Researchers from the University of Sheffield have found that rats deliberately change the position and movement of their whiskers depending on various different factors, such as familiarity with the environment and the risk of bumping into objects, much like how a human uses hands and fingers. The findings therefore suggest that such whisker movement is under active control and is purposefully carried out in order to seek information. The study has been published in Current Biology.
Vibrissae are long, thick hairs found on almost all mammals, except humans, that are specialized for tactile (touch) sensing. While they’re found on various parts of the body, those located on the face (whiskers) are most frequently studied.
Rats and other small mammals are known to perform rhythmic back and forth sweeps of their whiskers during exploration (“whisking”), which is assumed to convey some sensory advantage to the animal. While it is known that this behavior allows the animal to locate interesting stimuli, to what extent the animals can deliberately modulate whisker movement was unknown. In particular, researchers did not know whether whisker control changes according to context, such as the availability of visual cues, which is a hallmark of “active sensing.”
In order to find out more, scientists trained rats, some of which were functionally blind, for several days to run circuits for food and then filmed them using high-speed videography. The researchers then observed how whisking changed according to certain variables, such as environmental familiarity and risk of collision with obstacles.
They found that blind rats in new environments moved slowly and performed broad, exploratory sweeps with their whiskers, but as the environment became more familiar they moved faster around the course and directed their whiskers in front of them, making smaller whisker movements in order to avoid unexpected collisions. Furthermore, in environments with increased risk of bumping into objects, blind rats pushed their whiskers further forward and moved more slowly, suggesting they were aware of the obstacles and changed their strategy accordingly.
They also discovered that sighted rats changed whisker control strategy as they got used to the environment and also when visual cues were removed by placing them in darkness.
Lead researcher Tony Prescott likened this behavior to how humans use their hands to detect obstacles whilst navigating in the dark. If the environment is familiar, humans will move faster than in novel environments, holding out their hands in front of them to avoid crashing into unexpected objects.
“All mammals except humans use facial whiskers as touch sensors. In humans we seem to have replaced this sense, in part, by being able to use our hand and fingers to feel our way,” added Prescott. “The rat puts its whiskers where it thinks it will get the most useful information, just as we do with our fingertips.”