Loss of big mammals breaks alliance between ants and trees

Blogging on Peer-Reviewed ResearchThe natural world is full of alliances forged between different species, cooperating for mutual rewards. The relationship between ants and acacia trees was one of the first of these to be thoroughly studied. But new research suggests that this lasting partnership may be sundered by the unlikeliest of reasons – the decline of Africa’s large mammals.

Giraffe next to whistling-thorn acaciaAcacias are under constant attack from hungry animals, from tiny caterpillars to towering giraffes. In response, many species like the whistling-thorn tree (Acacia drepanolobium) recruit colonies of ants as bodyguards. Any hungry herbivores eager to chomp on the acacia’s leaves quickly get a mouthful of biting, stinging ants. The tree is a fair employer. In return for their services, its ant staff receive a sugary and nutritious nectar as food and hollow swollen thorns called ‘domatia’ as board.

But this pact is a fragile one. Todd Palmer from the University of Florida and colleagues from the USA, Canada and Kenya have found that it rapidly breaks down if the large animals that graze on the acacia disappear. Without the threat of chomping mouths, the trees reduce their investments in bodyguards to the detriment of both partners.

Palmer demonstrated this with plots of land in Kenya’s Laikipia Plateau, where fences have kept out large plant-eaters for over a decade. Since 1995, no herbivore larger than a small antelope has entered the four-hectare “exclosures” in an attempt to study the effect of these animals on the local ecology.

Within these 10 years, Palmer found that the majority of trees produced fewer domatia and less nectar and unexpectedly, the strongest alliances were hit the hardest. What were once happy partners quickly became selfish rivals.

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Envious capuchin monkeys react badly to raw deals

Blogging on Peer-Reviewed ResearchIn my last post, I wrote about two studies which showed that even bacteria cooperate towards a common goal and can be infiltrated by cheating slackers. In one of the studies, cheaters were eventually weeded out through natural selection because their rise to prominence created such havoc for the group that each individual bacterium suffered.

Envious capuchin monkeys react badly to raw dealsIn this scenario, slacking wasn’t punished but merely reduced over time. But more complex creatures, like humans, have the capacity to actually recognise unfairness and punish it directly. It turns out that we’re very keen on doing that; so strong is our innate sense of justice that we’ll often punish cheaters at our own expense.

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.

That suggested that the 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.

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Cooperating bacteria are vulnerable to slackers

Blogging on Peer-Reviewed ResearchAs a species, we hate cheaters. Just last month, I blogged about our innate desire to punish unfair play but it’s a sad fact that cheaters are universal. Any attempt to cooperate for a common good creates windows of opportunity for slackers. Even bacteria colonies have their own layabouts. Recently, two new studies have found that some bacteria reap the benefits of communal living while contributing nothing in return.

Cooperating bacteria are vulnerable to slackersBacteria may not strike you as expert co-operators but at high concentrations, they pull together to build microscopic ‘cities’ called biofilms, where millions of individuals live among a slimy framework that they themselves secrete. These communities provide protection from antibiotics, among other benefits, and they require cooperation to build.

This only happens once a colony reaches a certain size. One individual can’t build a biofilm on its own so it pays for a colony to be able to measure its own size. To do this, they use a method ‘quorum sensing’, where individuals send out signalling molecules in the presence of their own kind.

When another bacterium receives this signal, it sends out some of its own, so that once a population reaches a certain density, it sets off a chain reaction of communication that floods the area with chemical messages.

These messages provide orders that tell the bacteria to secrete a wide range of proteins and chemicals. Some are necessary for building biofilms, others allow them to infect hosts, others make their movements easier and yet others break down potential sources of food. They tell bacteria to start behaving cooperatively and also when it’s worth doing so.

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Ants herd aphids with tranquilisers in their footsteps

In your garden, there’s a fair chance that a farmer is currently tranquilising her livestock with a chemical cocktail she secretes from her feet. Don’t believe me? Look closer…

An ant receives honeydew from an aphidHumans aren’t the only species that farms other animals for food – ants do it too and their herds consist of aphids. They feed on plant sap and excrete a sweet and nutritious liquid called honeydew, which the ants drink.

In return, the ants run a protection racket, defending the aphids from predators like ladybirds. It seems like a nice two-way partnership that suits both partners, and aphid colonies tended by ants tend to be larger than unattended ones. But new research from two London universities suggests that ants are manipulating their herds more than previously thought. 

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Ancient plants manipulate insects for hot, smelly sex

Thrips flee a stinky cycad coneFor plants too, sex can be a hot and smelly affair. In most plant-insect partnerships, the pollinator seems to do most of the work by voluntarily transferring pollen from plant to plant in exchange for a meal.

But an ancient lineage of plants – the cycads – takes more active steps to ensure its future with a bizarre combination of heat and smells. In the afternoon, they use heat and a toxic stench to drive insects out of male cones only to lure them into female cones in the evening with a more alluring scent.

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Paper wasps – caring mothers evolved into selfless workers

The workers of many social insect colonies give up their chance to reproduce in order to raise their sisters and their nieces. A new genetic study in paper wasps, which are halfway down the road to this extreme altruism, tells us that worker selflessness evolved from motherly care.

Imagine that one day, you make a pact with your brother or sister, vowing to never have children of your own and instead spend your life raising theirs. You’ll agree to do the grocery shopping, cook for them, clean their rooms and bathe them, until you die.

A paper wasp foundress begins the task of building a hive.That seems like a crazy plan, but it’s one that some of the most successful animals in the world – the social insects – have adopted. It’s called ‘eusociality’ and it’s a puzzle for evolutionary biologists. Why should an animal forgo the chance to reproduce in order to help rear its siblings and their young?

The strategy makes sense if you share enough genes with your close relatives. In helping them, you indirectly ensure the transmission of your own genetic material. But even if this explains the existence of eusociality, it doesn’t explain how such an extreme form of co-operation evolved.

Now, Amy Toth and colleages at the University of Illinois have found a clue in the genes of the paper wasp, Polistes metricus, which suggests that their altruistic actions evolved from motherly behaviour.

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Altruistic chimpanzees clearly help each other out

Many scientists have argued that only humans show true altruistic behaviour. But a group of Ugandan chimps is set to change all that by showing clear signs of true selflessness, helping other unrelated chimps with no desire for reward.

Why do we help each other, instead of constantly looking out for ourselves? This is one of the most compelling questions in modern biology. Evolutionary and game theory alike predict that selfish behaviour should be the rule with altruism the exception, and animal experiments have largely supported this idea.

Nature, ‘red in tooth and claw’, is painted as a fierce competition between selfish individuals and their even more selfish genes. In this stark landscape, true altruism is a rare quality and some scientists believe that it’s one that only we humans possess.

Even our closest relatives, chimpanzees, are not exempt from this dividing line. Certainly, there is a large amount of anecdotal evidence of chimps helping each other or even saving each others’ lives. But some thinkers believe that this behaviour, along with other seemingly selfless animal acts, is actually self-serving in one of two ways.

The chimps could be helping their relatives in order to advanced the spread of its own genes, which family members are likely to share. Or they could be doing a favour for another individual, in the knowledge that it will be repaid later on. Either way, it’s the do-gooder that eventually benefits.

Humans, on the other hand, seem to flaunt this rule. We often help others who are not relatives and who are unlikely to repay the favour. We go out of our way to be helpful, and sometimes even risk personal harm to do so.

Two tests for altruism

Now, Felix Warneken and colleagues form the Max Planck Institute for Evolutionary Anthropology have found compelling evidence that we are not alone. Contrary to previous studies, they have found that chimps also behave altruistically in a very human way. They help out unrelated strangers without expectation of reward, and even go to great lengths to do so.

Warneken studied 36 chimps at Ngamba Island Chimpanzee Sanctuary, Uganda and looked at their willingness to help a human handler. To minimise the effect of any human-chimp bond, he only looked at chimps that were born in the wild, and used experimenters who the chimps had never seen before.

In the first test, the chimps saw a human unsuccessfully trying to reach a stick that they themselves could reach. Warneken found that chimps were all too happy to pass the stick across (video), regardless of whether they were rewarded with a banana or not. In fact, the only thing that affected their readiness to lend a hand was whether the human was struggling for the stick or just passively staring at it.

He found the same thing when he ran a similar set-up with a 36 eighteen-month-old human toddlers, but with toy cubes in lieu of sticks (video). At that age, a baby’s mental abilities are thought to similar to those of chimps, and indeed the only real difference between the two was that the babies were quicker with their assistance.

Passing a stick across is obviously fairly easy but would altruism persist if there was effort involved? Warneken tested this by changing the experiment so that the chimps had to climb over a raceway (video) and the toddlers had to walk past a series of obstacles (video). Those that helped in the first test were happy to do so in the second, again without any rewards.

The third and most important test

A skeptic might argue that this doesn’t show anything. During their stay at the sanctuary, the chimps could have learned that helping any one of their strange two-legged keepers was worth it. The acid test then, was to see if the chimps would help each other.

The first chimp – the subject – could only get into a room with food by lifting the chain attached to its door. But it couldn’t reach the chain – only a second chimp, the observer, could do that. And once again, the chimps proved their selflessness, lifting the chain for their fellow chimps the vast majority of the time (video).

This striking result flies in the face of other studies, which have failed to find altruistic behaviour between chimps. But in a related commentary, Frans de Waal, an international expert of ape behaviour, claimed that these were more tests of generosity than selfishness.

They created specific situations where chimps were motivated to look out for themselves and the species can’t be judged on these scenarios alone. It would be like claiming that all people are selfish after watching the self-interested behaviour of commuters. Failing to show altruism is not the same as proving that it doesn’t happen.

But it does happen – Warneken’s experiments are striking indicators of that. In the third test, the chimps were unrelated, the observer had no chance of getting a share of the food, and their roles were never reversed so there were no opportunities for payback. Clearly, humans are not alone in our desire to help each other. Chimps are now our fellows in altruism and it’s likely that our common ancestor did the same.

What this means for the altruism debate

It’s particularly fascinating that rewards in the first two tests didn’t affect the chimps’ behaviour. This suggests that chimps don’t continually analyse the pros and cons of helping their fellow – if they did, the reward would have motivated them to help even more often.

Instead, de Waal believes that the chimps have evolved psychological systems that steer them towards selflessness. In essence, natural selection has done the analysis for them and decided that altruistic behaviour works to its advantage in the long run. Selfless behaviour then, can evolve for selfish reasons, and that strikes to the very core of the debate on altruism.

Spend enough time reading about this field of research, and you could be forgiven for thinking that some scientists are taking cynical glee at ‘explaining away’ altruism. The extreme reductionist view is that discovering the evolutionary origins of selfless behaviour discredits that behaviour, somehow making it less worthy. As Robert Trivers put it, these models are designed to “take the altruism out of altruism”.

But this viewpoint is blinkered and too focused on the past. Evolutionary explanations can help us understand where an unusual behaviour like selflessness comes from, but they do not alter the value of those behaviours. They can tell us about how a behaviour arose, but not about an animal’s reasons for behaving in that way here and now.

Take sex. Its adaptive benefits are clear – it continues the line and promotes genetic diversity. But animals don’t consider the issue of reproduction every time they have sex and for the most part, humans actively deny it!

According to de Waal, we should now turn our attention to the psychological processes that foster altruism in chimps, and how they are different from those that work in our own minds. Do chimps share our strong sense of empathy, that fuels selflessness by letting us identify with the emotions and needs of others. Do their cultures, like ours, punish and vilify selfish behaviour?

References: Warneken, Hare, Melis, Hanus & Tomasello. 2007. Spontaneous altruism by chimpanzees and young children. PLOS Biology 5: e184.
de Waal. 2007. With a little help from a friend. PLOS Biology 5: e190.

Related posts on chimps:
Chimps show that actions spoke louder than words in language evolution
Not so unique – the chimpanzee Stone Age, and our place among intelligent animals
Cultured chimps pass on new traditions between groups
Chimpanzees make spears to hunt bushbabies

Related posts on altruism:
Army ants plug potholes with their own bodies

Images: from BBC, Nature, Jane Goodall Institute and Ngamba Sanctuary

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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The upside of herpes – when one infection protects against another

When people say that every cloud has a silver lining, they probably aren’t thinking about herpes at the time. Herpes may be unpleasant, but the viruses that cause it and related diseases could have a bright side. In mice at least, they provide resistance against bacteria, including the bubonic plague.

Herpes is one of a number of itchy, blistering diseases, caused by the group of viruses aptly-named herpesviruses. Eight members infect humans and cause a range of illnesses including glandular fever, chickenpox, shingles, some rare types of cancer and, of course, herpes itself.

Herpesviruses cause a range of diseases, including cold sores, but could they protect against many more?Almost everyone gets infected by one of these eight during their childhood. But herpesviruses are for life, not just for Christmas. After you body fights off the initial infection, the virus retreats into a dormant phase known as ‘latency’.

It remains hidden and causes no symptoms, but has the potential to reactivate at a later date. In this way, herpesviruses can seem like life-long parasites, ensuring their own survival at the cost of their host’s future health. In extreme cases, latent viruses can lead to chronic inflammation, which in turn can cause autoimmune diseases, or some types of cancer.

Parasite or partner?

But there is a bright side too. Erik Barton and colleagues from Washington University Medical School found that once infected mice entered the latent stage, they were surprisingly resistant to certain types of bacteria. Unlike their vulnerable uninfected peers, they even managed to ward off the deadly plague bug, Yersinia pestis.

At least in mice, latent herpesviruses turn out to be paying tenants rather than free-loading squatters – bacterial resistance is their rent. The latent stage is crucial to the resistance effect, and Barton found that a mutant herpesvirus that infect but doesn’t set up shop provides no benefits to its host.

The viruses work their magic by putting the immune system on high alert. The effect is similar to a raising of the terror alert creating a heightened level of security where the body is prepared to fight off any further threats.

How it works

An activated macrophage hoovers up a bacterial meal.The viruses trigger the release of high levels of immune system chemicals called cytokines. These molecules – including interferon-gamma (IFN-g) and tumour necrosis factor alpha (TNF-a) – help to co-ordinate the defence against infections.

These chemicals activate macrophages – a type of white blood cell. These cellular assassins (above) engulf invading bacteria, and sentence them to death by digestion. And in mice latently infected by herpesviruses, they are activated in bulk.

This sequence is similar to the way the immune system normally protects us against multiple bacterial invaders. But in Barton’s experiments, the protection was set off by viruses instead, and lasted for much longer than normal.

From mice to humans

All well and good for the mice, but do these viruses benefit us too? Barton thinks so. In his study, two very different strains – murine gammaherpesvirus 68 (gHV68) and murine cytomegalovirus (MCMV) – had the same effect. He believes that providing bacterial resistance is a general property of all herpesviruses.

The HSV-1 virus - could it make us resistant to bacterial infection?There is certainly growing evidence to support his claims. If many people, the latent viruses reactivate regularly, but not strongly enough to cause major symptoms. In these cases, doctors have seen higher levels of cytokines and long-term activation of the immune system, just like Barton saw in his mice.

Barton even suggests that herpesvirus infection may play a role in protecting against allergies. According to the ‘hygeine hypothesis’, infections during childhood prime the immune system against future threats. By depriving children of these experiences, overly clean homes can lead to naïve immune systems that react disproportionately to harmless things like pollen. Allergies are the result.

It isn’t clear what role herpesviruses play in priming the immune system. But at least one study found that people who are infected with the Epstein-Barr herpesvirus (EBV) are less likely to show sensitive antibody reactions to allergens in their environment. Clearly, the subject is a rich vein for further research.

Almost everyone has had an encounter with a herpesvirus of some kind. They cause a wide range of diseases, but could they be protecting us from many more?


Reference: Barton, White, Cathelyn, Brett-McClellan, Engle, Diamond, Miller & Virgin IV. 2007. Herpesvirus latency confers symbiotic protection from bacterial infection. Nature 447: 326-330.

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Beetle and yeast vs. bee – how American bees are losing the evolutionary arms race

In America, the parasitic small hive beetle has gained the upper hand in its evolutionary arms race against the honeybee. It has formed an alliance with a type of yeast, and together, they use the bees’ own chemical communications against them.

Bees release alarm pheromones that draw small hive beetles towards the hive.Bee hives, with their regularly arranged honeycombs and permanently busy workers may seem like the picture of order. But look closer, and hives are often abuzz with secret codes, eavesdropping spies and deadly alliances.

African honeybees are victimised by the parasitic small hive beetle. The beetles move through beehives eating combs, stealing honey and generally making a mess. But at worst, they are a minor pest, for the bees have a way of dealing with them.

They imprison the intruders in the bowels of the hive and carefully remove any eggs they find. In turn, the beetle sometimes fools the bees by acting like one of their own grubs, and gets a free meal instead of imprisonment. In Africa, both species have found themselves in an evolutionary stalemate.

But in 1998, American beekeepers spotted the beetle in hives of their local European-descended honeybees. These insects are gentler versions of their aggressive African relatives, and in them, the beetles found more vulnerable victims.

In the last decade, they have spread through hives on the East Coast, causing much more destruction than they normally get away with. In some cases, the damage is so severe that the bees are forced to abandon their hive. As the bees suffer, so do the economically vital crops they pollinate.

The small hive beetle detects bee alarm pheromones and brings a yeast into the hive.Now, scientists from the International Centre of Insect Physiology and Ecology and the University of Florida have uncovered the secrets behind the beetle’s destructive ability.

Small hive beetles (right) hunt down beehives by hijacking their communications. When bees are stressed or confronted by threats, they release alarm pheromones into the air to alert their hive-mates of impending danger. But the beetles can also detect these chemical signals and use the bees’ own early warning system to locate their hives.

Baldwyn Tonto and colleagues found that the beetles are sensitive to much lower levels of these pheromones than the bees themselves are, and can detect a much wider ranger of airborne chemicals from the hive. With their superior senses, the beetles can home in on beehives before the bees themselves can sense the alarm.

But that’s not the whole story. Tonto found that honeycombs infested by beetles, but free of worker bees, were emitting a strange smell. It mimicked the bees’ alarm pheromones and strongly attracted even more beetles. But it wasn’t coming from the parasites themselves.

Instead, the source of the smell was a type of yeast that hitches a ride with small hive beetles into the bees’ home. Tonto found that the fungus was fermenting the pollen collected by the bees, and releasing chemicals that closely mimic the beetle-attracting alarm pheromones.

Domesticated European honeybees have been bred into helplessness against the small hive beetle.The beetles’ keen sensitivity to the bees’ chemical messages allows them to initially home in on a hive. As they arrive, they bring the yeast along for the ride and distribute it among the hive’s pollen stores. The yeast ferments the pollen and releases chemicals that mimics the bee’s alarm pheromone, attracting even more beetles .

Soon, the infection reaches critical mass, and the bees are forced to abandon their homes. They leave behind a sizeable store of pollen and honey, ideal breeding grounds for the unwitting partnership of yeast and beetle.

But the yeast also exists in Africa, where it is similarly spread to hives by hive beetles. Why does the alliance not wreak such havoc there? Tonto believes that domestication is the answer. Because of years of selective breeding, the European honeybee is a slightly dopier version of the African bee – more docile and less prone to swarming.

It faces a larger number of pests and problems that prevent it from concentrating on imprisoning invading beetles. And its poor sensitivity to its own alarm chemicals allows the beetle-yeast alliance to gain a strong foothold before the bees recognise the threat.

With bee populations mysteriously dying off across America, the threat of the small hive beetle and its fungal partner may be even more pressing than before.


Reference: Torto, Boucias, Arbogast, Tumlinson & Teal. 2007. Multitrophic interaction facilitates parasite-host relationship between an invasive beetle and the honey bee. PNAS 104: 8374-8378.

Image: Photo of small hive beetle by Jeffrey Lotz

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Related stories about parasites and symbiosis:
Aphids get superpowers through sex
Hatena: when two cells are better than one
Too few genes to survive – a bacteria with the world’s smallest genome
Parasites can change the balance of entire communities

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In conflicts over beliefs and values, symbolic gestures matter more than reason or money

When battles are waged over values and ideologies, you can’t bribe or reason your way to peace. That’s the stark message from a new psychological study of people in the heart of the Israeli-Palestinian conflict.

Land - a solid resouce - can become a moral issue when prefixed by the word ‘holy’.The fight over the land of Israel/Palestine has raged for over a century and the peace process has been difficult, lengthy and often stagnant. All the while, lives continue to be lost in skirmishes and suicide attacks. Perhaps it’s time to put the situation under some scientific scrutiny.

A huge number of modern conflicts are fuelled by differences in opinions and beliefs, rather than grabs for power or land (at least on the ground level).

Even if the foundation of a dispute is not initially a moral issue, it can quickly become one. Land, for example, is a solid resource that can be completely transformed into something much more by adding the word ‘holy’ in front of it.

In these situations, people tend to forgo a rational weighing up of pros and cons in favour or making decisions with an intuitive moral compass. Jeremy Ginges and colleagues from the New School for Social Research studied the effects of this switch against the backdrop of the Israeli-Palestinian conflict.

Through a series of psychological experiments, they worked out that in these circumstances, the power of symbols is far greater than that of currency or logic.

They surveyed over 1,800 from three different groups: Jewish Israeli settlers, Palestinian refugees, and Palestinian students (half of whom were members of Hamas or related organisations).

Ginges asked the participants to consider hypothetical scenarios where they would have to compromise over issues that were particularly relevant to them. The Israeli settlers were asked about their willingness to exchange their land for peace, which would involved them having to relocate. The Palestinian refugees had to consider giving up their right to return to their former homelands. And the Palestinian students were asked to consider Palestine relinquishing sovereignty over Jerusalem.

Symbolic compromises are essential for engaging with moral absolutists.Unsurprisingly, all the participants were reluctant to compromise. But Ginges found that they fell into two camps based on the strength of their convictions.

The first, who he termed the ‘non-absolutists’ expressed no more than a strong preference against any compromises. The second group, the ‘moral-absolutists’, had elevated the respective issues to the status of sacred values and were more fervently against losing any ideological ground.

For example, about half of the Israeli settlers felt that the Jewish people should never cede part of the ‘Land of Israel’ not matter the cost or benefit. The other half opposed the loss of land but did not rule it out under extreme circumstances.

So far, so predictable. But the real surprise came when Ginges offered the participants the same compromise but with a rational incentive to sweeten the deal. The incentives included peace, resulting from the end of all hostilities, or money, in the form of substantial donations from the US or the European Union.

Faced with these added carrots, the two camps behaved very differently. The non-absolutists tended to take a pragmatic stance and softened their responses. After all, in rational terms, the extra benefits are better than nothing.

A Palestinian woman at MashaBut the moral-absolutists from both the Israeli and Palestinian sides showed even more outrage than before and an even greater number supported a violent response. For these people, bribes or appeals to reasons were just making things worse.

Symbolic incentives had the opposite effect. Moral-absolutists were more willing to compromise over their own sacred values if they saw that the other side was prepared to do the same.

For example, the Palestinian refugees were more prepared to recognise Israel’s right to exist if Israel would in return recognise the historic legitimacy of the refugees’ right to return. They were less angry about the thought of compromise and less supportive of violent responses or suicide attacks.

The swings in opinion were small but significant – elections in the Middle East have been settled by much smaller majorities.

It did not matter if these symbolic gestures altered the costs or benefits of the compromise, and indeed, in most cases they did not. It didn’t even seem to matter if the deals would be successfully carried out, and those surveyed were not confident that they would be.

The Israeli-Palestinian peace process is not amenable to reason or financial incentives.Amazingly, when you consider that this conflict regularly takes the lives of hundreds of people, it was the gesture that counted. It was the fact that one side showed willingness to even budge on matters of principle that prompted the other to do the same.

It just goes to show that when people promote their ideas to the rank of beliefs, they risk losing the ability to view those issues rationally.

Ginges has a reason for this. He believes we are almost programmed to avoid weighing things up in terms of costs and benefits when they concern our values or beliefs, preferring instead to rely on a moral compass. Indeed, we have such an inherent distaste for attempts to measure moral commitments in such a calculating way that such attempts are likely to be met by outrage and anger. So it was with the Israelis and Palestinians surveyed in this study.

In these situations, attempting to resolve the situations through logical arguments or financial bargaining can seriously backfire. Symbolic trade-offs hold much greater power in ensuring that peace can be achieved.

Reference: Ginges, Atran, Medin & Shikaki. 2007. Sacred bounds on rational resolution of violent political conflict. PNAS 104: 7357-7360.

Images: Photo of Palestinian woman and Israeli soldier by Justin McIntosh.


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Eavesdropping songbirds get predator intel from overheard calls

Animals gain valuable information on incoming predators by listening to the alarm calls of the communication of other species. New experiments with chickadees suggest that both alarm calls and the understanding of them are more complex than we had thought.

Humans are a funny lot. While we seem to be relentless voyeurs, we generally frown on eavesdropping as an invasion of privacy. But in the animal world, eavesdropping can be a matter of life or death.

The black-capped chickadee encodes information on predator size in its call.Animals rarely communicate in isolation. Often it pays for one species to monitor the dialogues of others, particularly when predator warnings are involved. Small animals in particular do well to pay attention to the alarms of other species, as they are often preyed upon by the same larger hunters.

Even very unrelated species can listen in and understand each other’s signals. Vervet monkeys respond to the alarm calls of superb starlings, while mongooses are well-versed in hornbill calls.

Alarm calls aren’t just a simple matter of shouting “Look out!”, and many species have different calls for different predators. But one of the most sophisticated alarm systems so far discovered is used by a small, unassuming bird called the black-capped chickadee

The chickadee acts as an inadvertent sentry for a multitude of bird species. Its name comes from its distinctive “chick-a-dee” alarm call, made in response to a perched bird of prey or a land predator.

The black-capped chickadee has one of the most complex alarm calls so far discovered.When this call sounds out, anywhere between 24 and 50 species of bird marshall together and mob the predator, robbing it of the element of surprise and harassing it from the area.

The chick-a-dee call is not a blunt warning, but a sophisticated piece of communication. By varying the acoustics of the call, the chickadee can warn others about not only the type of predator, but also its potential threat and size.

Smaller raptors, with their superior maneuverability, pose a greater danger to small birds and must be dealt with more carefully. When these hunters are near, the birds warn of their presence by shortening the gap between chick and dee, and add extra dees to the end.

Now, Christopher Templeton and Erick Greene from the University of Washington have found that other birds have learned to appreciate the subtle differences in the chickadee’s calls.

The red-breasted nuthatch has learned to understand the subtle variations in chickadee alarm calls.They recorded two variants of the chick-a-dee call, by exposing the birds to two species of owl, one large and one small, in controlled encounters. They then played the calls back to red-breasted nuthatches – common flock-mates of chickadees – from a speaker hidden in a tree. As predicted, the nuthatches mobbed the speaker in response to both calls.

But on closer inspection, they were clearly picking up on the greater threat suggested by the subtly different small-predator call. When that was played, they mobbed more frequently and for longer, approached the speaker more than twice as closely, showed more wing-flick displays (a sign of agitation), and were themselves more likely to call.

Conserving their energies for only the most dangerous predators could bring great benefits to the nuthatches, especially in winter, when they are most likely to be found in the company of chickadees. Their food sources are scarce, and keeping warm saps valuable energy. Understanding the subtle differences in the chickadee dialect could help them save their energy for the times of greatest need.

Templeton and Greene’s study shows that animals can pick up a surprisingly complex picture of their environment by listening in on other conversations.


Reference: Templeton & Greene. 2007. Nuthatches eavesdrop on variations in heterotrophic chickadee mobbing alarm calls. PNAS 104: 5479-5482.

Image: Nuthatch photo by Alan D Wilson.

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