COMMUNICATION OF INTENTIONS IN AGONISTIC CONTEXTS BY THE PIGEON GUILLEMOT, CEPPHUS COLUMBA by DOUGLAS A. NELSON1) (Museum of Zoology and Division of Biological Sciences. The University of Michigan, Ann Arbor, Michigan, U.S.A.) (With10 Figures) (Acc.28-VII-1983) Introduction In the last decade, beginning with MAYNARD SMITH & PRICE'S (1973) game-theory analysis of fighting behavior, there has been an increase of theoretical interest in the factors influencing animal aggression. Subse- quent papers by PARKER (1974), MAYNARD SMITH (1974, 1979), and MAYNARD SMITH & PARKER (1976) have led to predictions concerning when it should benefit animals to behave aggressively. They have also provided novel interpretations of the functions of the displays employed in agonistic situations. The models of agonistic behavior developed by MAYNARD SMITH and others distinguish between contest situations according to whether or not the contestants are evenly matched in such factors as fighting ability and the expected benefits to be gained from winning a contest. In "sym- metric" contests in which the opponents are equally matched, MAYNARD SMITH (1974) predicted that animals should adopt a "war of attrition" strategy which involves witholding information about intentions and per- 1) Research was supported by grants from the Frank M. Chapman Memorial Fund of the American Museum of Natural History; the Division of Biological Sciences, the Horace H. Rackham School of Graduate Studies, and the Museum of Zoology of the University of Michigan; and a predoctoral fellowship from the National Science Founda- tion. The real-time spectrum analyzer facilities were provided through a NSF grant (BNS 78-03178) to R. B. PAYNE.L. I. GROSSMANprovided access to the graphics calculator. The Point Reyes Observatory and San Francisco Bay National Wildlife Refuge made possible the field-work on the Farallones. I thank G. F. ESTABROOK,B. A. HAZLETT,R. B. PAYNEand R. W. STORERfor their comments on an early version of the manuscript, and L. LEE, N. ODEN,and D. SMITHfor help with data analysis. This is contribution No. 257 of the Point Reyes Bird Observatory. 146 sisting in a dispute for an unpredictable length of time. Symmetric con- tests are probably rare in nature as animals differ in fighting ability, age, size, prior ownership of resources, and the potential benefits realized from contesting a resource. In such "asymmetric" contests the models predict that using any asymmetric cues to settle contests without escala- tion is selectively advantageous (MAYNARD SMITH & PARKER, 1976; HAM- MERSTEIN, 1981; PARKER & RUBENSTEIN, 1981). The contestant which is stronger or stands to gain the most from a contested resource will be dominant. Escalation will tend to occur when information about asym- metries is imperfect. As a corollary these models suggest that animal displays in agonistic contexts function as mechanisms whereby opponents assess the probability of winning a fight (PARKER, 1974). Several studies have demonstrated an association between patterns of aggressive behavior and the potential energetic or reproductive benefits associated with contested resources (GEIST, 1966; RAND & RAND, 1976; RIECHERT, 1979; CLUTTON-BROCK et al., 1979; EWALD & ROHWER, 1980; RUBENSTEIN, 1981; SIGURJONSDOTTIR& PARKER, 1981; HAZLETT, 1982). However, fewer studies have examined the function and information content of agonistic signals with regard to recent theory (DAVIES & HALLI- DAY, 1978; CLUTTON-BROCK & ALBON, 1979; CARYL, 1979). At issue is the degree to which threat displays provide precise information about the ac- tor's tendencies or intentions. "Graded" displays are hypothesized to be potentially rich in motivational information because the continuous variation in the display's form relates to continuous changes in underly- ing motivational state (GREEN & MARLER, 1979). In line with this, MOR- TON (1977) has proposed a "motivation-structural rule" for vocal com- munication which predicts that high attack tendencies will be associated with low-pitched harsh sounds and that appeasing tendencies will be typified by high-pitched tonal sounds. In contrast, several authors have questioned whether it benefits in- dividuals to convey information about intentions (MAYNARD SMITH, 1974, 1979, 1982; DAWKINS & KREBS, 1978; CARYL, 1979). MAYNARD SMITH (1974) originally predicted in the context of the war of attrition that animals should not convey information regarding withdrawal inten- tions. This prediction was later generalized by CARYL (1979) and MAYNARD SMITH (1979) to include information about the level to which the signaller will escalate, i. e. , its "intentions" should not be com- municated, even in contests with pay-off asymmetries. Theoretical and empirical problems with this generalization will be discussed later (VAN RHIJN, 1980; HINDE, 1981; CARYL, 1982a). However, CARYL'S (1979) .