Acoustic Ecology Theory

The following summary is based on an informal presentation I gave to Tunghai University ecology field course students at Pasoh in mid-June 2008. References to the literature can be found in this short outline review (pdf format).

Contents

• Introduction
• Functions of animal calls
• Mechanisms of sound production
• Physical attributes of sound
• Sources of selective pressures
• Is there an acoustic niche?
• Open questions
• Some applications of bioacoustics

Introduction

Animals produce acoustic signals for various purposes. Their calls are presumably subject to natural selection. Acoustic ecology is the study of the selective pressures that act upon the calls that animals make.

Functions of animal calls

Animal calls are often (not always) used for intraspecific communication. Some exampes:

• Advertisement (long range) and courtship calls (shorter range), usually produced by males to attract the attention of potential mates.
• Aggressive signals, e.g. when there is mate competition. This may include territorial calls.
• Alarm and warning calls, to alert conspecifics to the presence of danger or predators.
• Release and distress calls, that animals produce when alarmed or threatened.

Advertisement calls are by far the most commonly heard in the forest, perhaps because their function is to attract attention. Single species often are capable of producing several different types of calls with different functions.

Mechanisms of sound production

The major calling groups of animals are: insects (especially orthopterans and cicadas), birds, anurans (frogs and toads), mammals, and even fishes (though underwater sound is not considered in this project). The methods that they use to produce sounds vary widely:

• Stridulation (Orthoptera) - like rubbing a pick across a comb,
• 'Tymbalization' (Cicadas) - rapidly 'popping' a flexible plate called the tymbal in its abdomen,
• Vocalization (vertebrates) - passing air over vocal cords which are stretched taut

Mechanisms of hearing also vary between species and groups.

Physical attributes of sound and engineering considerations

Sounds are longitudinal waves produced by compression and rarefaction of air molecules. Every sound has its own distinct wave form. However, we can decompose sound waves into component frequency spectra, producing a spectrogram, that shows what frequencies are dominant at what times. The temporal patterning (rhythm), time of calling, and the location from which a call is produed can also be considered secondary attributes like the frequency spectrum.

Sounds can be amplified and filtered, whether 'intentionally' by an animal, or simply as a consequence of the environment. Some mole crickets, for example, dig burrows in the ground of a certain length. These act as resonance tubes to amplify their calls. Sounds can also be filtered by the vegetation and geographic features in a habitat.

Sound filtering is different from sound masking. Filtering is also known as excess attenuation. 'Normal' attenuation is simply the reduction in sound amplitude as the sound wave spreads out from the source. Excess attenuation is additional attenuation on top of the normal attenuation; it is 'filtering' when certain frequencies are affected more than others, e.g. when vegetation tends to absorb high pitched sounds more than low pitched sounds. Sound masking, on the other hand, is when a sound signal is obscured by other sounds produced at similar frequencies. The source of masking is known as noise.

Sources of selective pressures

• Different kinds of habitat preferentially filter out different frequencies. The hypothesis that animals adapt their calls to the sound filtering of their preferred habitat is known as the Acoustic Adaptation Hypothesis.
• Masking can be the product of noise sources like watefalls and urban habitats. Urban birds, for example, have been found to call at higher frequencies thantheir rural conspecifics, because of the persistent low frequency rumble of urban areas.

• Other sound-producing species may mask the calls of a given species. 
• This may result in 'acoustic partitioning', one of the hypotheses that my project seeks to test.
• Species can avoid masking interference in several ways:
• Temporal partitioning (choosing to call at different times)
• Jamming avoidance (stop calling when one hears a call that would mask one's own)
• Choosing different calling sites
• Frequency partitioning (calling at less occupied frequency bands)
• One or a combination of these methods could result in acoustic partitioning.

• Eavesdropping predators and parasitoids - one can attract mates by calling, but also attract danger to oneself.
• Phylogenetic effect - animals are constrained by their physical form in what kinds of sounds they can make. This constraint is a product of development and phylogenetic history.
• Behavioral plasticity and mimicry - many animals (e.g. birds) are able to learn calls, the function of mimicry is not well known, but it shows that an individual call need not be determined completely by selective pressures.
• 'Inadvertent' evolution - when anatomy related to sound production is modified by other selective forces.

Is there an acoustic niche?

The niche concept states that different species occupy different environmental conditions as a result of competition between species. Niches are defined by the environment, the environment is modified by species, and the particular environmental parameter that is part of the niche definition has a direct bearing upon population growth or decline.

Acoustic space can be a resource niche because:

• The availability of acoustic space for animals to send signals is an environmental factor.
• The presence of species (and signals they produce) reduces that availability.
• If it is unavailable, the population growth will be reduced (for the case of mating signals).

The acoustic niche partitioning hypothesis that we are trying to test with my Pasoh data is based on the premise that signaling species seek to minimize masking interference by partitioning the frequency bands that their signals occupy - niche partitioning. The method we can use to study this is spectrogram analysis.

Open questions

The acoustic niche concept cannot explain everything. There are a few open questions that need other approaches to solve:

• How is the acoustic community assembled? (Is it more a function of minimizing acoustic interference, or do other factors such as predation and food availability come first, and adaptation of acoustic signals later?)
• What prevents an acoustic 'arms race' between species?
• Why is there a burst of sound at dusk and dawn? (The dusk and dawn choruses.)
• What determines acoustic competition in different habitat types?
• Are insects 'crowding out' birds at higher frequency bands?
• What is the function of acoustic mimicry?

Some applications of bioacoustics

Monitoring animal sounds is a useful proxy for monitoring animal populations directly, because it can be done remotely, is non-invasive, and can be done with automated equipment. The drawback for now is that a large amount of human labor is necessary to process the data, at least until computational methods become more sophisticated.

Sadly, local poachers also use 'bioacoustics' to hunt for birds. One common tactic is to set up a trap, and mimic the mating call of the bird of interest. Curious birds will come closer to find the source of the sound, and fall into the trap. Many songbirds are in high demand for the captive songbird trade, and are frequently trapped illegally in the wild.

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