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Tag: chimpanzee

Can’t Read My Monkey Face

(Yes, that was a Lady Gaga reference) I’m extremely late for January 2011’s paper-a-month blog post, but better late than never!

This month’s paper actually dates to a little over 3 years ago. And, it actually isn’t about monkeys – its about chimpanzees (but “monkey” and “poker” have the same number of syllables, whereas “chimpanzee” has one more and hence throws off the reference). But, in it, they describe a very interesting experimental design to look into a very bold question: how do chimpanzees play the ultimatum game?

First off, what’s the ultimatum game? The ultimatum game is considered to be a test of fairness vs. rationality. The basic idea is that you have a setup with two people – a offer-maker and an offer-taker. The offer-maker makes an offer to split a prize (be it money, or food, or something else that both the offer-maker and the offer-taker find valuable) between the offer-maker and the offer-taker (i.e., so the offer-maker could say that he/she gets 90% whereas the offer-taker gets 10%). The offer-taker then decides whether or not to accept the offer or to reject the offer – whereby rejection means both offer-maker and offer-taker get nothing. (refer to dinosaur comics below)


Why is this an interesting test of fairness vs rationality? If humans were perfectly rational, then the offer-taker would accept any offer better than 0 and, knowing this, the offer-maker would offer the bare minimum piece to the offer-taker and take the lion’s share for him/herself. The reason for this is that a rational offer-taker would realize that as long as the offer gives something better than 0 – which is what you get if you reject the offer no matter what — the offer-taker is actually better off just taking the offer.

However, study after study shows that human beings aren’t perfectly rational – that faced between a grossly unfair, but objectively better outcome and nothing, that most humans will prefer the latter. In fact, studies have shown that the offer-taker tends to reject offers where they receive less than 20%. There are many evolutionary, sociological, and psychological interpretations, but at the end of the day the combination of valuing fairness and punishing unfairness is something which lets people live and work together.

While there are all sorts of interesting scientific and philosophical work being done to explore how humans play the ultimatum game, it does beg the question: do other creatures value fairness the way we do? In other words, if we could get other animals to play the ultimatum game, how would they play?

The researchers here conducted a fascinating experiment where they “taught” chimpanzees, who are not only our closest genetic relative but also known to work collaboratively on tasks such as hunting and territorial patrol, to play a slight variation of the ultimatum game. The setup is shown below in Figure 1a:


This variation of the ultimatum game requires the offer-maker (or “proposer” in the language of the paper) to select between two possible distributions of the prize (in this case, different amounts of raisins), visible to both the offer-maker and offer-taker (or “responder”) by pulling on one of two different ropes. The offer-taker then decides whether or not to accept the offer by pulling on a rod which brings the raisins within reach of both chimpanzees to eat (see figures 1b and 1c below).

Also, to help get a sense for the relative value of fairness, in each “round” of the experiment, the offer-maker always had a chance to offer a 8/2 split (with the offer-maker getting 80% of the spoils and the offer-taker getting 20%), and depending on which experimental group, the other option was either: 10/0 (offer-maker gets 100%), 8/2 (same as the control), 2/8 (offer-maker gets 20%, offer-taker gets 80%), and 5/5 (even split). This gave the researchers a chance to see how offer-makers and offer-takers would view the relative merits of other offers against that control.


Very clever experimental design, in my humble opinion! To insure that the chimpanzees knew what was going on, they were trained for some time to make sure they understood how to operate the apparatus and that their actions had consequences for their partner (i.e., by having the chimpanzees operate the apparatus and then be allowed to enter the partner’s chamber to eat the raisins).

So, what were the results? See for yourself (below in Figure 2). The left side shows every experiment run (~50 for each comparison), how often the offer-maker in each group proposed 8/2 vs. the other option, as well as how often the offer-taker rejected the offer.


The first thing that jumped out to me is that, with the exception of the 10/0 offer where the offer-taker gets absolutely nothing, the offer-takers reject very few of the offers.  The second thing that jumped out to me is that, with the exception of the 8/2 vs 10/0 decision, the offer-makers strongly preferred the offer where they got more (they overwhelmingly selected 8/2 over 5/5 and 2/8), suggesting that they first value their own well-being, but are also cognizant that the other chimpanzee has minimal reason to accept a 10/0 offer where they get nothing. These two observations support the conclusion of the research group, that chimpanzees are mostly rational maximizers, and don’t place too much weight on fairness.

The third thing which jumped out to me is very confusing – the chimpanzees were actually more wiling to reject 8/2 deals when the alternative was equivalent or worse (another 8/2 deal or a 10/0 deal) than when they had the much better options of a 5/5 or a 2/8 deal. This is either random experimental noise or suggests that chimpanzees like having a fairer option even when they don’t get it?

If I were to suggest next steps for the team, I’d ask them to probe deeper into that third point – because it suggests either there is a very interesting behavioral quirk about chimpanzees that is not well-understood today or that the chimpanzees didn’t fully understand how to play the game. I’d also recommend them to try a more direct measure of finding out if chimpanzees value altruism (a concept related to but not exactly the same as fairness): I’d love to see if chimpanzees are Pareto efficient (are they willing to be generous if it doesn’t cost them anything): are they more likely to propose a 8/6 over a 8/2. A positive result there would reinforce the finding here, that chimpanzees are actually altruistic – but they are rational maximizers first and foremost.

(Image credit – Dinosaur Comics) (remainder from Figures 1 and 2)

Paper: Jensen et al., “Chimpanzees are Rational Maximizers in an Ultimatum Game.” Science 318: 107-109 (Oct 2007) – DOI: 10.1126/science.1145850


Crazy Y Chromosomes

A few weeks ago, I set myself some 2010 goals. One of which was to make sure that I fit in reading at least one paper every month. What I didn’t say though, was that I would try to do a quick blog post on each (to help keep me honest).

I forgot exactly how I found this paper, but the subject immediately caught my eye:

Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content

What? I had always been taught that the genomic differences between chimpanzees and humans were extraordinarily small, and since I’m male, I was also drawn to the fact that it would talk about a chromosome that was very near and dear to me.

What I read was pretty amazing. Despite the fact that we are very genetically similar to chimpanzees the Y chromosomes of our two species are actually very different. The best depiction of this that I can point to is from Figure 2 of the paper (below). The two charts below are dot plots which show where the human and chimpanzee chromosomes “line up”, so to speak. The right-hand chart shows how closely related the human chromosome 21 is to the analogous chimpanzee chromosome. You can see this from the nearly perfect diagonal line, showing that the two chromosomes are pretty close to identical as you move from one end of the chromosome to the other. The left-hand chart shows a similar comparison of a human Y chromosome and a chipmanzee Y chromosome. Notice the difference?


While it was fascinating to read what specifically was different (i.e. repeats, ectopic homologous recombination, introduction of nonsense mutations and open reading frames), what I found most interesting was the speculation as to why the Y chromosomes of two very similar species would diverge so much in such a narrow period of time. The research group’s hypothesis is that the Y chromosome holds a great deal of influence over sperm production, and because a Y chromosome will never have a “partner” chromosome the way that every other non-sex-determining chromosome does, changes in the Y chromosome are likely to have very significant changes in sperm characteristics. Because female chimpanzees oftentimes mate with multiple males, there is strong sperm competition and hence strong selective pressure for chimpanzees to have rapid evolution in the Y chromosome.

Of course, this is all just a guess. One way to test it would be to compare the human Y chromosome sequence with further primate species and see if primates where sperm competition is less intense have more similar chromosomes as humans. Another would be to see if any of the genetic changes resulted in clear sperm/testes genetic or transcriptional control differences.

But, all in all, a very cool start to what I’m hoping will be a fun 12 months!

(Figure 2 from paper)

Paper: Hughes, Jennifer F. et al. “Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content.” Nature 463, 536-539 (28 January 2010) – doi:10.1038/nature08700