The Theory of Honest Signalling

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Introduction: Part 1

Introduction: Part 2

The Basic Problem

The Basic Solution

Honest signalling in biology

Zahavi's handicap principle

Grafen's model

Attracting mates

Begging for food

Deterring predation

Contesting resources

Autumn color

Honest signalling in economics

Conspicuous consumption


The mathematics of honest signalling

Signalling as a game


Other resources

Carl T. Bergstrom

Using Mathematica

Contact Information

Department of Zoology
University of Washington
Box 351800
Seattle, WA 98195-1800

An Introduction to the Theory of Honest Signalling (Continued)

The gazelle's leaping behavior, the begging of the nestlings, the expensive suit - all of these things are signals. All are intended to convey information about a signaller, to a signal receiver. Among humans, for example, the suit - or a fast car, an expensive bottle of wine, or precious gemstone - all indicate the relative wealth or status of an individual, i.e., the "quality" of the signaller.

By their jumping or "stotting" displays, gazelles may be doing something similar. Biologists have argued that the stotting display serves as a signal of quality to potential predators. "Look," this display is implicitly saying, "I'm a strong and healthy gazelle. See how high I can jump? It would be a waste of your time and energy to chase after me." Similarly, baby birds, through their begging behavior, may be informing their parents about how badly they need to be fed. Additional examples are easy to find. Bull elks may signal their quality to rivals and to potential mates with a loud bugling call. A peacock's fine tail probably serves to advertise the high quality of its bearer to potential mates.

These signals share an additional feature: all are costly to produce and send. Expensive suits, fast cars, wine, or jewelery are not cheap in terms of that all-important human currency, money. Similarly, the stotting display is expensive - albeit in a different sort of currency - to the gazelle. By leaping up and down, a gazelle is expending the very energy that it will need desperately should a chase ensue. Squawking to one's parents takes energy and may also alert predators to the location of a nest; producing and bearing a huge flashy tail is also energetically expensive, not to mention risky when predators are around.

Why do so many different species of animals all use signals that are inherently expensive, in a wide range of different signalling contexts? Why not simply "whisper" the message to the intended receiver, rather than producing an elaborate and costly display? And why do these expensive signals seem to be so convincing to the intended signal receivers?

A field of evolutionary biology called signalling theory examines the nature of communication behavior, and attempts to answer these sorts of questions. This tutorial is intended to provide an introduction to this fascinating field, with a particular focus on the evolution and use of costly signals. Signalling theory takes a broad approach to the nature of signals, human or non-human, and as such will be of interest to researchers in a wide range of fields, including biology, economics, sociology, anthropology, political science, communication studies, linguistics, and psychology. For this reason, I have tried to keep the necessary background to a minimum, so that the text will be accessible to interested readers from all fields, irrespective of any prior background in animal behavior or evolutionary biology.

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Last modified September 4, 2002
Copyright © 2002 Carl T. Bergstrom