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Chapter 16: Signal Evolution

How does the whole process get started?

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Origins

• One or both parties (sender & receiver) must initially evolve some precursor of their eventual role in communication for other reasons.

• Other party subsequently takes advantage of this preadaptation.

• Sender vs receiver preadaptations

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Sender precursors

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Sender Preadaptation Scenario1. Signal (cue) / condition association (sender code)

2. Perception of cue (signal)

3. Association of signal with condition (receiver code)

4. Association between updated information and decision rule

5. Receiver response

Receiver benefitsSender benefits

Ritualization

Tuning

Code acquisition

Preferences

Refinement

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Ritualization• Refinement of an inadvertent cue into a

signal• Likely requires fitness benefits to sender

and receiver• Involves

– simplification or reduction of number of components

– Exaggeration of remaining components– Repetition of the display– Stereotypy during repeated renditions

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Ducks

Mallard Mandarin

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Preening & Courtship

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Food advertisement and pheasant courtship displays

Males give food calls and feed mates inBobwhite quail

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Sources of visual signals• Intention movements

– Principle of antithesis• Motivational conflict

– Ambivalence– Displacement behaviour– Redirected behaviour

• Autonomic Processes• Co-option of other displays• Incidental??

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Intention movements

• Preparatory phase of some action may become ritualized into signal of intent.

• Principle of antithesis– Signals with opposite meanings (e.g.

aggression/submission) should have opposite or contrasting form

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Intention movements

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Antithetical displays

Aggressive displays usually reflectattack preparation movements

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Motivational conflict

• Animal has several motivational systems – More or less independent sources of

motivation• System which is at the highest level

dictates the current behaviour• Hydraulic model• If it’s a tie, then what happens?

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Motivational conflict in wolves

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Ambivalence

• Intention movements from both motivational systems– Mate - intruder - mate – intruder…

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Ambivalent threat displays

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Displacement behaviour

• Something else altogether– Maybe I’ll have something to eat instead

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Displacement Acts

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Redirected behaviour

• Acts relevant to one of the dominant motivational systems, but directed toward inappropriate target– Aggressive gulls tearing at grass

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Autonomic Processes

• Turning red when angry• Hair standing on end• Eyes bugging out• Etc.

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Displays from autonomic responses

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Other displays

• If a similar response is beneficial to senders, co-opt a signal from another context.

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Display co-option

Subordinate male appeasement display mimics soliciting female

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Sender precursors of auditory signals

• Visual courtship displays– Woodcock, grouse displays

• Defensive antipredator acts– rattlesnake

• Foraging movements– Mosquitoes, woodpeckers, beaver

• Respiration– High tension vocal chords = whistle,

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Jerusalem cricket

Drums but doesn’t sing.

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Giant weta:

Scrapes legs against body, has ears.

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Antithetical vocalizations

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Sources of olfactory signals

• Dietary– E.g. insects that can handle plant defensive

compounds• Reproductive precursors and products

– E.g. hormone derivatives (wild boar drool)• Defensive chemicals

– E.g. schrekstoff or ant alarm signals• De novo production

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Bark beetle mating pheromones

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Receiver bias and feature detectors

• Feature detectors are receiver refinements that improve signal detection in noise

• Feature detectors allow for invariant responses and require no learning

• Provide explanation for sign stimuli and supernormal stimuli

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Feature detectors

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Innate releasing mechanisms

Herring gull chicks use a moving red spot on bill as a sign stimulus to recognize their mother.

A yellow stick with red spots acts as a super normal stimulus.

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Receiver precursors

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Receiver preadaptation scenarios

• Models of signal evolution that propose new signals arise as a result of changes in receiver processes.

• Sensory Drive• Sensory Exploitation• Sensory Trap

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Sensory Drive• Environmental factors shape sensory system of

receiver• This imposes constraints on the effectiveness of

different possible signals• Drives signals toward certain forms that are

compatible with the environmental/sensory conditions

• Like previous model but sender has limited choice for code– In a context like mate advertisement, both can benefit

by being more obvious to one another

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Sensory driveSignal (cue) / condition association (sender code)

Perception of cue (signal)

Association of signal with condition (receiver code)

Association between updated information and decision rule

Receiver response

Receiver benefitsSender benefits

Ritualization

Tuning

Code acquisition

Preferences

Refinement

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Signalling may vary with habitat

Phylloscopus warblers: species in brighter habitats use more colour.

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Physalaemus pustulosus, the tungara frog

Sensory Exploitation

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Sensory exploitation

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Sensory Exploitation• aka pre-existing bias or hidden preferences• Pre-existing or latent preferences in receivers

are “exploited” by senders– Does not specify origins of these hidden preferences,

but some theoretical models exist• Adjust signals to match receiver bias• Origins only

– Unless receivers benefit, they will be under selection to discriminate more precisely

– “exploitation” may be a misleading term

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Water mite courtship evolution

T = male trembling mimicscopepod preyN = female net stance

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Sensory Trap• Similar to sensory exploitation, but includes

origins of female preference– Red berries hypothesis– Water mites, pretty good evidence

• Also includes importance of benefit to receiver

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Female preferences evolved prior to male swordtails

Phylogeny based on morphological data

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or do they?

Phylogeny based on mtDNA sequence data

Black = swordtailsWhite = no swordHatched = ambiguous

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How can any of this work?

• Neural network models– Trained to discriminate a given set of signals– End up particularly sensitive to a more

extreme signal (that was never part of the training)

– Could be mechanism for super-normal stimuli• Could be analagous to natural processing

mechanisms in sensory/nervous system