DYNAMICS OF HIERARCHY FORMATION: THE SEQUENTIAL ...

Behaviour, volume 80, 218-240/ 1982

DYNAMICS OF HIERARCHY FORMATION: THE SEQUENTIAL DEVELOPMENT OF DOMINANCE RELATIONSHIPS by IVAN D. CHASE1 (Department of Sociology, State University of New York at Stony Brook, Stony Brook, N.Y. 11794, U.S.A.) (With 3 Figures) (Acc5-IV-I982) A central unresolved problem in the study of dominance concerns dynamics: what are the behavioral processes leading to hierarchy formation? For example, an experimenter brings a group of previously unacquainted animals together and a short while later observes a stable dominance hierarchy based upon agonistic behavior. What are the patterns of interaction used by these newly assembled animals in "deciding" their dominance relationships? How did they go from not knowing one another to clearly recognizing members to which they were dominant or subordinate? The goal of this paper is to investigate the dynamics of hierarchy formation and start answering questions such as these. This paper develops a general method for examining the theoretically possible sequences of relationship formation for animals "winning" dominance encounters, "losing" dominance encounters, and bystanders to the dominance encounters of others. Some of these sequences are congruent with the kinds 1 This study was supported by grants from the Harry Frank Guggenheim Foundation, the U.S. Public Health Service (Biomedical Research Support Grant 5 S07 RR 07067-13 to SUNY at Stony Brook), the University Awards Program, and the Research Development Fund of the State University of New York at Stony Brook. I thank Jack HAILMAN for his advice and encouragement during t h e first pan of this research and Jeffrey BAYI.IS, Shelly COHEN, Scott FELD, Mark GRANOVETTER, Erica JEN, Bob NORMAN, Gene WEINSTEIN, and Bill WIMSATT for providing criticisms of earlier papers and suggesting a variety of helpful ideas. I would like to thank Paul SIEGEL for help with my i n i t i a l efforts in the observation of dominance behavior; Gordon STEPHENSON for aid with the SSR System; Phil REESE and Jo Ann COMITO for assistance with the Apple microcomputer system; Gary SIMON for statistical consultation; Erica JEN for aid in data analysis; and Shelly COHEN, Sara EDWARDS. and Nancy RAFFETTO for assistance in data collection. Part of this study was completed at the Department of Zoology, University of Wisconsin-Madison, and I thank the department for its support.

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of hierarchy structures commonly found in many species while others are not. The method is illustrated by application to experiments on hierarchy formation in chickens. CHASE (1980) hypothesized typical strategies for winners, losers, and bystanders in dominance encounters and developed a method for the sequential analysis of successive, individual aggressive acts. This paper extends the method to the sequential development of dominance relationships and tests empirical support for the hypotheses. Part I of this paper introduces the method and uses it to analyze the dynamics of dominance relationship formation in groups of three chickens. Part II generalizes the method, uses it to analyze hierarchy formation in groups of four chickens, points out the implications for explaining typical hierarchy structures in chickens and other species, and reviews the available data in other species. Part I

Animals and methods. Animals. I used 27 two-year old white Leghorn hens, 24 hens in the original design and three to replace animals that died or became ill during the course of the experiment. The hens were purchased at one week of age from a commercial hatchery, housed in a common cage until approximately four months old, and then kept in individual cages (74 x 77 x 43 cm). The hens had met in pairs for a previous dominance experiment, but had been separated for several months before this experiment. Hens separated for two or three weeks appear to forget a previous dominance relationship (SCHJELDERUP-EBBE, 1935; GUHL, 1975; MAIER, 1964 but cf. SMITH & HALE, 1959 for chickens with relationships modified through aversive sti mu li) . Experimental apparatus. I tested the hens in a separate observation room using a neutral cage 1 m in diameter and 71 cm high. Partitions divided the cage i n t o three sectors of equal area and were used to keep the animals apart until the experiment began. The cage had one food and one-water dispenser and each was available ad libitum. Experimental procedure. After approximately 10 minutes in the observation cage, I removed the partitions separating the chickens and began to record data. After four hours of observation, the chickens were returned to their home cages. The author and a research assistant took alternate one hour shifts of data collection; the pretest interobserver reliability was 89%. (formula: number of interobserver agreements/number of interobserver agreements + number of interobserver disagreements.) The observer sat in a low chair at chicken-eye level and recorded data with an SSR Keyboard. (See STEPHENSON el at., 1975 and STEPHENSON, 1979 for a description of the SSR Keyboard.) All occurrences of the following four aggressive contact behaviors were recorded: (1) Peck: one hen strikes or grabs another with its beak or pulls another's leathers; (2)Jump On: one chicken jumps into the air s t r i k i n g another chicken with one or both of her feet; (3) Claw: one chicken scratches another with the claws of one foot while keeping the other foot on the substrate; (4) Nib-

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ble: one chicken lightly brushes the feathers or wing identification badge of another with its beak (any beak contact to the head or neck, no matter how light in intensity, was coded as a Peck). The SSR Keyboard automatically provided the real time of each action. A combined total of 2801 aggressive acts were recorded in 96 hours of observation time; the average rate of aggression was 29.2 acts per hour or 116.7 acts per triad.

Results Dominance c r i t e r i a . I used two criteria to determine the formation of dominance relationships. One animal was considered to dominate another if she: (1) delivered any combination of three strong aggressive contact actions (pecks, jump ons, and claws) to the other and (2) there was a 30 minute period following the third action during which the receiver of the actions did not attack the initiator. The involvement of either the initiator or the receiver with other chickens or the length of time necessary to complete three actions was not considered in these criteria. Once formed, a dominance relationship could be reversed if a subordinate fulfilled the criteria over a previous superior. Although somewhat arbitrary, these criteria have support as a valid indicator of stable dominance relationships; only two out of 52 dominance relationships initially formed were later reversed. The average time required to fulfill the criteria in the 52 initial relationships was 82.6 minutes; only two relationships were formed less than 30 minutes before the end of an observation period. There was, then, a sufficient time period after the formation of most relationships for reversals to occur. Sequences of dominance r e l a t i o n s h i p formation. There are four and only four possible configurations for the first two dominance relationships formed in a triad. By convention, I will call the chicken becoming dominant in the first relationship A, the initial subordinate B, and the bystander C. After the initial dominance relationship: (1) the initial dominant can dominate the bystander (Double Dominance or DD), (2) the bystander can dominate the initial subordinate (Double Subordinance or DS), (3) the bystander can dominate the i n i t i a l dominant (Bystander Dominates I n i t i a l Dominant or BDID), or (4) the i n i t i a l subordinate can dominate the bystander (Initial Subordinate Dominates Bystander or ISDB). These four possible sequences are shown in Fig. 1. If the dominance relationships in a triad are formed randomly, i.e., if there is a .5 probability of either animal dominating in a pair, then each sequence has an equal .25 probability of occurrence. Although each pat-

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tern is equally probable in theory, the Double Dominance sequence occurred in the great majority (17 out of 23 triads or 74%) of triads (dominance relationships were not formed in one triad and it was dropped from this analysis). Of the remaining three sequences, Double Subordinance occurred four times (17%) and BDID and ISDB were quite rare indeed occurring once each (4%). (χ2 = 30.4, d.f. =3, p