Adaptive variation in the acoustic sensitivity of California ground squirrels (Otospermophilus beecheyi) living in higher- and lower-elevation habitats

California ground squirrels (Otospermophilus becheeyi) exhibit adaptive variation to their local environments based on antipredator, immunological, and social-communication evaluations. The following pair of experiments investigated the acoustic and behavioral responses of California ground squirrels from the higher-elevation Sierra Valley (alt. ∼1900 m) and from the lower-elevation Sunol region (alt. ∼270 m). Analyses of their genetic distance suggest that these populations diverged during the Late Pleistocene. The first experiment compared small groups from these populations on the basis of their auditory brainstem responses (ABR) to a range of sound frequencies. The ABR audiograms of Sierra Valley squirrels showed greater acoustic sensitivity to frequencies lower than 4 kHz than the Sunol squirrels. Both groups exhibited similar audiograms for higher frequencies. Such an adaptation would be useful for coping with dangerous circumstances possibly masked by inclement weather. The second experiment compared the squirrel groups for several 9-hr observation periods focusing on the number of egress bouts from nest boxes centered in a laboratory room equipped with sound playback equipment to simulate far-field naturalistic sounds. Three sound treatments were compared: 1) ambient room sound, 2) temperate forest, and 3) rainstorm. Analyses indicated that the higher-altitude Sierra Valley squirrels were reliably less likely to leave the nestbox to explore the room than were low-altitude Sunol squirrels under the rain treatment. The results of both studies suggest that the enhanced low-frequency hearing of Sierra Valley squirrels is a sensory adaptation for coping with the reduced sound transmission of low frequencies at higher altitudes.