For most of us humans (and other species), the ability to recognize faces comes so naturally that we don’t even think about it. However, many other animals lack the ability to distinguish between individual faces, even when they are faces of their own species. Even closely related species within a single genus sometimes display variability in this type of individual recognition. So what determines whether an animal can learn to identify the faces of its conspecifics?

New research in Science suggests that face learning may be shaped by the specific ecology of individual species. To study this, researchers used Polistes fuscatus, a species of paper wasp, that is excellent at facial recognition. P. fuscatus has all the ingredients necessary for this cognitive ability: they have excellent vision, their nests are well-lit, their facial features are highly variable, and (perhaps most importantly) they have multiple queens.

This life history detail may be the key to their facial recognition skills. Since there is more than one queen in each nest, having a hierarchy becomes important; wasps must learn who is who in order to avoid conflict and keep the colony stable.

A group of researchers put 12 P. fuscatus females through their paces in order to determine exactly how good their face learning abilities are. The wasps were put into a T-maze where each arm of the maze was associated with a different image. The floor was electrified, with just a small “safe zone” left unelectrified in one arm of the maze. This let them test how good the wasps were at associating images with the safe zone.

The first set of images used were two photos of different P. fuscatus wasp faces. Each female was tested forty times, and judged on the speed and accuracy of her choices. The P. fuscatus females were very good at learning to distinguish between their conspecifics’ faces. Amazingly, they were better at recognizing faces than they were recognizing simple patterns, such as triangles and crosses.

The wasps did, however, have trouble learning faces without antennae, and faces that had been digitally rearranged, suggesting that both antennae and facial configuration are important cues.

Then, the researchers tested a different species of wasp, Polistes metricus. This species is very similar to P. fuscatus, but with one important difference: each nest has just one queen. The researchers hypothesized that, with less evolutionary pressure for the wasps to recognize each other, P. metricus might not have the same ability to recognize faces.

The researchers' suspicion was right. When tested with the same protocol, P. metricus females didn’t perform any better than chance on the facial recognition task. They did, however, do relatively well when asked to distinguish between simple patterns. Interestingly, P. metricus were slightly better at identifying P. fuscatus faces than their own conspecifics, suggesting that the facial features of P. fuscatus might be more readily identifiable.

Overall, the two species were equally good at learning to distinguish simple patterns; only when it came to face learning did their abilities diverge. It seems likely that the lifestyle of P. fuscatus wasps has led to a specialization in the ability to recognize faces, via both their cognitive abilities and their variable facial features.

It’s likely that many animals with an aptitude for facial recognition may have life history traits in common, such as complicated social systems. Their ability to distinguish between individual faces have most likely evolved independently, but the selective pressures that drove these specializations may be similar.

Science, 2011. DOI: 10.1126/science.1211334 (About DOIs).