Human noises may change reproductive behaviour. For example, it has been suggested that male ovenbirds (Seiurus aurocapilla
) on quieter territories may be more likely to have a mate than those on louder ones. If females base mate choice on male territory noise levels, instead of size or age, anthropogenic sounds may negatively affect male mate pairing success (Habib et al. 2007
The way animals communicate can also be changed by anthropogenic sounds. Certain species of acoustically active, pond-dwelling frogs decrease their call rate when exposed to airplane flyby or motorcycle engine playbacks (Sun & Narins 2005
). This finding suggests that frogs changed their calling behaviour to avoid acoustic masking.
Animals may also increase antipredator vigilance in the presence of loud noises. For instance, California ground squirrels (Spermophilus beecheyi
) in areas with loud wind turbines exhibited higher rates of vigilance after hearing conspecific alarm calls than those in quieter areas (Rabin et al. 2006
We suspect predation risk assessment may also be affected by anthropogenic noise, and we are aware of only one study conducted to assess this. Karp & Root (2009)
found that loud ecotourist conversation increased alertness and flight initiation distance in hoatzin birds (Opisthocomus hoazin
). However, they did not provide a mechanism for this observed pattern.
We evaluated two hypotheses that may explain how anthropogenic sounds affect risk assessment. The first is that the anthropogenic noise masks auditory cues of the approaching threat. The second is that the anthropogenic sound reallocates an animal's finite attention, effectively distracting it and preventing it from responding to predatory threats.
Attention is the process that filters out all but a few stimuli from an individual's environment, letting in only as much as it can process (Bushnell 1998
; Dukas 2004
). Individuals can process only a finite amount, though total attention can be divided or reallocated among various tasks or stimuli (Washburn & Taglialatela 2006
). The choice of what to focus attention on is both voluntary and involuntary; distraction is commonly understood to be the animal's attention suddenly shifting involuntarily. Many animals must split their attention (Dukas 2004
) or time (Lima & Bednekoff 1999
) between a necessary task (e.g. foraging) and antipredator behaviour (e.g. vigilance). Consequently, distracting animals could enhance vulnerability to predation (Dukas 2004
), and thus be detrimental.
We investigated whether anthropogenic noise affects a model species, the Caribbean hermit crab (Coenobita clypeatus
), and if so, how. Terrestrial hermit crabs are an ideal species with which to study the effects of anthropogenic stimuli for several reasons. First, terrestrial hermit crabs are conspicuous and locally abundant. Second, they have an unambiguous and easy-to-measure antipredator behaviour (they hide in their shells). The same logic used to study flight initiation distance (Cooper & Frederick 2007
) can be applied to study what we define as hiding initiation distance (HID) because, as an alternative to flight, hiding is the hermit crab antipredator response. Finally, they rely on both sight and sound (Burggren & McMahon 1988
), and thus we could test risk assessment using both modalities.