As living beings, we are constantly interacting with our environment and modifying our behavior in response to it. For instance, if it is raining outside, we may want to grab an umbrella while leaving the house. If we really analyze, every behavior of ours, from simplest to most complex, is dependent on our environment, or rather, our perception of it. In simple terms, perception is the process of gathering sensory information from the environment to build a meaningful representation of it in the mind. At any given moment, even now as you are reading this sentence, you are using your senses like vision, audition or touch to construct a mental representation of your surroundings. This process is so effortless and instantaneous that we often don’t give much thought to how crucial perception is for us humans, and in fact all animals, to navigate our surroundings. As I just mentioned, perception, as a neural process, is certainly not restricted to humans. While we all can agree to the above fact, one error that many of us make is that non-human animals perceive the world in the same way as humans. Not all animals care about the same parts of the sensory world as us.
In 1934, Jacob von Uexkull published his book A Foray into the Worlds of Animals and Humans, in which he argued that every living organism perceives the world in a different way. Thus, even though residing in the same environment, different organisms have their unique perceptual world- their special umwelt (german word for “surrounding environment”). The idea of differential perception or unique perspectives has often made intuitive sense to us in philosophical or sociological contexts. But in a biological context, we less commonly acknowledge that how we see, hear, smell, taste and feel the world can be drastically different than our beloved pets, or even our closest evolutionary relatives, the primates.
In the following sections, I will focus on how animals perceive the world around them by addressing two broad questions that have captured the interest of scientists across multiple disciplines like neuroscience, ecology, animal behavior, evolution, psychology and computer sciences. The first question I will explore is ‘how different animals come to have different umwelts’. While pondering over this question, we will see, first, how animals are equipped with diverse sensory systems to detect information from the surroundings, and secondly, how animals could use simple rules to organize the sensory information to ultimately build a meaningful representation of the surroundings. The second question I will explore is ‘why different animals have different umwelts’. Here, we will see perception through the lens of evolution and examine how evolution has designed the animals to function just to maximise their survival and reproduction. Finally, I will wrap up with a humble reminder on why understanding the process of perception is important.
1. [HOW] different animals have different umwelts?
While addressing this question, we should understand that perception can be decomposed into two key processes- detecting sensory information and organizing it to construct a meaningful percept of the environment. An alteration in any of the two processes can result in an altogether different perceptual product. Let us see how similar/dissimilar animals are in the way they perform these two processes.
1.1. Detecting sensory information
An animal’s sensory organs are its first window into the world. Some common sensory organs, most of us know about, are- eyes, ears, nose, tongue and skin. Each of the sensory organs are equipped with cells, called sensory receptors, that are specialized to respond to certain physical and chemical stimuli from the environment like light, sound, heat or chemicals. At a deeper level, these cells are connected to the endings of neurons. In the presence of certain sensory stimuli, the sensory cells respond by releasing neurotransmitters. In a simple sense, neurotransmitters are molecules that are used by the nervous system to transmit information. Some common neurotransmitters that you may be familiar with are adrenaline, dopamine and serotonin. The neurotransmitters released by sensory cells initiate an electrical signal (see action potential) in the connected neurons. These neurons then transmit the sensory information all the way to specific regions in the brain.
Broadly, senses across different animals and modalities (like visual, auditory, olfactory etc.) follow the same framework- specialized receptor cells to gather sensory information and a neuronal network to transmit the information from sensory organs to the brain. In spite of this commonality, animals exhibit vast differences in the makeup of their sensory organs and nervous system which results in different animals perceiving the same environment differently.
1.1.1 Vision
Let us take the case for vision first, as it is the first sense that usually comes to our mind. Besides humans, multiple animal forms- from simplest to most complex, use their sense of vision to navigate their surroundings, communicate with each other, and, find mates and food items. The light around us is responsible for the sense of vision and the sensation of light begins at the eye. The eye morphology itself is astonishingly diverse across the animal kingdom (Fig. 1.). For example, the eye of a turbellarian worm (Fig. 2a) lacks a lens, an otherwise common feature of visual systems, and instead comprises a cluster of light receptor cells arranged in a cup-shaped fashion. This architecture ensures that the receptors receive light only from one direction. Due to this simplistic design, the visual sensation of the turbellarian worm is limited to sensing just the intensity of light as opposed to forming an image of the source, as in the case of humans and many other animals. You might have noticed my special emphasis on the word limited. It may seem intuitive for us to declare that some animals are limited or constrained or less advanced in their sensory capabilities compared to us humans. But are they really?
We will come back to this later when we discuss the “WHY” questions about animal senses.
Figure 2 : The structure of a) Turbellarian worm eye, b) Human eye and c) Fruit fly eye. (Source: Understanding evolution)
While some animals may have apparently simple visual apparatus, other animals, our beloved fruitfly, for instance, have fascinatingly complex eyes comprising hundreds of “ommatidia” or light-sensing units, each having its own lens and visual receptors (Fig. 2b). This design equips the animal with a greater capability to track moving objects. These eyes are called “compound” eyes and they provide a stark contrast to the single-lens, “simple” eyes that humans and many other vertebrates are equipped with. Further, the compound eyes of a bee can detect UV light which our human eye can not detect because our lens blocks light components that have short wavelengths like ultraviolet. As such, a flower may appear very different to bee than us (Fig. 3). Many birds can also “see” ultraviolet.
Figure 3: The same flower as seen through a human eye (left) and bee’s eye (right). Image credit: Martin Stevens
Finally, besides eye morphology animals also show tremendous diversity in the nature of visual receptors they have. Some stomatopods have one of the most complex eyes in the entire animal kingdom. For instance, they have 16 different visual pigments, compared to our three visual pigments (Read more the visual feats of stomatopods here). The diverse forms of visual perception, we just discussed, are by no means representative of the entire animal kingdom, and by describing them, I am merely scratching the surface on the discovered diversity of vision across the animal kingdom. I highly encourage you to supplement this section with this informative video and review paper.
1.1.2 Audition
Another dominant means through which we sense the world is sound. We use our ears to gather information about various auditory events in the environment like the siren of an ambulance, chirping of birds or a thunderstorm. Besides, our auditory perception capabilities also allow us to communicate across different languages and appreciate music. Just like vision, there is a humongous variety of animal forms that rely on sound. Animals use sounds to communicate with each other, find a mate, detect a prey or food item and even escape a potential predator. Here too, ears are the first window into the auditory world. Human ear, for example, has a cartilaginous outer ear that directs the sound to the eardrum. From the eardrum, sounds travel into an inner ear organ called “cochlea”, via an ear canal (middle ear). Cochlea is a coiled structure that has sound receptor cells, also known as hair cells, that mechanically vibrate in response to sound (bonus: watch this fun video showing a vibrating hair cell). The design of cochlea is such that different sound frequencies stimulate different cochlear regions. This set-up breaks the incoming sound into its constituent frequencies (sort of similar to “fourier transform”, read more about it here, if you are interested) and is one of the early steps that ultimately lead to our perceptual experience of pitch. From having different shapes and sizes of the outer ear to completely lacking any outer or inner ear, animals show tremendous diversity in the sound reception organs. The shape of the outer ear (Fig. 4.) can greatly impact the frequency and intensity of sound that reaches the inner ear as well as our judgment of the location of the sound source. Other animals like frogs and birds completely lack an outer ear, and instead have an eardrum-like membrane (tympanic membrane). Crickets, katydids and grasshoppers actually have the tympanum on their legs, as opposed to the head. Differences in the architecture of the tympanic membrane, the hair cell and the underlying sensory neurons across animals leads to vast differences in the range of hearing. The ears of many animals filter out many frequencies that us humans can hear. On the other hand, many animals like bats, some moths, rats and many underwater mammals can hear frequencies that are way beyond our hearing range (Read more about ultrasonic communication in animals here). Consequently, the very same sounds can be perceived very differently by different animals (Also, watch this cool video about how a popular pop song may sound like, to a salmon). Multiple sound waves that are excluded from our perceptual world may form an indispensable part of another animal in your proximity. There is a whole different sonic world that isn’t visible, rather audible to us.
Figure 4: Diversity of ear shape and morphology across animals. (credits: Sea Lion: © iStock/LFStewart; Squirrel: © Erik Mandre/Shutterstock; Frog: ©Frank B. Yuwono/Shutterstock; Owl: ©XNature.Photography/Shutterstock; Lizard: ©Andrew Wijesuriya/Shutterstock; Bat: © iStock/GlobalP; Ostrich: © Jamen Percy/Shutterstock; Dog: ©Annette Shaff/Shutterstock; Lynx: © Dmitri Gomon/Shutterstock)
1.1.3 Other senses and beyond
Needless to say, we can find multiple such differences across other senses as well. Many times we will find animal forms that are completely devoid of the senses we have. At the other end of the spectrum, many animals can sense through modalities that are not available to us. Some examples, absolutely worth mentioning here, are the use of electric (as in the case of electric fish), and magnetic (as in some bird species) senses in animals.
1.2 Organizing sensory information
The next step to perception is organizing the detected sensory information. Let’s spend some time pondering what we mean by perceptual organization. The ultimate goal of perception is to build a meaningful representation of the world. Generally, this meaningful representation consists of different discrete elements called “objects”. For example, your visual perception of a room should equip you with telling a table apart from a chair or bed. Similarly, your auditory perception of a busy street must equip you with the capability of telling apart the sounds of a car with different people walking by. In fact, at any point of time, you are perceiving the world as a collection of objects. The fact that we do it effortlessly doesn’t mean it is a trivial task. Our senses do not detect the object as a whole, rather a collection of physical properties of the object. For example, walking down a busy street, we encounter multiple visual objects like people, shops, streets, birds and so on. What our eyes are gathering are light waves reflected from each of these objects. How do we then determine which light wave belongs to which object? A simple solution to this problem is organizing the mixture of sensory information so that all the light waves belonging to the same object are grouped together and those not belonging to the same object are segregated. We do similar organization along other senses too. For example, walking down a busy street, we encounter different sound sources like vehicles, birds, people (fig. 5.). What our ears gather is a mixture of different sound frequencies belonging to each of the sound sources. We decompose this auditory clutter to group the sounds belonging to the same sound source and segregate the ones belonging to different sources. How do humans and animals perform this perceptual organization? Studies on visual and auditory perception reveal that human perception employs simple rules to group or segregate sensory elements. For example, in a visual scene, our perception tends to group visual elements that are proximal or similar to each other. Parallel rules are also followed in auditory object perception. Perceptual organization has not been studied as extensively in other non-human animals but some recent studies show interesting similarities between how non-human animals and humans perceptually organize sounds and visual input. So even though animals show such diversity in what sensory information they gather, they seem to organize it in broadly similar ways. You can read more about perceptual organization here.
Figure 5: In any auditory scene, sound waves from different sound sources add up and form a composite sound wave that impinges on our ears.
1. [WHY] different animals have different umwelts?
Our extensive discussion about animal senses leads to the obvious question as to WHY animals are so diverse in the way they perceive the world. As I discussed previously, one might also be tempted to conclude that certain animals have either limited or extraordinary sensory capabilities compared to humans. Before making such a conclusion, it might be worth pondering over how sensory systems come to be. The design of any sensory system is largely determined by the evolutionary history of the organism and the evolutionary adaptations in response to a given environment. By evolutionary history, I specifically mean the sensory capabilities of the recent evolutionary ancestors of an organism. Frogs, for example, have a specific anatomy of the auditory system because their recent ancestors had similar auditory systems. So one reason why different animals have differently designed sensory systems is simply because they have descended from ancestors having diverse sensory systems. Now let’s discuss how evolutionary adaptations determine sensory system design. In addition to the evolutionary descent, an organism can further acquire modifications in sensory systems in order to survive and reproduce in a given environment. Now note that in the very same environment different animals may need to do very different things for survival and reproduction. That leads to very different requirements on the sensory system of different animals. And thus sensory systems evolve to achieve just what is required . So an animal may be equipped with eyes specialized to detect only orange color because the prey it needs to catch is orange in color. Similarly, a different animal may be equipped with eyes specialized to detect green color because their potential mates have that coloration. Similarly bats do not have ears that are sensitive to frequencies that human ears are sensitive to simply because the kind of sounds that are biologically relevant to them do not share any frequencies with human range. So the answer to WHY animals have different umwelts is that what they require to perceive in order to reproduce and survive are different. I would like to conclude this by revisiting the point we raised at the very beginning of this section. Evolution has tinkered every animal’s sensory system to perform just what it needs to perform and thus it may be an unfair comparison to say that a given animal has limited or extraordinary sensory capability than the other.
2. Summary
Through this article, I wanted to bring home the point that the very same surroundings can be perceived quite uniquely by different animals- each having its own special umwelt. This fascinating discovery can lead to a set of equally fascinating questions about how and why these unique umwelts come out to be. In pursuing the how questions, one is encountered with an astonishing diversity of sensory capabilities across the animal kingdom, by virtue of which the same environmental elements may seem, sound, feel or smell very differently to different animals. In pursuing the why question, one goes on a quest to understand how ultimately animals are devices that are constantly tinkered by evolution to perform, in a given environment, only what it needs to survive and reproduce. I hope the next time you go on a long walk out in nature, you would be prompted to think about the multiple worlds that exist beyond your perceptual realms. I also hope this humble wonderment leads you in pursuit of finding the special umwelt of your favourite animal. I always wonder about perceiving the world through the senses of an ant. What about you?
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