Stop-and-go approach by a predator: a novel predation risk factor for the phrynosomatid lizards Sceloporus virgatus and Callisaurus draconoides

in Amphibia-Reptilia
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As an immobile prey monitors an approaching predator, the predator may move at a constant speed directly toward the prey or on a path that bypasses the prey. These scenarios have been studied extensively. Economic escape theory successfully predicts flight initiation distance (FID = predator-prey distance when escape begins). However, predators often alter their speed and may exhibit stops and starts during approaches. Empirical studies have shown that prey rapidly adjust assessed risk to a predator’s changes in approach speed and direction, but effects of interrupted (stop-start) approach are unknown. Because a prey is likely to assess that a nearby predator that resumes approaching has detected it and is attacking, escape theory predicts that assessed risk is greater at a given predator-prey distance when approach resumes than is continuous. Therefore, we predicted that FID is longer when a predator approaches, stops nearby, and renews its approach than when it approaches continuously. Second, although assessed risk increases as duration of the predator’s stop nearby increases, as indicated by latency to flee, we predicted that pause duration does not affect FID because prey interpret resumed approach as attack. Field experiments with two lizards, Sceloporus virgatus and Callisaurus draconoides, verified the predictions: FID was longer for discontinuous than continuous approaches and pause duration did not affect FID. We also observed distance fled and probability of entering refuge, escape behaviors for which theory is undeveloped. Distance fled was unrelated to continuity of approach in both species, as was refuge entry in S. virgatus.


Publication of the Societas Europaea Herpetologica



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  • Fluxogram of decisions made by prey when a predator approaches continuously or pauses during its approach. If the predator approaches continuously, the prey’s flight initiation distance (FID) is determined economically based on costs of not fleeing and cost of fleeing. If the predator stops during its approach, the prey flees before the predator resumes moving if the predator stops long enough to meet the economic criteria for latency to flee. If the predator resumes its approach before that time, FID is again determined economically, but is longer a response to an increase in assessed risk that occurs when the predator resumes approach.

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  • Flight initiation distance by Sceloporus virgatus was longer for approaches interrupted by pauses during a predator’s approach than for continuous approaches. C: continuous approach, I5: 5 s pause, I15: 15 s pause, I30: 30 s pause. Bars represent means; vertical lines represent 1.0 SE.

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  • Flight initiation distance by Callisaurus draconoides was longer for approaches interrupted by pauses in a predator’s approach than for continuous approaches. C: continuous approach, I5: 5 s pause, I30: 30 s pause. Bars represent means; vertical lines represent 1.0 SE.

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