Anthropogenic noise affects risk assessment and attention: the distracted prey hypothesis

Abstract

Many studies have focused on the effects of anthropogenic noise on animal communication, but only a few have looked at its effect on other behavioural systems. We designed a playback experiment to test the effect of noise on predation risk assessment. We found that in response to boat motor playback, Caribbean hermit crabs (Coenobita clypeatus) allowed a simulated predator to approach closer before they hid. Two hypotheses may explain how boat noise affected risk assessment: it masked an approaching predator's sound; and/or it reallocated some of the crabs' finite attention, effectively distracting them, and thus preventing them from responding to an approaching threat. We found no support for the first hypothesis: a silent looming object still got closer during boat motor playbacks than during silence. However, we found support for the attentional hypothesis: when we added flashing lights to the boat motor noise to further distract the hermit crabs, we were able to approach the crabs more closely than with the noise alone. Anthropogenic sounds may thus distract prey and make them more vulnerable to predation.

Figure 1.

Spectrograms (512 point FFT, 75% overlap) of the top 40 dB of boat motor sounds broadcast to hermit crabs.

Figure 2.

The response (average ± s.e. of FRD or average ± s.e. of HID) of hermit crabs to approaching threats. (a,b) A human approached with either boat motor noise or silence. (c) A looming object (a black t-shirt put over an inflated donut) approached with either boat motor noise or silence. (d,e) A human approached with either boat motor noise combined with flashing lights or boat motor noise only. (a) d-score = 0.814, p = 0.003; (b) d-score = 0.709, p = 0.008; (c) d-score = 0.532, p = 0.027; (d) d-score = 0.717, p = 0.084; (e) d-score = 0.898, p = 0.030.