The Auditory Sensitivity and Active Space of the Northern Grasshopper Mouse (Onychomys leucogaster) in Relation to Woody Plant Encroachment

Vocal communication is a critical component of mate choice and territoriality in many animals. The efficacy of communication depends on signal transmission through an often- cluttered environment. Anthropogenic-induced changes in vegetation structure may impact sound propagation and thus habitat quality, but few studies have explored this hypothesis. In the American Southwest, fire suppression and cattle grazing have facilitated displacement of grasslands by pinyon-juniper woodlands. Northern grasshopper mice (Onychomys
leucogaster) produce long-distance vocalizations to advertise their presence to rivals and potential mates and inhabit regions impacted by juniper encroachment. Here, we coupled acoustic recordings and physiological measurements of hearing sensitivity in the laboratory with sound transmission experiments in the field to estimate the active space of grasshopper mouse calls. We found that receivers are sensitive to loud (ca. 85 dB re: 20 μPa at 1 m) 11.6 kHz calls at 30 dB and that calls travel 30 m (range: 20 – 60 m). However, we found no effect of shrub encroachment on call propagation as juniper and grass density were inversely associated and both present barriers to a 9 cm mouse advertising at ground level. Our findings suggest that juniper encroachment does not affect sound propagation but may influence habitat quality through alternative mechanisms.