Monday, 10 December 2012

Traffic noise causes communication breakdowns

In a perfect world, communicating animals would always be capable of hearing each other's signals and responding appropriately. In the real world, however, the environment is filled with interfering sounds such wind, rainfall, other species' vocalizations, and, increasingly, anthropogenic noise. Under these conditions, lucky animals may be able to respond to a signal when they should, and avoid responding when they shouldn't. However, some individuals might accidentally miss out on hearing an important signal, or react to one that hasn't even been given. While these mistakes might not always have repercussions, there might be times--especially when a predator is involved--when they could mean the difference between life and death.

(A Stephens' kangaroo rat, Dipodomys stephensi. Image courtesy of activerain.)

This is a possibility that has only rarely been studied experimentally and (as far as this author knows) has never been investigated in mammals--until now. A team of American researchers has just reported findings from a field study of endangered Stephens' kangaroo rats (Dipodomys stephensi) living near noisy roadsides in California's Southwestern Riverside County Multispecies Reserve. Their results indicate that human noise pollution can cause the rats to make both of the communication errors mentioned above. As a result, the researchers worry that these nocturnal animals, which rely on acoustic signals to "coordinate social interactions...in a variety of contexts," may suffer from the introduction of human noise pollution into their habitats.

The researchers explored the effects of acoustic pollution by recording both signals and noise at roadside rat burrows. In this case, the signals were footdrums, or thumps made by the rats as they rapidly stamp one foot against the ground; a series of these is known as a "footroll." These low-pitched sounds travel well through both air and soil, and are therefore audible to rats that are outdoors and to those that are holed up in their subterranean burrows. In order to compare the acoustic properties of footdrums to those of traffic noise, the researchers also recorded car traffic at distances of both 10 and 25m from the edge of the road. They then used acoustic analysis software to quantify a variety of characteristics in both sets of recordings (minimum, maximum, and fundamental frequency, plus total bandwidth), and investigate similarities in the two types of sound. In other words, they examined the likelihood that noise pollution might "mask" kangaroo rat communication.

(A view of the Southwestern Riverside County Multispecies Reserve. Image courtesy of Gossamer Tapestry.)

The scientists also conducted a series of playback experiments during which wild rats were exposed to recordings of footrolls. These signals were played during three different types of ambient noise conditions: a no-noise control, traffic noise, and cricket chirps (high-pitched noise that should not mask rat footdrums, but should still elicit any interesting responses to noise in general). Rat behavior was observed throughout each trial so that the researchers could determine whether the animals were impacted by any type of environmental noise (i.e., traffic and crickets vs. control), and also whether one type of noise was particularly disruptive to communication (i.e., traffic vs. crickets).

Across the 39 footdrum recordings collected, all four frequency characteristics were similar to those measured for the traffic recordings. For example, the minimum and maximum frequencies of footdrums were found to be 126 and 352 Hz (respectively); for traffic noise, these values were 118 and 371 Hz. In other words, both signal and noise have quite similar acoustic properties, increasing the likelihood that kangaroo rats will make mistakes while communicating.

Indeed, rats exposed to traffic sounds during the playback experiments seemed aware of the fact that the noise pollution could impair their ability to hear. Even before any footdrumming recordings were broadcast into the experimental chamber, the noise-exposed individuals spent more time alert; they also delivered more signals, themselves. While rats in the control and cricket treatments immediately responded to footdrums when they were broadcast, animals in the traffic treatment tended not to alter their behavior; they also continued to footdrum for no apparent reason. Thus, they not only failed to detect actual signals amidst the din of noise pollution, but they also sometimes interpreted the footdrum-like traffic noise as a signal from another rat that wasn't even there.

(A Stephens' kangaroo rat in the entrance to its burrow. Image courtesy of the L.A. Times--which also featured an article about whether the rats act as a "barrier to development.")

Cumulatively, these results suggest that kangaroo rats should--and do--have difficulty discriminating between signals and traffic noise. This could have several negative impacts on both individual animals and the entire population. Rats that accidentally interpret traffic as signal might waste energy producing a footdrum; further, this highly audible response might make them more obvious to predators such as owls or cats. Additionally, animals whose signals are not heard may fail to defend their territories or attract mates, and may therefore have lower reproductive success.

This is bad news given that the rats have already suffered significant habitat losses. Until these recent findings were reported, researchers had thought that roadsides were actually beneficial to this species since they offered individuals the perfect substrate for digging burrows. Noise pollution, however, may cause this anthropogenic habitat to serve as a trap rather than a refuge. One glimmer of hope is offered by previous studies of avian species living in similarly noisy habitats: In order to escape acoustic masking, some birds can improve communication by altering their signal properties (e.g., pitch, volume, timing). Though it may not be easy or likely, it is possible that the Stephens' kangaroo rats might eventually learn to drum more rapidly or more intensely; given enough time, the species might even evolve better hearing so that noise-exposed individuals are more capable of discriminating between traffic and footdrum.

Shier, D.M., Lea, A.J., and Owen, M.A. 2012. Beyond masking: endangered Stephens' kangaroo rats respond to traffic noise with footdrumming. Biological Conservation 150:53-58.

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