Trading Food and Grooming for Sex from a Behavioral Neuroscience Perspective

A lot of people on ScienceBlogs are talking about this paper, Hockings et al., which shows that male chimps will trade food for sex. The food in this case is papayas stolen from nearby farms; foraged food is apparently not traded in particularly large amounts. The chimps will also give papayas to other males as a means of advertising their prowess and reproductive fitness or possibly as a means of reinforcing social bonds in the raiding party.

Now Greg Laden and Afarensis analyze the anthropological aspects of this paper, and they have much more articulate things to say on that subject.

However, I want to talk about this paper from a behavioral neuroscience perspective: namely, how are two values for different goods converted in the brain?

It seems to me that there have been a lot of papers about food and grooming trading for various social benefits -- of which sex is one -- in the primate literature of late. There was also a paper that appear in Animal Behavior last month by Michael D. Gumert that looked at grooming-for-sex exchanges. (It was covered here in New Scientist, but sadly both articles are behind a subscription wall.)

Gumert describes the theoretical framework under which such exchanges take place in terms of biological marker theory. To wit (from his Animal Behavior paper):

When applied to the social realm, biological market theory models social trade as if payment for social acts or partners occurs (Noe 2001). The model states that individuals trade social behaviour in a social market, and in this market there are differing classes of social partners that are distinguished by what they can offer to others (i.e. social commodities: Noe 2001). One class holds access to a social commodity (i.e. holding class) and another class seeks access to that commodity (i.e. demanding class). The dynamic between the two classes is the basis for a system of exchange where the demanding class offers something to the holding class to gain access to a social commodity. This system of trade will follow basic principles of economics. Factors such as supply, demand, advertisement and partner value will influence exchange in situations where the trade partner is not forced to cooperate ([Noe and Hammerstein, 1995] and [Noe, 2001]). For example, the relative abundance of potential partners in the holding class should alter how much individuals in the demanding class will offer them for access to the commodity they hold, if the demanding class is trading and not just taking the social commodity based on their social power. When the holding class is scarce, the demanding class should offer more, and when abundant, should offer less.

Biological marker theory implies that animals possess a host of cognitive capacities, some of which were previously only ascribed to humans. For example, I posted a case where monkeys were even trained to use little plastic wafers as currency. Just some of the capacities implied by biological marker theory are:

• The ability to assign value to abstract items such as social class.
• The ability to detect changes in supply and demand for valuable goods.
• The ability to convert values from one valuable good to another in terms of a standard value -- in essence, the ability to understand price.

What are the neurological machinery that allows assignment of value and conversions to take place?

Well, Geoffrey Schoenbaum from Hopkins has shown that reward assignment probably occurs in a part of the prefrontal cortex called the orbitofrontal cortex (OFC). (It is right above your eye-balls in the brain.) In his work, mostly in rats, he has shown that OFC neurons have the ability to form arbitrary associations between particular responses and the reward received. They can also respond specifically to very precise stimuli such as particular odors. (I don't have time to look up these papers. Look on Google scholar for Schoenbaum and OFC. There are like a bagillion of them. Here is a good review of the functions of the OFC.)

In any case, it appears that the reward assignment function occurs at least in part in the OFC. This conclusion is corroborated by the fact that humans with OFC damage -- much more common than you would think -- have difficulty with decision making and assessing long-term reward. (Reviewed here.)

(Just to note, this is a gross over-simplification since there are clearly other brain areas that form response-reward associations -- such as the striatum. Likewise, there are cases where the response becomes divorced from the receipt of reward such as habitual learning.)

How is this reward-response machinery leveraged into complex behaviors like trading? Well, here is where I start to speculate. It is possible the internal circuitry of the OFC makes this conversion possible. The activation of one representation in this scenario is transitive and can activate alternative equally valuable representations. Another possibility is that there is some competitive process by which the power of two representations is compared to determine their relative value. This probably occurs in a different brain region. Candidates have been proposed including the dorsolateral prefrontal cortex in humans and primates (and probably the PL-IL in rats). The third possibility is that reward-response pairings are daisy-chained. A particular reward could become a stimulus for another reward, signaling its own response which could itself be rewarded.

I think the last one -- the instrumental learning explanation -- is the most likely with respect to the chimpanzees in this case. I would speculate that the reason that male chimps are handing over fruit is because they receive consistent gratification -- so to speak -- for doing so. Whether or not they know this when they are stealing the fruit -- meaning whether they have a representation of the future papaya-for-sex exchange prior to having the actually papaya -- is equally speculative, but I would argue that they do. Numerous species show a remarkable ability to learn complex and multi-part tasks. (Remember the ninja squirrel.) If the monkey can learn the elaborate process of stealing the food, I assert that it can represent what it plans to do with it afterward.

In any case, this is just a behavioral neuroscience perspective. The fact that these animals show such complex behaviors is in itself remarkable. They are responding to changes on the ground such as increased human habitation in utilizing what in hindsight are very clever and rational strategies. However, from my perspective, the more interesting question is how their brains perform the complex calculation of trading values. This is the fundamental question of neuroeconomics, and I hope more papers attempt to deal with this aspect.