Abstract
Nearly all animals face a tradeoff between seeking food and mates and avoiding predation. Optimal escape theory holds that an animal confronted with a predator should only flee when benefits of flight (increased survival) outweigh the costs (energetic costs, lost foraging time, etc.). We propose a model for prey risk assessment based on the predator's stage of attack. Risk level should increase rapidly from when the predator detects the prey to when it commits to the attack. We tested this hypothesis using a predator – the echolocating bat – whose active biosonar reveals its stage of attack. We used a prey defense – clicking used for sonar jamming by the tiger moth Bertholdia trigona– that can be readily studied in the field and laboratory and is enacted simultaneously with evasive flight. We predicted that prey employ defenses soon after being detected and targeted, and that prey defensive thresholds discriminate between legitimate predatory threats and false threats where a nearby prey is attacked. Laboratory and field experiments using playbacks of ultrasound signals and naturally behaving bats, respectively, confirmed our predictions. Moths clicked soon after bats detected and targeted them. Also, B. trigona clicking thresholds closely matched predicted optimal thresholds for discriminating legitimate and false predator threats for bats using search and approach phase echolocation – the period when bats are searching for and assessing prey. To our knowledge, this is the first quantitative study to correlate the sensory stimuli that trigger defensive behaviors with measurements of signals provided by predators during natural attacks in the field. We propose theoretical models for explaining prey risk assessment depending on the availability of cues that reveal a predator's stage of attack.
Highlights
Most animals face a tradeoff between avoiding predation and seeking food and mates
Discriminating real and false threats To determine whether it is possible for a moth to differentiate real and false threats, bat call pulse intervals and intensities for five attacks on five tethered, soundless noctuid moths and nine attacks on nearby soundless moths were plotted (Figure 4)
We compared the degree of overlap in call intensities between real and false threats for each echolocation phase using the area under the Receiver Operating Characteristic (ROC) curve (AUR), which equals the likelihood a randomly chosen call from the real threat group has a higher intensity than a randomly chosen call from the false threat group
Summary
Most animals face a tradeoff between avoiding predation and seeking food and mates. The dogbane tiger moth Cycnia tenera has been a model for the study of moth clicking for many decades [37,38,39] It detects attacking bats using the acoustic cues of call intensity and call repetition rate (or its inverse, call pulse interval) [37], [40]. C. tenera is most sensitive to calling rates that bats use in the approach phase of attack This is when most tiger moths click in response to bats [35]. We studied the cues that stimulate a clicking response in B. trigona to test our hypothesis that prey assess predator risk by determining the stage of the predator’s attack. We tested this prediction by 1) determining the pulse interval and intensity thresholds that elicit clicking by broadcasting simulated bat calls to tethered moths in a sound chamber; 2) characterizing the sounds moths hear in the field when being attacked by bats (‘‘true threat’’) and when bats are attacking a nearby moth (‘‘false threat’’); and 3) comparing the acoustic properties and flight trajectories of bat passes that did and did not elicit a clicking response from B. trigona in the field
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