Specialized sensory systems, perfected over years of evolution, enable animals to exploit various information in the environment. By extracting salient information from their surroundings, animals can adjust their behavior in ways that promote their survival and reproduction. Behaviors such as mate identification, foraging and predator evasion all rely on the successful acquisition of relevant stimuli. Noise, both anthropogenic and natural, masks environmental signals thus rendering the relevant stimulus in-discriminable.
Prey animals, unable to detect a looming predator, will perceive noisy environments as risky and therefore reduce their foraging behavior and increase visual vigilance. In the wild, moving water, waterfalls and rapids are examples of natural sources of noise that have been found to reduce foraging behavior in listening animals. In the modern age, however, anthropogenic noise has become a more ubiquitous source of noise. Human encroachment introduces noise which is considerably different from the natural noise profile that has shaped risk aversion behavior for millennia. Do animals respond to anthropogenic noise as they do to natural noise? What acoustic properties of noise make an environment perceptually risky?
Sweet et al (2019) investigated the influence of both natural and anthropogenic noise on foraging behaviors among song sparrows (Melospiza melodia). Although previous studies had already shown that both sources of noise have similar effects on foraging-vigilance trade-offs, there was yet a direct comparison across the same species. Furthermore, in order to make relevant predictions on how animals might respond to novel anthropogenic noise, the researchers studied the effects of quantifiable noise characteristics including amplitude, frequency and temporal structure.
First, the researchers compared song sparrow foraging behavior and vigilance under automobile traffic noise and whitewater river noise. They found that despite the origin of noise, song sparrows perceived intense, noisy environments to be risky. On average, birds had shorter head down durations in intense noise environments. Although their pecking rate did not change in noisy environments, they spent less time foraging suggesting that high intensity noise led to reduced caloric intake.
The researchers also compared the effects of unedited whitewater river noise to similar noise that was shifted upwards in frequency. They hypothesized that lower frequencies were useful in identifying predators from long distances. In this case, higher frequency noise, which does not mask the low frequencies necessary for auditory surveillance, could be perceived as less risky. Results showed that indeed, raising the frequency of the river noise resulted in more foraging (defined by number/rate of pecks) and less vigilance (defined by number and duration of visual scans).
Finally, the team of researchers investigated the effects of the temporal profile of noise on foraging behavior. River noise, unlike traffic noise, has minimal temporal variation in loudness. Traffic often has intermittent periods of low intensity noise which could allow for ‘dip listening’. By giving river noise a similar temporal profile to traffic, they explored whether noise with a constant high intensity led to more vigilance relative to noise with intermittent periods of low intensity. No significant differences were found between continuous river noise and amplitude modulated river noise.
The results of this study showed that both natural and anthropogenic noise can shape antipredator behavior in similar ways. It is worth noting that although natural and anthropogenic noise were shown to have similar effects, anthropogenic noise continues to become more extensive and intense. This is likely to alter animal distributions leading to drastic changes in ecosystems. Further work is needed to explore the specific effects of various acoustic characteristics of noise in shaping behavior.
K A Sweet, B P Sweet, D G E Gomes, C D Francis, J R Barber, Natural and anthropogenic noise increase vigilance and decrease foraging behaviors in song sparrows, Behavioral Ecology, 2021;, arab141, https://doi.org/10.1093/beheco/arab141