Monkeys (and their neurons) are calculating statisticians

Using a simple psychological test, scientists have found that monkeys can use simple statistical calculations to make decisions. They even managed to catch individual neurons in the act of computing.

Say the word ‘statistician’ and most people might think of an intelligent but reclusive person, probably working in a darkened room and almost certainly wearing glasses. But a new study shows that a monkey in front of a monitor can make a reasonably good statistician too.

Rhesus macaques can compute statistics in a simple psychological task.Tianming Yang and Michael Shadlen from the University of Washington found that rhesus macaques can perform simple statistical calculations, and even watched their neurons doing it.

Psychologists often train animals to learn simple tasks, where the right choice earns them a reward and the wrong one leaves them empty-handed or punished. But real life, of course, is not like that.

Mostly, there are risks and probabilities in lieu of guarantees or right answers. Animals must weigh up the available information, often from multiple sources, and decide on the course of action most likely to work out in their favour.

A simple psychological test

Yang and Shadlen tested this decision-making ability in two rhesus macaques using a variation of the well-known weather prediction task used to test human volunteers. In the human version, people are shown a series of cards that represent various probabilities of good or bad weather. After some training, they are shown combinations and asked to predict the likely weather from these.

The monkeys had a slightly simpler task – they had to look at either a green or a red target. If they picked the right one (which changed from trial to trial), they were rewarded with a tasty drink. To help the monkeys choose, Yang and Shadlen showed them a series of shapes that represented the probability that the rewarding target was red or green.

For example, a square strongly indicated that the red target was the rewarding one, while a triangle strongly favoured the green one, and an hourglass only slightly favoured the green. The monkeys were shown four shapes out of a possible ten, and to get the right answer, they had to add up the probabilities indicated by these shapes.

Monkey see, monkey decide

Yang and Shadlen saw individual neurons performing computations.And that is exactly what they did. They learned to base their decisions on the combined probabilities of the four shapes, and chose the appropriate target. It did, however, take them a while to learn (or two months of training with over 130,000 trials to be exact). Any statisticians reading this don’t need to fear about being replaced by monkeys any time soon.

They weighed up the strength of the evidence too. When the shapes strongly suggested one colour, the monkeys almost always went with that colour. When the summed probability lay between the two extremes, they chose either target but still favoured the one indicated by the shapes.

With 715 different combinations of shapes, the experiment’s design makes it highly unlikely that the monkeys simply memorised the ‘answers’ for different mixes. And because the shapes only dealt in probabilities, it was still possible to choose the wrong target, even if the monkey strictly adhered to the shapes’ advice. They were clearly reasoning with probabilities, and in pretty subtle ways.

Calculating neurons

For their next trick, Yang and Shadlen visualised this reasoning directly by looking at 64 neurons in the monkeys’ lateral intraparietal area (LIP). This part of the brain is responsible for several higher functions like mathematical skills. Other studies have found that the LIP collects data from the visual cortex, and helps to process what the monkey sees.

Monkeys can calculate the sums of different probabilities.When the monkeys saw a shape, the activity of their LIP neurons was proportional to the probability indicated by that shape. As the four shapes were shown in sequence, the neurons altered their rate of firing to account for the new information. As the evidence was building up, the monkeys were busy doing sums in their heads. Yang and Shadlen were seeing arithmetic in action.

Of course, monkeys are living things and not fuzzy calculators, and they were not equally good at statistical reasoning. One was clearly better than the other, and Yang and Shadlen put this down to differences in their neurons.

Each neuron varies slightly in its typical firing rate, and summed together, these variations can lead to biases in how the monkeys deal with calculations. This explains why the monkeys sometimes did different things when shown the same combination of shapes.

Their confusion was particularly apparent when the shapes gave no strong inclination to pick one target or another. We can certainly relate to that – after all, it’s certainly harder to make a decision, when neither option seems particularly better than the other.

Yang and Shadlen believe that human brains use similar methods to make decisions. Cues about probabilities are funnelled into the brain’s control centres (like the LIP), which act like calculators powered by the firing of neurons.

Reference: Yang & Shadlen. 2007. Probabilistic reasoning by neurons. Nature (doi:10.1038/nature05852)

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The evolution of animal personalities – they’re a fact of life

Animals have distinct personalities and temperaments, but why would evolution favour these over more flexible and adaptible mindsets? New game theory models show that animal personalities are a natural progression from the choices they make over hwo to live and reproduce.

Any pet owner, wildlife photographer or zookeeper will tell you that animals have distinct personalities. Some are aggressive, others are docile; some are bold, others are timid.

An animal’s reproductive decisions can determine if it is a hawk…In some circles, ascribing personalities to animals is still a cardinal sin of biology and warrants being branded with a scarlet A (for anthropomorphism). Nonetheless, scientists have consistently found evidence of personality traits in species as closely related to us as chimpanzees, and as distant as squid, ants and spiders.

These traits may exist, but they pose an evolutionary puzzle because consistent behaviour is not always a good thing. The consistently bold animal could well become a meal if it stands up to the wrong predator, or seriously injured if it confronts a stronger rival. The ideal animal is a flexible one that can continuously adjust its behaviour in the face of new situations.

And yet, not only do personality types exist but certain traits are related across the entire animal kingdom. Aggression and boldness toward predators are part of a general ‘risk-taking’ personality that scientists have found in fish, birds and mammals.

Life decisions affect personality

Max Wolf and colleagues from The University of Groningen, Netherlands, have found a way to explain this discrepancy. Using game theory models, they have shown that personalities arise because of the way animals live their lives and decide when to reproduce.

For an animal, success is measured achieved through living long enough to reproduce, and individuals constantly gamble their current success against their future one. They could reproduce now, or defer it to a later time when resources are more abundant.

The crux of Wolf’s theory is that those with a stable, assured future have more to lose by gambling, and are likely to be more risk-averse. Those with little to lose can afford to live fast and die young.

A model personality

Wolf tested this idea by using a mathematical model to simulate these choices and their consequences. The protagonist of his model is a fictional animal that lives in an area with many territories, some rich in food and others lacking it.

The animal can choose how thoroughly it wants to explore its habitat. If it is adventurous, it could find a lush and bountiful territory, but it will have less energy to raise young, and must postpone this to the following year. That may not be so bad – its new home will give it a ripe, long life and it will have many opportunities for breeding.

or a dove.He found that simulated animals picked one of two stable strategies. Some decided to explore thoroughly and hope for greater reproductive success in the future. Others decided to stay put, have young now and make the best of things, poor resources be damned.

Wolf then modelled how these two groups would react to decisions about risk, in a classic hawk-dove experiment. When faced with a predator or a rival, the animal could run away or back down (dove), which takes time and could lose it feeding opportunities or its territory. If it stood and fight (hawk), the likelihood of death or injury was greater but so were the rewards.

Sure enough, the explorers who were investing on future success, consistently evolved to be docile, timid and risk-averse, while those who reproduced immediately consistently became bold and aggressive. These patterns held up under a wide range of simulated conditions. Over time, they gave rise to stable individual differences and behaviour traits that were consistently linked with each other, the foundations of personality.


In New Scientist’s coverage of this story, Judy Stamp from the University of California, Davis, criticises Wolf’s work for only explaining extremes of personality. Obviously, animals are not always black hawks or white doves, but many shades in-between.

But Wolf’s study answers this too. In the most advanced version of his model, he accounted for the fact that behaviours are governed by many heritable genes. This generated a much more realistic and continuous spectrum of personalities. Even with this more plausible model, the same principle applied – the more an animal had to lose, the fewer risks it was prepared to take.

Wolf is now keen to see his theory tested in the field. He suggests that many other behaviour traits could be linked to aggression or boldness. Individuals that invest heavily in the present may be more likely to guard nests, care for their young or woo mates with conspicuous and costly displays.


Reference: Wolf, van Doorn, Leimar & Weissing. 2007. Life-history trade-offs favour the evolution of animal personalities. Nature 447: 581-584.

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Orang-utan study suggests that upright walking may have started in the trees

A common theory of human evolution says that after our ancestors descended from the trees, they went form walking on four legs to two. But a new study in orang-utans could overturn that theory, by suggesting that our ancestors evolved a bipedal walk while they were still in the trees.

Did and upright posture evolve in a tree-dwelling ancestor?Walking on two legs, or bipedalism, immediately sets us apart form other apes. It frees our arms for using tools and weapons and is a key part of our evolutionary success. Scientists have put forward a few theories to explain how our upright gait evolved, but the ‘savannah theory’ is by far the most prolific.

It’s nicely illustrated by this misleading image that has become a mainstay of popular culture. It suggests that our ancestors went from four legs to two via the four-legged knuckle-walking gait of gorillas and chimps. Dwindling forests eventually pushed them from knuckle-walking to a full upright posture. This stance is more efficient over long distances and allowed our ancestors to travel across open savannahs.

But this theory fails in the light of new fossils which push back the first appearance of bipedalism to a time before the forests thinned, and even before our ancestors split from those of chimpanzees. Very early hominins, including Lucy (Australopithecus afarensis) and Millennium Man (Orrorin) certainly ambled along on two legs, but they did so through woodland not plains.

Our arms provide a further clue. Even though our ancestors’ back legs quickly picked up adaptations for bipedalism, they steadfastly kept long, grasping arms, an adaptation more suited to moving through branches. To Susannah Thorpe at the University of Birmingham, these are signs that bipedalism evolved while our ancestors were still living in trees.

Two legs good, four legs bad?

Orang-utans can go bipedal and our ancestors may well have done the same in the trees.But there is a snag – an adaptation must provide some sort of benefit. And, as many children painfully discover, it is hard to imagine how walking on two legs could benefit sometime in a tree.

But Thorpe has an answer to this too. She spent a year in the Sumatran jungle, studying the orang-utan – the only great ape to spend the majority of its life in the trees.

She carefully documented over 3,000 sightings of wild orang-utans moving through the treetops. On large sturdy branches, they walk on all fours (below right), and on medium-sized ones, they start to use their arms to support their weight.

But on the thinnest and most unstable branches, the apes use a posture that Thorpe calls ‘assisted bipedalism’ (below left). They grip multiple branches with their long, prehensile toes and use their arms to balance and transfer their weight. And unlike chimps which bend their knees while standing up, bipedal orang-utans keep their legs straight, just like humans do.

An orang-utan used both two-legged and four-legged postures.

It’s a win-win posture – the hands provide extra safety, while the two-legged stance frees at least one hand to grab food or extra support. With it, the apes can venture onto the furthest and thinnest branches, which provides them with several advantages.

As Thorpe says, “Bipedalism is used to navigate the smallest branches where the tastiest fruits are, and also to reach further to help cross gaps between trees.” That saves them energy because they don’t have to circle around any gaps, and it saves their lives because they don’t have to descend to the ground. “The Sumatran tiger is down there licking its lips”, she said.

A new view of ape & human evolution

With these strong adaptive benefits, it becomes reasonable to suggest that bipedalism evolved among the branches. Based on this theory, Thorpe, along with Roger Holder and Robin Crompton from the University of Liverpool, have painted an intriguing new picture of ape evolution.

It begins in the same way as many others – with the rainforests of the Miocene epoch (24 to 5 million years ago) becoming increasingly patchy. For tree-dwelling apes, the gaps in the canopy started becoming too big to cross. But in Thorpe’s view, these ancestral apes were already using a bipedal stance, and different groups took it in separate directions.

Our ancestors were bipedal long before they came down from the trees.The ancestors of orang-utans remained in the increasingly fragmented canopy and became specialised and restricted there. The ancestors of chimps and those of gorillas specialised in climbing up and down trees to make use of food both in the canopy and on the ground. The postures used in vertical climbing are actually very similar to those used in four-legged knuckle-walking and this became their walk of choice on the ground.

The ancestors of humans abandoned the trees altogether. They used the bipedal stance that served them well on thin branches to exploit the potential of the stable land environment. Over time, they brought in further adaptations for efficient walking, culminating in the human walking style that we now neglect by sitting at a computer all day.

Thorpe’s reconstruction is delightfully non-human-centric. It suggests that in the evolution of movement, we were conservatives who relied on a walk that had been around for millions of years. Chimps and gorillas with their fancy new knuckle-dragging gait were the true innovators.

Reference: Thorpe, Holder & Crompton. 2007. Origin of human bipedalism as an adaptation for locomotion on flexible branches. Science 316: 1328-1331.

Image: Black and white image from Science magazine.

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Why music sounds right – the hidden tones in our own speech

The keys on a piano are a physical representation of the sounds of our speech.Have you ever looked at a piano keyboard and wondered why the notes of an octave were divided up into seven white keys and five black ones? After all, the sounds that lie between one C and another form a continuous range of frequencies. And yet, throughout history and across different cultures, we have consistently divided them into these set of twelve semi-tones.

Now, Deborah Ross and colleagues from Duke University have found the answer. These musical intervals actually reflect the sounds of our own speech, and are hidden in the vowels we use. Musical scales just sound right because they match the frequency ratios that our brains are primed to detect.

Interlude – speaking for beginners

When you talk, your larynx produces sound waves which resonate through your throats. The rest of your vocal tract –your lips, tongue, mouth and more – act as a living, flexible organ pipe, that shifts in shape to change the characteristics of these waves.

What eventually escapes from our mouths is a combination of sound waves travelling at different frequencies, some louder than others. The loudest frequencies are called formants, and different vowels have different ‘formant signature’. Our brains use these to distinguish between different vowel sounds.

The first two formants, the ones with the lowest frequencies, are the most important. The brain pays particularly close attention to these and uses them to identify vowels. If they are artificially removed from a recording, the speaker becomes impossible to understand. On the other hand, getting rid of the higher formants does no such thing.

(This spectrogram shows the different frequencies that make up three different vowels. Frequency goes up the vertical axis. The darker the image, the louder that particular frequency is. For each vowel, the first two formants (the lowest dark bands) are marked.)

A spectrogram showing formants for three common vowels.

Despite the wide variety of sounds in different languages, and the even greater variety in people’s voices, the formants of their vowels fall into narrow and defined ranges of frequencies. The first one always has a frequency of 200-1,000 Hz, while the second always lies between 800 and 3,000 Hz.

Hidden musical intervals

Ross analysed the formants of English vowels by asking 10 English speakers to read out thousands of different words and some longer monologues. Amazingly, she found that the ratio of the first two formants in English vowels tends to fall within one of the intervals of the chromatic scale.

When people say the ‘o’ sound in rod, the ratio between the first two formants corresponds to a major sixth – the interval between C and A. When they say the ‘oo’ sound in booed, the ratio matches a major third – the gap between C and E. Ross found that every two in three vowel sounds contain a hidden musical interval.

Her results didn’t just apply to English either. Ross repeated her experiments with people who spoke Mandarin, a vastly different language where speakers use four different tones to change the meaning of each word.

Even so, Ross still found musical intervals within the formant ratios of Mandarin vowels. The distribution of the ratios was even similar – in both languages, an octave gap was most common, while minor sixth was fairly uncommon.

How hidden intervals shape our musical tastes

Music sounds right to us because intervals match the frequency ratios of our vowels.Ross believes that these hidden intervals could explain many musical curiosities. For example, the musical preferences of a certain culture could reflect the formants most commonly used in its language.

Hardly any music uses the full complement of 12 semitones, and European music usually limits itself to just 7 – the so-called ‘diatonic scale’ represented by a piano’s white keys. Music from other parts of the world tends to use the ‘pentatonic scale’ where the octave is split into just 5 tones.

Ross found that the 70% of the chromatic intervals in her data were included in the diatonic scale, and 80% were found in the pentatonic one. She reckons that these scales are so widely used because they reflect the most common formant combinations in our speech.

She now wants to see if the link between formants and intervals can explain why music in a major key instinctively sounds happier and more upbeat than music in a minor key.

Formants are common to the vast majority of languages and cultures, which explains why the twelve-semitone chromatic scale is so universal. Regardless of our cultural differences, it is heartening to realise that in some ways, we are all the same.

Reference: Ross, Choi and Purves. 2007. Musical intervals in sounds. PNAS 104: 9852-9857.

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Army ants plug potholes with their own bodies

Army ants are wonderful examples of animal co-operation. In one species, workers use their own bodies to fill potholes in the paths of their sisters, leading to easier journeys and more food for the colony.

Imagine that you’re driving along a country lane. As often happens, the road suddenly transforms from a well-paved street to a pothole-ridden nightmare. As your suspension and your stomachs start to tire, your friends in the back suddenly force you to stop the car.

To your amazement, they jump out and lie across the potholes, beckoning you to drive your car over them. It may seem like a far-fetched scenario, but if you were an army ant, such selfless behaviour would be a matter of course.

An army ant worker plugging a pothole.Army ants are some of the deadliest hunters of South America. Amassing in legions of over 200,000 ants, they become a massive predatory super-organism that fan out across the jungle floor leaving dismembered prey in their wake.

Behind the killing front, the corpses of the ant’s prey are taken back to the nest by foragers. But the route back home is not a smooth one. At an ant’s size, small twigs and leaves can be the equivalent of a bumpy, unpaved motorway.

Scott Powell and Nigel Franks from the University of Bristol found that at least one species of army ant (Eciton burchellii) solves this problem with living paving. Certain workers stretch their bodies over gaps in the forest floor, allowing their food-carrying sisters to march over them.

Eciton burchellii, a deadly predator, but a highly co-operative one.The ants carefully size-match to the holes that they plug. Powell and Franks stuck planks with different sizes of hole in the path of the ant column, and found perfect matches between ant and hole.

By smoothing the trail home, they ensure that other workers can return food to the colony as fast as possible. Powell and Franks calculated that this increase speed means that the colony as a whole gets more to eat, even thought the plugging ants cannot carry any food themselves.

It may seem that the plugging ants have a hard lot in life. But they are ultimately rewarded for their temporary sacrifices. When the foraging trip finally ends, the pluggers can look forward to a hearty meal when they return home. By taking on a specialised role, these ants improve the performance of the colony as a whole.

Reference: Powell & Franks. 2007. How a few help all: living pothole plugs speed prey delivery in the army ant Eciton burchellii. Animal Behaviour doi:10.1016/j.anbehav.2006.11.005

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Tracks provide evidence of swimming dinosaurs

It’s amazing how much you can learn about an animal from the tracks it leaves behind. In the case of dinosaurs, tracks that have lasted for millions of years tell us how fast they moved or whether they travelled in groups. Now, a unique set of tracks discovered in Spain tell us that at least some types of dinosaur could swim*.

The track in question is preserved in the sandstone of the Cameros Basin, one of the richest known sources of dinosaur tracks from the Cretaceous period. It stretches across 15 metres but consists of just six pairs of footprints; their maker was clearly a large animal.

A drawing of a swimming theropodThe ‘footprints’ are few in number, but their size and shape speak volumes. Each is actually a series of two or three long, slender scratch marks. That rules out a walking animal or a tip-toeing crocodile, both of which would have produced a broader, flatter print.

Ruben Ezquerra from the Fundación Patrimonio Paleontológico de La Rioja, who discovered the tracks, thinks that they are clear signs of a paddling carnivorous dinosaur.


During the late Cretaceous, these sandstone flats would have been submerged under metres of water. As the predator swam through the lake, its torso would have floated near the surface while its legs propelled it along. As it swam, the tips of its toes lightly scratched at the sediment, creating the tracks that exist today.

Each of its paddling strides spanned about 2.5 metres; this was a large animal. Even so, its tracks suggest that it swam with exaggerated walking motions, in the same way that modern (and less fearsome) water-birds do.

The tracks even tell Ezquerra that the predator was swimming against the current. They are asymmetric with the right prints angled forty-five degrees to the left. These were caused by the animal pushing harder with its right foot, while its body was slightly angled against upriver.

Baryonyx, a fishing dinosaur from Cretaceous Spain - could it have left the Camperos tracks?In a way, we shouldn’t be surprised. The dinosaurs filled ecological vacancies that modern mammals now inhabit, and many large mammals from bears to (surprisingly) elephants prove to be surprisingly capable swimmers.

Some dinosaur species were even thought to be specialised fishermen and one of these, Baryonyx (above), lived in Spain during the early Cretaceous. Could it have made the tracks that Ezquerra found?

Reference: Ezquerra, Doublet, Costeur, Galton, Perez-Lorente. 2007. Were non-avian theropod dinosaurs able to swim? Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain). Geology 35: 507-510.

Drawing: by Guillaume Suan, University Lyon.

*Note that prehistoric marine reptiles, like plesiosaurs and icthyosaurs, were not dinosaurs. All dinosaurs were land-living creatures.

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Babies can tell apart different languages with visual cues alone

Most of us could easily distinguish between spoken English and French. But could you tell the difference between an English and a French speaker just by looking at the movements of their lips? It seems like a difficult task. But surprising new evidence suggest that babies can meet this challenge at just a few months of age.

The shapes of our mouths when we speak provide valuable clues that we can use to understand language.Young infants can certainly tell the difference between the sounds of different languages. Whitney Weikum and colleagues from the University of British Columbia decided to test their powers of visual discrimination.

They showed 36 English babies silent video clips of bilingual French-English speakers reading out the same sentence in one of the two languages. When they babies had become accustomed to these, Weikum showed them different clips of the same speakers reading out new sentences, some in English and some in French.

When the languages of the new sentences matched those of the old ones, the infants didn’t react unusually. But when the language was switched, they spent more time looking at the monitors. This is a classic test for child psychologists and it means that the infants saw something that drew their attention. They noticed the language change.

Weikum found that the babies have this ability at 4 and 6 months of age, but lose it by their eighth month. During the same time, other studies have found that infants become worse at telling apart consonant and vowel sounds from other languages, and even musical rhythms from other cultures.

Babies can distinguish between two languages from birth.It seems that initially, infants are sensitive to the properties of a wide range of languages. But without continuing exposure, their sensitivities soon narrow without continuing exposure to both languages, the babies’ sensitivities soon narrowed to the range that is most relevant for their mother tongue.

To test this idea, Weikum repeated his experiments on bilingual infants. Sure enough, at 8 months, these babies could still visually tell the difference between English and French speakers.

We normally think of lip-reading as a trick used only by deaf people. But this study suggests that the shapes our mouths make when we talk provide all of us with very important visual clues.

From a very early age, infants are programmed to sense these clues, and this so-called ‘visual speech’ may even help them to learn the characteristics of their native tongue.

Reference: Weikam, Vouloumanos, Navarra, Soto-Faraco, Sebastian-Galles & Werker. 2007. Visual language discrimination in infancy. Science 316: 1159.

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