Introduction

The interaction between a predator and its prey represents a dramatic example of animal behaviour in which the capabilities of nervous systems are stretched to the limit. A hunting animal faces the fundamental problems of detecting and localising the prey, and it must solve them on the basis of purely passive information given out inadvertently by the prey. This is a formidable task and it has led to the evolution of some remarkably sophisticated neuronal systems in species that are adapted for hunting.

If one is asked to name a hunting species, the natural choice is a suitably complex animal such as a large cat or a hawk. These animals do, indeed, possess central nervous systems with the necessary sophistication to handle the complex task of tracking prey, but this sophistication makes most birds and mammals unsuitable as subjects for neuroethological research. However, the difficulty can be overcome by looking at species with a highly specialised method of hunting, based on a sensory system that is dedicated to the specialised method of prey detection and localisation. It then becomes easier to correlate the properties of particular neurons in that system with the particular behavioural task (see section 1.2).

Such dedicated systems are found in two groups of animals that employ hearing as a means of tracking prey, namely owls and bats, which use specialised auditory systems to hunt at night when visually guided predators are at a disadvantage. Owls are able to locate small animals on the ground by listening for the tiny rustling noises made by an animal moving among fallen leaves and twigs. On most nights, an owl's hearing is used in conjunction with its excellent eyesight, but on very dark nights some species can hunt by sound alone.

Insectivorous bats both hunt and find their way around exclusively by sound, using a method that is akin to human sonar and is technically known as echolocation. This method involves the bat emitting loud pulses of sound and then analysing the returning echoes in order to find out what lies ahead. Echolocation is a characteristic of bats in the suborder Microchiroptera, a diverse group that is quite distinct from the non-echolo-cating fruit bats, the Megachiroptera (cf. Macdonald, 1984).

The basic properties of sound and their variation over time determine what is possible by way of detection and localisation for a hunting owl or bat. Sound is created when air molecules are set in motion by a vibrating structure such as a loudspeaker. The vibrations of the speaker generate alternating waves of compression and rarefaction of the air, which propagate out from the speaker at the speed of sound. The molecules involved in propagating the sound move back and forth from regions of high pressure into regions of low pressure, which thereby become regions of high pressure, and so on.

The pressure generated by a sound wave is technically expressed as sound pressure level and is measured on a logarithmic scale. The unit most often employed in studying animal sounds is the decibel (dB), which is equivalent to an increase or decrease of about 12.2 per cent in relative sound pressure. Absolute pressure levels are described relative to a reference sound pressure, usually 20 ^Pa (= 2 X 10~5 N m 2), which is roughly the threshold of human hearing to 1 kHz sound. The interval from a given point on one sound wave to the equivalent point on the next sound wave is the wavelength, which is usually expressed as a frequency (the reciprocal of the wavelength) and measured in cycles per second (Hertz). Most sounds that animals produce or listen to have frequencies of thousands of cycles per second, or kiloHertz (kHz).

Neuroethological research on owls has concentrated on the owl's ability to localise a sound source in space by measuring these basic properties of sound. Most research on bat echolocation has paid more attention to the bat's ability to determine the distance from which an echo has returned and to the suitability of different bat sounds for different sonar techniques. Taken together, these studies on bats and owls are providing valuable insight into the neuronal basis of some sophisticated behaviour patterns.

Figure 6.1 The hunting technique of an owl, drawn from photographs of Tengmalm's owl (Aegolius funereus) in its natural habitat. (a) The owl about to strike prey with its talons, after flying down from an observation perch. (b) The owl on its perch immediately before striking, with a diagram showing the errors involved in localising prey by hearing. The prey (O) is observed at a shallow angle (a), with the result that a given angle of error converts into a greater distance along the ground for a vertical (elevation) error than for a horizontal (azimuth) error. (Modified after Norberg, 1970, 1977.)

Figure 6.1 The hunting technique of an owl, drawn from photographs of Tengmalm's owl (Aegolius funereus) in its natural habitat. (a) The owl about to strike prey with its talons, after flying down from an observation perch. (b) The owl on its perch immediately before striking, with a diagram showing the errors involved in localising prey by hearing. The prey (O) is observed at a shallow angle (a), with the result that a given angle of error converts into a greater distance along the ground for a vertical (elevation) error than for a horizontal (azimuth) error. (Modified after Norberg, 1970, 1977.)

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment