Theory of mind in animals

The Knower-Guesser method has been used in many studies relating to animal ToM.[6]

The competitive feeding paradigm approach is considered by some as evidence that animals have some understanding of the relationship between "seeing" and "knowing".[1]

In one suggested protocol, chimpanzees are given first-hand experience of wearing two mirrored visors. One of the visors is transparent whereas the other is not. The visors themselves are of markedly different colours or shapes. During the subsequent test session, the chimpanzees are given the opportunity to use their species-typical begging behaviour to request food from one of the two humans, one wearing the transparent visor and the other wearing the opaque. If chimpanzees possess ToM, it would be expected they would beg more often from the human wearing the transparent visor.

A method used to test ToM in human children has been adapted for testing non-human animals. The basis of the test is to track the gaze of the animal. One human hides an object in view of a second human who then leaves the room. The object is then removed.[14]

There is controversy over the interpretation of evidence purporting to show ToM in chimpanzees.[16]

Chimpanzees were unable to follow a human's gaze to find food hidden under opaque bowls, but were able to do so when food was hidden in tubes that the experimenter was able to look into. This seems to suggest that chimpanzees can infer another individual's perception depending on the clarity of the mechanism through which the individual has gained that knowledge.[17]

Attempts to use the "Goggles Method" (see above) on highly human-enculturated chimpanzees failed to demonstrate they possess ToM.[9]

In contrast, chimpanzees use the gaze of other chimpanzees to gain information about whether food is accessible.[7] Subordinate chimpanzees are able to use the knowledge state of dominant chimpanzees to determine which container has hidden food.[18]

Young chimpanzees were shown to reliably help researchers perform tasks that involved reaching (such as picking up dropped items that the researcher struggled to retrieve), without specific prompting. This suggests that these chimpanzees were able to understand the researcher's intentions in these cases and acted upon them.[19][20]

In a similar study, chimps were provided with a preference box with two compartments, one containing a picture of food, the other containing a picture of nothing. Neither were actually related to the contents of the box. In a foraging competition game, chimpanzees avoided the chamber with the picture of food when their competitor had chosen one of the chambers before them.[21][22]

Captive bonobos such as Kanzi have been reported to show concern for their handlers’ well-being.[23] Bonobos also console other bonobos who are victims of aggressive conflicts and reconcile after participating in these conflicts.[24] Both of these behaviors suggest some semblance of ToM through an attribution of mental states to another individual.

Chimpanzees have passed the False Belief Test (see above) involving anticipating the gaze of humans when objects have been removed. Infrared eye-tracking showed that the chimpanzee subjects’ gaze were focused on where the experimenter would falsely believe the object /subject to be, rather than focusing on its actual location of which the chimps were aware. This seems to suggest that the chimpanzees were capable of ascribing false belief to the experimenter.[25]

Rhesus macaques selectively steal grapes from humans who are incapable of seeing the grape compared to humans who can see the grape.

In one approach testing monkeys, rhesus macaques (Macaca mulatta) are able to "steal" a contested grape from one of two human competitors. In six experiments, the macaques selectively stole the grape from a human who was incapable of seeing the grape, rather than from the human who was visually aware.[26] Similarly, free ranging rhesus macaques preferentially choose to steal food items from locations where they can be less easily observed by humans, or where they will make less noise.

The authors also reported that at least one individual of each of the species showed (weak) evidence of ToM.[27]

In a multi-species study, it was shown that chimpanzees, bonobos and orangutans passed the False Belief Test (see above).[25]

In 2009, a summary of the ToM research, particularly emphasising an extensive comparison of humans, chimpanzees and orang-utans,[28] concluded that great apes do not exhibit understanding of human referential intentions expressed in communicative gestures, such as pointing.[29]

Grey parrots (Psittacus erithacus) have demonstrated high levels of intelligence. Irene Pepperberg did experiments with these and her most accomplished parrot, Alex, demonstrated behaviour which seemed to manipulate the trainer, possibly indicating theory of mind.[30]

Ravens adjust their caching behaviour according to whether they have been watched and who was watching them.

Ravens are members of the family Corvidae and are widely regarded as having complex cognitive abilities.[31][32] Other studies indicate that ravens recall who was watching them during caching, but also know the effects of visual barriers on what competitors can and can not see, and how this affects their pilfering.[33]

Ravens have been tested for their understanding of "seeing" as a mental state in other ravens.[34] The researchers further suggested that their findings could be considered in terms of the "minimal" (as opposed to "full-blown") ToM recently suggested.[13]

Using the Knower-Guesser approach, ravens observing a human hiding food are capable of predicting the behaviour of bystander ravens that had been visible at both, none or just one of two baiting events. The visual field of the competitors was manipulated independently of the view of the test-raven.[35]

Western scrub jays may show evidence of possessing theory of mind

Scrub jays are also corvids. Western scrub jays (Aphelocoma californica) both cache food and pilfer other scrub jays' caches. They use a range of tactics to minimise the possibility that their own caches will be pilfered. One of these tactics is to remember which individual scrub jay watched them during particular caching events and adjust their re-caching behaviour accordingly.[36] One study with particularly interesting results found that only scrub jays which had themselves pilfered would re-cache when they had been observed making the initial cache.[37] This has been interpreted as the re-caching bird projecting its own experiences of pilfering intent onto those of another potential pilferer, and taking appropriate action.[8][38]