Nesting and Brood Care

Although brood care is best known among the social insects, other insects exhibit maternal care of offspring and even maternal tailoring of habitat conditions to enhance survival of offspring. Primitive social behavior appears as parental involvement extends further through the development of their offspring.

Several environmental factors are necessary for evolution of parental care (E. Wilson 1975). A stable environment favors larger, longer-lived species that reproduce at intervals, rather than all at once. Establishment in new, physically stressful environments may select for protection of offspring, at least during vulnerable periods. Intense predation may favor species that guard their young to improve their chances of reaching breeding age. Finally, selection may favor species that invest in their young, which, in turn, help the parent find, exploit, or guard food resources. Cooperative brood care, involving reciprocal communication, among many adults is the basis of social organization (E.Wilson 1975).

A variety of insect species from several orders exhibit protection of eggs by a parent (Matthews and Matthews 1978). In most cases, the female remains near her eggs and guards them against predators. However, in some species of giant water bugs (Belostoma and Abedus), the eggs are laid on the back of the male, which carries them until they hatch. Among dung beetles (Scarabaeidae), adults of some species limit their investment in offspring to providing protected dung balls in which eggs are laid, whereas females in the genus Copris remain with the young until they reach adulthood.

Extended maternal care, including provision of food for offspring, is seen in crickets, cockroaches, some Homoptera, and nonsocial Hymenoptera. For example, females of the membracid, Umbonia crassicornis, enhance offspring survival by brooding eggs, cutting slits in the bark of twigs to facilitate feeding by nymphs, and defending nymphs against predators (T. Wood 1976). Survival of nymphs with their mother present was 80%, compared to 60% when the mother was removed 2-3 days after egg hatch and 10% when the mother was removed prior to making bark slits. Females responded to predators or to alarm pheromones from injured offspring by fanning wings and buzzing, usually driving the predator away (T.Wood 1976).

A number of arthropod species are characterized by aggregations of individuals. Groups can benefit their members in a number of ways. Large groups often are able to modify environmental conditions, such as through retention of body heat or moisture. Aggregations also increase the availability of potential mates (Matthews and Matthews 1978) and minimize exposure of individuals to plant toxins (McCullough and Wagner 1993, Nebeker et al. 1993) and to predators (Fitzgerald 1995). Aggregated, cooperative feeding on plants, such as by sawflies and bark beetles, can remove plant tissues or kill the plant before induced defenses become effective (McCullough and Wagner 1993, Nebeker et al. 1993). Groups limit predator ability to avoid detection and to separate an individual to attack from within a fluid group. Predators are more vulnerable to injury by surrounding individuals, compared to attacking isolated individuals.

Cooperative behavior is evident within groups of some spiders and communal herbivores, such as tent-building caterpillars and gregarious sawflies. Dozens of individuals of the spider Mallos gragalis cooperate in construction of a communal web and in subduing prey (Matthews and Matthews 1978). Tent-building caterpillars cooperatively construct their web, which affords protection from predators and may facilitate feeding and retention of heat and moisture (Fitzgerald 1995). Similarly, gregarious sawflies cooperatively defend against predators and distribute plant resin among many individuals, thereby limiting the effectiveness of the resin defense (McCullough and Wagner 1993).

Primitive social behavior is exhibited by the woodroach, Cryptocercus punc-tulatus; by passalid beetles; and by many Hymenoptera. In these species, the young remain with the parents in a family nest for long periods, are fed by the parents, and assist in nest maintenance (Matthews and Matthews 1978). However, these insects do not exhibit coordinated behavior or division of labor among distinct castes.

The complex eusociality characterizing termites and the social Hymenoptera has attracted considerable attention (e.g., Matthews and Matthews 1978, E.

Wilson 1975). Eusociality is characterized by multiple adult generations and highly integrated cooperative behavior, with efficient division of labor, among all castes (Matthews and Matthews 1978, Michener 1969). Members of these insect societies cooperate in food location and acquisition, feeding of immatures, and defense of the nest. This cooperation is maintained through complex pheromon-al communication, including trail and alarm pheromones (Holldobler 1995, see Chapter 3), and reciprocal exchange of regurgitated liquid foods (trophallaxis) between colony members. Trophallaxis facilitates recognition of nest mates by maintaining a colony-specific odor, ensures exchange of important nutritional resources and (in the case of termites) of microbial symbionts that digest cellulose, and may be critical to colony survival during periods of food limitation (Matthews and Matthews 1978). Trophallaxis distributes material rapidly throughout a colony (M. Suarez and Thorne 2000). E.Wilson and Eisner (1957) fed honey mixed with radioactive iodide to a single worker ant and within 1 day detected some tracer in every colony member, including the two queens. Such behavior may also facilitate spread of pathogens or toxins throughout the colony (J. K. Grace and Su 2000, Shelton and Grace 2003).

Development of altruistic behaviors such as social cooperation can be explained largely as a consequence of kin selection and reciprocal cooperation (Axelrod and Hamilton 1981, Haldane 1932, Hamilton 1964, Trivers 1971, E. Wilson 1973, Wynne-Edwards 1963,1965, see also Chapter 15). Self-sacrifice that increases reproduction by closely related individuals increases inclusive fitness (i.e., the individual's own fitness plus the fitness accruing to the individual through its contribution to reproduction of relatives). In the case of the eusocial Hymenoptera, because of haploid males, relatedness among siblings is greater than that between parent and offspring, making cooperation among colony members highly adaptive. The epitome of "altruism" among insects may be the development of the barbed sting in the worker honey bee, Apis mellifera, that ensures its death in defense of the colony (Haldane 1932, Hamilton 1964). Termites do not share the Hymenopteran model for sibling relatedness. Genetic data for termites indicate relatively high inbreeding and relatedness within colonies and kin-biased foraging behavior for some species (Kaib et al. 1996, Vargo et al. 2003). However, Husseneder et al. (1999) reported that DNA (deoxyribonucleic acid) analysis of colonies of the African termite, Schedorhinotermes lamanianus, did not indicate effective kin selection through inbreeding or translocation complexes of sex-linked chromosomes that could generate higher relatedness within than between sexes. They concluded that ecological factors, such as predation and food availability, may be more important than genetics in maintaining termite eusociality, at least in this species.

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