BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. COORDINATION AND REGULATION IN ANIMALS

17.8. Innate Behavior

17.8.3. Motivation

The Nature and degree of expression of any behavioral response are influenced by various factors, the most important of which is motivation. The same stimulus does not always elicit identical responses from a given individual. Such variation is invariably driven by internal and external factors. For example, a hungry animal will respond to food quite differently than a satiated one. Between these two extremes, responses of varying intensity may occur depending on the degree of hunger. However, even a hungry animal may withhold from eating if it risks predation; as a rule, it will suppress its feeding drive until the danger has passed.

A motivational component is present in many reproductive behaviors. For instance, in many mammal species, females respond favorably to male courtship only at specific times of the year. These periods coincide with estrus. This synchronization has high adaptive value, as it increases the chances of Fertilization and, consequently, the birth of offspring during the season most favorable for infant survival. Periodic shifts in behavioral responses are known as biological rhythms, which are discussed in detail in section 17.8.5. In many animals, the level of motivation, or "drive," in males and females aligns, whereas in other species, a specific signaling system between the sexes is required to express motivational intensity. For example, in many primates, the onset of estrus in females is accompanied by Swelling and color Changes in the Skin around the genitals, which the female displays to the male (Fig. 17.58). This behavior reduces the likelihood of the male attempting to mate when the female is not yet ready. Signals that alter behavior are called key signals or sign stimuli. Depending on their origin and function, they are classified into "motivational" (affecting motivation), releasing, and terminating signals.

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Fig. 17.58. A female chimpanzee displaying her readiness to mate to a male.

Motivation-affecting signals

Signals of this category can be either external (e.g., lengthening daylight hours, which trigger territorial behavior and courtship in birds) or internal (e.g., the depletion of nutrient reserves during hibernation, leading the animal to wake up and forage for food). "Motivational" signals generate a "need" (or "goal"), preparing the Organism for activity that can be triggered by the second type of signal—releasing signals.

Releasing signals («releasers»)

A releaser is either a simple signal or a complex sequence of signals originating from one individual that elicits a behavioral response in another individual of the same species. The term "releaser" was introduced by Lorenz, and The Role of such stimuli in behavior was extensively studied by Tinbergen.

The action of a releaser was demonstrated by Tinbergen in his studies on chick feeding in herring gulls. Typically, a chick pecks at the red spot on the parent's yellow lower Mandible, stimulating the parent to regurgitate food, which the chick immediately swallows. In a series of experiments using models of an adult gull's HEAD, Tinbergen and Perdeck established that any contrasting spot on the parent's beak serves as a releaser for this begging response. As shown in Fig. 17.59, the potency of this stimulus is such that a pointed stick with alternating colored stripes elicited an even stronger response from the chicks than their own parent's beak.

Fig. 17.59. A. Horizontal bars with numbers correspond to the number of pecks by herring gull chicks at various Cardboard Models of their parent's head with gray beaks, where the spots differ in color (Hinde R.A., 1966 after Tinbergen N., 1951). B. A pointed red stick with three white rings elicited 20% more pecks than an exact three-dimensional model of an adult herring gull's head with a red spot on a yellow bill. (Tinbergen, Perdeck, 1950, Behaviour, 3.1.)

Terminating signals

As their name suggests, these signals bring a behavioral response to a close. They can be either internal or external. For example, the sight of a completed nest (an external stimulus) causes birds to cease nest-building activity; the internal "satisfaction" accompanying ejaculation in males leads to the cessation of mating; and a full Stomach causes an animal to stop eating.

Other Examples of key stimuli and their role in behavioral responses are discussed in sections 17.8.4-17.8.9.