Decision Theory Journal Club: The failure of rationality when foraging

When an animal forages for food, it leaves it current location for what it hopes is a better locale.  We like to believe that this foraging decision is made when the animal expects to get more food if it leaves than if it stays.  Simple and obvious, right?  Unfortunately for our intuition, this doesn’t seem to be the case.  When I was at the Foraging workshop at Cosyne, one of the main themes of the day seemed to be that animals aren’t optimal foragers and they don’t look like our idealized rational economic agents, either.

Let’s consider an animal that is forced to choose between different sources of food.  Life isn’t easy, so it takes some time to gather the food from each source and after gathering it must leave the area before its allowed to gather food there again.  The way this particular experiment happens to be set up, if our animal was an Optimal Forager that was trying to get as much food as possible, it would apportion its time solely on the easiest food source, ignoring the rest.  Anything else is a waste of time and energy.  And yet – and yet! – the animal doesn’t do this; it still will go gather food at one location, move to the next and gather food there, and move on again.  Look at how often (pWait) an animal will wait at a food source versus how often they should wait.  It doesn’t exactly look rational to me.

Rats aren't rational economic actors

Okay, so maybe Optimal Foraging isn’t the way to go; maybe an animal doesn’t apportion its time based on maximizing food rate but uses some other economic criterion.  Maybe it matches its investment and reward so that the more reward it gets from a food source, the more time it spends there?  Data says nope.  Maybe it is a temporal discounter and prefers rewards that happen sooner over rewards that happen later?  Nope again.

Let’s consider another option: maybe the animals get attached to the different options, or just like sampling different opportunities.  What if we introduce a parameter into these different theories that represents an unwillingness to reject potential food options?  This helps a little with the temporal discounting model but where it really shines is when added to the Optimal Foraging model.   But foragers aren’t just always opposed to rejecting a food option.  Rather, as the environment becomes more resource-rich, they are more willing to reject a food option when the environment is resource poor (low opportunity cost of time) than when it is resource rich (high opportunity cost of time).

Rats don't like to give up an opportunity

But does this tell us what we think it does?  It seems like much of the effect is explained by the length of time the animal must wait for the longest food source; the length of the other options don’t seem to affect whether an animal will reject those options.  This gives us another possible option: the task was just too easy for the animals.  I would like to see whether the usefulness of the discounting model changes as everything gets harder and the environment becomes sufficiently ‘resource poor’, and also how animals behave when the basic optimal foraging model predicts something other than “always go to the easiest option”.

References

Wikenheiser, A., Stephens, D., & Redish, A. (2013). Subjective costs drive overly patient foraging strategies in rats on an intertemporal foraging task Proceedings of the National Academy of Sciences, 110 (20), 8308-8313 DOI: 10.1073/pnas.1220738110