Charles Darwin wondered if animals were aware of themselves. Allowed to visit a rare orangutan in the London Zoo, he brought a mirror and observed the ape apparently make faces at its own reflection. It’s hard to say for sure that the orangutan really was aware that its reflection was its own. Over a century later, a scientist named George Gallup turned Darwin’s idea into a more rigorous test. He would secretly put a mark on an animal’s forehead and see if it noticed the difference the next time it passed a mirror.
Human adults pass this test, but young children don’t, suggesting that our self-recognition takes time to develop. Some chimpanzees (our closest relatives) seem to pass this mirror test, but others fail it. Orangutans also show mixed results. Beyond the primates, studies have indicated that magpies, dolphins, and elephants pass the mirror test. In a new paper in PLOS One, Luis Populin of the University of Wisconsin publish what may be the first compelling evidence that monkeys pass the mirror test too.
It’s a surprising result because people have tried to find evidence of self-recognition in monkeys before. Most scientists failed. The Harvard primatologist Marc Hauser claimed in 1995 that the cotton-top tamarin could pass the mirror test, but that paper was one of several that Harvard now claims were tainted by Hauser’s misconduct. Populin and his colleagues came across their first clues of self-recognition by accident. They had implanted electrodes in the skulls of rhesus monkeys for a different study. They keep mirrors in the monkey cages just to stimulate the animals, and they noticed that the monkeys started spending a lot of time looking at themselves in the mirrors after surgery.
To see if the monkeys were really aware that their appearance had changed, the scientists put different mirrors into the cages. Some were big and some were small. Some were made of ordinary glass, while others were painted glass. The monkeys looked much more often into the real mirrors than the blackened ones. The scientists then introduced an even bigger mirror into the cages, which allowed the monkeys to see their whole bodies. The mirror hung from the top of the cage so they could turn it around. The video above shows a monkey without an implant inspecting one of these big mirrors. It doesn’t use any of the gestures it might if it met another monkey. Instead, it seems to be inspecting its body. Every nook and cranny, in fact.
I asked two experts on self-recognition, Lori Marino and Frans de Waal, both of Emory University, what they thought of the paper.
Marino was enthusiastic:
I’ve been reading this article over and over again all morning and have looked at the videos. Here is my reaction: I think that this is potentially a very important study. The videos are absolutely convincing. At first I thought it was the saliency of the implanted head device that made a difference but the videos show that monkeys without the device also use the mirror and, even more importantly, the monkeys are using the mirror to explore OTHER parts of their bodies, i.e. genitals. That can’t be explained away in some sort of ’stimulus saliency’ explanation, as far as I can tell. There are two areas that I wish I had more information about. First, exactly what was their prior mirror exposure? Is there anything they can report about how long that was and if there were any interactions with the monkeys during that prior time that could have made a difference in terms of the outcome of this study? Second, they report that the monkeys “failed the mark test” but there is no information given about the methods used in the mark test. It would be important to know how that was conducted.
With all that said, this is – by far – the most compelling evidence for MSR in monkeys to date. I have been trying to find an alternative explanation for the results – and haven’t come up with one yet. I’ll continue to return to it to see if any come to mind. I think that these findings show that self-awareness is not an all-or-nothing phenomenon as Gallup has asserted. I don’t find this conclusion particularly surprising because it has been extremely difficult for anyone to come up with an explanation for the supposed “discontinuity” between great apes and monkeys. I think the reason for this is that there is none.
De Waal was more circumspect:
It is hard to say what is going on as the cap may be seen as a “supermark,” as the authors call it, but it is of course a mark that is not only seen but also felt. As a result, the monkeys have two sources of feedback at the same time, the image in the mirror and the sensation of something new on their head. This is different from the classical mark test, which has only one source of information (visual). It seems clear that the convergence of these two sources of perception is helpful to achieve self-inspection, and this is an interesting finding, but the authors still need to explain why rhesus monkeys apparently cannot do the same with just the visual information.
The hallmark of the mark test is its spontaneity, purely on the basis of visual input, so in this sense this is different. Rhesus monkeys have been tested many times and never pass the test. I think you need to talk to Dr. Gordon Gallup and see what he thinks. It is unclear to me what to conclude, but this study is quite different from e.g. the magpie study, which applied a purely visual test.
The idea that mirror self recognition is a black & white distinction (you either have it or you don’t) was first challenged in another monkey study that we conducted, in which we showed that capuchin monkeys do not seem to see a stranger in the mirror: they seem to distinguish the monkey in the mirror from another monkey, strange or familiar. As a result, we proposed a gradual scale of self awareness. The piece of intriguing information presented here may support this view, but I am sure many scientists would want more tests and more controls.
[Update: New Scientist reports that Gallup shares De Waal's reservations.]
This back-and-forth is not just interesting in itself, both for what it says about monkeys and what it says about ourselves. It also says something about science. If you read some of the more extreme comments about the Hauser affair, you’ll find some people trying to indict the entire line of research into how human and animal minds evolved. This new paper, and its reception, shows just how absurd that radical rejection is. Science is bigger than individual scientists, and the mirror test will survive.
[Update: PLOS One paper linked fixed]