Page 106 - The Welfare of Cattle

sense and sensIbILItY 83

auditory cues alone. Cattle use vocalizations to communicate with each other, to find mates, and
express distress. Hearing is one of the senses that humans caretakers have an easier time under-
standing or recognizing the behavioral response to because in most cases the human and the animal
she is working with both hear the sound. When working with cattle (and other livestock species),
the position of their ears can provide a helpful clue to understand what noise they are listening to
and identifying.

Inner Workings of an ear

Ears, or pinnae, come in difference shapes and sizes. Humans have short, fixed pinnae while
cattle and other mammals have pinnae that extend farther from the head and usually have some
degree of mobility with the ability to alter orientation. Unlike olfaction and gustation, hearing is a
mechanical sense. A sound source creates waves or pressure changes in the air and it is these altera-
tions in pressure that the animal senses. The ear has several distinct parts: the outer ear, the middle
ear, and the inner ear. The pinnae (outer ear) function to help funnel the sound into the ear. The
sound waves travel through the outer ear into the auditory canal which is often elongated in mam-
mals. The ear drum is at the start of the middle ear and it responds to the sound wave with vibra-
tions. The auditory canal amplifies sound and the ear drum is very sensitive to changes in pressure
both of which act as natural hearing aids. The sound wave transformed into the vibration of the ear
drum transfers to three bones in the middle ear that propagate the information further to the oval
window. The oval window propagates the sound information into the inner ear which is a maze of
tubes and fluid. The cochlea is the structure in the inner ear in which the sound wave information is
transferred into electrical impulses that send information to the brain via the auditory nerve. In the
fluid-filled cochlea there are membranes that contain microscopic hair-like fibers (hair cells) that
are connected to the auditory nerve. The fibers move in response to the movement of the cochlear
fluid in response to the sound and depending on the degree of movement different fibers will react
and transduce information to the auditory nerve.

Sound Localization

Sound localization is a significant component of hearing. There is a large variation in sound
localization acuity between species. The location of a sound is described by its azimuth (direction
left to right relative to a listener), elevation (direction up and down relative to the listener), and
distance from the listener. The directionality of an animal’s pinnae provide information that assist
in vertical sound localization, reduce front–back confusion and assist in locating in the horizontal
plane (Butler, 1975; Musicant & Butler, 1985).
Mammals use several binaural acoustic cues, i.e., cues based on the comparison of the signals
received by the left and right ears, to help identify the location of the sound. To identify sounds
moving in the vertical dimension, the comparison of sound energy across different sound wave
frequencies arriving at each ear, i.e., spectral analysis, is utilized (Grothe, Pecka, & McAlpine,
2010). Typically, mammals use differences in time of sound arrival (interaural temporal difference,
ITD) and differences in received sound pressure level (interaural level differences, ILD) (Heffner &
Heffner, 1992a). The ITD is the difference between the times sounds reach the two ears; the sound
is “heard” in one ear and after it travels around the surface of the head it is “heard” by the other
ear. The ILD is the difference in pressure reaching the two ears. This occurs because the head of
the person/animal hearing casts an acoustic shadow which changes the intensity of the sound that
reaches each of the ears. This is most applicable with high-frequency sounds as the “shadow effect”
is more pronounced. Head width determines at which sound frequencies the ILD becomes signifi-
cant (Grothe & Pecka, 2014). Humans are able to locate sound within 1° of its location as compared
with cattle that localize sound within 30° of its actual location (Phillips, 2002). There was some