The majority of behavioral interactions between animals involve communication using signals or displays

 

Behavioral displays:

Threat displays

Territorial or advertisement displays

Association or "contact" displays

Begging displays

 

Structural displays:

Coral Snake	The Red-lipped Parrotfish (Scarus rubroviolaceus): Males develop large, bulbous snouts with age.
	Adult male. . . . . . . . . . . . Young fish or female

 

Many displays involve both behavior and structure

e.g., the Anolis dewlap display

 

Effective communication is the product of natural selection, so we expect selection for specific kinds of signals to evolve, given the context of the communicative bout.

Sorting out what factors influence the evolution of signals can be a challenge:

For example, check out the various types of auditory signals that animals generate when communicating with sound: Visit the Cornell Lab of Ornithology's Macaulay Library for bird song examples, as well as many other animals.

 

Three basic ways that selection may influence signals

 

Ecological constraints that arise from specific physical environmental conditions

 

Physiological constraints on the ways an animal may produce or receive signals

 

Social constraints that arise from the response of recipients to displays

Defining communication: displays or actions by one individual (actor) that modify the behavior of another (reactor)

Many possible examples using this definition

 

For this class: the process in which specially designed signals or displays modify the behavior of recipients of the signal or display.

 

By inference, this involves the provision of information by a sender that can be utilized by a receiver during a response.

Ecological constraints and communication - think chemistry and physics

Communication proceeds along one or more sensory modalities

 

Chemicals (smell and taste): Dispersal of organic chemicals

Requires individual molecules to move the entire distance between sender and receiver (slows transmission relative to other forms of communication)

Currents, diffusion, boundary layers.

Molecular vibration: Propagation of pressure waves through a molecular medium (no sounds in a vacuum).

Sound (longitudinal pressure waves) vs Touch (both transverse and longitudinal pressure waves... can include heat)

Characteristics of sound: amplitude and frequency (speed of propagation depends on medium)

Attenuation (more ordered than diffusion), interference, reflection, refraction, and absorption

Doppler shifts (not significant for most communication)

Linearity of sound within liquid and gas allows recipients to extract a specific signal from many sounds (complex waveforms can be broken down into component waveforms)

Light (vision): Propagation of a type of electromagnetic energy

Characteristics of light: frequency or wavelength (speed of propagation is constant), intensity (brightness)

As with sound: reflection, refraction, and diffusion

Straight line transmission.

"Visible" light and the spectrum of light energy as a function of wavelength

Polarization - the orientation of the wave form

Electrical (aquatic habitats only... air insulates)

Characteristics of electrical signals: Charge, potential, polarity... the production of fields.

Field shapes (monopoles and dipoles)

Production of electrical signals limited to vertebrates

 

To understand communication, we must consider how these signals are produced, propagated, and received... again, know some basic physics and chemistry

We will examine sound, sight, and olfaction (smell/taste)

 

Sound

Sound production involves three steps.

1) Producing vibrations

 

Monopoles, dipoles, tetrapoles and directionality

Pressure (amplitude) depends on vibration speed and volume (size)

Muscle contractions are generally < 1 kHz....

Frequency multipliers

 

2) Modifying vibrations

 

Modification via resonators:

Source Driven

Response Driven

 

3) Coupling vibrations to the propagating medium

 

In air, thin membranes

What about water?

 

Sound propagation

Global attenuation (inverse square rule for all frequencies)

Medium absorption (frequency dependent)

Interference and reinforcement

 

Sound reception

Translating physical energy to electrochemical signals.

Particle detectors (many invertebrates: highly directional)

Pressure detectors (many vertebrates, non directional for single receiver)

Pressure differential detectors: (small vertebrates: can provide directionality)

Sight

With visible light rays present, an organism's presence and location can be detected with no active signal production.

Light production:

Color production via absorption and reflection of specific frequencies.

Pigments

Iridescence via interference

Scattering

Bioluminescence

 

Light propagation between sender and receiver

Attenuation and background noise relative to available light

No transmission past opaque objects

 

Light reception:

Contrast (color and spatial pattern), coupled with intensity allow detection vs a background.

Sensitivity (maximized by aperture)

Resolution (maximized by focal length)

Ability to focus(accommodation by the lens)

Depth perception

 

Olfaction

Chemical production

Pheromones

Scent glands/ducts or packaged in waste: Urine/Feces

"Packaging"

How quickly does the molecule degrade? Volatility/Solubility (molecule size)

How is the molecule produced in time (rate): e.g., Pulse vs continuous production

 

Chemical propagation

Diffusion

Concentration

Durability

Still vs moving currents

 

Chemical reception

Sensitivity vs concentration

Specificity

 

Beyond understanding the physics of communication modalities, animal communication requires knowledge about:

 

The physiological structures used by animals to send and receive signals

 

Selection upon both sender and recipient with regard to the information being communicated (next lecture)

 

Both of these may constrain the way in which signals are sent and received

 

Physiological structures for recieving signals are many and varied: e.g.,

 

Eyes (visible signals)

 

Ears and other pressure sensitive structures like hair and feathers (for hearing and touch),

 

Noses and tongues (for chemical signals),

 

Lateral lines (for pressure waves or electrical signals).

 

Similar physiolgical structures have evolved for sending signals, and their design will influence the type of signals that can be sent.

 

This, in turn, may cause selection for specific types of receivers (e.g. Cricket Frogs).

 

For example, consider structures for producing sounds in vertebrates:

 

Sounds produced by air being pushed through a tube past a membrane that is induced to vibrate by four basic forces

 

Mammals produce sounds with a larynx via the following steps:

1) Muscles pull the vocal chords (glottis) into the center of the larynx and blocks air flow.

Note: exhalation is "passive" (exhaled air comes from the relaxation of the diaphram, a muscle sheet below the lungs)

2) Air pressure builds up behind the vocal chords and then is released when it becomes too great.

3) Bernoulli forces quickly bring the vocal chords back together, creating periodic air puffs (pressure waves) that travel out the mouth or nose.

4) Period (pitch) is set primarily by vocal chord thickness (130 pulses/sec for men, 220/sec for women)

5) Such sounds have many harmonics, allowing resonant filtering and selective amplification to modify the final signal.

From: Bradbury, J.W. & Veherencamp S.L. 1998 Principles of Animal Communication (p 95)

Here is a video clip of human vocal chords in action

Exceptions to these basic patterns:

Echolocating bats have additional membranes "upstream" of the vocal chords

Porpoises and whales have air sacs in the head: air passing back and forth between lungs, trachea, and these sacs moves across additional membranes.

 

Frogs and Toads (anurans) also use a larynx to produce sounds: two differences from mammals

 

1) Another set of thin membranes before the glottis acts as the main source of sounds (similar to echolocating bats).

These vibrations are independent of the size of the opening of the glottis and they vibrate and a much higher frequency (0.5 - 2.0 kHz vs. 100-200 Hz).

This allows more complicated sound generation, as intial sounds from the vocal chords (first membrates) are modified by the opening of the glottis.

 

2) Unlike mammals, these vibrations are passed into the throat (buccal) sac.

This can force the air back into the lungs and allows additional control on rates of airflow across the vocal chords

The buccal sac may also act as a resonating chamber for propagating the signal into the surrounding air

From: Bradbury, J.W. & Veherencamp S.L. 1998 Principles of Animal Communication (p 98)

Birds sounds are produced differently than by anurans and mammals.... as a result they are remarkably more complex.

 

1) Exhalation is "active"; muscle contractions expel air (= greater control on the rate and force of exhaled air).

Additional, inspiratory muscles allow "mini breaths" during songs.... this allows much longer songs.

By itself, this allows for more complicated sound production.

 

2) Sound production occurs in a modified junction of the bronchi called the syrinx

Missing or reduced cartilage rings are replaced by membranes that may be forced into the airflow by muscles, air pressure, and Bernoulli forces.

 

3) The location of these vibrating membranes varies among species: e.g. chickens, vs. song birds, vs. cuckoos and penguins.

From: Bradbury, J.W. & Veherencamp S.L. 1998 Principles of Animal Communication (p 102)

Thus, most birds can sing two songs at once. Environmental conditions limit the range of signal possibilities

Lots more information on bird vocalizations

 

Physiological constraints (costs) may further limit the degree to which certain signals are expressed

 

Whether signals may be easily turned on or off (e.g. sounds, movements) as opposed to always on (e.g. plumage patterns) may also be important.

Type of Signal

	Chemical	Auditory	Visual	Tactile	Electrical
Spatial range	Long	Long	Medium	Short	Medium
Rate of signal degradation	Slow (relatively)	Fast	Fast	Fast	Fast
Ability to pass obstacles	Good	Good	Poor	Poor	Medium
"Locatabililty"	Variable	Medium	High	High	Medium
Energetic cost	Low	High	Low	Low	Medium

Evidence for environmental constraints:

 

Correlation between birdsong type and habitat (open grassland vs. forest)

Some examples of a grassland bird