Complex Animal Societies

Factors Contributing to Social Evolution

Why do complex societies evolve? What do individuals gain from grouping? Lions can increase their lifetime reproductive success by living in groups, rather than as isolated individuals (see Figure 1.14). Thus, lions with gregarious tendencies are positively selected relative to individualists because they can hunt more successfully and provide better care for their offspring. Other factors favoring the evolution of complex sociality involve enhanced foraging (e.g., group hunting in chimpanzees), improved defense against predators (e.g., alarm calls of ground squirrels), and efficient infant care (e.g., group care in lions). However, additional elements must be considered, such as the genetic relatedness among individuals in a society, group selection, and division of labor, among others discussed here.

Few fields in the study of animal behavior display the level of fascinating discovery and theoretical development as the study of complex societies, especially in insects. These are shown in the next sections, which provide a glimpse of the complexity of sociality as well as a survey of the theoretical attempts at explaining such complexity.

Subsociality and Eusociality

Eusocial Species

Entomologists interested in the evolution of social behavior in insects have traditionally classified species as subsocial and eusocial. Subsociality occurs in species that exhibit parental care and overlapping generations. In addition to insects, subsociality occurs in birds and mammals. In eusociality, a third feature is added, namely, a division of reproductive duties

Reproductive and Social Behavior 437 according to which one (or a few) individuals produce viable eggs while the rest form sterile castes of nonreproductive individuals that care for the eggs and defend the nest. Depending on the species, nonreproductive individuals can be sterile or retain the ability to procreate, which is expressed under appropriate conditions. Thus, eusocial species display parental care, overlapping generations, and sterile castes. Apart from human societies, the most complex animal societies are those of eusocial species.

Eusociality is known in species from two distantly related animal phyla, arthropods and vertebrates. Among arthropods, it is found in insects, including ants, wasps, bees, termites, gall thrips, bark beetles, and aphids, and in crustacean sponge-dwelling shrimps. Among vertebrates, it has been described only in naked mole rats and Damaraland mole rats. Because of the remote relationship of arthropods and vertebrates, and the presence in each of them of many closely related species that are not eusocial, it is agreed that eusociality has evolved independently several times, even among arthropods. Eusociality can be considered both rare and extremely successful.

Among the Hymenoptera, the distribution of eusociality is estimated to be as follows:

• • Ants: all known species, about 11,000 species of an estimated 20,000 species that may exist.
• • Bees: an estimated 1,000 species of about 30,000 living species.
• • Wasps: approximately 900 species of about 7,000 living species.

These estimates yield about 26% of eusocial species among the hymenopterans. The vast majority of eusocial species are hymenopterans, but when one considers that the estimated number of insect species is about 900,000, only about 2% of all insects display eusociality. However, eusocial insects account for about half the total insect biomass. With colony sizes varying from a few dozen individuals to an estimated 20 million in some ant species, eusociality is a successful strategy.

A few examples of hymenopteran eusocial organization provide a sense of their complexity (Hblldobler & Wilson, 1990). Some ants have evolved an extreme degree of dependency from other species following a parasite-host system. The Amazon slave-making ant Polyergns mfescens “steals” eggs from a host species and carries them to their own nest. Slave ants will eventually take over all the activities of maintenance in the colony without which Polyergns ants would not be able to survive. Some species are also known to practice agriculture. Ants of the genera Atta and Acromyrmex collect plant material and carry it into their nests, where leaves are cut in small pieces, chewed, and mixed with anal secretions. These pieces of pulpy plant material are then deposited in fungus gardens. Each ant species may have a specific association with a particular fungus symbiont, the two depending on each other. The fungus acts in fact as an ancillary digestive tract, providing nutrients and some enzymes that these ants no longer produce. In return, the fungus symbionts depend on the ants for care and cultivation. Ants protect specific species of fungi in a variety of ways, such as eliminating unwanted fungus species, fertilizing the substrate with nutrients to which their fungus symbionts have become adapted, and producing a hormone that stimulates fungal growth. Weaver ants of the genus Oecophylla team in large groups to create cavities with leaves. These ants live in trees and build their nests by pulling together from the edge of a leaf until they can bind the leaf to produce an enclosure. Workers form a living chain to pull one edge closer to the other, and then rows of workers keep the two edges together while a second group attempts the binding from the inside of the leaf. Binding is achieved by using a special silk secreted only by the larvae. Workers bring larvae to the building site and stimulate them to produce the silk.

Table 11.2 Coefficients of relatedness (r) for diplodiploid and haplodiploid species

Note: Haploid fathers produce only one set of genes shared by all the haplodiploid daughters produced by the same father. This leads to an unusually high r of 0.75 among sisters.