light (for humans) as falling within the range of approximately 400–700 nm (Figure 2.5).
Within the visible spectrum of an animal, sensitivity to light is not the same across all wavelengths. Light at some wavelengths can be detected at lower intensities, while others can only be detected at relatively higher intensities. When visual sensitivity has been determined in a particular species at a range of wavelengths a spectral sensitivity curve can be constructed. The curve for the Rock Dove (Feral Pigeon) Columba livia at daytime light levels is shown here. It indicates that the eye is most sensitive to light in the yellow-orange wavelength range and that sensitivity falls rapidly in the reds and in the greens and blues, but there is a slight rise in sensitivity in the violet range.
At a glance spectral sensitivity functions convey a lot of information about the average basic vision of a species. These functions generally show a broad domed shape. This indicates that there is lower sensitivity (high-intensity lights are needed for their detection) to both longer and shorter wavelength lights, and that the highest sensitivity (lights of lower intensity can be detected) occurs in a mid-range. However, the position of the peaks in sensitivity and the shapes of spectral sensitivity functions are usually not symmetrical, and in some animal species more than one peak can occur.
If spectral sensitivity functions are available for a number of species, then differences in their vision can be comprehended readily by comparing the functions. Therefore, they are a valuable tool for characterising and comparing vision across species. They are also used as a clinical tool in humans to detect different types of vision loss.
Perhaps the most familiar of these are the differences that can occur in humans in their sensitivity to sounds, in our individual audiograms. If we have reason to have our hearing tested the results will be compared with a normal or average audiogram for humans. This shows how our own hearing may be more or less sensitive to particular frequencies than the hypothetical ‘normal’ individual. However, an inevitable process of ageing is that people start to lose their sensitivity, particularly to higher-frequency sounds. This is referred to as differential hearing loss, and the details of this loss describe how much our own hearing may have changed over time. However, it is worth bearing in mind that these changes can be the result of natural processes, although they can result from disease or physical damage caused by exposure to loud sounds.
When audiograms are compared across a wide range of animal species (mammals, birds, reptiles, and fish) very notable differences in sensitivity to sounds of different frequencies are found between these main animal taxa, as well as between individuals within a taxon. Some animals are able to detect sounds of very low frequencies, others are able to detect sounds with very high frequencies. The same applies to vision, where some species are able to detect light of short wavelengths and others light of relatively long wavelengths. Sensitivity to these shorter wavelengths of light is referred to as vision in the ultraviolet part of the spectrum. In hearing, sounds of low and high frequency are referred to as infrasounds and ultrasounds.
These infra- and ultra- labels are derived from comparisons with the range of light and sound that humans can normally detect. Although in an anthropocentric world it seems natural to use humans as the base comparator species, seeing and hearing outside the range of humans is nothing special, it is just different. Indeed, we might well ask why human hearing is stuck in the middle and does not reach these ultra- and infra- ranges. Many of the mammals that we share our everyday lives with hear sounds that are well outside the range that we can detect (Figure 2.5).
FIGURE 2.5 The spectrums of light and sound, showing the general range of frequencies and wavelengths that humans and other animals are able to detect. Infrasounds have frequencies below those that humans can hear, ultrasounds have frequencies higher than those that can be detected by humans. However, there are many animals species which can detect sounds within the infrasound and ultrasound frequency ranges. Similarly the electromagnetic, light, spectrum has a range of wavelengths that humans detect as spectral colours (the colours of the rainbow) from violets to reds, but some animals can detect light at shorter wavelengths, in the ultraviolet part of the spectrum, and some can detect light in the infrared part of the spectrum.
Costs and trade-offs in senses
Humans are all too keen to think that they ‘know’ the world, that the world is as they perceive it through their senses. However, even the above brief mention of infrasounds and ultrasounds, of infrared and ultraviolet light, tells us that the world contains far more information than we can directly receive. By definition these are sources of information about the world that we cannot directly access. These are sounds that we cannot hear, lights that we cannot see, but other species can. This means that it is arrogant to believe that humans should be the comparator of all things – but it is equally true that no organism can fulfil that role. Put simply, it is not possible for any one species to be able to detect everything that is going on in its environment. It is not possible for any one species to ‘know’ comprehensively how the world actually is.
Trade-offs between senses
The world of any one species is no more important or special than that of another. All sensory worlds have equal importance. They have been shaped by natural selection to extract information for the efficient conduct of the life of each species. For individual species there will be important constraints on how their sensory organs can perform. This is because there are costs and trade-offs in sensory capacities within a sense, and also between senses. The trading off of information between senses is something that will be discussed a number of times in this book. One particularly dramatic example is found in some ducks which, unlike most other birds, have comprehensive vision of the world about their head. This is only possible, however, because their foraging has become controlled by touch and taste information, so that they do not need to see where their bill is. This has resulted in a trade-off between vision, touch, and taste that has given rise to particular diets and behaviours.
An important constraint on how animals detect their environments comes from the metabolic costs of operating different sensory systems. Vision is particularly costly. Not only are eyes demanding of support and protection in the skull, but their actual running costs are high. There is a rapid and constant turnover of materials and large amounts of neural processing are necessary to extract information from visual input, and neural processing is demanding of energy. Eye size is a fundamental factor in both visual resolution (the amount of detail that can be extracted from a scene) and sensitivity (the minimum amount of light necessary for the extraction of information). As a general rule the larger the eye, the higher its sensitivity and resolution.
The eyes of most birds are small, but there are plenty of species that have large eyes – for example owls, albatrosses, raptors, hornbills, and penguins. In all of these species larger size would seem to be the result of natural selection for either high sensitivity or high resolution, or even both. But not all nocturnal species have large eyes. We might reasonably predict that large eyes could have easily evolved in kiwi species, because they are flightless and weight should not be a problem – but in fact the eyes of a kiwi are similar in size to those of a small passerine. The answer to this apparent paradox lies in the fact that kiwi conduct many tasks guided by information derived from non-visual senses, most notably smell, hearing and tactile cues from the bill tips. This is another striking example of how information from one sense can be traded off or complemented by information from another. In the case of kiwi, the result of these trade-offs is that their sensory world, their reality, is far removed from those of other nocturnal birds.
Trade-offs within a sense
When attempting to understand the behaviour of particular species the above examples of complementarity
