Category: Monkeys

The Bruce effect – why some pregnant monkeys abort when new males arrive

By Ed Yong | February 23, 2012 2:00 pm

On a grassy Ethiopian plateau, revolution and death are underway. The plateau is home to a group of geladas – shaggy, grass-eating, and occasionally terrifying relatives of baboons. They’re like a cross between a cow, Animal from the Muppets, and your nightmares.

Geladas live in units where a single dominant male lords over several related females, whom he monopolises as mates. It’s an enviable position, and males often have to fend off takeover bids by eager bachelors. If a newcomer ousts the chief monkey, it’s bad news for the group’s females. A wave of death sweeps through the unit, as the new male kills all the youngsters whom his predecessor fathered. Indeed, babies are 32 times more likely to die after a takeover than at any other time.

But that’s not all. Eila Roberts from the University of Michigan has found that the new male’s arrival triggers a wave of spontaneous abortions. Within weeks, the vast majority of the local females terminate their pregnancies. It’s the first time that this strategy has been observed in the wild.

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Larger monkey groups lose fights because they contain more deserters

By Ed Yong | December 27, 2011 9:30 am

In the Battle of Rorke’s Drift, 150 or so British troops defended a mission station against thousands of Zulu warriors. At the Battle of Thermopylae, around 7,000 Greeks successfully held back a Persian army of hundreds of thousands for seven days. Human history has many examples of a small force defeating or holding their own against a much larger one.

Among animals too, the underdogs often become the victors. One such example exists in the rainforests of Panama. There, capuchin monkeys live in large groups, each with its own territory. The monkeys often invade each other’s land. Numbers provide an obvious advantage in such conflicts, but small groups can often successfully defend their territory against big ones. Unlike human underdogs, they don’t win because of superior tactics or weapons. They win because their rivals are full of deserters.

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English monkey gives itself a pedicure with self-made tools

By Ed Yong | July 7, 2011 6:00 pm

Animals use tools to get food, communicate with one another, defend themselves or even have a scratch. But in Chester Zoo, England, one monkey uses tools to give itself a pedicure.

Riccardo Pansini and Jan de Ruiter from Durham University watched a 18-year-old mandrill called JC clean his toenails out using small splinters. He made them himself, fashioning them from wood chips and twigs on the floor his enclosure, and honing them till they were small and sharp.

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What is the point of pruney fingers?

By Ed Yong | June 28, 2011 10:03 am

The common wisdom is that your fingers wrinkle when they’re wet because they absorb water. But Mark Changizi thinks there’s more to it than that. According to him, pruney fingers are an adaptation to help humans, and probably other primates, get a better grip during wet conditions. They act like the rain treads on tyres. Mark lays out his hypothesis in a wonderful paper that I wrote up as a news story for Nature News.

Here’s the start; click through for the whole thing.

The wrinkles that develop on wet fingers could be an adaptation to give us better grip in slippery conditions, the latest theory suggests.

The hypothesis, from Mark Changizi, an evolutionary neurobiologist at 2AI Labs in Boise, Idaho, and his colleagues goes against the common belief that fingers turn prune-like simply because they absorb water.

Changizi thinks that the wrinkles act like rain treads on tyres. They create channels that allow water to drain away as we press our fingertips on to wet surfaces. This allows the fingers to make greater contact with a wet surface, giving them a better grip.

Scientists have known since the mid-1930s that water wrinkles do not form if the nerves in a finger are severed, implying that they are controlled by the nervous system.

“I stumbled upon these nearly century-old papers and they immediately suggested to me that pruney fingers are functional,” says Changizi. “I discussed the mystery with my student Romann Weber, who said, ‘Could they be rain treads?’ ‘Brilliant!’ was my reply.”

Reference: Changizi, Weber, Kotecha, & Palazzo. Brain Behav. Evol. (2011).

Our closest relatives – a visual tour of the primates

By Ed Yong | March 17, 2011 5:00 pm

Few groups of animals hold such special significance for us as the primates – the apes, monkeys, lemurs and more. This is the group that we are a part of. Its members are familiar and charismatic, but our evolutionary history is tangled and occasionally controversial.

Now, Polina Perelman has provided the most comprehensive view of the primate family tree to date. Her team sequenced genes from over 186 species, representing 90% of all the genera that we know of. Her tree confirms some past ideas about primate evolution and clarifies other controversies. It’s a story of island conquests, shrinking bodies, tangled branches and ancient relics. This slide show tells that story.

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Playing by the same rules reduces the differences between humans, chimps and monkeys

By Ed Yong | February 7, 2011 3:00 pm

You’ve been asked to compete against some of your friends in a game of skill, but you realise something is amiss. They’ve been given precise instructions and details about the game’s mechanics. You’ve been given a couple of pieces and left to figure things out on your own. On this uneven playing field, no one could fairly compare your performance with that of your friends. This seems obvious, but it’s a problem that plagues a lot of research into the behaviour of humans and other animals.

Scientists will often test monkeys and apes with tweaked versions of psychological games that were originally designed to test humans. The goal is simple: understand the similarities and differences between our mental abilities and those of our closest relatives.

But these comparisons are tricky. Frans de Waal, who studies the behaviour of apes and monkeys, says, “Humans are tested by their own species and the apes by a different species (us). Humans understand everything the experimenter says or explains, whereas the ape needs to figure these things out based on experience. The paradigm really doesn’t permit the comparisons that have been made, especially the negative assessment of ape capacities.”

Sarah Brosnan form Georgia State University agrees. “As humans, we surely design tasks that are more intuitive to us than to other species. We don’t know whether humans perform differently from other species absent these advantages. Are human-specific abilities, including language, added ‘on top’ of other primate abilities, making us fundamentally similar in our outcomes. Or are we fundamentally different from the rest of the primates?”

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Monkey see, monkey facepalm

By Ed Yong | February 2, 2011 5:56 pm

A group of English mandrill monkeys has started doing facepalms. The monkeys live in a zoo in Colchester, England, and eight of them frequently raise one or both of their hands to cover their eyes. They might be the only ones in the world who perform this distinctive gesture.

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Do new discoveries ever “rewrite evolutionary history”?

By Ed Yong | September 1, 2010 9:00 am


You can’t go for a month without seeing a claim that some new discovery has rewritten evolutionary history. If headlines are to be believed, phylogeny – the business of drawing family trees between different species – is an etch-a-sketch science. No sooner are family trees drawn before they’re rearranged. It’s easy to rile against these seemingly sensationalist claims, but James Tarver from the University of Bristol has found that the reality is more complex.

Tarver focused on two popular groups of animals – dinosaurs and catarrhines, a group of primates that includes humans, apes and all monkeys from Asia and Africa. Together with Phil Donoghue and Mike Benton, Tarver looked at how the evolutionary trees for these two groups have changed over the last 200 years. They found that the catarrhine tree is far more stable than that of the dinosaurs. For the latter group, claims about new fossils that rewrite evolutionary history (while still arguably hyperbolic) have the ring of truth about them.

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Boom-boom-krak-oo – Campbell's monkeys combine just six 'words' into rich vocabulary

By Ed Yong | December 7, 2009 5:06 pm

Many human languages achieve great diversity by combining basic words into compound ones – German is a classic example of this. We’re not the only species that does this. Campbell’s monkeys have just six basic types of calls but they have combined them into one of the richest and most sophisticated of animal vocabularies.

By chaining calls together in ways that drastically alter their meaning, they can communicate to each other about other falling trees, rival groups, harmless animals and potential threats. They can signal the presence of an unspecified threat, a leopard or an eagle, and even how imminent the danger is. It’s a front-runner for the most complex example of animal “proto-grammar” so far discovered.

Many studies have shown that the chirps and shrieks of monkeys are rich in information, ever since Dorothy Cheney and Robert Seyfarth’s seminal research on vervet monkeys. They showed that vervets have specific calls for different predators – eagles, leopards and snakes – and they’ll take specific evasive manoeuvres when they hear each alarm.

Campbell’s monkeys have been equally well-studied. Scientists used to think that they made two basic calls – booms and hacks – and that the latter were predator alarms. Others then discovered that the order of the calls matters, so adding a boom before a hack cancels out the predator message. It also turned out that there were five distinct types of hack, including some that were modified with an -oo suffix. So Campbell’s monkeys not only have a wider repertoire of calls than previously thought, but they can also combine them in meaningful ways.

Now, we know that the males make six different types of calls, comically described as boom (B), krak (K), krak-oo (K+), hok (H), hok-oo (H+) and wak-oo (W+). To decipher their meaning,  Karim Ouattara spent 20 months in the Ivory Coast’s Tai National Park studying the wild Campbell’s monkeys from six different groups. Each consists of a single adult male together with several females and youngsters. And it’s the males he focused on.

With no danger in sight, males make three call sequences. The first – a pair of booms – is made when the monkey is far away from the group and can’t see them. It’s a summons that draws the rest of the group towards him. Adding a krak-oo to the end of the boom pair changes its meaning. Rather than “Come here”, the signal now means “Watch out for that branch”. Whenever the males cried “Boom-boom-krak-oo”, other monkeys knew that there were falling trees or branches around (or fighting monkeys overhead that could easily lead to falling vegetation). 

Interspersing the booms and krak-oos with some hok-oos changes the meaning yet again. This call means “Prepare for battle”, and it’s used when rival groups or strange males have showed up. In line with this translation, the hok-oo calls are used far more often towards the edge of the monkeys’ territories than they are in the centre. The most important thing about this is that hok-oo is essentially meaningless. The monkeys never say it in isolation – they only use it to change the meaning of another call.

But the most complex calls are reserved for threats. When males know that danger is afoot but don’t have a visual sighting (usually because they’ve heard a suspicious growl or an alarm from other monkeys), they make a few krak-oos. 

If they know it’s a crowned eagle that endangers the group, they combine krak-oo and wak-oo calls. And if they can actually see the bird, they add hoks and hok-oos into the mix – these extra components tell other monkeys that the peril is real and very urgent.  Leopard alarms were always composed of kraks, and sometimes krak-oos. Here, it’s the proportion of kraks that signals the imminence of danger – the males don’t make any if they’ve just heard leopard noises, but they krak away if they actually see the cat. 

The most important part of these results is the fact that calls are ordered in very specific ways. So boom-boom-krak-oo means a falling branch, but boom-krak-oo-boom means nothing. Some sequences act as units that can be chained together to more complicated ones – just as humans use words, clauses and sentences. They can change meaning by adding meaningless calls onto meaningful ones (BBK+ for falling wood but BBK+H+ for neighbours) or by chaining meaningful sequences together (K+K+ means leopard but W+K+ means eagle).

It’s tempting to think that monkeys have hidden linguistic depths to rival those of humans but as Ouattara says, “This system pales in contrast to the communicative power of grammar.” They monkeys’ repertoire may be rich, but it’s still relatively limited and they don’t take full advantage of their vocabulary. They can create new meanings by chaining calls together, but never by inverting their order (e.g. KB rather than BK).  Our language is also symbolic. I can tell you about monkeys even though none are currently scampering about my living room, but Ouattara only found that Campbell’s monkeys “talk” about things that they actually see.

Nonetheless, you have to start somewhere, and the complexities of human syntax probably have their evolutionary origins in these sorts of call combinations. So far, the vocabulary of Campbell’s monkeys far outstrips those of other species, but this may simply reflect differences in research efforts. Other studies have started to find complex vocabularies in other forest-dwellers like Diana monkeys and putty-nosed monkeys. Ouattara thinks that forest life, with many predators and low visibility, may have provided strong evolutionary pressures for monkeys to develop particularly sophisticated vocal skills.

And there are probably hidden depths to the sequences of monkey calls that we haven’t even begun to peer into yet. For instance, what calls do female Campbell’s monkeys make? Even for the males, the meanings in this study only become apparent after months of intensive field work and detailed statistical analysis. The variations that happen on a call-by-call basis still remain a mystery to us. The effect would be like looking at Jane Austen’s oeuvre and concluding, “It appears that these sentences signify the presence of posh people”.

Reference: PNAS doi:10.1073/pnas.0908118106

More on monkey business (clearly, I need more headline variation):

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Envious capuchin monkeys react badly to raw deals

By Ed Yong | December 5, 2009 12:00 pm

This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.

Two years ago, Sarah Brosnan and Frans de Waal at the Yerkes National Primate Research Center found that brown capuchin monkeys also react badly to receiving raw deals. Forget bananas – capuchins love the taste of grapes and far prefer them over cucumber. If monkeys were rewarded for completing a task with cucumber while their peers were given succulent grapes, they were more likely to shun both task and reward.

Envious capuchin monkeys react badly to raw dealsThat suggested that the human ability to compare own efforts and rewards with those of our peers evolved much earlier in our history than we previously thought. Of course, animal behaviour researchers always need to be careful that they’re not reading too much into the actions of the animals they study.

It’s easy to suggest that the monkeys were motivated by envy, fuelled by directly weighing up their rewards with those of others. But they could equally be driven by greed of frustration. They could simply have coveted the better reward regardless of the fact that it was given to their partner. Alternatively, they could have been frustrated at being given grapes in previous trials and having to contend with cucumbers.

To rule out these alternative explanations, de Waal and Brosnan tasked graduate student, Megan van Wolkenten with repeating their earlier study with subtle tweaks. Their new results firmly show that monkeys can indeed spot unjust deals and respond with envy and apathy.

The trio worked with 13 capuchins who were asked to hand over a small granite rock in exchange for a cucumber or grape reward. They tested the monkeys in pairs, sat in adjacent wire cages so that each individual could see what its partner was getting.

If both partners were rewarded equally, they completed the task about 90% of the time, regardless of whether they were given grapes or cucumbers. Even if they were shown their future rewards before the experimenters reached for the rock tokens, they didn’t make any special efforts to earn the grapes.

That suggests that they’re not being greedy after all and are more than happy to work for a cucumber reward if their peers are rewarded equally. However, if monkeys were given cucumbers while their partners received grapes, they only cooperated 80% of the time and as the trials continued, they were more and more likely to refuse.

The researchers also found that monkeys were just as likely to hand over the tokens, regardless of whether they received a grape or a cucumber in the previous round. That effectively discounts the frustration angle, which suggests that cucumbers fail to meet the lofty expectations set by grapes.

The trio of researchers also found that the monkeys weren’t just fussed about rewards. They also compared their efforts to those of their partners and were less likely to cooperate if they had gone to more trouble to get their rewards.

If one monkey exchanged tokens for cucumbers while their partner got one for free, it was still happy to complete the task 90% of the time. But if it had to hand over three rocks for the same reward, it only complied 75% of the time. The monkeys became even more indignant if their slacker partners were given grapes for slacking. Now, they were making more effort and getting poorer rewards and their tendency to hand over rocks fell to new lows.

However, if both partners were given grapes, they were willing to do whatever it took to get them, inequity be damned. It seems that capuchins aren’t willing to act disdainfully in the face of really good rewards.

Together, these new results show that capuchins react negatively to unequal rewards and are motivated neither by greed nor frustration. Capuchins hunt squirrels as a team and once food is found, they willingly share it out among the group. Their intolerance for unequal handouts would foster greater cooperation among monkey troupes by preventing any individuals from monopolising the spoils.

In other studies, pairs of capuchins who cooperate for unequal rewards do better in the long run if they swap who gets the lion’s share. De Waal speculates that this need to share the spoils of a hunt could be the origin of our own disdain for inequality.

Even so, de Waal notes that the monkeys’ aversion to injustice isn’t on a par with humans. They don’t like getting less than their peers, but they don’t react to getting more. If anything, this worsens any inequality since monkeys that do badly end up shunning the task and its reward altogether, while the one that’s better off continues to be rewarded.

It may be that in a more realistic situation, monkeys that were ripped off could just leave and find other social partners, but only further research would tell.

Reference: van Wolkenten, M., Brosnan, S., de Waal, F. (2007). Inequity responses of monkeys modified by effort.Proceedings of the National Academy of Sciences, 104(47), 18854-18859.

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