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. 1999 Oct 26;96(22):12644-9.
doi: 10.1073/pnas.96.22.12644.

Signaling of need, sibling competition, and the cost of honesty

R A Johnstone  1
Affiliations

Signaling of need, sibling competition, and the cost of honesty

R A Johnstone. Proc Natl Acad Sci U S A. .

Abstract

Young birds and mammals frequently solicit food by means of extravagant and apparently costly begging displays. Much attention has been devoted to the idea that these displays are honest signals of need, and that their apparent cost serves to maintain their honesty. Recent analyses, however, have shown that the cost needed to maintain a fully informative, honest signal may often be so great that both offspring (signaler) and parent (receiver) would do better to refrain from communication. This apparently calls into question the relevance of the costly signaling hypothesis. Here, I show that this argument overlooks the impact of sibling competition. When multiple signalers must compete for the attention of a receiver (as is commonly the case in parent-offspring interactions), I show that (all other things being equal) individual equilibrium signal costs will typically be lower. The greater the number of competitors, the smaller the mean cost, though the maximum level of signal intensity employed by very needy signalers may actually increase with the number of competitors. At the same time, costs become increasingly sensitive to relatedness among signalers as opposed to relatedness between signalers and receivers. As a result of these trends, signaling proves profitable for signalers under a much wider range of conditions when there is competition (though it is still likely to be unprofitable for receivers).

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Figures

Figure 1
Figure 1
Equilibrium signal cost (or intensity) as a function of beneficiary need for different numbers (as indicated on the respective curves) of competing beneficiaries. Relatedness among all individuals is 0.5; i.e., r = s = 0.5.
Figure 2
Figure 2
Mean signal cost incurred by individual beneficiaries per benefit transferred, and as a function of r (signaler/receiver relatedness) and s (signaler/signaler relatedness) for different numbers of competing beneficiaries. Darker shading indicates lower cost, paler shading indicates higher cost.
Figure 3
Figure 3
Profitability of signaling for donors as a function of r (signaler/receiver relatedness) and s (signaler/signaler relatedness) for different numbers of competing beneficiaries. Unshaded regions are those in which donors enjoy higher inclusive fitness at the signaling equilibrium than they would in the absence of signaling. Shaded regions are those in which donor fitness is lower when signals are given.
Figure 4
Figure 4
Profitability of signaling for beneficiaries as a function of r (signaler/receiver relatedness) and s (signaler/signaler relatedness) for different numbers of competing beneficiaries. Unshaded regions are those in which beneficiaries enjoy higher inclusive fitness at the signaling equilibrium than they would in the absence of signaling. Shaded regions are those in which beneficiary fitness is lower when signals are given.
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