Debating about Animal Signals & Communication

Oren Hasson

Letter from John Maynard Smith: September 7^th, 2000

The following is the relevant part of John Maynard Smith’s last letter in this correspondence, on September 7^th, 2000. It summarizes our discussion regarding the distinction between amplifiers and indices, and gives his conclusions:

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Dear Oren,

Thanks for the letter. I don’t think there is any disagreement between us about evolutionary mechanisms. If we do disagree, it is about terminology. As I see it, your term “amplifer” refers to a signal that provides information to a receiver about quality of the signaler. It has two characteristics. It makes differences in quality easier to perceive, and it cannot easily be faked. I think (?) you chose the term because the signal increases (=amplifies) the fitness differences (your S component) between signalers of high and low quality. Your paper (1992) with Shmida [Hasson Cohen & Smida 1992 – OH] presented a model to explain why signalers of low quality should make a signal, although it costs them.

“You contrast this with a signal that makes quality appear “higher” – eg arching the back, lowering the larynx. Such a signal is unreliable when first appears, but is soon fixed in the population, and the reliability of the amplifier is restored. In Taylor et al 1999 you refer to such a signal as an “index” (I’m not sure when you first used this term in this sense).

“I hope I have understood this right. Our difficulty is that, when we first used the term index (in our 1995 paper), we used it in almost precisely the sense that you had earlier used amplifier. Our emphasis was on the “cannot be faked” aspect of an index, because our main aim was to distinguish such signals from handicaps (which = at least in Grafen’s version – are not faked because it would be too costly to do so). I still think this is the distinction that most needs making. After all, “making easier to perceive” is a feature of most signals (tho’ not all) – it is what Guilford & Dawkings mean by “efficacy”. Unfortunately, I had not appreciated at the time that you were using amplifer to mean this type of signal – I should have, because I had seen your 1994 “Cheating Signals” paper. Had we done so, we would certainly have pointed out that we intended “index” to have the same meaning as “amplifier”. However, I think we would still have introduced the new term. It has the advantage of being defined in semiotics as “signifying in virtue of a causal relation between sign and object”. (Although my recent reading in semioticians convinces me that they are inconsistent in their use of terms ethologists! [I suppose that JMS meant “as are ethologists” or something of that sort – OH]. In some cases they use index to mean what you would call a “pointer”). Also, for me “amplifier” does not readily convey the meaning of being hard to fake. Incidentally, I think Enquist’s (1985) phrase “performance-based signal” has the same meaning of “hard to fake”.

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Here, I (OH) omitted from the letter parts that relate to Maynard Smith's plans for the book, and names of peoples he corresponded with. The letter ends up as follows:

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We are very grateful for the trouble you have taken to make things clear... ...As you said in your first letter, there is no way we can lay down an "official" terminology, but at least I hope we will not add to the confusion. Our exchange of letters has been very helpful to us.

With all best wishes,

John