The Theory of Honest Signalling

This is an old version of the site. Unless you have some particular reason to want the old veevrsion, you'll probably do best to head over to the
new one.


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

Honest signals and sexual selection:
The peacock's tail

How can animals use costly signals to attract mates?

Consider the following hypothetical scenario. In a particular species of birds, female-choice sexual selection is operating: females pick mates from a pool of males of varying qualities. The process of choice is not entirely straightfoward, however. Females are unable to assess directly the quality of the males. Instead, males advertise their quality via a display of some sort, for example, with bright plumage and a long flamboyant tail. This advertisement is a handicap in the sense that it is energetically costly and possibly dangerously conspicuous as well. As a male adopts brighter colors and a longer tail, his chance of surviving declines.

But the cost of producing a long tail varies among males. A weak and sickly male can scarely afford to put energetic resources into production of mere ornaments, and moreover he would have a hard time escaping from a predator if his flight was hindered by a long tail. A strong and healthy male, by contrast, can afford the energetic expenditure necessary to produce bright colors and a long tail, and, moreover, can usually escape a predator even when his flight is somewhat restricted by the length of his tail.

In this population of birds, females prefer males with bright colors and long tails. High quality males choose to produce these bright colors and extravegent plumes, to ensure that they are chosen as mates by females. Low quality males cannot afford to do so, and so they will produce duller colors and shorter tails. (Perhaps next year they will be stronger and thus will be able to be more ambitious.)

And so in this population of birds, the bright colors and long tails are honest signals of male quality, used by females to choose their mates. This is the basic idea behind the use of the costly signals in a sexual selection context. Of course, the costly signal need not involve bright coloration and extended tail feathers; a big rack of antlers, an elaborate song, a captured prey offered as a gift, or any number of other expensive ornaments or gestures could serve equally well. Nor, for that matter, must the male sex be the signalling sex. In some cases, females may use costly signals to advertise their own qualities to male suitors.


Nur and Hassen (1984) lay out this basic argument with an elegant graphical approach. Grafen (1990a,b) provides a mathematical demonstration that such a system is evolutionarily stable. Johnstone (1995) reviews the empirical evidence for this type of sexual selection signalling. In addition, numerous other papers consider the various subtleties of costly signalling in a sexual selection context.

[ Previous Page ] [ Next Page ]

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