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