Chapter 10

Leaf-cutter ants. Through an extensive division of labor, leaf-cutter ants work together to bring pieces of leaves back to their nest. In this photo, a large worker ant carries a cut piece of leaf while a much smaller worker ant rides on top to discourage parasitoid flies from attacking the worker.

Photo by Gavriel Jecan/age fotostock.

The leaf-cutter ant is an extraordinary farmer. Living in colonies of several million individuals, these ants leave the colony each day to harvest leaves from the surrounding forest. Using their sharp mandibles, they slice through leaves to cut off pieces that are many times larger than their bodies. They then carry the pieces back to the nest, which can rise several meters out of the ground and sink tens of meters below the ground. Back at the nest, the ants consume the sap in the leaves, but they do not eat the leaves. Instead, they use the leaves to grow a specialized species of fungus that they consume.

There are more than 40 species of leaf-cutter ants, which live primarily in Mexico, Central America, and South America. Like honeybees, the leaf-cutter ants form enormous societies of cooperating individuals. An ant colony normally has a single queen that can live for 10 to 20 years. Early in her life, the queen participates in a mating flight with males. The sperm she receives then are held inside her body and remain viable for the rest of her life. She uses them sparingly when fertilizing eggs to make daughters; in some species, the queen releases only one or two sperm per egg. Occasionally she lays unfertilized eggs to make sons whose only function is to mate with other queens. The millions of individuals in the nest are daughters of the same queen and sisters to each other. They all forgo reproduction.

Daughters in this ant society are the workers in the colony and the division of labor among the workers is amazingly complex. Scientists estimate that there are nearly 30 different jobs for the worker ants and they have determined that different workers are suited to different jobs. The worker caste is composed of several subcastes, known as minims, minors, mediae, and majors. Ants in each subcaste differ dramatically in size and shape. The largest workers, the majors, can be 200 times more massive than the smallest workers, the minims. The differences in subcastes are thought to be a phenotypically plastic response to different diets the ants receive as larvae. In addition to size and shape differences, the jobs of a given worker can also change during its lifetime. For example, when workers are young, they spend most of their time inside the nest where they build tunnels, air condition the nest, and raise the larvae. When large leaf pieces arrive at the nest, other workers cut them into smaller pieces and then even smaller workers mash them into tiny bits. The smallest workers bring a strand of fungus to the mashed bits and tend the fungus gardens, a job that includes removing undesirable species of fungi that can be pathogenic to the ants.

The oldest and largest ants act as soldiers and go out of the nest to collect leaves. The process of leaf collection is complex. Some ants climb into trees and cut large pieces of leaves that drop to the ground, while others relay leaf pieces back to the nest. When workers first begin leaf cutting, they possess razor-sharp jaws, known as mandibles, that are very effective at cutting tough leaves. Researchers have recently discovered that these mandibles become dull over time, causing older workers to take twice as long to cut a leaf. When an ant’s cutting performance declines, it shifts to the task of leaf carrying. This change in job allows older individuals to continue contributing to the ant society. The division of labor also helps the group to maintain a high foraging efficiency.

While the ants benefit from living in a very large group, the large colonies and conspicuous foraging behavior make them quite noticeable to their enemies. For example, a species of parasitoid fly that is specialized to hunt large leaf-cutter ants lays its eggs inside the necks of foraging ants. To reduce the risk of these attacks, leaf-cutter ants have evolved several tactics. Smaller individuals, which are less attractive to the fly, forage during the day. At night, when the flies do not hunt, the larger, more effective foragers go out to collect leaves. Small workers also serve as guards by riding on leaves that larger ants carry. When a fly approaches, these tiny guards prevent the fly from laying its eggs on the neck of the large ant that is carrying the leaf. When not repelling attacks, these hitchhiking individuals clean the leaf of undesirable microorganisms.

The leaf-cutter ants illustrate an extreme case of social behavior and living in groups. As we will see in this chapter, being social is a common occurrence in the animal world and the ecological conditions under which species live have important consequences for the costs and benefits of social living.

In the course of a lifetime, an individual typically interacts with many members of its species. Interactions with mates, offspring, other relatives, and unrelated individuals in one’s species are known as social behaviors. Like most behaviors, social behaviors have a genetic basis and are therefore subject to natural selection. As a result, many types of social behaviors have evolved to favor the cohesiveness of family groups and populations and to constrain antagonism among conspecifics.

Although the study of social behavior typically focuses on animals, many other organisms interact with conspecifics in ways that might be considered to be social. For example, bacteria and protists can sense the presence of individuals of the same species, often through chemical secretions, and react in “friendly” or “aggressive” ways. During parts of their life cycles, free-living slime molds respond to others when they aggregate to form large fruiting bodies. Even plants communicate with one another. When one plant is damaged by herbivores, it emits volatile compounds. Other individuals detect the compounds and respond to them by producing chemical or structural defenses against future herbivore attacks.

In this chapter, we will focus on the social behaviors of animals, which play important roles in the ecology of individuals, populations, communities, and ecosystems. Social interactions pervade nature and form an important part of the environment to which populations adapt. We will explore some of the implications of interacting within social groups, and describe various ways that individuals manage social relationships. We will also examine how different ecological conditions affect the evolution of social behaviors.