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.
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.
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.
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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.
