exists because monitoring an approach is costly. Hypothesized causes are increase in assessed risk and decrease in obtainable benefits while monitoring as starting distance increases. We propose the delay risking emergence and avoid dying (DREAD) hypothesis: hiding time in refuge increases as starting distance increases because prey use risk assessed during approach to estimate risk upon emerging. In the lizard Callisaurus draconoides, FID increased as standardized starting distance increased at faster approach speeds, supporting the FEAR hypothesis. In its first test, the DREAD hypothesis was supported: hiding time in the lizard Sceloporus virgatus increased as standardized starting distance increased. No large benefits were attainable, suggesting that dynamic increase in assessed risk accounts for these findings.