origins of investigating senses
The origins of thinking about the problem of understanding human and animal senses go back more than 2000 years in western thought. However, it is only during the last 200 years that we have gained real insight into the diversity of sensory information across species. The Darwin–Wallace ideas of natural selection, and Alexander von Humboldt’s ideas about the interrelationships of organisms with their environment, provided two big ideas which now provide broad frameworks for thinking about the senses of animals. They give us many reasons for expecting that senses, and the information that they provide, will differ between species. Importantly they provide ways to account for these differences in terms of their evolution and their ecological functions. We can now appreciate why sensory information might differ between species, and how it is linked to specific behaviours and ecologies.
Epicurus and Sextus
Ideas about the challenges of understanding the sensory world of humans were first thought through and described by the Greek philosopher Epicurus (341–270 BCE). The writings of Epicurus, and the elaboration of his ideas by later philosophers, have given rise to the substantial body of thought known today as Epicureanism. Many prominent thinkers through to the present day have characterised themselves as Epicureans. Epicureanism has a number of key ideas, and prominent among them is being prepared to accept the limitations on the information that is available to guide actions.
Epicurus was among the first to write explicitly about the way that our senses place very real constraints on our overall understanding of the world. He was the first western philosopher–scientist to recognise that the information we receive changes from moment to moment. He also argued that information, even though it is concerned with the same object, is radically different depending upon the sense involved. What becomes recognised by us as a particular object is constructed from many different, and ever-changing, sources of information received from different sensory systems.
Some 500 years later, the Roman philosopher Sextus Empiricus (160–210 CE) took these human-focused ideas of Epicurus and saw their implications for all animals (Figure 2.1). Sextus argued that non-human animals are also constrained by the information that their senses provide and, crucially, that different animal species are constrained in different ways, so that they cannot possibly be living in identical worlds. This was probably Sextus’ most important insight. Today we have a wealth of scientific data to support such a notion.
FIGURE 2.1 The Greek philosopher Epicurus (left) and the Roman philosopher Sextus Empiricus (right). Their ideas laid the foundations for a comparative approach to studying the senses of animals. They viewed sensory capacities as linked to the environment in which an animal lives and to the tasks that it performs. Shaped by modern ideas about evolution, their approach to understanding the worlds of animals is manifest today in the science of sensory ecology. (Image of Sextus Empiricus from Wikimedia Commons, public domain.)
It might not have been unreasonable to surmise that while birds experience a different world to that of humans, all bird species experience the world in the same way. Thus, it would have been possible to explain that bird species exhibited different behaviours simply because of differences in their anatomy and bodily structures. That is, it was only because of the differences in their bills, wings, legs, etc. that birds behaved differently, not because of any differences in the information that they had about the world. Sextus argued that this was not the case, and that while species might share the same environment the information that they had about it was different.
Sextus argued that the anatomical and structural differences between species went hand-in-hand with differences in the information that each species received to control their behaviour. Having established this as a key idea, it can be seen that in essence sensory science, and especially sensory ecology, has for two millennia been filling in the details – an enterprise that has been boosted significantly by the evolutionary and ecological frameworks that emerged in the last 200 years.
Sextus’ insights and their role in the evolution of fundamental ideas about the world have been profound. This is because they lead to a quite unsettling position. They inevitably lead to the question of what the ‘real’ world might be like. Is there such a thing, or are there many different worlds depending upon the information available to particular species? It was questioning in this way that laid the foundations of Scepticism – the idea that we must neither accept any idea as true nor any idea as false, but we must always question. Scepticism is based upon the recognition of how difficult it is to be sure of the world when it can be based only on the information that the senses provide. Scepticism, in turn, led to Empiricism and the system of enquiry that underpins the modern scientific approach to understanding the world through experimentation and hypothesis testing.
Questions about senses: differences and dimensions
Questions about the sensory world of birds are legion. Every reader will have his or her own set of questions. Questions may be focused around particular bird species, but with almost 11,000 species, specific answers are not always available. Some questions have straightforward answers, some answers will be nuanced, and many will not have an answer … yet. We might ask, for example, about how the ability to see details in a scene differs between species, or how visual fields might differ between species. If we know the answers to such questions, we might be able to account for a raptor’s ability to capture its prey, or why species differ in their vigilance behaviour. If we know something about hearing, we can ask about the sounds used by birds to advertise their presence in a territory and whether birds can accurately pinpoint a singing rival. If we know about smell and taste, we can ask about their role in helping birds to locate profitable foraging locations.
Such questions lead to the three core challenges of sensory ecology:
1. to measure sensory performance so that it is possible to properly compare like with like across species;
2. to understand the anatomical and physiological differences that are responsible for differences in sensory capacities;
3. to propose ideas about what drives these differences from an ecological and behavioural perspective.
Devising ways of comparing sensory performance is a key challenge because we need to be confident that the same thing is being measured in different species. Despite differences in the size, general structure, behaviour, and ecology of bird species, it is necessary to get information about the same sensory capacities in each species. Furthermore, while it is possible to ask in general terms what a bird can see, hear, smell, taste, etc., each sensory capacity is highly complex (Figure 2.2).
FIGURE 2.2 A photographic montage of bird species from a wide range of orders and families. Each species has a unique biology, and this montage could well be used to show diversity of bill structures and sizes and how they can be related to different diets and foraging techniques. However, aspects of the senses, especially vision, in all of these species have been investigated. These show that these species also differ in the information that their senses extract from the environment. Furthermore, it has been shown that these are intimately linked with both the ecology and the behaviour of these species. In short, the eyes and vision of these birds vary as much as their bills.
The only way to characterise and quantify each sensory capacity is to subdivide it into particular dimensions. It is these subdivisions which can be measured and compared with confidence. For example, when investigating vision, comparisons across species are usually made with respect to a number of
