From Science:
Classical philosophers called humans “the rational animal.” Clearly, they never looked closely at ants. A new study suggests that ant colonies avoid irrational decisions that people and other animals often make.
Consider the following scenario: You want to buy a house with a big kitchen and a big yard, but there are only two homes on the market–one with a big kitchen and a small yard and the other with a small kitchen and a big yard. Studies show you'd be about 50% likely to choose either house–and either one would be a rational choice. But now, a new home comes on the market, this one with a large kitchen and no yard. This time, studies show, you'll make an irrational decision: Even though nothing has changed with the first two houses, you'll now favor the house with the big kitchen and small yard over the one with the small kitchen and big yard. Overall, scientists have found, people and other animals will often change their original preferences when presented with a third choice.
Not so with ants. These insects also shop for homes but not quite in the way that humans do. Solitary worker ants spread out, looking for two main features: a small entrance and a dark cavity. If an ant finds an outstanding hole–such as the inside of an acorn or a rock crevice–it recruits another scout to check it out. As more scouts like the site, the number of workers in the new hole grows. Once the crowd reaches a critical mass, the ants race back to the old nest and start carrying the queen and larvae to move the entire colony.
More here.