Bonobo Week continues! I'm donating whatever proceeds I receive from my blogging shenanigans for the entire month of June to help the bonobos at Lola Ya Bonobo.

ResearchBlogging.orgPrimate researchers used to think that only humans voluntarily share their own food with others. At the time, it was a reasonable conclusion to make, since lots of studies indicated that chimps don't. But that was before anyone checked to see if bonobos were willing to share their food with others.

So Brian Hare and Suzy Kwetuenda tested pairs of the bonobos from the Lola Ya Bonobo sanctuary in DRC. In all cases, the two participating bonobos were unrelated, and in some of the cases, were from different social groups entirely. This was done to remove any form of kin-directed altruism from confounding the experiment.

The rooms are set up so that there are three different rooms with adjoining doors. The participant bonobo is in the center room, with a bowl of food. On one side is one room and door; and on the other side, the other room and door. The walls are actually fences made of vertical bars 10cm apart, so you can clearly see into the adjoining rooms.

First, the first bonobo (the participant) is introduced to the testing environment, and shown that he or she can unlock the doors by pulling on a rope, which removes a wooden block, and which allows the door to be opened. The bonobo achieves door-unlocking so many times, and then the experiment begins.

Before the morning meal (so the bonobos were likely hungry), the two bonobos are brought into the building. The entire experiment takes place in one of the night buildings (at Lola, the bonobos hang out outside in wide swathes of forest, but at night they sleep inside). They specifically picked bonobos who had experience sleeping in that particular night building, so that they'd be comfortable and familiar with the surroundings. The participant bonobo is in the center room with the food, and the recipient bonobo is in one of the adjacent rooms, behind the locked door. Of course, the room in which the second bonobo is found was counterbalanced. The doors could only be unlocked by the participant, not by the recipient.

If bonobos are capable of voluntarily sharing their food, they should choose to open the recipient's door before opening the door to the empty room, and before finishing all of the food. Random door opening, or aggression towards the recipient subsequent to opening the door, would suggest that it wasn't an act of voluntary food sharing, but a lack of inhibition for opening the doors.

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Figure 1: Experimental set-up. Experimental condition above, control condition below.

The experimental condition was conducted five times over two days, and was always followed on a different day with five trials of the control condition. The control condition was the same as the experimental condition, except that in the adjacent room was an additional bowl of food instead of another individual. This tested several things: (1) could the subjects eat the same amount of food, by themselves, that they had initially shared, and (2) whether the subjects could inhibit opening a door when something attractive (the second bowl of food) was behind it. If they could eat the same amount of food that they had initially shared, then they weren't simply sharing because they knew they couldn't finish everything. If they could finish the food in their bowl before going and retrieving the second bowl of food, then, when in the experimental condition, they didn't simply open the doors because something interesting was in the next room.

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Figure 2: Click to embiggen. Source.

Results

In the experimental condition, participants had a significant preference to open the recipient's door instead of the alternative door. When released, the recipients always obtained preferred fruits. The recipients spent more time co-feeding per trial (80.6%) than time that they were excluded from feeding (19.4%). No form of aggression was ever observed between the two individuals. The sharing didn't change between the first session of the day, and the last session of the day.

In the control condition, subjects ate all the food, and also preferred to open the door with the additional fruit than the door to the empty room.

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Figure 3: Mean+SEM number of trials when participant opened either door during the experimental condition, top. Time until first opening, comparing conditions, below.

Comparing conditions, subjects opened the recipients' door (average 68.8 seconds to opening) significantly faster than the door to the additional food (average 261.14 seconds to opening).

So what can we learn from this? The sharing wasn't due to non-social factors like satiation (the sharing didn't change between the first session, when they were hungry, and the last session of the day, when they weren't). It also wasn't due to an inability to inhibit opening doors, since their door opening was specific, and also because they did inhibit opening the doors in the control condition.

It can be inferred that sharing represented some sort of cost, since subjects ate all the food in the control condition, but shared in the experimental condition; the sharing wasn't simply a matter of having more than enough food. However, the sharing can't be explained by social factors such as harassment (since the subject had to explicitly allow access to the recipient), kinship (since none of the pairs were related), or by reciprocation for previous favors (since sharing was observed even in pairs from entirely different social groups). It is possible that participants shared so that they could receive favors in the future, or it could reflect a more genuine form of altruism. More research is necessary to address this question. What is particularly awesome, according to Hare, about this finding is that in other contexts, bonobos are particularly averse to food loss and try to minimize it (e.g. this study and this study). So the fact that they voluntarily share, in this situation, is striking.

Hare, B., & Kwetuenda, S. (2010). Bonobos voluntarily share their own food with others Current Biology, 20 (5) DOI: 10.1016/j.cub.2009.12.038