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Why do people and other primates share food?

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Matt Walker Matt Walker | 14:00 UK time, Monday, 11 July 2011

Male baboon eating (image: Andrew King/ZSL Tsaobis Baboon Project)

A male baboon tucks in (image: Andrew King/ZSL Tsaobis Baboon Project)

We like to sit down and break bread with one another, share a platter and join around a table to tuck into a hearty meal.

Consider ourselves human, and the idea of sharing food this way seems utterly reasonable.

Remember that we are primates, however, and it becomes a little harder to explain.

The reason is that, in evolutionary terms, it doesn’t make a huge amount of sense for primates to voluntarily give up food and have another benefit from it.

But two new pieces of research are helping to finally explain why some apes and monkeys willingly share food, and others don’t.

Questioning what motivates other primates to split a meal in this way can help inform what spurred our distant ancestors to similarly share meals, as well as revealing something about primate behaviour in general.

The first study, by Adrian Jaeggi and Carel Van Schaik of the Anthropological Institute and Museum at the University of Zurich in Switzerland, aims to explain the occurrence of food sharing across many primate species.

They reviewed 173 studies related to food sharing in primates, looking for evidence that it occurs and between which groups of primates.

Chamca baboons share with friends more than relatives (image: Andrew King/ZSL Tsaobis Baboon Project)

Chamca baboons share with friends more than relatives (image: Andrew King/ZSL Tsaobis Baboon Project)

One aspect of food sharing is relatively easy to explain: food is given to offspring for the same reason that offspring are had in the first place; a parent wants its offspring to survive and prosper to enable it to pass on its genes.

This form of parental investment is part of a set of evolutionary strategies to pass on genes that biologists call kin selection.

But even here, not all species share food with their offspring to the same degree.

Jaeggi and Van Schaik’s research shows that it is possible to predict which primate species will share food with its young (during and post weaning) by the degree of so-called “extractive foraging” it does.

In simple terms, extractive foraging involves using tools or other sophisticated foraging methods, such as a chimp using a stick to extract termites from a nest.

It seems that adult primates that forage this way are more likely to share their spoils, perhaps because younger members would not yet have learnt how to get to the food themselves.

The researchers also eliminated another hypothesis that has been proposed: which states that primates may be more likely to share high quality foods with their offspring compared to low quality foods.

The idea here is that by giving high quality foods, a parent can encourage weaning and boost the growth rates of their babies.


Sharing with younger primates is key (image: Anup Shah / NPL)

But Jaeggi and Van Schaik found no evidence this occurs.

So food that is difficult to get is passed onto offspring, but not food of especially high nutritional value.

The scientists then tested a more difficult problem: why unrelated adult primates might share food with one another.

This is harder to fathom and requires another evolutionary explanation.

The first thing Jaeggi and Van Schaik found was that feeding their offspring influences whether adult primates feed each other; in fact they discovered that feeding offspring may even be a precondition for food sharing among adults to have evolved.

Then comes some further tasty morsels.

Food sharing evolved between the sexes as a way to influence the choosing of partners.

In short, food is exchanged for sex, and food is exchanged for support.

In the food for sex exchanges, male primates share with females to attract them, and influence their choice of male partner.

In species that form coalitions, males and females swap food to strengthen their bonds.

Among unrelated adults, the type of food seems less important. Monkeys and apes share trivial foods as well as rich foods such as meat or large fruits, the researchers report in the journal Behavioral Ecology and Sociobiology.

The second recently published study investigated food sharing among chacma baboons.

Andrew King of the Royal Veterinary College in London, UK, and colleagues followed 14 baboons as they wandered the Namib Desert on foraging trips.

Over two seven-month study periods, they recorded around 5000 separate foraging events, as part of the Tsaobis Baboon Project run by the Zoological Society of London.

Baboons grooming (image: Andrew King/ZSL Tsaobis Baboon Project)

Food helps reinforce social bonds (image: Andrew King/ZSL Tsaobis Baboon Project)

King’s team was able to study a complex network of who dined with whom in the troop, the first study of foraging in a large primate social network.

While the researchers expected to find baboons sharing food most readily with relatives, the monkeys actually shared most with their grooming partners.

Food it seems is a great way to maintain close social bonds, backing up the “food for support” strategy revealed in Jaeggi and Van Schaik’s wider study.

Most of these strategies were likely employed by early human forager societies, in which food sharing is a universal feature.

So our early ancestors also likely traded food for sex, and food for support, if the behaviour of modern primates is anything to go by.

Next time you cook a meal for someone who isn’t family, remember why your ancestors did similar.

It was probably because they wanted to have sex with their dinner date, or to form or strengthen a coalition with one.

Now that’s something to chew on.


  • Comment number 1.

    Groan, here we go again. BBC Science reporting fail.

    "The reason is that, in evolutionary terms, it doesn’t make a huge amount of sense for primates to voluntarily give up food and have another benefit from it."

    Yes it does. This is a well established, documented and demonstrated part of reciprocal altruism.

    Food is often either very scarce or over abundant in different times and places. If chmp1 finds more food than he can eat now (and can't store it) it makes sense to share it. chimp2 benefits now but owes chimp1 one in the future. This sharing effectively works as a store of value for the future for chimp1 when things aren’t so good.

    These two chimps working together (sharing the same gene) will use food more efficiently than two chimps not sharing food. They are therefore more likely to survive and so the gene is more likely to survive and propagate into the population.

    It makes perfect sense. The selfish gene is producing cooperative chimps.

  • Comment number 2.

    Excellent exposure of an article whose ramifications are on so many different levels and carry forward to so many other fields, e.g. anthropology, human evolution etc as the article cleverally hints at. I think primate research is far too undervalued if we consider how much we can learn ultimately about ourselves albeit from much effort put in to analyse animals which make very difficult experimental models!

  • Comment number 3.

    #1 MonkeyNews
    Reciprocal altruism is not as clear cut as you state. According to Jaeggi and Schaik's review, there are two major theories to explain non-kin sharing in primates: harassment and reciprocal exchange, with harassment often assumed to be the more parsimonious explanation.

    Essentially other primates beg for food, or try to steal it, and it is often less costly to give it up than try to retain it. An adjunct to this is that food is often shared to avoid social costs, rather than direct harassment costs. These social costs could include the withdrawal of mating rights, as shown by female orangutans.

    Reciprocal altruism is harder to accept, as it requires primates to have an understanding of its delayed benefits, and to take the risk that those benefits will actually occur.

    That does not mean it doesn't happen of course, and there is evidence for it among individual primate species.

    But in Jaeggi and Schaik's words: "while sharing with offspring can be explained by kin selection, explanations for sharing among unrelated adults are more controversial."

    Their review was an attempt to tease out how common reciprocal exchange might be, and is the first to look at the occurrence of reciprocal exchange across primate species.

  • Comment number 4.

    Probably tool-using fish, recently reported in the literature, share food.

  • Comment number 5.

    "Reciprocal altruism is harder to accept, as it requires primates to have an understanding of its delayed benefits, and to take the risk that those benefits will actually occur." Matt Walker.

    Not so, this misunderstands the nature of genetic selection for behaviour. The individual does not need to have any understanding of their behaviour as long as a tendency towards a behaviour has a survival advantage. Genetic selection then makes it more likely that that behaviour will spread in a genetic population. Evolution cannot be a conscious act, as consciousness is a result of evolution.

  • Comment number 6.

    Matt Walker's understanding of reciprocal altruism (RA) is sounder than that of some of the commenters. If it is such an obvious explanation for food-sharing then why is it not more widespread in the animal kingdom? Very few clear-cut cases have been shown (I think vampire bats are one example). True, there are methodological difficulties in demonstrating its operation, but notwithstanding these difficulties, it is rather obvious to anyone who actually watches them that most mammals do not actively share food except with close kin.
    The problem is that reciprocal altruism is highly vulnerable to cheating: if I share food with you now, what's to stop you withholding food from me in the future? This is where "the understanding of its delayed benefits, and ... the risk that those benefits will [not] actually occur" comes in, meaning that RA is likely to be a cognitively demanding operation if one is to avoid being cheated.
    In my opinion RA could not have evolved except in conjunction with kin selection in a viscous population: this means that if I give+reciprocate, it is likely to be with people who are related to me and therefore are likely to give+reciprocate too. RA therefore must have evolved in conjunction with cognitive mechanisms for recognising (real or imagined) kinship.

  • Comment number 7.

    I put it down to mercy in the heart.


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