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 well as by predators that are unable to hear the higher frequencies (Ramsier et al., 2012). Examples of animals that can hear ultrasound include cats, dogs, bats, mice, and rats (Figure 1). Through technological advances, we have been able to detect, observe, study, and utilize these signals found outside our perceptual capabilities (Arch and Narins, 2008). By investigating different animals that can hear ultrasound, we better our understanding of the physiological and anatomical mechanisms behind their ability to perceive these high-frequency sounds.
The Auditory Pathway and Ultrasound
The auditory system provides animals with the ability to detect and perceive sounds over a wide range of frequen- cies and intensities. Sound waves travel through the outer and middle ear before being transferred to the cochlea in the inner ear. The cochlea deconstructs sounds of differ- ing frequencies and intensities into electrical signals that can be interpreted by the brain. These electrical signals travel up the auditory pathway from the cochlea, passing through the brainstem, until eventually being relayed by the nuclei in the thalamus to their final destination, the auditory cortex.
Neurons in the auditory cortex are generally arranged according to the frequency at which they respond with the greatest sensitivity, namely their characteristic fre- quencies. In many animals, the characteristic frequencies of neurons progress linearly along the cortical surface as a tonotopic map (Moerel et al., 2014). This organization allows the identification of neurons responsible for con- veying specific kinds of information such as ultrasound. As such, it is important to consider where these specific neurons for encoding ultrasonic frequencies are found within the cortices of terrestrial mammals and what the relevance and benefits associated with the ability to detect ultrasound might be.
Measuring the Audible Frequency Range
Audiometry experiments can provide insight into the ultrasonic abilities of different species. The point at which a sound is detected is known as the audibility threshold. As described in a previous issue of Acoustics Today (Dent, 2017), psychophysical approaches are often employed to measure perceptual thresholds in nonhuman species. Psy- chophysical approaches encapsulate experimental designs where a physical stimulus is presented to a subject and the neural and/or sensory responses evoked by the stimulus
are examined. Psychoacoustics, one form of psychophys- ics, analyzes the relationship between auditory stimuli and neural events by employing various conditioning tech- niques (Dent, 2017). The results of the different conditions tested are often depicted using an audiogram (Figure 2).
Biological Importance
Small rodents such as rats and mice emit and perceive ultrasonic sounds to communicate with conspecifics for a variety of social communicative interactions, including courtship and mating, aggression and territoriality, repro- duction, and to alert conspecifics (Arch and Narins, 2008).
Male mice produce ultrasonic vocalizations between 48 and 79 kHz in response to female pheromones to attract them as a potential mate (Gourbal et al., 2004) and emit vocalizations up to 75 kHz when sniffing or mounting female mice (Matsumoto and Okanoya, 2016). Further- more, mouse pups produce isolation calls with frequencies between 50 and 80 kHz when they are separated from their nest (Hofer et al., 2001). Because mice can hear frequen- cies between 1.5 and 92 kHz at 60 dB sound pressure level (SPL; Heffner et al., 2001), the pup vocalizations reliably elicit a retrieval response from the mothers (Dunlap and Liu, 2018). The frequencies used in courtship vocalizations
  Figure 2. A behavioral audiogram for the mouse (Mus musculus; Ehret, 1974), ferret (Mustela putorius furo; Kelly et al., 1986), human (Homo sapiens; Sivian and White, 1933), rat (Kelly and Masterson, 1977), and cat (Felis catus; Heffner and Heffner, 1985). Dashed vertical line, beginning of the ultrasonic range (20 kHz). Data represent the lowest sound level detected at each frequency.
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