Causation

Sensory-motor mechanisms

At this level of analysis, questions concern the physiological machinery underlying an animal’s behaviour. Behaviour is explained in terms of the firings of the neural circuits between reception of the stimuli (sensory input) and movements of the muscles (motor output). Consider, for example, a worker honeybee (Apis mellifera) flying back to her hive from a field of flowers several kilometres away. The sensory processes the bee employs, the neural computations she performs, and the patterns of muscular activity she uses to make her way home constitute some of the mechanisms underlying the insect’s impressive feat of homing. In the course of exploring these mechanisms and those underlying other forms of animal behaviour, physiologists have learned an important lesson regarding the mechanisms underlying behaviour: they are special-purpose adaptations tailored to the particular problems faced by an animal, but they are not all-purpose solutions to general problems faced by all animals. Linked to this lesson is the realization that the physiology of a species will have limitations and biases that reflect individuals’ need to deal only with certain behavioral problems and only in specific ecological contexts. In behaviour, as in morphology, an animal’s capabilities are matched to its expected environmental requirements, because the process of natural selection shapes organisms as if it were always addressing the question of how much adaptation is enough.

Consider first the sensory abilities of animals. All actions (such as body movements, detection of objects of interest, or learning from others in a social group) begin with the acquisition of information. Thus, an animal’s sense organs are exceedingly important to its behaviour. They constitute a set of monitoring instruments with which the animal gathers information about itself and its environment. Each sense organ is selective, responding only to one particular form of energy; an instrument that responds indiscriminately to multiple forms of energy would be rather useless and similar to having none at all. The particular form of energy to which a sense organ responds determines its sensory modality. Three broad categories of sensory modalities are familiar to humans: chemoreception (exemplified by the senses of taste and smell but also including specialized receptors for pheromones and other behaviorally important molecules), mechanoreception (the basis for touch, hearing, balance, and many other senses, such as joint position), and photoreception (light sensitivity, including form and colour vision).

The capabilities of an animal’s sense organs differ depending on the behavioral and ecological constraints of the species. In recognition of this fact and of the equally important fact that animals perceive their environments differently than do humans, ethologists have adopted the word Umwelt, a German word for environment, to denote an organism’s unique sensory world. The umwelt of a male yellow fever mosquito (Aedes aegypti), for example, differs sharply from that of a human. Whereas the human auditory system hears sounds over a wide range of frequencies, from 20 to about 20,000 Hz, the male mosquito’s hearing apparatus has been tuned narrowly to hear only sounds around 380 Hz. Despite its apparent limitations, a male mosquito’s auditory system serves him perfectly well, for the only sound he must detect is the enchanting wing-tone whine of a female mosquito hovering nearby, a sound all too familiar to anyone who lingers outdoors on a midsummer’s evening.

Pit vipers, colubrid snakes from the subfamily Crotalinae, which include the well-known rattlesnakes, provide another example of how the umwelt of a species serves its own ecological needs. Pit vipers possess directionally sensitive infrared detectors with which they can scan their environment while stalking mammalian prey, such as mice (Mus) and kangaroo rats (Dipodomys), in the dark. A forward-facing sensory pit, located on each side of the snake’s head between the eye and the nostril, serves as the animal’s heat-sensing organ. Each pit is about 1 to 5 mm (about 0.04 to 0.2 inch) deep. A thin membrane, which is extensively innervated and exquisitely sensitive to temperature increases, stretches from wall to wall inside the pit organ, where it functions like the film in a pinhole camera, registering any nearby source of infrared energy.

Human umwelt is not without its own limits and biases. Human eyes do not see the flashy advertisements to insects that flowers produce by reflecting ultraviolet light, and human ears do not hear the infrasonic calls of elephants or the ultrasonic sounds of bats. Furthermore, human noses are limited relative to those of many other mammals. Moreover, humans completely lack the sense organs for the detection of electric fields or of Earth’s geomagnetic field. Sense organs for the former occur in various species of electric fishes (such as electric eels and electric catfish), which use their sensitivity to electric fields for orientation, communication, and prey detection in murky jungle streams, while the latter exist in certain birds and insects, including homing pigeons and honeybees, which use them to navigate back to the home loft or hive. At the same time, unlike most animals, humans are endowed with superb visual acuity and colour vision as a result of having evolved large, high-performance, single-lens eyes.

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Each species’ nervous system is an assemblage of special-purpose devices with species-specific and sometimes sex-specific capabilities. These capabilities become even more apparent when investigating how animals use their sense organs to acquire information for solving behavioral problems, such as territory defense or prey capture. Although an animal may possess diverse sensory organs that enable it to receive a great deal of information about the environment, in performing a particular behavioral task, it often responds to a rather small portion of the stimuli perceived. Moreover, only a subset of available stimuli reliably provides the information needed to perform a particular task. Ethologists call the crucial stimuli in any particular behavioral context “sign stimuli.”

A classic example of sign stimuli comes from the behaviour of male three-spined sticklebacks (Gasterosteus aculeatus) when these fish defend their mating territories in the springtime against intrusions from rival male sticklebacks. The males differ from all other objects and forms of life in their environment in a special way: they possess an intensely red throat and belly, which serve as signals to females and other males of their health and vigour. Experiments using models of other fish species have shown that the red colour is the paramount stimulus by which a territory-holding male detects an intruder. Models that accurately imitated sticklebacks but lacked the red markings were seldom attacked, whereas models that possessed a red belly but lacked many of the other characteristics of the sticklebacks, or even of fish in general, were vigorously attacked.

Similarly, the brain cells of some toads (Bufo) are tuned to pick out those features of the environment that reliably match the toads’ natural prey items (such as earthworms). Experiments were conducted in which a hungry toad was presented with cardboard models moving horizontally around the individual at a constant distance and angular velocity. The research revealed that just two stimuli, the elongation of the object (that is, making the cardboard model longer to increase resemblance to prey) and movement in the direction of the elongation, were sufficient to initiate the toad’s prey-catching behaviour. Subsequently, the toad jerked its head after the moving model in order to place it in its frontal visual field. Other stimuli, such as the colour of the model and its velocity of movement, did not influence the toad’s ability to distinguish worms from non-worms, even though toads possess good colour and form vision. Even the broadly tuned human sensory system operates in a highly selective, yet adaptive, manner. For instance, a person hunting white-tailed deer seeks the prey almost exclusively by watching closely for deerlike movements amid the stationary trees of a forest, not by straining to sense the deer’s shape, smell, or sound.

As with sensory systems, the neural mechanisms by which animals compute solutions to behavioral problems have not evolved to function as general-purpose computers. Rather, the central nervous system (that is, the brain and spinal cord of a vertebrate or one of the segmental ganglia of an invertebrate) performs specific computations associated with the particular ecological challenges that individuals face in their environment. A helpful illustration of this point is the startle response of goldfish (Carassius auratus). If a hungry predatory fish strikes from the side, the goldfish executes a brisk swivelling movement that propels its body sideways by about one body length to dodge the predator’s attack. How does the goldfish’s central nervous system process information from the sense organs to instantaneously decide the correct direction (right or left) to move? The key neural element in the startle response of the goldfish is a single bilateral pair of neurons, called the Mauthner neurons, located in the goldfish’s hindbrain. Each neuron on the left or right receives input from the lateral line system (a row of small pressure sensors that are triggered by the disturbances caused by nearby moving objects) located on the left or right side of the goldfish’s body. Each neuron sends output to neurons that activate the musculature on the opposite side of the body. There is strong, mutual inhibition between the left and right Mauthner neurons; should the left one fire in response to a mechanical stimulus from the left side of the body, for example, the right one is inactivated. Inactivation prevents it from interfering with the crucial, initial contractions of the trunk muscles on the goldfish’s right side. The net effect is that 20 milliseconds after sensing danger the goldfish assumes a C-like shape with the head and tail bent to the same side and away from the attacker. This reaction is followed 20 milliseconds later by muscle contractions on the other side of the body so that the tail straightens and the fish propels itself sideways, away from the danger. Thus, the two Mauthner neurons of the goldfish’s nervous system function exquisitely for processing information regarding predator attacks, and solving this crucial behavioral problem appears to be the only task that they perform.

Small-brained creatures, such as fishes, are not the only species whose nervous systems have evolved to solve tasks in a limited—but ecologically sufficient—way that turns difficult problems of computation into more tractable ones. For example, take the task of a human computing an interception course with a flying object, such as when a baseball player runs to catch a fly ball. In principle, the task could be solved with a set of differential equations based on the observed curvature and acceleration of the ball. What happens instead, evidently, is that the fielder finds a running path that maintains a linear optical trajectory for the ball. In other words, the player adjusts the speed and direction of his movement over the baseball field so that the trajectory of the ball appears to be straight. Unlike the more complicated differential equation approach, the linear trajectory approach does not tell the player when or where the ball will land. Consequently, the player cannot run to the point where the ball will fall and wait for it. If he did, complicating factors such as wind gusts diverting the ball might mean that he would end up in the wrong place. Instead, the player simply keeps his body on a course that will ensure interception.

Once an animal has received information about the world from its sense organs and has computed a solution to whatever behavioral problem it currently faces, it responds with a coordinated set of movements—that is, a behaviour. Any particular movement reflects the patterned activity of a specific set of muscles that work on the skeletal structures to which they are attached. The activity of these muscles is controlled by a specific set of motor neurons that in turn are controlled by sets of interneurons connected to the animal’s brain. Thus, a given behaviour is ultimately the result of a specific pattern of neural activity.

Sometimes neural control takes the form of a simple sensory reflex, in which the activity in the motor neurons is triggered by sensory neurons. This activity can be achieved directly or via one or two interneurons. Other times, as in the case of rhythmic behaviour (such as with birds flying or insects walking), a central pattern generator located in the central nervous system produces rhythms of activity in the motor neurons. Central pattern generators do not depend on sensory feedback. Feedback, however, commonly occurs to modulate and reset the rhythm of the motor output after a disturbance to the animal’s behaviour, as in the case of air turbulence disrupting the wing movements of a flying bird.

Most commonly, the neural control of behaviour takes the form of a motor command in which the initiation and modulation of activity in the motor neurons is produced by interneurons descending from the animal’s brain. The animal’s brain is where inputs from multiple sensory modalities are integrated. In this way, a sophisticated tuning of the animal’s behaviour in relation to its internal condition and its external circumstances can occur. Often the control of an animal’s movements involves an intricate synthesis of all three forms of neural control: patterned neural activity, simple sensory reflex, and motor command. As in all aspects of behavioral physiology, an immense diversity exists among animal species and behaviour patterns in the way the components of behavioral machinery have been linked over time by natural selection.