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

Costly signalling from trees to pests:
Autumn coloration of deciduous trees

Are bright autumn leaves a costly signal from trees to pests?

One of the most recent - and most exciting - applications of the handicap principle purports to explain the bright autumn colors displayed by many deciduous trees.

Sam Brown and the late W. D. Hamilton propose that bright leaf coloration is not just a side-consequence of chlorophyll reabsorption. Rather, autumn color may be a signal from trees to aphids (and other pests) that are looking for places to lay their eggs. Their argument is presented in three parts, each with substantial empirical support.

  • Autumn color (red, in particular) is nutritionally costly to trees.
  • Aphids, a particularly damaging group of tree pests, appear to preferentially avoid trees with bright red or yellow leaves when colonizing trees in autumn.
  • Tree species with bright (red or yellow) autumn leaves have more specialist aphid pests than do trees lacking bright autumn coloration.
The evidence is certainly suggestive of costly signalling. But what, precisely, are the trees communicating with their signals? Brown and Hamilton argue that they may be communicating the degree of to which they have invested in chemical anti-pest defense, a character which varies widely both within and among tree species.

Brown and Hamilton do not claim that their study is the last word on autumn coloration. Rather, their study beautifully illustrates one of the most important roles of theory in biology: to generate testable predictions. They have provided a set of predictions, and now we will have to wait and see whether they are borne out by further empirical study. If so, autumn coloration will certainly stand as one of the most spectacularly visible instances of the handicap principle in nature.


The autumn color hypothesis was proposed by Brown and Hamilton (2001). The full citation is listed in the references.

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