Inside the colourful world of animal vision

As humans, we live in a colourful world, but differences in visual systems means that not all animals see the world in the same way.
The mantis shrimp has 12 types of visual cones. Klaus Stiefel/Flickr, CC BY-NC
Unlike other aspects of an object such as size or mass, colour is not an inherent property of an object but a result of the sensory system of the viewer. In other words, colour is a construct of the mind.

The eye’s retina contains specialised cells called photoreceptors, which convert light that bounces off objects into signals that the brain processes into visual images. Two types of photoreceptors are rods and cones.

In humans, there are three types of cone cell that are responsible for the early stages of colour vision. Each type of cone cell is maximally absorbent in a different part of the spectrum – short, medium and long wavelengths of light. These are typically named blue, green and red cones, respectively, to describe how humans perceive light at each cone’s peak absorbency.

When light hits the eye, the cones are stimulated differentially according to their type, and the relative excitation of each type underlies colour sensations. In a process known as colour opponency, the outputs are then compared against each other in various permutations. This information is then sent to and interpreted by the brain, which provides the final sensation of colour.
Inside the colourful world of animal vision

How do other animals see colour?

Animals vary in the number and sensitivity of cones present, so visual processing can result in very different colour sensations, even before differences in brain processing are taken into account.
An example of how the world looks to a dichromatic (left) and trichromatic (right) primate. PLOS ONE, CC BY
Most mammals are dichromatic – they have only two cone types (blue and green sensitive). Humans have three types of interacting cones and so are trichromatic, although there is at least one documented case of a female having four cones.

Apes and Old World monkeys also have trichromatic vision, but New World monkeys have variable colour vision that is also sex-linked, meaning that males and females of the same species can have different numbers of cone type. Generally, females are trichromats whereas males are dichromats as they lack the photoreceptor that is sensitive to red wavelengths of light.

In several species of New World monkey, such as marmosets and tamarins, all males are dichromatic but females may be either dichromatic or trichromatic. Trichromacy may offer a foraging advantage by enabling green and red food items to be easily distinguished, but may also be useful in signalling amongst individuals of the same species, whereas dichromacy may be advantageous when foraging for camouflaged food or in low light levels.

Bees are also trichromatic, but they can see ultraviolet (UV) light as they have a UV sensitive receptor, as well as blue and green sensitive receptors. In contrast, most birds, fish, and some insects and reptiles are tetrachromatic, having four (but sometimes even five or more) types of cone cell. In many cases in tetrachromats, the fourth photoreceptor allows the animal to perceive UV light.
Spectral sensitivity curves for a typical tetrachromat (such as a bird). L. Shyamal/Wikipedia
Despite not having a specific UV receptor, it was recently discovered that reindeer in the Arctic Circle see UV light. While the mechanism of this ability is still under investigation, it is thought that UV vision evolved due to the UV rich snowy conditions that the reindeer live in.

Lichens, which are a major source of food for reindeer, absorb UV light, as does urine – a good indicator of the presence of predators or potential mates. These appear black against the UV reflecting snow and are likely easier to see.

Do more photoreceptors result in better colour vision?

Theory predicts that a visual system comprised of around five photoreceptor types is plenty for encoding colours of the visual spectrum in day-to-day life.

The mantis shrimp (Haptosquilla trispinosa) by far exceeds this, with 12 photoreceptors. It was thought that the 12 types of photoreceptor in this marine crustacean allowed them to see a spectacular array of colours that we, as humans, could not imagine.

A recent study examining this hypothesis tested the limits of the mantis shrimp’s ability to discriminate between two colours. If more photoreceptors enable heightened colour perception, then the shrimp should be excellent at distinguishing between similar colours. Surprisingly, however, the mantis shrimp performed worse than humans.

The shrimp seem to have evolved a novel way of encoding colour, as the photoreceptor outputs do not undergo any opponent processing. The outputs appear to be sent directly to the brain where they may be compared to a “mental template” of colours. This type of vision may be advantageous as light requires less processing in the eye and is therefore likely to be more rapid. However, nothing is yet known about how the brain processes these inputs.

In truth, we can probably never know how a shrimp, or any kind of animal, perceives colour. Not only is it difficult for us to imagine colour vision more with more dimensions than our own, but we also need to account for how the brain interprets such information. That being said, there is still much left to learn about the colourful world of animal vision.

Laura Kelley, Research Fellow, University of Cambridge

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Monkey minds: what we can learn from primate personality

Carla Litchfield, University of South Australia

Every human is different. Some are outgoing, while others are reserved and shy. Some are focused and diligent, while others are haphazard and unfussed. Some people are curious, others avoid novelty and enjoy their rut.

This is reflected in our personality, which is typically measured across five factors, known as the “Big Five”. These are:

Fascinating #research by @UniversitySA #primates, just like us have unique personalities. Some are chilled, while others are chaotic, some are curious while others are conservative. Some are the life of the jungle party, others shy!

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• Openness – intellectual curiosity and preference for novelty
• Conscientiousness – the degree of organisation and self-discipline
• Extraversion – sociability, emotional expression and tendency to seek others’ company
• Agreeableness – degree of trust or suspicious of others and tendencies towards helpfulness and altruism, and
• Neuroticism – emotional stability or volatility.

But did you know that our primate cousins – other apes (chimpanzees, bonobos, orangutans, gorillas and gibbons) and monkeys – also exhibit a similar personality profile? Some are bold, others shy. Some are friendly, other aggressive. Some are curious, while others are conservative.

But they also differ from us in some interesting ways. And it’s in teasing out these differences that we can learn a surprising amount about the way they live, and how they have evolved.
Purple-faced Langur Semnopithecus vetulus

Social influence

Comparative psychologists have long adapted personality tests to measure the personality of other species, including pets, big cats, and our “hairy” primate relatives.

Since nonhuman animals cannot fill out a questionnaire, a human who knows them well – perhaps a caregiver, zookeeper, owner, researcher or park ranger – rates their personality for them.

Chimpanzees, it turns out, are remarkably similar to us in their personality make-up. They have been found to have the same five personality factors that we have. However, they also have a sixth Dominance factor. This includes features such as: independent, confident, fearless, intelligent, bullying and persistent.

Why do chimps have a Dominance factor and we don’t? It appears to be due to the kind of society that chimps live in. Understanding the dominance hierarchy of male chimpanzees – who is powerful and who is not – is a matter of survival and well-being for every chimpanzee in a community.

Other primates also show interesting variations in personality that correspond to their social dynamics.
Do I look conscientious or neurotic? Rod Waddington/Flickr, CC BY-SA

Macaque machinations

The 22 species of macaque monkeys are the only primates that are as widespread in their distribution as we are. Along with their disparate habitats, they also have a wide variation in the structure of the societies, which appears to have influenced the evolution of their personalities.

A team of researchers, led by Mark Adams and Alexander Weiss of Edinburgh University, investigated personality and social structure in six species of macaque and found some interesting variation.

There are four main categories of social style, ranging from Grade 1 “despotic” to Grade 4 “tolerant”, depending on how strict or relaxed their female dominance hierarchies are.

Grade 1 species showed strong nepotism or favouritism towards kin and high ranking monkeys. These species include rhesus macaques, a species commonly used in laboratories and sent into space before humans, and Japanese macaques, which include the famous snow monkeys who soak in hot springs.

Lion tailed macaque

At the other end of the spectrum, the Grade 4 species showed more tolerance in social interactions between unrelated females. This includes Tonkean macaques, which are found in Sulawesi and the nearby Togian Islands in Indonesia, and Crested macaques, which are critically endangered.

(A wild crested macaque received international attention when he stole a wildlife photographer’s camera and then photographed himself. This could be an example of a “bold” and “curious” personality.)
Red-cheeked Gibbon Nomascus gabriellae
In the middle of the social tolerance scale are the Grade 2 and 3 species. This includes Assamese macaques, which are sometimes found at high altitudes in Nepal and Tibet, and Barbary macaques, which include the infamous “apes” of Gibraltar (actually monkeys, not apes), who are often overweight and aggressive because tourists overfeed them.
Do I look aggressive to you? Michelle Bender/Flickr, CC BY-NC-ND

Personality differences between macaque species

Interestingly, the individual species of macaques didn’t all have the same personality factors. The Japanese, Barbary, crested and Tonkean macaques had only four, while the Assamese had five, and rhesus monkeys had six factors.

All of the species exhibited the dimension of Friendliness. This seems to be a personality factor unique to macaques, and is a blend of chimpanzee Agreeableness and human Altruism.

Tonkean macaques also had a Sociability personality factor. Just like chimpanzees and humans, this species of macaque uses affiliative contacts (i.e. friendship) to reinforce bonds. Only crested macaques did not show the personality factor of Openness (i.e. curiosity), usually found in humans and other primates. The factors Dominance and Anxiety were found for rhesus and Japanese macaques.

The old and the new

The study also showed the fascinating connections between personality and social style. Grade 1 despotic species – Japanese and rhesus macaques – were rather similar, and so were Grades 2, 3 and 4, including the more tolerant species such as Assamese, Tonkean and crested macaques.

On the evolutionary scale, African primates, such as the African Barbary macaque, are “older”. Therefore, they represented the “ancestral” social behaviours for macaques.

Barbary macaque personality has a Dominance/Confidence factor, which is related to social assertiveness, an Opportunism factor, which relates to aggression and impulsivity, a Friendliness factor, relating to social affiliation, and an Openness factor, relating to curiosity and exploratory behaviour.

Rhesus and Japanese macaques, on the other hand, are “younger” on the evolutionary scale. Therefore, the Dominance and Anxiety factors seen in these species must have evolved later.
Psst. You’re disagreeable. jinterwas/Flickr, CC BY
Understanding the personality of an individual animal or species can help in animal management and welfare. Rhesus macaques, for example, display an Anxiety personality factor. These monkeys are also most commonly used in bio-medical laboratory research. Knowing that some individuals may be prone to anxiety means that researchers must make extra efforts to alleviate any potential distress.

The findings that some Barbary macaques may be especially socially assertive, aggressive, impulsive, curious and exploratory may also help us convince tourists to keep their distance from these monkeys in Gibraltar to avoid conflicts!

Such studies of animal personality also shed light on our own personality dimensions. Our lack of a Dominance factor suggests that our ancestral environment was perhaps more egalitarian and less characterised by high social stratification, which is also borne out by anthropological and palaeontological studies.

Ultimately, we can learn a lot from our primate cousins, not only about their personalities, but about personality itself – not to mention learning a thing or two about ourselves and the social environment in which we evolved.

Carla Litchfield, Lecturer, School of Psychology, Social Work and Social Policy, University of South Australia

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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