ABSTRACT

We review mating–systems theory and conflict theory and apply them to the analysis of intrasexual and intersexual conflicts. We distinguish three types of sexual interaction, persuasive, coercive, and forcing, that are used by members of each sex to gain control over fertilization and reproductive resources, and we discuss the nature of male–male, female–female and intersexual interactions with respect to these types of behavior. The mating sequence from pair formation to the end of parental care provides the context for analyzing sexual conflict and confluence of interest; events during any given stage in the sequence may affect the dynamics of other events. We suggest that three main sets of variables influence the evolution of mating systems: (1) resource and mate distributions; (2) the presence and extent of transferrable genetic and material benefits; and (3) the degree of control exerted by each sex over events at different stages in the mating sequence. The former two sets of variables are more or less predictable from ecology, demography and life–history adaptations not directly related to mating behavior; the latter is intimately related to the first two, but should often be unpredictable owing to the idiosyncracies of lineage–specific events.

INTRODUCTION

Insects and arachnids display a diversity of reproductive strategies unparalleled among animals; this diversity has put research on these creatures at the forefront of the study of animal mating systems (Blum and Blum 1979; Thornhill and Alcock 1983; Gwynne and Morris 1983; Smith 1984).

Sexual behavior and social behavior are profoundly alike in that both involve one set of individuals more or less willingly providing a limiting resource to another set (Queller 1994). Thus, in sexual interactions females provide resource–rich ova and other parental investment to males, and in social interactions workers provide labor to queens. In both situations, the parties are virtually always in conflict over the allocation of the resources, but their interests also partly coincide: eggs must be fertilized and offspring produced, and a new generation of reproductives must be successfully protected and reared. The complex mixtures of conflict and cooperation that thus typify sex and sociality make them among the most endlessly fascinating and difficult topics in ecology and evolution.

This book, and its companion (Choe and Crespi 1997), explore the intricacies of sexual and social competition. We have drawn together, for each of these topics, a set of authors whose expertise is both taxon–deep and broadly based in the theory that guides interpretation of natural history. Our goal has been to bring theory and observation together, to find parallels and convergences between disparate taxa, and to sketch out the patterns of engagement that will allow us to understand how conflicts and confluences of interest evolve together.

ABSTRACT

Two sympatric species of Zoraptera in central America, Zorotypus barberi Gurney and Z. gurneyi Choe, exhibit distinctively divergent mating behaviors before, during, and after copulations. The mating system of Z. barberi is essentially polygynandry in which both males and females mate with multiple mates. Males of Z. barberi perform elaborate precopulatory courtship involving courtship gifts, provide extra stimulation during copulation, and continue to court for additional copulations. Females appear to exert almost exclusive control over mating by being able to reject males at any point during the entire mating episode. In addition to deciding with whom to mate and how long each copulation lasts, Z. barberi females also control the frequency of copulations. They can mate with one male repeatedly, with different males, or both. Among the hypotheses examined, the material–benefit, postcopulatory female choice, sperm–supply, and fertilization–enhancement hypotheses, or some combinations of them, appear to provide good explanations for the observed mating variations in Z. barberi. Alternatively, repeated mating may be a result of ‘parceling’ of courtship gifts by males to guard females for a longer period of time. In Z. gurneyi, males gain considerable control over mating by establishing dominance hierarchies among themselves and thus predetermining female mating decisions to a certain degree. Zorotypus gurneyi males display no apparent precopulatory courtship; once genital coupling is made, they are able to prolong copulations by everting much of their internal genitalia into the female. After copulations, dominant Z. gurneyi males continue to protect their harems of females from other males.

The purpose of this book is to explore the causes of some of the most spectacular transitions in evolutionary biology, the shifts between cooperation and competition, commensalism, or parasitism. In particular, we seek to explain under what conditions such behavior as parental care, allo – parental care, and other forms of altruism among adults have evolved from the individually selfish life histories so common in animals, and how such behavior, once evolved, can be lost. Insects and arachnids provide the most varied and numerous instances and forms of the evolution of cooperation in animals; hence, they represent the most useful, though often idiosyncratic, database for analyzing social evolution.

We approach our main question, understanding the causes of the diversity of animal social systems, from the viewpoint of behavioral ecology: explaining and predicting behavior from ecology. In the context of studying sociality ‘ecology’ encompasses resource distribution in time and space, interactions with other species, and aspects of demography such as voltinism, survivorship and fecundity. Social ‘behavior’ can be defined broadly as the presence of cooperation and some form of organization, whereby the actions of individuals are coincident and coordinated or communicative in some way (Wilson 1971, p. 469). Our unifying framework for analysis in behavioral ecology is the gene's eye view of inclusive fitness theory, coupled with optimization theory (see, for example, Oster and Wilson 1978).

ABSTRACT

The colony founding phase is a unique period in the life cycle of social insects in which young reproductives have an option to be either solitary or social. We review the literature on pleometrosis in ants with a special emphasis on the founding processes by Azteca constructor and A. xanthacroa nesting in Cecropia trees. Azteca colonies are established in three distinct ways: by single queens, by single–species groups of queens, and by mixed–species groups. Pleometrotic colonies of both single species and mixed species tend to outcompete haplometrotic colonies by (1) producing much larger first worker cohorts; (2) migrating upward within the Cecropia tree earlier; and (3) eventually taking exclusive control of the tree. The fact that mixed– species Azteca colonies are nearly as successful and cooperative as their single–species counterparts provides a clear–cut case where inclusive fitness is not a prime selective factor for foundress association. Although many studies of pleometrotic ants measure the number of workers produced and the timing of first worker emergence, such measures do not account for reproduction because workers are merely a means by which a queen produces reproductive offspring. Colonies with larger and more rapidly produced worker forces tend to survive better, largely because they are likely to prevail over smaller colonies via interference or exploitation competition.