Unit Eight. The Living Environment
Organisms as diverse as bacteria, cnidarians, insects, fish, birds, prairie dogs, whales, and chimpanzees exist in social groups. To encompass the wide variety of social phenomena, we can broadly define a society as a group of organisms of the same species that are organized in a cooperative manner.
Why have individuals in some species given up a solitary existence to become members of a group? We have just seen that one explanation is kin selection, where groups may be composed of close relatives. In other cases, individuals may benefit directly from social living. For example, a bird that joins a flock may receive greater protection from predators. As flock size increases, the risk of predation may decrease because there are more individuals to scan the environment for predators.
In insects, sociality has chiefly evolved in two orders, the Hy- menoptera (ants, bees, and wasps) and the Isoptera (termites), although a few other insect groups include social species. These social insect colonies are composed of different castes, groups of individuals that differ in size and morphology and that perform different tasks, such as workers and soldiers.
Honeybees In honeybees, the queen maintains her dominance in the hive by secreting a pheromone, called “queen substance,” that suppresses development of the ovaries in other females, turning them into sterile workers. Drones (male bees) are produced only for purposes of mating. When the colony grows larger in the spring, some members do not receive a sufficient quantity of queen substance, and the colony begins preparations for swarming. Workers make several new queen chambers, in which new queens begin to develop. Scout workers look for a new nest site and communicate its location to the colony. The old queen and a swarm of female workers then move to the new site. Left behind, a new queen emerges, kills the other potential queens, flies out to mate, and returns to assume “rule” of the hive.
Leaf-Cutter Ants The leaf-cutter ants provide another fascinating example of the remarkable lifestyles of social insects. Leaf-cutters live in colonies of up to several million individuals, growing crops of fungi beneath the ground. Their moundlike nests are underground “cities” covering more than 100 square meters, with hundreds of entrances and chambers as deep as 5 meters beneath the ground. The division of labor among the worker ants is related to their size. Every day, workers travel along trails from the nest to a tree or a bush, cut its leaves into small pieces, and carry the pieces back to the nest. Smaller workers chew the leaf fragments into a mulch, which they spread like a carpet in the underground fungus chambers. Even smaller workers implant fungal hyphae in the mulch (recent molecular studies suggest that ants have been cultivating these fungi for more than 50 million years!). Soon a luxuriant garden of fungi is growing. While other workers weed out undesirable kinds of fungi, nurse ants carry the larvae of the nest to choice spots in the garden, where the larvae graze. Some of these larvae grow into reproductive queens that will disperse from the parent nest and start new colonies, repeating the cycle.
In contrast to the highly structured and integrated insect societies and their remarkable forms of altruism, vertebrate social groups are usually less rigidly organized and cohesive. Each social group of vertebrates has a certain size, stability of members, number of breeding males and females, and type of mating system. Behavioral ecologists have learned that the way a vertebrate group is organized is influenced most often by ecological factors such as food type and predation.
African weaver birds, which construct nests from vegetation, provide an excellent example to illustrate the relationship between ecology and social organization. Their roughly 90 species can be divided according to the type of social group they form. One set of species lives in the forest and builds camouflaged, solitary nests. Males and females are monogamous; they forage for insects to feed their young. The second group of species nests in colonies in trees on the savanna (figure 37.18). They are polygynous and feed in flocks on seeds. The feeding and nesting habits of these two sets of species are correlated with their mating systems. In the forest, insects are hard to find, and both parents must cooperate in feeding the young. The camouflaged nests do not call the attention of predators to their brood. On the open savanna, building a hidden nest is not an option. Rather, savanna-dwelling weaver birds protect their young from predators by nesting in trees, which are not very abundant. This shortage of safe nest sites means that birds must nest together in colonies. Because seeds occur abundantly, a female can acquire all the food needed to rear young without a male’s help. The male, free from the duties of parenting, spends his time courting many females—a polygynous mating system.
Figure 37.18. Savanna-dwelling African weaver birds form colonial nests.
Key Learning Outcome 37.13. Insect societies are very structured and are composed of different castes. The extent of vertebrate sociality is affected by environmental conditions.