Seyfarth and Cheney provide a commentary on a recent study in the Proceedings of the National Academy of Sciences (PNAS). The topic of the study under review (Proops et al. 2008) is individual recognition and cross-modal perception in horses. Cross modal perception involves assimilating information from one sensory mode to another. Individual recognition is, for lack of better terms, putting a face with a name. So the experiment: when hearing an individual's 'voice' (in horses it's a whinny), how aware are others of who this whinnier is?
We humans do this all the time, but researchers across many fields have been interested in whether other animals are capable of such behavior. In this study, Proops et al. tested this by having 2 horses observe another move behind a barrier. When this horse was no longer visible, the researchers played back the pre-recorded whinnies of the now-hidden horse or some other random philly. Apparently, most often if the whinney and the hidden horse didn't match up, the other horses seemed 'surprised.' Thus, though a fictional horse who ate apples or something in order to look like he was talking, Mr. Ed and his fellow equines are capable of putting a face with a name. Or a horse-face with a whinny.
Similar studies have been done in non-human primates such as baboons, and the results are similar. The primate ones I've heard of, though, seem much cooler. One study by Thore Bergman and Jacinta Beenher (now here at UM) and Cheney and Seyfarth, in baboon societies with complex social systems, involved confusing the established hierarchy--i.e. a pretending a low-ranking individual threatens a higher-ranking one (Bergman et al. 2003). In other studies, when playing back a juvenile's cry, unrelated mothers often direct their attention to that juvenile's mother. What these studies show is that these non-human primates are quite cognizant of others' social and kin relations.
These are really neat results, which suggest to Seyfarth and Cheney, and maybe Proops et al--I didn't read their paper--that cross modal perception and individual recognition have deep evolutionary roots. After all, the last common ancestor of humans and baboons existed probably over 25 million years ago. And primates and horses diverged probably around 60 million years ago. The fact that these three distantly related animals are capable of this behavior suggests their last common ancestor may have been capable of this, too. But it seems to me that these behavioral capabilities could have evolved independently, under selection in a social system. Horses (at least those in this study) lived in herds of around 30 individuals, and primates generally (but by no means all) live in large social groups. In such circumstances, being able to recognize individuals, and their relations to you, aurally would conceivably be very adaptive. So that's another hypothesis that remains to be tested: that these cognitive behaviors evolved in animals as a result of social living. Could lead to a chicken-egg thing. But maybe it could be tested by looking at animals that live in smaller groups or are solitary.
Of course, what I would have done in this horse situation, since I have no soul, is once the one horse moved behind the barrier, I would have played back the whinny of one of the observing horses. That could freak them out, really mess with their reality.
Bergman et al. 2003. Hierarchical classification by rank and kinship in baboons. Science 302: 1234-1236.
Proops et al. 2008. Cross-modal individual recognition in domestic horses (Equus caballus). Proc Nat Acad Sci USA 106: 947-971.
Seyfarth and Cheney. 2009. Seeing who we hear and hearing who we see. Proc Nat Acad Sci USA 106: 669-670.