EFFECTS OF NOISE ON WILDLIFE

Introduction

The effects of noise on animals vary due to the animal's hearing ability, which varies considerably among animal species. Each species has adapted, physically and behaviourally, to fill an ecological role within a community; an animal's hearing ability often reflects this role. Animals rely on hearing to avoid predators, to obtain food, and to communicate with members of their own species and other members of the community.

If sound has been a determinant in the evolution of behaviour and morphology, its production and use have also depended on other aspects of the external environment. While specializations such as echolocation entail an integrated evolution of mechanisms of sound production and sound reception, the evolution of one is not always dependent on the evolution of the other. Sound production is not confined to animals with well-developed sound receptors, nor do all animals in which sound perception is well-developed produce sound themselves.

Sound production by animals also varies considerably. For example, mammalian vocalizations range in frequency from 50 to 100 Hz in the horse up to 150 kHz in some bats. High-frequency sounds are extremely directional and attenuate quickly with distance. Low-frequency sounds attenuate slowly with distance and are relatively omnidirectional. The transmission properties of a vocalization depend on environmental factors, such as temperature, humidity, landscape, and vegetation. Range of vocal signal is influenced by intensity of the source, background noise levels, rates of signal degradation, and the perceptual abilities of the receiver. Vocal communication in social animals helps maintain group cohesiveness by giving cues to individual identification and the next possible action of group members. Noise impacts could potentially disrupt a species' ability to communicate, either vocally or by disturbing its behavioural patterns.

The literature concerning hearing ability of animals includes studies of hearing mechanisms and determination of hearing thresholds (audiograms), through primarily behavioural responses to various noise levels in laboratory experiments. Knowledge of specific audiograms for even domestic species is scant; however, a number of studies have been conducted since the mid-1970's on the hearing ability of various wildlife species. Comparisons between groups of species within the same habitat have revealed a wide variety of tolerance to noise levels.

Amphibians

Sound unquestionably influences the activities of most anurans (i.e., frogs and toads) and plays a significant role in the reproductive behaviour of many, but not all, species. However, literature concerning hearing ability of anurans is lacking for most species. The anuran ear shows a complex, frequency-dependent directionality; the wide coupling of the two middle cavities via the mouth leads to acoustic interactions that enhance interaural time and intensity differences. Modifications of the ear of anurans may be associated with specializations in the sound transmitting apparatus.

The vocalizations of closely related anuran species, or even local populations of those with disjunctive distributions, are known to differ in dominant frequencies, relative intensities of the harmonics, duration of individual calls, their rates of repetition, and trill or pulsation rate. A positive motor response seemingly depends more on pulse rates or utterance rates rather than on dominant frequencies or harmonics, although the intensity of the sound may determine the maximum distance at which the sound is an effective stimulus. Previous research measured sound levels of bullfrog choruses at about 20 dB SPL in the 1.5- to 2.5-kHz frequency band up to 965 m above small ponds; however, sound travels upward much farther and more predictably than along the surface. To be effective, the sound serving as the stimulus probably must be within relatively narrow limits of variation in one or more characteristics peculiar to the voice of individual species.

Acoustic avoidance behaviour was demonstrated in a natural population of the neotropical treefrog. The threshold for acoustic avoidance at different frequencies varied from 230 to 3,420 Hz. Tones of 605-2,000 Hz were uniformly above threshold when presented at 60-70 dB SPL. Below 665 Hz, threshold dropped at 14 dB per octave to a maximum sensitivity of 41 dB SPL at 230 Hz. Tones of 3,420 Hz (approximately the third harmonic of the first note of the advertisement call) failed to elicit a response even at high levels (over 81 dB SPL in one case).

Fish

Although the hearing of several species of fish has been studied, the effects of aircraft or nonaircraft noise on fish have not been well investigated.

Birds

Psychophysical investigations of hearing in a number of avian species over the last two decades have added significantly to the knowledge of hearing capabilities of this vertebrate group. Behavioural measures of absolute auditory sensitivity in a wide variety of bird species show a region of maximum sensitivity between 1 and 5 kHz, with a rapid decrease in sensitivity at higher frequencies. On the basis of this general measure, birds fall between two other major vertebrate groups: reptiles and mammals. Discrimination and masking data from birds include measures of frequency, intensity, and duration difference thresholds (critical ratios, critical bands, and psychophysical tuning curves). Data are also available on temporal summation, temporal resolving power, and temporary threshold shift from noise exposure. Taken together, these data suggest that, in the region of 1-5 kHz, birds show a level of hearing sensitivity similar in most respects to that found for the most sensitive members of the Class Mammalia, with avian performance clearly inferior above and below this range of frequencies. Possible exceptions to this general picture include the echolocating oilbird and growing evidence that pigeons are sensitive to infrasound at moderate intensity levels. The relation among critical ratio, critical band, and intensity difference threshold measured in the parakeet is similar to that described for the human, but the pattern of masking as a function of frequency is dramatically different from that observed in mammals (Dooling and Searcy 1981).

Wild Rats

Only a few studies of the physiological effects of noise on rodents have involved wild animals. A field study involved two populations of house mice near the end of a runway at Memphis International Airport. Adult mice also were collected from a rural field 2.0 km from the airport field. Background noise levels at both fields were 80-85 dB. Noise levels of incoming and outgoing aircraft at the airport field averaged 110 dB, with the highest reading reaching 120 dB. Total body weights and adrenal gland weights of mice from the fields were measured. Additional mice were captured from the rural field, placed in the laboratory, and exposed to 1 minute of 105-dB recorded jet aircraft noise every 6 minutes to determine if noise was the causative factor. Control mice were not subjected to noise. After 2 weeks, the adrenals were removed and weighed. Adrenal gland weights of male and female mice from the airport field were significantly greater than those of mice from the rural field. The noise-exposed mice in the laboratory study had significantly greater adrenal gland weights than the control mice. After ruling out stress factors, such as population density the conclusion was that noise was the dominant stressful factor causing the adrenal weight differences between the two feral populations.

Reptiles

Sound perception appears to be subordinate in importance to vision or chemoreception in the activities of most reptiles. Sound-producing mechanisms are absent in the majority of species, but occur in some or all members of the four orders of reptiles. Studies have shown that certain desert reptiles are sensitive to low-intensity sound. Sounds may be of more adaptive significance for nocturnal species, such as crocodilians, the tuatara and nearly all geckos, because full use cannot be made of photoreceptors or vision. The optimal acoustical sensitivity of various species of desert iguanid lizards varies between 700-3,000. Critical environmental sounds are often of relatively low intensity (e.g., movement of insect prey and predators such as snakes and owls); sensitive hearing acuity is essential to the survival of these desert vertebrates. The temperature of maximum auditory sensitivity of lizards varies as a function of the natural thermal preference for each species. Sensitivity decreases as temperature varies either above or below the range of preferred temperatures for normal activity. Average sensitivity loss of 10-20 dB/10 ·C can be found in the region of maximum sensitivity. Hearing appears to be best at ecologically optimal temperatures.

The effects of ORV noise (114 dB for 1 and 10 hr) on the desert iguana (Dipsosaurus dorsalis), whose optimal acoustical sensitivity is between 900 and 3,000 Hz, were studied in the laboratory. Animals tested for acoustical sensitivity immediately after sound exposure had a greater loss of hearing sensitivity than those tested 7 days later. Results indicated that a shift in the hearing threshold had occurred. Permanent sensitivity losses were experienced by lizards exposed to both 1 and 10 hours of sounds. Animals exposed for 10 hours suffered the greatest permanent losses. The destructive dose was less than 1 hour. The time it took for sensitivity loss to recover exceeded 7 days. Duration of exposure affected the degree of recovery.

The Mohave fringe-toed sand lizard inhabits the quiet, protected dunes of the California desert and has also evolved the ability to hear low-intensity, low-frequency sounds. Researchers tested the effects of dune buggy sounds on the hearing of this lizard in the laboratory. Lizards were exposed to taped dune buggy sounds of 95 dB, representative of a dune buggy at 5 m. All noise-exposed lizards suffered actual hearing loss after exposure to 510 seconds of 95-dB dune buggy sounds. Shallow burial in sand was not considered an adequate escape from ORV sounds. Because the animals were sacrificed immediately after the experiment, it could not be determined whether they had suffered a permanent or temporary threshold shift. Dune buggies can penetrate deep into the interior of sand dune ecosystems. Therefore, many areas within the dune periphery are exposed to repeated episodes of high intensity sounds. Also, intense ORV activities in the spring and summer coincide with the reproductive season of all three Uma species occurring in California, which may have adverse biological effects. Research suggested that during times of stress, such as drought and subsequent lack of food and shelter retreats, all unnecessary disturbances, including ORV activity, mining, repeated low-altitude jet overflights, and gunnery, should be restricted from the immediate area of the dune systems.

References:

“EFFECTS OF AIRCRAFT NOISE AND SONIC BOOMS ON DOMESTIC ANIMALS AND WILDLIFE: A LITERATURE SYNTHESIS” NERC 88/29 AFESC TR 88.14 JUNE 1988

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