Introduction
In alpine regions, cows are often equipped with a bell throughout the summer season to ensure that farmers can locate their animals on the wide alpine pastures, many areas that are obstructed from view. The chime of these cowbells is characterized by high and varying amplitudes from 90 to 113 dB at a distance of 20 cm, the approximated distance between the bell and the cows’ ears (1). Goats have been found to show higher behavioral arousal when being exposed to the playback of a bell compared to the playback of a uniform sinusoidal sound, indicating that the bell sound might be more aversive to goats than the uniform sound. With repeated exposure, goats habituated to both stimuli (2).
So far, little research has been conducted investigating the effect of noise on the hearing capacities of animals. Kenneled dogs that were constantly exposed to noise between 100 and 108 dB for 6 months developed hearing loss as indicated by measurements of the auditory brainstem response (ABR) (3). In mice, ABR recordings showed that a single exposure to noise of 100 dB for 2 h induced temporary hearing loss (4), and an exposure to noise of 110 dB for 60 min even induced permanent hearing loss (5).
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Noise-induced hearing loss is one of the most common causes of exogenously acquired sensorineural hearing loss in adult humans (6). Although anatomic differences among mammal species lead to differences in hearing capacities (7), the basic physiologic processes underlying the detection and sensation of sound are essentially identical between humans, dogs, cattle, and mice (8-11). Considering that cows can hear sounds between 23 Hz and 35 kHz, with the highest sensitivity at 8 kHz, and are able to detect sounds at −11 dB, i.e., amplitudes the human ear cannot detect (11), the continuous exposure to bells during pasturing season might impair the cows’ hearing capacity.
Behavioral indicators such as the acoustic startle response (12-15) or avoidance reactions (16, 17) have been used as an indirect but non-invasive test of hearing capacity in earlier studies. The acoustic startle response is an electromyographic response, which in rodents is elicited by stimuli with an amplitude of more than 80 dB (18, 19), and the latency is very short [5-10 ms for the electromyographically measured response in different muscles (20-22)]. Behaviorally, a startle response is defined as a cross-species response to an intense and abrupt stimulus (23) and as any first reaction of any part of the body, such as body movements, movements of limbs and facial movements, or any first behavioral reaction to sound stimulation (24). Therefore, the latency to the first behavioral reaction, e.g., sudden head movements in response to an acoustic stimulus can be used as a proxy for the induction of a startle response. In addition, avoidance reactions in response to an acoustic stimulus, e.g., increasing the distance between the source and oneself, indicate that the stimulus is perceived as aversive by the animals (25-27).
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In addition, cardiac parameters such as heart rate and heart rate variability can be used to assess arousal induced by noise (2, 28). If cardiac parameters indicate an arousal due to noise exposure, it can be assumed that the animal perceives the noise as aversive (23). Noise exposure is often accompanied by an increase of heart rate in humans (29, 30). Lee et al. (31) evaluated instant responses of the autonomic nervous system to short-duration noises using heart rate variability analysis. The results indicated that, compared with background noise of 38 dB, exposure to noise between 50 and 80 dB increased sympathetic activity as indicated by a higher ratio of low-frequency (LF) to high-frequency (HF) spectral power. For humans, “hazardous” noise is defined as sounds that exceed 85 dB over a typical 8-h workday (32, 33). It has been shown that constant exposure to such hazardous noise can result in irreversible hearing loss and that even a single intense sound event can cause hearing loss and tinnitus (32, 33).
A widespread solution used to protect humans and also horses from noise exposure by using hearing protection devices such as earplugs (34, 35). Commercially available earplugs for horses are made of memory foam (35). In cattle, acoustic earphones were inserted into the ear canal and were held in position with either silicone earplugs or earplugs of compressible foam while measuring brainstem auditory evoked potentials (BAEPs) (36-38). Such earplugs occlude the ear canal and attenuate background noise.
Although some studies on the general hearing capacity of cows are available, to our knowledge, no studies exist on hearing capacities of cows that have been exposed to noise in general. Bells seem to be a relevant noise factor for cows considering that cows are exposed routinely and for a longer period of time to the chime of bells (1). The aim of this study is to test whether routine bell exposure affects the reactivity to a noise stimulus and be associated with hearing impairment in cows. Behavioral and cardiac indicators were used as indirect measures for the assessment of cows’ hearing capacity. Thus, we examined the reactivity toward noise of low (65 dB) and high (85 dB) amplitude in bell-experienced and bell-inexperienced cows on 24 Swiss dairy farms. We additionally tested whether mimicking hearing impairment using earplugs would reduce the reactivity to the sounds. We hypothesized that cows that had been exposed regularly to a bell on alpine pastures (bell-experienced cows) would show reduced reactivity toward these sounds (increased latency of the first behavioral reaction, reduced avoidance and heart rate, and increased heart rate variability) contrarily with cows that were only equipped with a bell as heifers or never before (bell-inexperienced cows). Contrary, we expected that cows would show increased reactivity in response to a stimulus of high amplitude (decreased latency of the first behavioral reaction, increased avoidance and heart rate, and reduced heart rate variability) compared to a stimulus of low amplitude. Further, we expected that cows without earplugs would also show increased reactivity toward these sounds (decreased latency of the first behavioral reaction, increased avoidance and heart rate, and reduced heart rate variability) compared to cows with earplugs. Altogether, if earplugs do not diminish the reaction of a given cow, this might be an indicator of either a low-reactive animal, well habituated to noise or hearing impairment.
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