Overview of the topic: Recent theoretical and em pirical work on social behavior has emphasized the complex nature of animal societies, in which individual behavior combines elements of conflict and cooperation. Many studies have explored how aggression mediates conflicts over access to resources, such as food or reproductive output. We propose that a different perspective on aggression merits increased attention: the role of aggression in organizing group behavior. The unifying theme of this session will be that intra-group fighting ca n provide an important mechanism for establishing social structure and regulating division of labor among group members.

The session will present examples of empirical studies on a wide diversity of animal species. Bringing together speakers that share interests in social behavior and aggression, and who are familiar with different groups of animals, will encourage the application of new approaches to research on social aggression in diverse animal societies. We hope to identif y common themes that apply across different taxa, and to recognize how a species' social structure and its evolutionary history might shape the expression and the function of social aggression. A second objective is to compare species that exhibit different levels of social complexity, which may yield insights into evolutionary transitions in the role of aggression. One interesting possibility is that aggressive interactions may be modified by selection on colonies or other kin groups, such th at division of labor and group fitness are optimized. Aggressive interactions may become increasingly ritualized, functioning more in regulating task performance and less in direct competition.

Dr. Scott Creel

Department of Biology, Montana State University, Bozeman, Montana 59717 (406) 994-2460

Title: Aggression, Dominance and Social Stress in Vertebrates

In most cooperatively breeding birds and mammals, reproductive rates are lower for social subordinates than for dominants, and it is common for reproduction in subordinates to be completely suppressed. Early research conducted in captivity showed that losing fights can increase glucocorticoid (GC) secretion, a general response to stress. Because GCs can suppress reproduction, it has been widely argued that chronic stress might underlie reproductive suppression of social subordinates in cooperative breeders. Contradicting this hypothesis, recent studies of cooperative breeders in the wild show that dominant individuals have elevated GCs more often than subordinates do. These findings complicate the conventional view of social stress: elevated GCs can be a consequence of subordination or a cost of dominance, with broad ramifications for the evolution of dominance and reproductive suppression.

Dr. Robin Foster (co-or ganizer)

Department of Psychology, University of Puget Sound, Tacoma, Washington 98416 (253) 879-3759

Title: Worker Reproductive Conflict and Division of Labor in a Simple Society- Bumble Bees

In primitively eusocial bumblebees (Bombus), workers frequently lay unfertilized eggs which develop into males. Direct competition for the opportunity to reproduce appears to influence the frequency of aggressive interactions among nest mate workers withi n a colony. This study examines correlations between levels of aggression, worker reproductive condition as measured by ovary maturation, and tasks performed by workers. Important tasks accomplished by workers include foraging, nest defense, brood care, brood thermoregulation, egg-cell construction and egg laying. Dominant workers engage in the most aggressive interactions, and they are also less likely to leave the nest to forage and more likely to construct brood cells and lay eggs. Theref ore, while aggression is likely correlated with individual bee's reproductive condition and behavior (clearly most agonistic interactions take place at the site of egg-laying), it also may also directly affect a colony's social organization by mediating division of labor.

Dr. Deborah M. Gordon

Department of Biological Sciences, Stanford University, Stanford, California 94305-5020 (415) 725-6364

Title: The Regulation of Activity in So cial Insects

Social insect colonies adjust the numbers of workers engaged in different tasks, so as to allocate more effort when needed to a particular task. The basic question about task allocation is how, without any direction or central control, individuals decide to perform a task. Recent work from a variety of social insects suggests that individuals use cues based on the rate of, or waiting time between, some simple, repeated social interaction. In harvester ants, an impo rtant cue in task allocation is the ant's recent history of brief antennal contacts with other ants. Rates of encounter help regulate activity in several tasks. One example is the onset of foraging activity each day, which appears to be regulated by interactions between patrollers, who return to the nest after searching around outside, and foragers waiting inside the nest entrance. In many animal groups, the function of a particular interaction between 2 individuals, from a brief antennal cont act to an aggressive act, may be as an element of a pattern of interactions. The pattern of interactions, rather than the interaction itself, may regulate social behavior in the group.

Dr. Sean O'Donnell (co-organizer)

Department of Psychology, Animal Behavior Program, University of Washington, Seattle, Washington 98195 (206) 543-2315

Title: Evolutionary Modification of Aggression and its Role in Division of Labor

Soc ial aggression is thought to be closely related to conflict over direct reproduction in simple paper wasp societies. I will present evidence from adult-producing (post-emergence) colonies of the wasp Mischocyttarus mastigophorus that, as expected, queens direct their aggression toward female nest mates with high reproductive potential, but that workers and males do not. Worker and male aggression instead appear to be based on competition over food. Queen aggression functions in reproduc tive suppression, while in contrast, worker and male aggression may play an important role in regulating task performance by the worker force. Swarm-founding paper wasps evolved from relatives with small, simple societies like Mischocyttarus, but swarm-founders have larger colonies and greater social complexity. Like their primitively social ancestors, swarm-founding wasp workers frequently engage in biting interactions. I found that that individual variation in rates of giving and re ceiving biting did not correspond to variation in body size or in ovary development in the swarm-founding wasp, Polybia occidentalis. Receiving biting was associated with foraging behavior (leaving the nest to collect food and building materials), and being bitten may generally increase individuals' activity levels. Therefore, social biting in Polybia was at least partly independent of direct reproductive conflict, but biting appeared to play a role in organizing task performance and division of labor among workers.

Dr. Wendy Saltzman (with co-author Dr. David H. Abbott)

Wisconsin Regional Primate Res. Center, University of Wisconsin, 1220 Capitol Court, Madison, Wisconsin 53715-1299 (608) 263-3563

Title: The Role of Aggression in Reproductive Competition among Common Marmoset Monkeys (Callithrix jacchus)

The common marmoset, a cooperatively breeding, New World monkey, lives in multi-male multi-female groups in which, typically, only a single, dominant female breeds. Socially subordinate females usually undergo suppression of reproductive physiology and inhibition of sexual behavior and help to provide care for the offspring of the dominant female. Agonistic interactions are an important determinant of reproductive roles. First, in new groups, dominant females selectively direct aggression at subordinates undergoing ovulatory cycles rather than at anovulatory subordi nates. Second, in established groups, ovulation suppression appears to occur only if a clear dominance relationship exists among females; overt aggression, however, is not necessary for the occurrence of reproductive suppression. Third, on the rare occasions when two females breed in the same group, infanticide usually occurs, with the breeding females implicated in killing each other's infants. Thus, agonistic interactions mediate reproductive competition among female marmosets and contribu te at numerous levels to maintenance of singular cooperative breeding.

Dr. Stanley Schneider (with co-author Dr. Gloria DeGrandi-Hoffman)

Department of Biology, University of North Carolina, Charlotte, North Carolina 28223 (704) 547-4053

Title: The Vibration Signal of the Honey Bee: Competition, Cooperation and Modulation

The vibration signal is one of the most commonly occurring communication signals in honey bee colonies and is associated with a wide array of tasks. It represents a type of "modulatory communication" that may help to integrate the behavior of different worker groups that engage in interdependent tasks sets. Thus, under most circumstances the vibration signal is associated with the organization of cooperative, colony-level activities. However, the signal may also play a role in reproductive conflict, which can occur under two circumstances. First, when colonies become permanently queenless, some workers' ovaries develop and there is a concomitant increase in aggession associated with conflict over oviposition. Second, during temporary polygyny (when multiple virgin queens are present within a colony) there can be intense queen-queen and worker-queen conflict over which new queen will inherit the nest. We present evidence that the vibration signal is utilized in both circumstances. Under queenless conditions, it may be used by workers with undeveloped ovaries to interfe re with the oviposition of more developed nestmates. During temporary polygyny, the signal may give workers the ability to interrupt queen aggressive interactions and influence the outcome of queen rivalry. The vibration signal may therefore contain elements of aggression and may provide a model behavior for investigating how conflict is regulated in honey bee societies and perhaps how aggressive signals may evolve into displays associated with cooperation.

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