ABSTRACT

Burying beetles (Coleoptera: Silphidae: Nicrophorus) exhibit elaborate biparental care. Males and females independently search for small vertebrate carcasses, which serve as the sole food source for developing young. Both parents prepare the carcass for burial, excavate a cavity in the carcass within which the young feed, provision the young during the early stages of their development, and protect them from conspecific and interspecific predators and competitors. Carrion is an extremely nutrient–rich resource, but it can vary greatly in quantity and quality, and owing to its rarity and ephemeral nature, its occurrence is highly unpredictable. We propose that many of the sexual and parental behaviors of Nicrophorus can be regarded as adaptations to the unique problems posed by these resource features. Competition for carrion is intense; consequently, traits that help reduce or eliminate competition, such as carcass burial, should increase reproductive success.

Competition among burying beetles is manifest in inter– and intraspecific aggressive interactions that can escalate into damaging fights. However, losers of contests over carcasses can adopt alternative reproductive tactics: subordinate females can leave some young to be cared for by a dominant female, and subordinate males can sire offspring by surreptitiously mating with the resident female. Males can also inseminate females without having found a carcass, but the number of offspring resulting from these matings is small relative to that of parental males on carcasses.

ABSTRACT

Unilateral postzygotic paternal care is extremely rare among animals. The giant water bug family Belostomatidae contains most of the arthropod species known to exhibit this unusual behavior. In the subfamily Lethocerinae, males brood eggs laid on emergent vegetation. Brooding in this group involves watering eggs, shading them, and defending them against predation. In the subfamily Belostomatinae, males employ a variety of behavior patterns to aerate eggs attached to their backs by their mates. Brooding is obligatory in all belostomatid species studied; unattended eggs invariably die if left in the open air or submersed.

This chapter explores the biology, phylogeny and fossil record of the Belostomatidae and related taxa in an attempt to discern the selection forces, the constraints, and the sequence of historical events responsible for the evolution of this unusual behavior and its subsequent diversification. Selection for large bug size, in order to take advantage of vertebrate prey, together with the dual phylogenetic constraints of Dyar's Law and the apparent inability of heteropterans to add molts, coupled egg size to body size. Thus selection for large bugs also produced large eggs: too large to develop unattended submersed in water. A past history of eggs being laid in water left these larger eggs lacking the necessary adaptations to survive desiccation when laid unattended in the open air. Consequently, large eggs created selection for an innovation to lift egg–size limitations on imago size. Ergo, emergent brooding evolved in the lethocerine lineage.

ABSTRACT

Cockroaches show the entire range of reproductive modes: oviparous, ovoviviparous, viviparous, and intermediate stages. Postparturition parental care is likewise diverse, ranging from species in which females remain with neonates for a few hours, to biparental care that lasts several years and includes feeding the offspring on bodily fluids in a nest. Both ovoviviparity and parental care arose a number of times in the taxon. Evolution of reproductive mode seems most influenced by predators, parasites and cannibalism. Ovoviviparity, aggregation behavior of young nymphs, and diet are suggested as factors influential in the evolution of postparturition parental care. Females regulate parental investment via absorption of oocytes, abortion, cannibalism and brood reduction. The developmental status of cockroaches at hatching ranges along an altricial–precocial spectrum and is correlated with the presence and type of parental care. In several subsocial species neonates are blind, poorly sclerotized, and dependent for food, while in the sole viviparous cockroach nymphs hatch in an advanced state of development and require fewer molts to adulthood than any known cockroach. Association with microorganisms in both the digestive system and the fat body is suggested as one factor influential in the repro – ductive versatility of cockroaches. In particular, the endosymbiont flavobacteria which mediate the storage and recycling of nitrogenous waste products may allow for the variety of modes of postovulation provisioning of offspring.

ABSTRACT

Most members of the Passalidae live in rotting wood. They occur in family groups including male and female parents, eggs, larvae, pupae, and teneral and mature offspring. All stages must eat the feces of the mature adults. Feces are comprised of wood that is fragmented, digested, inoculated with bacteria and fungi from the digestive tract of the adults, and further decomposed after being excreted, an example of an external rumen. Larvae and adults cooperate in pupal case construction and teneral adults repair pupal cases of siblings. The selective advantages of adults and offspring staying together may include: (1) gaining protection and food from the log in which egg–laying occurred; and (2) adults supplying the larvae with shredded wood and feces, which speeds juvenile development and consequently increases the degree of overlap of generations.

The selective value of cooperative pupal–case construction is apparently protection of the easily damaged pupa. The value of emergence of the adult offspring while their exoskeleton is still soft may be that they become inoculated with the wood–decomposing bacteria or fungi and help in pupal case repair before migrating. Hypotheses for the selective advantage of the male staying with the family include: (1) contribution to feeding his progeny; (2) his sperm, delivered in repeated copulations, stimulates egg production and may nourish the female; and (3) defense of his mate against other males.

ABSTRACT

I discuss phylogenetic relationships, nesting and feeding habits, kinship and ecological determinants of social behavior in the second largest taxon exhibiting eusociality, the termites. Contrary to previous hypotheses, cladistic analyses indicate that the woodroach Cryptocercus is probably not a sister taxon to the termites. Focusing on the reproductive consequences of termite societies, I place termite species along points on the spectrum of eusociality according to the reproductive potential of their offspring. The tendencies of workers to maintain their reproductive options in some species but not others are explained, in part, by termite nesting and feeding habits of four life types. Each life type is associated with a level of resource stability (nest and food resources) that sets the upper limit on the extent of worker altruism because resources influence maximum colony longevity. Cycles of inbreeding may yet be important in genera whose colonies are typically headed by sibling–mated supplemental reproductives. I discuss how reproductive–replacement strategies might minimize the effects of reduced genetic variability. Direct and indirect benefits explain why single–site and many multiple–site nesters provide alloparental care. Only indirect benefits, i.e. helping kin, appear to explain why centralsite nesters and numerous multiple–site nesters stay and help. I suggest that the cumulative indirect fitness benefits (the potential benefits over time) earned per altruist may best explain individual decisions in termites throughout the social spectrum.

ABSTRACT

We review key ecological and behavioral mechanisms underlying the origin and maintenance of larval sociality in the Lepidoptera. Using communication contexts of group defense, cohesion and recruitment as a framework, we relate social complexity among gregarious caterpillars to three patterns of foraging: patch–restricted, nomadic, and central–place. A review of the incidence of larval gregariousness in the Lepidoptera demonstrates that sociality is widespread in the order, occurring in twenty or more families representing thirteen ditrysian superfamilies, and it is likely to have evolved numerous times in response to different selective pressures. We specifically address the role of sociality in larval defense and resource use, with a focus on (1) signal enhancement in communication systems, (2) differential larval vulnerability, and (3) ant association. Larval Lepidoptera experience the greatest likelihood of mortality in the earliest instars; larval sociality enhances defensive and resource–exploitation signals in these instars, positively influencing survivorship and larval growth. Disease, predation and parasitism, nutrition, and inclusive fitness are discussed in terms of costs and benefits of group living. Finally, we identify two areas where additional research will contribute significantly to an understanding of social evolution in the Lepidoptera: (1) comparative phylogenetic studies, using ecological and communicative characters to trace the origins of caterpillar societies and transitions among them, and (2) larval behavior and ecology, focusing on kin discrimination abilities, assessment of colony genetic structure, and most importantly on the means and contexts of caterpillar communication.

ABSTRACT

We review the causes of the evolution of social systems and the methods used in their analysis. First, we discuss the roles of genetics, phenotypic traits, ecology (basic necessary resources and natural enemies) and demography in the origin and evolution of sociality, and synthesize the effects of these conditions in a comparative assessment of the predictions of optimal skew models. The models provide a useful framework to explaining and predicting social systems, but would benefit from expansion in the range of their assumptions and more explicit connection to ecological and demographic selective pressures. Second, we review the purposes and usefulness of alternative social system lexicons. We conclude that the trade–off between universality and taxon–specific precision of terms can usefully be addressed by explanation of social terms for each comparative test coupled with striving for recognition of convergence across the broadest possible taxonomic range. Finally, we provide an overview of current adaptationist methods used for analyzing social systems, focussing on approaches that utilize phylogenetic information. Integration of comparative with behavioral–ecological methods, especially experimentation, promises to lead to the next series of insights and critical data for tests of theory.

INTRODUCTION

This volume has had three main objectives. First, we have tried to bring together the widest possible diversity of social insects and arachnids, to elucidate necessary and sufficient conditions for the origin and maintenance of different social systems. In this chapter, we first review the evidence for associations between social systems and genetic, phenotypic, ecological and demographic variables.

ABSTRACT

I propose two hypotheses regarding the relationships between halictine bee demography and social behavior and the environment: the ‘Mating Limitation Hypothesis’ (MLH), that a female's social role (‘caste’) is dependent on whether she mates while young (and thus dependent on male demography); and the ‘Environmental Control Hypothesis’ (ECH), that the decision to lay eggs of one sex or another (thus defining male demography) is dependent upon the temperature and/or photoperiodic conditions experienced at the time the egg is laid, in temperate species. Published demographic and behavioral data typically lack the precision needed for truly conclusive analysis and review, and there are some important recently discovered phenomena that were historically overlooked, and extremely difficult to document. Nonetheless, a review of the available data reveals many notable features that can help evaluate the proposed hypotheses, and offers some guidance for future research. An older hypothesis that is also addressed here is that sexual selection is expected to promote protandry, for many reasons, yet the data on halictine bees suggests that protandry is absent in social populations, although it is characteristic of solitary populations.

Geographic variation in demography and behavior is common, and this variation appears to correlate with the seasonality of the habitat, in a manner consistent with the predictions of the two proposed hypotheses.