Drive, or motivation, stands out among the most interesting concepts in animal behavior. What causes an animal to stop one activity and start another? Why do animals behave one way in the spring and another in the winter? Ethologists developed concepts to answer these questions, but they are now best understood on physiological and neurobiological levels.

The ethological concept of drive

Early ethologists thought of animals as having a supply of energy, which could be allocated to tasks. Over time this energy accumulates, and ultimately is released through performance of a behahavioral act. Energy models of drive gave ethologists tools for explaining how animals prioritized their behavior.

Excess energy, particularly if an intended behavior is blocked, might be dissipated in redirected behavior. A person who is prevented from attacking another person, and instead hits an inaminate object, such as a wall, has redirected their behavior. Musth bull elephants present an extreme example of redirected aggression; often an elephant in this condition will attack any object or animal that happens to be in the way.

If a conflict occurs between allocating energy to two or more behaviors (such as approach-avoidance conflict), energy might be dissipated in an irrelevant act, such as grooming; this is called displacement behavior.

Modern concepts of homeostatic regulation

As scientific understanding of the hormonal and neurobiological regulation of behavior has increased, animal behaivorists have shifted away from talking about motivation and drive. Behavior is the result of physiological needs (e.g. thirst, hunger) or biological imperatives (e.g. reproduction) regulated via neurochemical and hormonal mechanisms.

Many behaviors directly improve an animal's physiological condition and have direct effects on internal homeostasis. By seeking the right environmental conditions an animal can avoid dessication and maintain its body temperature. Behavioral thermoregulation provides a simple example of how behavior and physiological homeostasis can interact.

However, motivation and drive remain useful concepts in the discussion of how animals balance conflicting needs and imperatives. Analyzing the strategies used by animals in prioritizing behavioral possibilities remains an important area of inquiry for scientists. These priorities are, of course, the result of neurobiological processes and researchers will soon be able to identify the biochemical processes and interactions which enable animals to choose between conflicting needs and imperatives.

Balancing demands: how animals budget their time

An animal's ultimate goal is to successfully reproduce. Reproduction, though, involves much more than mating and bearing offspring. Animals must maintain themselves by feeding, must protect themselves from predation and parasitism, and must spend time finding appropriate environmental conditions (temperature, humidity, and so on) for their survival. They may also need to store food reserves, either internally as fat or externally (such as honey in a bee hive) to support their future offspring. Finally, successful mating and reproduction may require investment of time in obtaining and defending a territory. By balancing these time demands an animal can optimize its chances of successfully reproducing. Analyses of behavioral time budgets give insight into how animals prioritize various demands for survival and reproduction.

Developmental and seasonal changes

Behavioral changes during growth and development are key to animal's survival. Young animals are sometimes dependent on others for survival. Juveniles may behave in ways which enhance bonding and cooperative behavior with parents and sibs. Play serves key roles in developing motor skills and the formation of social bonds. Steroid hormones, such as estrogen and testosterone, help to regulate behavioral development in vertebrates.
Hormonal changes drive striking seasonal patterns in sexual receptivity, conflict and aggression, and parental behavior. In vertebrates, behavioral changes of this type are strongly associated with the levels of steroid hormones, such as testosterone and estrogen. Seasonal changes are equally marked in some invertebrates, but their physiological bases are less well understood.

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copyright ©2001, 2002 Michael D. Breed, all rights reserved