In a post last year on Honest Signals I mentioned that a book on Animal Signals by John Maynard Smith and David Harper was scheduled for publication.

This has now been published in the series Oxford Studies in Ecology and Evolution. The details are:

John Maynard Smith and David Harper: Animal Signals Oxford University Press, 2003. ISBN 0 19 852685 7.

This is not a popularisation, but a scholarly monograph. At a price of about $45 or £25 for a slim (160-page) paperback it is not cheap. But for anyone with a serious interest in the subject it will be valuable for its clarity and breadth of coverage.

The main problem in the field is to explain how signals can be reliable, given that there are often incentives to ‘cheat’. The most fashionable theory in recent years has been the Handicap Principle of Amotz Zahavi, which maintains that signals can only be reliable if it is costly to produce them, in such a way that it is more costly for a dishonest signal than an honest one. Maynard Smith and Harper argue for a more pluralistic approach. They show that there are several circumstances in which signals can be reliable without cost (beyond that necessary for effective transmission):

a) where the signaller and receiver place the possible outcomes of the signal in the same rank order
b) where dishonest signals are punished
c) where although signaller and receiver prefer different outcomes, they
share an overriding common interest (such as avoiding an escalated conflict)
or
d) where the individuals concerned interact repeatedly.

Where these circumstances do not apply, the Handicap Principle is one way of ensuring reliability, but there is sometimes an alternative. Maynard Smith and Harper distinguish between a Handicap, which they define as a signal whose reliability is ensured because its cost is greater than required by efficacy requirements, and an Index (plural Indices), which they define as a signal whose intensity is causally related to the quality being signalled, and which cannot be faked. A good example of an Index is the practice of tigers marking their territory by standing on their hind legs against a tree and scratching the bark as high up as they can, which gives a reliable signal of their height. An Index may or may not be costly to produce: its reliability depends on its causal relationship with the quality signalled, not on its cost.

Maynard Smith and Harper recognise that in practice it may be difficult to distinguish between an Index and a Handicap. Whether or not a signal is fakeable depends on circumstances, which may change. If tigers learned to stand on boxes (and had a supply of boxes) the tree-scratching signal would no longer be reliable. But fakeable signals may also become unfakeable. For example, in displaying its chest a bird may exaggerate its size by fluffing out its feathers, but if all birds do this, the display becomes a reliable indicator of size again.

The authors believe that both Handicaps and Indices exist among animal signals, but they are inclined to ascribe to the Index principle many examples (such as antelopes’ stotting) that Zahavi would regard as Handicaps. Zahavi would probably argue that there is no such thing as an unfakeable signal, and it is only the extra cost of faking that ensures reliability. But the distinction does seem valid in principle, and the authors' discussion of problem cases is persuasive.

In Googling for a paper referred to the book’s bibliography, I came across the following recent paper by Lachmann, Bergstrom and Szamado: Cost and conflict in animal signals. This is also very useful on the subject.