Social Behavior

 

Animals may live in groups or they may be solitary. Sooner or later, however, all animals must make contact with others to reproduce. In each animal species, social behaviors have evolved that optimize individual fitness. Some patterns of interaction follow:

1. Agonistic behavior (aggression and submission) originates from competition for food, mates, or territory. Because most agonistic behavior is ritualized, injuries and time spent in contests are minimized.

 

2. Dominance hierarchies indicate power and status relationships between individuals in a group. Established hierarchies minimize fighting for food and mates.

- Pecking order is a more or less linear order of status often used to describe dominance hierarchies in chickens.

 

3. Territoriality is the active possession and defense of the territory in which an animal or group of animals (often related) lives. Territories insure their owners adequate food and a place to mate and rear their young.

 

4. Altruistic behavior is seemingly unselfish behavior that appears to reduce the fitness of the individual. It commonly occurs when an animal risks its safety in defense of another or sacrifices its reproduction to help another individual (of the same species) rear its young. However, altruistic behavior actually increases inclusive fitness, the fitness of the individual plus the fitness of relatives (who, in fact, share a percentage of identical genes with the altruist). Evolution of these behaviors occurs by kin selection, a form of natural selection that increases inclusive fitness.

- Belding’s ground squirrels give alarm calls (whistles) that warn other squirrels that a predator is approaching. In doing so, the squirrel risks safety by revealing her presence (an altruistic behavior) but increases the survival of other squirrels with which she lives. As it turns out, these squirrels live in groups of closely related females (daughters, mothers, sisters, and aunts). Thus, it is an example of kin selection.

- Bees live in colonies that consist of a queen and female daughters (worker bees). Since only the queen reproduces, the fitness of worker bees is zero, that is, they produce no progeny of their own. Bees are an example of a haplodiploid reproductive system: queens and female workers are diploid, while male bees (drones) are haploid. In the spring, males are born from unfertilized eggs. One male mates with the queen, who, in turn, stores the sperm to fertilize her eggs throughout the year. Following mating, the males are driven from the hive and die. In a normal diploid reproductive system, mother and offspring, as well as sisters, are related, on average, by only 50% of their genes. However, because half of the chromosomes of all sister bees are the same (from their haploid father), they share, on average, 75% of their genes. Thus, the inclusive fitness of a worker bee (in terms of the number of genes she contributes to the next generation) is greater if she promotes the production of sisters (by nurturing the queen) than if she were to produce offspring herself. Thus, kin selection favors sterile workers in a haplodiploid society.