Telling the truth about intentions

J. theor. Biol. (1982) 97,679-689

Telling the Truth about Intentions P. G. CARYL Psychology Department,

University of Edinburgh, Edinburgh, Scotland

7 George Square,

(Received 4 January 1982, and in revised form 6 April 1982) In this paper I discussMoynihan’s hypothesis of the “sceptical recipient”, showing that it is compatible with predictions from games theory, and discuss how it might be tested in contests in which the opponents differ in resource holding potential. I also comment on evidence which suggests that in agonistic encounters information is transferred by unritualized actions, showing that these actions do not allow better prediction of behaviour than is allowed by ritualized agonistic displays, and that analysis of the interactions requires an approach based on games theory. 1. Introduction Moynihan (1982) has argued that in agonistic interactions signals of intention should be perceived readily but analysed sceptically by the recipient. I point out that this hypothesis is consistent with predictions from games theory, rather than incompatible with these predictions as Moynihan believes. Applied to cues about asymmetries such as those in resourceholding potential (RHP), rather than to signals of intention, the hypothesis of the “sceptical recipient” appears to make sense of some striking observations in the literature; I discuss experiments in which it might be tested further. Finally I comment on the implications of evidence that unritualized behaviour may transmit information about aggressive intention. 2. Should Contestants Use Information

about the Opponent’s Intentions?

Maynard Smith (1974) argued that we should not expect animals to evolve (ritualized) displays which gave accurate information about the contestant’s intentions because such a situation would not be evolutionarily stable. This prediction arose from consideration of a model in which encounters between equally matched contestants were assumed to be settled by display alone; in this situation it was found that the average costs of the display required would be equal to the average gain from victory. This is of course 679

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uneconomic. If animals indicated by the intensity of their initial display the effort they were prepared to make to win the resource, it would be less wasteful if the loser-to-be retreated immediately on seeing that its rival had shown the more intense display. But a population of animals which took the information proffered at its face value would be open to invasion by a mutant which always displayed at the highest possible intensity, no matter what its real intentions. Because of the possibility that an individual giving a very intense display might be cheating in this way, we should not expect animals to take such displays at their face value. On the contrary, a population which ignored the intensity of their opponent’s display would be evolutionarily stable. In a population that behaved in this way, there would be no selective forces leading to the evolution of displays which communicated accurately the exact degree of effort intended by highly aggressive animals. While the opponent may be cheating when it supplies information which appears to place it at an advantage, it is unlikely to be doing so when the information places it at a disadvantage. In his original paper, Maynard Smith (1974) showed that a strategy which ignored information indicating above-average persistence in fighting, but exploited information indicating below-average persistence (taking it at its face value) would be better than one which ignored all information from the opponent. On the basis of games theory, we should expect animals to make use of information which places their opponent at a disadvantage, and such information might well be available from intention movements. However, since it would not be in the sender’s interests to communicate such information, we should not expect the evolution of ritualized displays to make it available to the recipient, although evolution might well increase the recipient’s ability to detect and act on slight cues from the opponent’s behaviour. (We must distinguish clearly between changes in the clarity of the information proffered by the sender, and changes in the effectiveness with which the recipient can extract information, and it is changes in the latter which might be expected.) Thus, the prediction from games theory involves two lines of argument, one applicable when the intended effort is above average, the other when it is below average; but in neither case would the evolution of a truthful and accurate communication system be expected. 3. The Sceptical Recipient? The hypothesis of the “sceptical recipient” is difficult to test in agonistic contests in which the opponents differ only in intention; yet unless it suggests

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ways in which it could be tested, a hypothesis is of little value. In this section, I shall discuss the way in which it might be applied to asymmetric contests, and examine how it could be tested in this context. In asymmetric contests, transfer of information is to the advantage of both opponents. Games theorists have shown that animals would be expected to transfer information about differences in strength, etc., which affected the probability of winning a contest (e.g. Maynard Smith, 1979; Parker & Rubenstein, 1981) while at an empirical level, these differences are known to influence the results of contests in many species (e.g. Clutton-Brock & Albon, 1979; Davies & Halliday, 1978). Hence animals are expected to evolve signals which make conspicuous such differences in RHP, and recipients are expected to utilize whatever cues to RHP they can extract. For a discussion of the distinction between RHP and intentions in aggressive encounters, see Maynard Smith (1982). In some species, several cues carry information about RHP, and the recipient would be expected to be sensitive to all of these. But how should the recipient respond if the different cues provide information which is inconsistent? I suggest that analogy with the model discussed by Maynard Smith (1974) leads to the following rule of thumb: “An opponent may try to cheat by exaggerating cues, but is unlikely to gain any advantage by underemphasizing cues to RHP. Hence, where cues to RHP are inconsistent, it is likely to be better to believe the least impressive information that the opponent provides”. Some of the most striking observations on the response to inconsistent cues of RHP have been made by Rohwer (1977) and Rohwer & Rohwer (1978). In Harris sparrows, Zonatrichia quereula, variations in dominance are marked by the variation in pigmentation of the feathers of the throat, breast and crown. Dominant birds have dark feathers, while subordinate birds have light feathers. Rohwer found that subordinates whose throats and crowns were experimentally darkened to look like dominants (producing dominants who were “cheats”) were persecuted by the legitimate dominants. Apparently their behaviour led them to be attacked, because when it was modified by testosterone injections given in conjunction with the experimental change in plumage, the darkened birds were not attacked. In this context, where plumage and behavioural cues provided inconsistent information the birds responded to the behavioural cues. Rohwer found that when he bleached the plumage of legitimately dominant birds, their conspecifics now responded to the morphological rather than to the behavioural cues. The bleached birds could not get their opponent to retreat by threat alone, but only after a real fight, which was

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sometimes very lengthy. (The results of this fight confirmed that their natural dominant markings were associated with real fighting ability.) Rohwer noted that the morphological and behavioural cues were taken at their face value when they were consistent, but not when there was incongruity between them. In its original form, his “incongruence hypothesis” does not explain why behavioural cues should be disbelieved in one context and morphological cues in another. But if animals are using the rule of thumb “When in doubt, pay attention to the least impressive cue-that way you will not get cheated”, it becomes clear why we should expect the results that Rohwer obtained. The new hypothesis also makes it unnecessary to postulate a tendency to “punish cheats” (a tendency which is difficult to account for in terms of advantage to the individual, although it would clearly to good for the species; Maynard Smith, 1979). 4. Testing the Hypothesis

While the hypothesis of the “sceptical recipient” provides a convenient post hoc explanation of Rohwer’s puzzling observations, it must be tested in other contexts. Davies & Halliday’s (1978) work on toads is at least superficially compatible with the hypothesis. The ability of a male toad which is clasping a female to resist a takeover attempt by a rival male depends on its size. The owner’s size can be assessed by the pitch of its croak, given when attacked by a rival: small males have high-pitched croaks. Davies and Halliday showed that in experiments in which every contact was followed by a burst of croaks at the owner’s natural pitch, small males which owned a female were frequently attacked by a spare male placed in the tank, whereas large owners were rarely attacked. When the information from depth of croak was experimentally varied independently of size, change of either cue from the “large size, deep croak” condition led to an increase in the frequency of attack, and neither cue was fully dominant. It could be argued that the spare male, perceiving a high-pitched croak, treated sceptically the other cues to its rival’s size; while perceiving its rival to be small, it treated sceptically the misleading information about size provided by its deep croak. However, it is not clear that Davies and Halliday’s data fully support the hypothesis of the “sceptical recipient”. The effects of changing the pitch of the croak were quantitatively different from those of varying the size from the “large, deep” condition. For a convincing test, it would be preferable to frame the hypothesis in a quantitative form. As alternatives to this hypothesis, it is possible that the different cues would both contribute to the response, perhaps showing heterogeneous summation (Seitz, 1940;

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see also Heiligenberg, 1976) of their effects; or that one cue would be dominant, perhaps because it was more conspicuous, or a better predictor of success in contests. If the hypothesis of the “sceptical recipient” is to be supported over such competing hypotheses, it will be necessary to frame it in a strong enough form to be tested against them. Several authors have experimentally removed morphological cues used in agonistic display, and have shown that this hampers the bearer in aggressive interactions. For example, in Calcinus laevimanus, a Diogenid hermit crab, agonistic displays reveal a white area on the outside of the major chela, and the size of the white patch is correlated with the size of the individual. Dunham (1978a,b) blacked out this area of white, on the assumption that it would provide a cue to size which would be important in settling disputes, and showed that this hampered the modified crabs in agonistic interactions. Large, darkened crabs had to fight for much longer than usual to get their rival to submit; the fact that they did win eventually indicates that the natural differences in area of white reflect real differences in fighting ability. Dunham did not study the effect of increasing the area of chela white. If the hypothesis of the “sceptical recipient” is applicable in its strongest form, we should expect that when the area of white was increased, the rival should begin to place reliance on behavioural and other cues. Thus, the strong form of the hypothesis would predict that the advantage, if any, of artificially increasing the white area would be quantitatively less than the penalty conferred by an equal reduction in its area. Similarly, in Davies and Halliday’s experiments, the strong form of the hypothesis would predict that when the owner was of medium size, the decrease in attacks produced by, for example, a 20% increase in its apparent size as indicated by the pitch of artificial croaks would be less than the increase in attacks produced by a 20% decrease in its apparent size, again produced by manipulation of the croak. Two other manipulations which deserve to be re-examined with respect to this hypothesis are the effects of enlarging the black chest-patches of the golden hamster (Mesocricetus auratus ; see Grant, Mackintosh & Lerwill, 1970; Payne & Swanson, 1972) and the effect of colouring female chaffinches Fringilla coelebs to look like males (Marler, 1955, 1956). It should be stressed that support for the strong form of the hypothesis is at present equivocal. 5. Do Unritualiied

Signals Convey Intention?

Maynihan (1982) raises the possibility that if ritualized displays do not transmit information about intention, this information may be transmitted

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by other, unritualized, behaviour such as intention movements, and I agree with him that information may be extracted from such cues. van Rhijn (1980) has also argued that in aggressive interactions, “subtle actions” might be informative although “elaborate action patterns” were not, citing an important paper by Bossema & Burgler (1980). It is worth looking at this evidence in detail, because although it is not strictly relevant to the previous arguments (which concern the evolution of ritualized displays), it illustrates clearly the need for an approach from games theory in this context. Displays vary, of course, in the extent to which they have been modified during the course of evolution, and so there must be a continuum between the most highly ritualized displays, and wholly unritualized intention movements. But to simplify the discussion below, I shall assume that a clear distinction can be made between ritualized displays, and the unritualized behaviour which Moynihan and van Rhijn suggest might convey information about intentions. There are two questions at issue: (1) Do unritualized cues provide information about intention which is more precise than that provided by ritualized displays (see review by Caryl, 1979). (2) Do the recipients take the information from unritualized cues at face value; do they respond in a way which is to the advantage of the signaller? Bossema & Burgler (1980) looked at interactions between dominant and subordinate jays at a food source. Threats by the dominant were separated into monocular and binocular fixations of the opponent, and into those which occurred when the subordinate was at a long or a short distance from the dominant. The probability of attack by the dominant was greater at short than at long distances, and greater after binocular than after monocular threats. The subordinates’ readiness to retreat (as shown by the “hop back” response, the clearest escape reaction) increased across these four signal-situations in the same rank order as the dominant’s probability of attack. As Bossema and Burgler, and van Rhijn, point out, this suggests communication between dominant and subordinate, although not by ritualized displays. In one respect, the data resemble the data on ritualized displays reviewed by Caryl (1979): prediction of attack was not reliable. (The dominant’s probability of attack depended on the subordinate’s response to the threat. But even considering the response which most readily elicited it, attack could not be predicted with an accuracy of better than about 40% .) Treating the four signal situations as a single sequence confounds a difference in the dominant’s internal state (reflected in the type of fixation) with a difference in distance (a contextual factor which is evident to both opponents, and about which communication need not occur). To analyse

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the consequences for the dominant of proffering information about its internal state (monocular vs. binocular fixation) it is necessary to separate these distinct factors. It is also interesting to look at other possible actions by the subordinate, and in particular “look at” (a binocular fixation which certainly indicates reluctance to escape, and from the description and published figure may well be a form of threat by the subordinate to the dominant). Table 1 shows the data from Bossema and Burgler’s paper, with categories combined so that each contingency table shows the effect of a single factor on a single action by the subordinate. 1 The effect of distance between individuals, and the dominant’s behaviour, on two acts by the subordinate t TABLE

Hop (a) The frequency to distance Long

with

which

subordinate

back shows

Other “hop

back”

in relation 759 (713) 854

& 135

Short

W3) (b) The frequency with to dominant’s fixation Monocular

which

subordinate

shows

Look (c) The frequency to distance Long

(989) “hop

back”

in relation 514 (493) 1099 (1121)

(ii) 131 (109)

Binocular

with

which

subordinate

at shows

Other “look

at”

(136) (d) The frequency with to dominant’s fixation Monocular Binocular

which

subordinate

shows

act

in relation 703 (675) 823 (853)

(170:) 166

Short

act

“look

at”

in relation 498

(& 202 (170)

(464) 1028 (1060)

t The frequency of subordinate acts at different distances and after two types of fixation by the dominant (expected values in brackets). Data from Table 1 of Bossema & Burgler (1980).

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The effect of monocular and binocular threat on “hop back” is shown in Table l(b). When the dominant showed binocular fixation, the subordinate was more likely to hop back than expected by chance. It was also more likely than expected to show “hop back” when the dominant was at a short distance [Table l(a)]. When we compare the strength of these effects, we can see that the discrepancy between observed and expected frequencies is much greater in Table l(a) than in Table l(b); distance had the stronger effect on “hop back” by the subordinate. Table l(c) and (d) shows that when the dominant showed binocular fixation, and when it was at a short distance, the subordinate showed “look at” more frequently than expected. In this case, the dominant’s action had a stronger effect on the subordinate’s action than did the distance apart of the opponents, since the discrepancy between observed and expected frequencies is greater in Table l(d) than in Table l(c). The size of the discrepancy between observed and expected frequencies provides only a crude measure of the influence of different factors on the two actions by the subordinate, but the effects can be quantified using information theory. For instance, the extent to which uncertainty about the subordinate’s “hop back” response is reduced by knowledge of the distance between the opponents can be expressed quantitatively in terms of bits; this is the information transmission T(X; Y) between distance and “hop back” (Steinberg, 1977). Since the amount by which uncertainty can be reduced will depend on the overall uncertainty about the subordinate’s action, it is appropriate to normalize the measure by dividing by the overall uncertainty of the response, H(Y) (Steinberg, 1977). Table 2 shows the values or normalized information transmission for the four contingency tables in Table 1. The only large value for the normalized information transmission is that for the effect of distance information on escape. But the effect of intentions on “look at” is actually slightly stronger than its 2

TABLE

Transmission of information to subordinate from dominant’s behaviour, and from context Effect Hop Distance Intention

back

0.067 0.016

on Look

at

0.012 0.018

Values of normalized binary transmission (Steinberg, 1977), t(X; Y) = T(X; Y)/H(Y), are shown for data from Table 1.

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effect on escape, and examination of Table 1 shows that “look at” occurs more frequently than expected by chance after binocular fixation. When the dominant proffers information about its intentions by binocular fixation, the most powerful effect is not on its opponent’s escape but on its more aggressive response; rather than reducing aggression and enhancing escape, an aggressive signal by the dominant enhances the probability of an aggressive response by the subordinate. This example emphasizes the point made by Hinde (1981), who argued that the social releasers used in threat elicited attack as well as flight from other individuals, and that exaggeration of the probability of attack might increase the probability or viciousness of an attack on the displaying bird. So even subtle aggressive signals do not simply elicit retreat. If binocular fixation reveals aggressive intentions, then to reveal intentions is to use a two-edged sword! These data also illustrate how the process of interaction in aggressive encounters, which Hinde stresses, could be analysed in a quanttiative way. It is clear that in assessing the benefit to the dominant of monocular or binocular threat, it is not enough to consider just their effect on avoidance reponses. Instead, we must consider their effect on the probability distribution across the whole range of possible responses, together with the costs or benefits of the dominant’s replies to these responses. For example, “look at”, the response which is more strongly affected by the type of threat used by the dominant, is a response which is very likely to elicit attack by the dominant, no matter at what distance it is shown. It is possible that although more frequent use of binocular threat might bring benefits to the dominant in the form of more frequent retreat by the subordinate, it would bring even greater penalties in terms of responses which would oblige the dominant to follow the threat up with an attack. From the subordinate’s point of view, the response it shows to a particular threat is presumably dependent on the probability distribution of the dominant’s reactions to each particular response, and the costs and benefits that these entail. The simplest interpretation of these interactions (which appears to be the one favoured by van Rhijn, and by Bossema and Burgler themselves) is that of a simple communication system in which a truthful message brings a straightforward escape response. (It pays A, the dominant individual, to warn truthfully of its intention to attack, and-except in special circumstances, for instance when it is very hungry-it pays B, the subordinate, to react to this warning with avoidance, indicating that the warning signal has been understood.) But this interpretation, with its assumption that there will inevitably be a single best action for the dominant and subordinate to

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perform, is inadequate. Instead of a simple communication system in which a truthful message brings a straightforward response, we have a situation in which there is no single best action, and A’s best choice of action will depend on B’s probable choice of response, which in turn will depend on A’s likely actions given this response, etc. Games theory is clearly the most appropriate tool to use in analysing this situation. 6. Discussion

I agree completely with Moynihan (1982) that recipients would be expected to test the reliability of different bits of information by comparison with other bits, treating sceptically the information proffered by the opponent. I believe that this view is in accord with the predictions of games theory. We must be clear that the predictions to which Moynihan refers concern symmetrical contests (between individuals which differ only in the effort that they intend to invest in the particular contest) rather than asymmetric contests involving differences in class membership, such as that involved when a territory owner meets an intruder. In the latter case, there may well be differences in the displays given, and differences in the effort made to win if the opponent does not retreat in response to display alone. But these differences are both consequences of the superordinate difference in class membership. Thus while at a purely behavioural level we might say that the different calls of territory owning and intruding wagtails Motucifla alba (Davies, 1981) communicated differences in aggressive intention, it is important to appreciate that games theorists would explain both differences in terms of the owner-intruder dichotomy, and would not apply the theory of symmetric contests which we are discussing here. Where RHP differences and other asymmetries between the contestants are concerned, I expect that this process suggested by Moynihan should lead to honest communication, and in this context it should be possible to test the hypothesis of the “sceptical recipient”. It is important that this hypothesis should be tested, since several plausible alternatives exist. Games theorists have not as yet discussed in detail the use of unritualized cues in agonistic encounters. The evidence discussed here indicates that there is some transfer of information using these cues, and I have suggested that the process could best be understood if modelled in terms of games theory. But unritualized cues do not appear to provide precise information about the intention to attack, and the recipients do not respond to the unritualized cue which is a predictor of attack by simply retreating. Hence the data do not invalidate my criticisms of what I have termed the “tradi-

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tional ethological view” of agonistic communication Hinde, 1981; Caryl, 1982).

(Caryl, 1979; see also

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