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Animal and insect behavior

On earth millions of organisms live in the large diversity of natural environments that still exist, a small number is also capable to live in anthropogenic habitats. Animal exhibits a variety of behaviors such as simple reflexes and innate programmed behaviors. Behaviors can also be modified by learning and memory in contexts of classical and operant conditioning making use of sensory clues and own actions. The most complex behaviors are social behaviors. This section describes the mechanisms of the simplest form of behavior, such as reflexes and innate programmed behaviors from an ethological point of view. Although this platform focuses on invertebrates, the mechanisms of expression of the instinctive behavior, such as reflex and innate programmed behaviors are common to insects and vertebrate. This section describes the bases of such behaviors.

1. Animal behavior

Animal behavior is a result of individual activity and interaction with conspecifics, individual of other species, and the environment. Mankind has been interested in animal behavior since the ancient Greek philosopher Aristotle or even earlier. More systematic approaches such as represented by the behaviorists and ethologists have been established in the early to mid 20th century. . Most prominent among the founders of ethology have been the Austrian Konrad Lorenz (who later moved to Germany ), Niko Tinbergen, a Dutch later relocating to England, and the German Karl Ritter von Frisch.

At the beginning of 20th century it was thought that animal behavior emerges as a chain of reflex behaviors following the work of Pavlov. However, the ethologists revealed that animal behavior is species-specific and to a significant extent genetically fixed. They showed that animals are endowed with the species-specific behavioral patterns that are incorporated genetically (or genetically programmed), and that these behavioral patterns are released as a chain of movements by a specific stimulus (releaser; Lorentz, 1935). Such a chain of motor actions released by a stimulus can be recognized as a behavioral unit.

The term ethology was derived from the ancient Greek ‘ethos’, which means habit, trait, or character. The purpose of ethology is to investigate the relationships between animal behaviors such as innate behavior, and the external environment. Ethology is concerned with four main aspects: 1) mechanisms of the emergence of behavior, 2) meaning of behaviors to the animals (i.e. survival value), 3) relationships between development of behavior and heredity, 4) evolution of behavior. The field concered with the analysis of the mechanisms of the generation of behavior at the neuronal level is commonly called neuroethology. Neuroethology analyzes how stimuli that release a certain behavior are transformed to neuronal signals and how these signals are processed in a nervous system and release the behavior. The field investigating on the survival value, heredity, learning and development of a certain behavior is called behavioral ecology. Sociobiology is concerned with social animals and their social interactions.

2. The mechanisms of the generation of behavior

2.1 Key stimulus


Just as much as the morphology (body shape, color, …) is an intrinsic property of an animal species, animal behaviors are essentially species-specific. A certain animal behavior is triggered by a certain releaser that includes a key stimulus, i.e. some specific sensory information. Example for key stimuli is certain shapes, colors, sounds, odors, gestures of other animals. The innate behavior of threespine stickleback investigated by Tinbergen should serve here to describe a key stimulus in detail.

Threespine stickleback males undergo a conspicuous color change in the breeding season, displaying a red coloration of the belly.In females, the belly swells considerably. Males construct nests and display territorial behavior around their nests, defending it agains conspecific males but being permissive to females. To find out what causes this difference in behavior, Tinbergen displayed two types of models (lures) in the shape of threespine sticklebacks to a male that owned a territory. One of them had a red belly while the other was in normal coloration. The territorial male attacked models of various shapes, provided they had the apparance of a red belly. Models with blue eyes and metallic back are not attacked. Besides, the shape and size of the models did not affect the agressive behavior. These lines of evidence showed that the red belly feature is the most important stimulus parameter. The releaser of male threespine stickleback attacking behavior is the red belly of conspecific males.

On the other and, when a male encountered a conspecific female in his territory, it exhibited a zig-zag swimming display. If the female followed the zig-zag movements,the male guided the female to his nest. In this case, the swollen belly is the key stimulus that releases male zig-zag swimming. At the same time, the males zig-zag display functions as key stimulus for the female’s male-following behavior. An important finding of these experiments was that a specific attribute of the body aspect was sufficient to release male agressive behavior.

2.2 The pattern of reflex and innate behavior

Innate behaviors range from reflexes released via simple neural circuits to behaviors released by more complex programs generated in the central nervous system.

Reflexes, typified for example by the knee-jerk reflex in humans, are released via simple reflex arcs in which sensory neurons directly connect to motor neurons A reflex is a simple behavior that shows a high stimulus dependency is while being little dependent on CNS processing.

Innate behaviors are preprogrammed in the central nervous system and are also is released by a key stimulus that is essentially genetically detemined. It cannot be interrupted once initiated. It can be likened to the discharge of a condenser in that when it discharges once it cannot be stopped. The innate behavior is also called a consummatory behavior. An animal that exhibits an innate behavior sometimes shows vacuum activity. An example of such an activity in the rat is the display of nest-making behavior beyond a certain age even if there is no suitable material for nest construction in the rat cage. Vacuum activity is a phenomenon that is released without a key stimulus. It is released because the behavioral threshold is lowered when a drive factor is increased drastically. The vacuum activity allows us to infer that an innate behavior is released by a central program that is incorporated genetically.

2.3 The predatory activity of the toad

Many animal behaviors that we can observe are combinations of reflexes and innate behaviors. Take the predatory activity of a toad for example, it consists of a consecutive sequence of a reflex and an innate behavior. First of all, when a toad detects motion of a prey such as an insect or worm, 1) it orients towards the prey, 2) fixates it within its binocular field , 3) rapidly sticks out its tongue whereby catching the prey, 4) swallowing it, and finally 5) wiping its mouth with a foreleg. Interestingly, the behavioral sequence continues when the prey is removed shortly before the toad catches it but not when it is removed as late as in the fixation phase. These findings allow us to infer that the sequences 1) to 3) mentioned above are a chain of reflexes and 3) to 5) are a central programmed innate behavior.

2.4 The hierarchical model of behavior

Many behaviors of animals are thought to be genetically programmed. Tinbergen proposed the so-called hierarchical model of behavior that postulates animals have a hierarchically organized nerve center that releases behaviors. When an animal attains a certain physiological condition the highest nerve center is activated. In this period the second highest nerve center can be activated but still is inhibited so without a certain key stimulus a lower nerve center can not be activated. The mechanism removing this inhibitionis called the innate releasing mechanism (IRM). When the key stimulus is perceived by the animal, it releases a hierarchy-specific behavior. This mechanism is called ‘innate’ because the key stimulus is genetically programmed as well as the behavior that it releases .

Consider the mating behavior of the male threespine stickleback as an example described in terms of the hierarchical model of behavior.

1) In spring threespine sticklebacks move to shallow water near the shore (Spring movement). Day length or rise of water temperature activates their endocrine systems in turn activating the genitospinal centers. As a consequence, the bellies of male threespine sticklebacks turn red and the female display swollen bellies.

2) The highest nerve center releasing the spring movement activates the next highest center responsible for the territoral behavior but the behavior is still under the control of inhibition mechanisms. However, shallow and warm water as a key stimulus activates IRM to suppress the inhibition and transmit a signal to activate the nerve center for territorial behavior.

3) A conspecific male entering the territory cancels the inhibition of the lower center responsible for agression and then the center of attack behavior activates IRM and unlocks the inhibition of the lower behavioral center.

4) Following the same mechanisms, higher centers activate other lower centers and accordingly, various chains of behaviors are released.

This hierarchical model of behavior is not based on neurophysiological experiments but offers an important working hypothesis for elucidating the neural mechanisms of behavior.



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