Need some real estate advice? Ask an ant.

What does it mean to be rational?

In a biological context, rationality means that when animals (including humans) are making a decision, they choose the option that maximizes their fitness benefit. For example: my cat, Mr. Minou, prefers to eat canned food rather than grass. Canned food provides him with more nutrients, more protein, and more energy than grass: it has a fitness benefit for Mr. Minou. Even if he is presented with a third option, say, dry cat kibbles, Mr. Minou stills prefers canned food, because it still has the highest fitness benefit (and, obviously, kibbles taste gross).

Pretty straightforward so far. So, being the advanced species we are, humans must be rational beings, right?

Wrong.

Here is another example. Let’s say you’re shopping for a new house. You have two equally important criteria for your new house: it must have big windows to get lots of natural light in, and it must have a big garage to store all your stuff. There are two houses on the market. House A has big windows but a small garage. House B has a big garage but small windows. Rationally, humans in this position have a 50% chance of picking either house.

Suddenly, a third house is made available on the market. House C has big windows, but NO garage. In a situation like this, humans overwhelmingly put rationality aside and shift their pick to house A with the large windows but the small garage, because the perceived value of house A increased when comparing it with other available houses. However, houses A and B have unchanged value!

The reason for this shift is that as decision-makers, we don’t assign absolute values to options, we assign relative values. We like to compare. And comparing can be misleading.

Recently, two American researchers wondered about the rationality of collective animals, such as ants. The researchers figured that just as choices we make result from the complex interactions of many brain cells, the decisions that an ant colony makes might similarly stem from a complex network of interacting insect. Ant societies act as unitary decision-makers, jointly deciding on things like a single travel direction, or a nest site. The researchers decided to take advantage of the ants’ nest-seeking strategies to test their rationality.

The ants in the study live in natural holes like hollow branches, and are able to emigrate to a new nest if their current nest is damaged. Colonies seeking a new nest reach consensus on the better site among the new options based on entrance size, cavity dimensions, interior light level, etc. The way a colony reaches consensus is fascinating: a few scout ants head out to assess the quality of potential homes. When a scout finds a potential new home, it leaves to recruit more scouts, who will then recruit more scouts, and so on. The strength of this technique lies in the key fact that the higher the quality of the nest an ant finds, the faster it will recruit other ants. Eventually, a threshold of recruiting is reached, and non-scout ants are recruited and eventually the entire ant colony is moved.

The researchers first established that ant colonies prefer nests that have small openings and low inside light levels. They then assessed the susceptibility of ant colonies to irrationality by comparing the colonies’ preference for new nests with different attributes in a very similar way to my house example: nest A has a dim interior but a large entrance size, and nest B had a brighter interior but a small entrance size. In this case, the ant colonies showed no preference for either site, which is very rational of them.

The researchers then added one of two decoy nests. Decoy nest A2 was just as dim as nest A, but had an even larger entrance diameter. Decoy nest B2 had the same entrance diameter as B but was even brighter than B. In summary, each decoy nest had a good feature equivalent to that of A or B, and the other feature was worse.

Well, the study suggests that we should turn to ants for real estate advice: the presence of either decoy did not affect the proportion of colonies choosing A or B. This means that even with the decoy, the ant colonies recognized that A and B had equal fitness values, and that the option of the decoy did not change the fitness values of the original nest sites.

So what is the ants’ secret for being so rational? The most plausible explanation is that for the most part, each scout ant only visits one site. If it’s good, it recruits, and if it’s crummy, it moves on. No comparing with the one next door. In this case, the fact that individuals in the decision-making strategy lack either the opportunity or the ability to compare all the options offers some protection against irrational, fitness-reducing errors.

Is this relevant for us, other than the piece of humble pie we must eat when realizing that ants can be more rational than we are? Well, when faced with a decision, it can be helpful to remember to evaluate each option for its absolute value, and not its relative value.

Mr. Minou himself occasionally forgoes rationality and chooses to eat grass. I suspect he only does it for the pleasure of watching me wash puke from the floor afterwards. Maybe in some twisted way, making sure I clean up after him confers him some kind of fitness benefit…

Mr. Minou being irrational

Reference: Rationality in collective decision-making by ant colonies. Edwards SC, Pratt SC. Proc Biol Sci (2009) Jul 22 [Epub ahead of print]