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produced by male mice to attract females shows the sex- specific relevance of ultrasound production and hearing.
Similar to mice, adult rats have two main purposes for emitting ultrasonic vocalizations as a form of commu- nication: alarm calls at 22 kHz to warn conspecifics of danger and calls at 50 kHz for social cooperation and affiliative behavior (Wright et al., 2010). Rats generally emit vocalizations with frequencies that fall within their hearing range (between 250 Hz and 80 kHz). For exam- ple, infant rats can emit vocalizations between 40 and 65 kHz when they are separated from their nest, and adult rats can emit ultrasonic calls to solicit sexual behavior from the opposite sex (Portfors, 2007). On hearing the 50 kHz vocalizations from male rats, females display a series of attracting behaviors, increasing the likelihood of the male approaching and copulating (Portfors, 2007). Rodents therefore rely on ultrasound for their survival whether it is for communicating with conspecifics, attracting mates, or evading predators.
Unlike rodents, there are only limited data available on the evolution and biological importance of ultrasonic hearing in carnivores. Carnivores, aside from carnivo- rous rodents like the northern grasshopper mouse (Onychomys leucogaster), are seldom known to pro- duce or use ultrasonic frequencies for communication (Brown et al. 1978; Farley et al., 1987). Even so, many carnivores can perceive sounds with ultrasonic frequen- cies. It is thought that perhaps, at one point in history, the common ancestor of carnivores used ultrasound for prey detection (Heffner and Heffner, 1985; Kelly et al., 1986). However, as discussed in Rodents, prey (such as mice or rats) primarily communicate at frequencies above the hearing range of carnivores (Kelly and Masterton, 1977).
Phillips and colleagues (1988) determined that ferrets (Mustela putorius furo) can detect sounds from 40 Hz to approximately 40 kHz. Ferrets provide a useful model for investigating the development, organization, and plastic- ity of the auditory cortex because the onset of hearing in ferrets occurs late compared with other mammals (Moore, 1982). Before their ear canals open, newborn fer- rets, known as kits, produce high-frequency vocalizations often above 16 kHz. Lactating female ferrets respond to these kit vocalizations (Shimbo, 1992) similar to the rodent behavior described in Rodents. Overall, ferrets provide useful models for investigating different aspects of hearing and hearing loss, given that their hearing range largely overlaps that of humans (Fritz et al., 2007).
Another common carnivore model used for auditory research is the domestic cat (felis catus). The sensitive hear- ing range of cats is commonly believed to be between 5 and 32 kHz, although there are notable discrepancies in the literature regarding their hearing range limits (Figure 3). The literature agrees that cats can hear ultrasonic frequen- cies, but the full extent of their perception remains unclear. The lower limit of hearing is generally reported as approxi- mately 125 Hz, but the upper limit is not well defined.
Most sources report the upper limit as the maximum fre- quency tested. As such, the upper hearing limit of cats is not commonly described as greater than 60 kHz (Figure 3), and, in some cases, the reported upper limit corre- sponds to the highest frequency of sound tested in the respective study. This is true for both electrical stimula- tion experiments, where electrical impulses are applied to neurons in the auditory pathway, and behavioral experi- ments. One exception is a study by Heffner and Heffner (1985) who tested frequencies up to 92 kHz and reported the upper hearing limit as 85 kHz. Therefore, it is possible
  Figure 3. Various reported hearing ranges for cats compared with the range of frequencies of the presented stimuli in each respective study, all of which are cited in References.
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