The ears of weasels are well adapted for the profession of hunting rodents in confined spaces. Each pinna, or direction-finding flap, is large and flat to the head, since a high pinna, like that of a bear or horse, would collect soil and get in the way underground. The external ear canal is also very large. As in many burrowing mammals, the sensitivity of the ear is magnified by the greatly enlarged tympanic bulla (cavity) of the middle ear, which is braced inside with trabeculae, or bony struts.
One would expect all weasels to be very sensitive to the high-pitched squeaks of mice, which range up through the ultrasonic to 92 KHz. Tests in captivity show that the range of hearing in the least weasel runs from 52 Hz to at least 60 KHz, and is best at 1 to 16 KHz (Heffner & Heffner 1985). Wild, long-tailed weasels, and presumably the others, are able to hear clearly the ultrasonic sounds of voles and mice (Powell & Zielinski 1989) and insects (Willey 1970). At the other end of the scale, the least weasel's sensitivity to low-frequency sounds is among the best of all mammals, and unusually good for a small animal.
Gillingham (1986) established a 3x3 m indoor grassland into which he could release least weasels and prairie and meadow voles to watch hunting behavior of the weasels. By lighting the grassland with red light only, he could eliminate use of sight; by flooding the area with white noise, he could eliminate use of hearing; and by flooding the area with odors, he could eliminate use of smell. The responses of his five least weasels to various deprivations of senses suggest strongly that they hunted primarily by sound, less so by smell, and least by sight. Longtails moved so fast in the grassland that data were difficult to collect. Nonetheless, longtails, too, appeared to hunt predominantly by sound.
High-frequency sounds are strongly directional, and the value for a small hunter of a keen ability to locate sounds is obvious. In this case, a weasel's ability to hear high-frequency sounds is not so much an adaptation as the natural consequence of having small, close-set ears (Heffner & Heffner 1985), but all weasels certainly make the most of it. A weasel sitting up with alert, sparkling eyes could be actually hearing around, not looking around.
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