Chimpanzees both revealed a substantially higher frequency of general mirror-related behaviors & engaged in significantly more and longer behaviors indicating self recognition when provided with small mirrors

Small mirrors do the trick: A simple, but effective method to study mirror self-recognition in chimpanzees. Kathrin S. Kopp et al. Accepted for publication in Animal Behavior and Cognition (October 17 2020). https://pure.mpg.de/rest/items/item_3261664/component/file_3261665/content

Abstract: Mirror self-recognition (MSR) is considered an indicator of self-awareness. Standardized mirror tests reveal compelling evidence for MSR in a few non-human species, including all great apes. However, substantial inter-individual variation of MSR within species resulted in an ongoing methodological controversy, questioning the appropriateness of standard MSR tests for cross-species comparisons. Especially lack of motivation is discussed as one possible cause for false negative results. Here, we compare the spontaneous behavioral response of 47 zoo-housed chimpanzees (Pan troglodytes) to (i) standard body-sized, stationary mirrors and (ii) small, portable hand mirrors. We predicted that the monopolizability and maneuverability of small mirrors increase the chances of identifying MSR across a larger proportion of individuals. Chimpanzees both revealed a substantially higher frequency of general mirror-related behaviors and engaged in significantly more and longer behaviors specifically indicating MSR when provided with small mirrors compared to a large mirror. Handheld mirrors provide a more sensitive measure for MSR within and likely between primate species than the traditional large mirrors, nd thereby are a potentially valuable tool for studying self-awareness across species.

Keywords: MSR, comparative cognitive research, primates, self-awareness, mirror test, cognition

Discussion

In the current study, we compared the traditional MSR test setup using a stationary, body-sized

mirror with an alternative setup using several portable, small hand mirrors. Corroborating our predictions,

chimpanzees interacted more with the small mirrors than with the large mirror, reflected by a higher

proportion of time spent with mirror-related behaviors and self-exploration. We found 12.8% more

individuals engaging in SE with the small mirrors than with the large mirrors in the short period of only

two hours for each condition. Taken together, our results support the hypothesis that this method

provides a more sensitive measure to detect MSR ability in chimpanzees and potentially other species

capable of maneuvering a mirror than the traditional approach. Including small, portable mirrors in

standardized comparative MSR tests will likely help to verify and potentially re-assess variation in MSR

previously found across and within species (de Veer & van den Bos, 1999; Gallup Jr. & Anderson, 2019).

As predicted in (P1) and corroborating early scarce descriptions of the behavior of capuchins and

chimpanzees (Anderson & Roeder, 1989; Köhler, 1926), our study revealed a substantially higher tendency

of chimpanzees to interact with maneuverable hand mirrors compared to a stationary body-sized mirror.

Being provided with unfamiliar, portable objects increases exploration and manipulation behavior in

captive primates(Paquette & Prescott, 1988; Westergaard & Fragaszy, 1985). Carrying a mirror away from

other chimpanzees to a place of their choice might facilitate intense mirror exploration with limited social

distraction. This opportunity might be reduced with a large, stationary mirror either because dominant

individuals can potentially monopolize it or because it is harder to position one’s body to observe difficultto-see body parts in a large mirror than to maneuver the handheld mirror to observe stationary body

parts. Related to the former, one might argue that the difference found was a result of having less

opportunity to get access to the large stationary mirror, especially for bigger social groups, than to one of

several small mirrors. However, our observations do not support this explanation. While small mirrors

were frequently picked up, carried around and explored, the area in front of the large mirror remained 

completely unoccupied on average 73% of the test time across groups. This suggests that the observed

difference is much more likely to be due to a greater interest in portable and maneuverable reflective

objects compared to a large stationary reflective surface, rather than to limited access to the large mirror

because of monopolization by particular individuals. Additionally, stationary mirrors force individuals to

spend time in a predefined area to engage with them, something not all individuals are keen to do. Mobile

mirrors allow for taking them to preferred spots and engaging with them at one’s own pace.

Furthermore, the different size of the two mirror types in relation to the body size of a chimpanzee

might have had an influence on their MRB. Unlike large mirrors, small mirrors do not reflect the whole

body, thus, the social stimulus properties of mirror-images (Anderson & Roeder, 1989) should be less

strong and therefore, less likely to trigger aggressive or fearful responses, which might constrain mirrorrelated interactions including SE (de Veer & van den Bos, 1999). Our finding that aggressive responses

were directed only towards large mirrors, predominantly by adult males, supports this interpretation. This

is importantto consider when examining MSR abilities in species with aggressive tendencies towards rivals

or potential mates.

In accordance with our prediction (P2), we observed more individuals engaging in SE and for a

greater proportion of time with the hand mirrors compared to the large mirrors. The chimpanzees were

exploring and interacting more frequently and extensively with the small mirrors compared to the large

ones. The small mirrors were directly accessible and could actively be manipulated and moved relative to

one’s own body. In doing so, chimpanzees had potentially more and varying opportunities to investigate

the reflective properties of the small mirrors and to detect contingencies between their own or other’s

movements and the reflection compared to large mirrors (Gallup Jr., 1994; Reiss & Morrison, 2017). This

in turn might have resulted in more spontaneous SE in the Small mirrorsthan in the Large mirror condition,

an indicator of MSR in great apes (Anderson & Gallup Jr., 2015; Gallup Jr., 1970, p. 1970; Lethmate &

Dücker, 1973; Povinelli et al., 1993). The objective of our study, however, was not to assess MSR abilities 

in chimpanzees, which has been done elsewhere (e.g., de Veer et al., 2003; Gallup Jr., 1970; Lin et al.,

1992; Mahovetz et al., 2016; Povinelli et al., 1993). Hence, we did not include control conditions to

determine MSR abilities, such as presenting mirror sized object without a reflective surface. Our aim was

to compare the effects of the Large mirror condition and the Small mirror condition on mirror-related

behaviors and thereby their suitability for MSR studies particularly in primates.

In that respect, we suggest that maneuverable hand mirrors offer a better tool to identify

spontaneous SE. When chimpanzees used the small mirrors for SE, they did not only touch otherwise not

or barely visible body parts (the classic criterion for SE), but held the mirror, moved and adjusted its

position, presumably to see the target body part. These directed mirror movements in combination with

manual exploration or actively opening of the mouth while looking at the reflection and following the

mirror with the gaze, are less ambiguous criteria to distinguish SE from not-mirror-guided self-directed

behavior (Heyes, 1994). Furthermore, the maneuverable small mirrors themselves had a highly reflective

and a non-reflective side. This offered a possibility to control whether self-directed behavior was mirrorguided or not by analyzing instances of self-directed behavior while looking at one side or the other.

Given the short period of total mirror exposure in our study compared to other studies (e.g.,

Povinelli et al., 1993), it is notable that we observed spontaneous SE in 42.6% of the chimpanzees (with

four immatures too young to show SE included in sample). However, we would not claim that we

demonstrated compelling evidence for MSR in all these individuals, especially because some of them

engaged in SE only once or twice and we did not include explicit control conditions. The point we want to

make here is that those individuals that showed SE only once and especially with a small mirror are likely

to demonstrate compelling evidence for MSR when studied with hand mirrors for a longer period of time.

Our results revealed an increased interest in the mirrors in the second compared to the first

condition, i.e., in the Small mirrors condition compared to the Large mirror condition, but a drop of 

interest in interacting with a mirror in the second session compared to the first session within conditions.

These findings are in line with our expectations and previous reports of novelty effects, resulting in higher

interest in interacting with novel objects and decreasing interest in the mirror over time in chimpanzees

and other primates (Anderson & Roeder, 1989; Gallup Jr., 1994; Povinelli et al., 1993). In contrast, they

do not suggest a substantial impact of a possible familiarity effect. We therefore argue that the greater

number of individuals demonstrating SE and the higher proportion of time spent with SE in the Small

mirrors condition compared to the Large mirror condition is more likely due to the substantially higher

amount of interactions with the mirror – and thereby increased opportunity to learn about the mirror –

than due to familiarity with the reflection accumulated during the presentation of the large mirror.

However, future studies on MSR abilities need to consider these potential influences and include

appropriate control measures.

Finally, there are practical advantages of using small hand mirrors. The method is easy to apply in

both simple mirror tests and the mark test (Gallup Jr., 1970). It is applicable in a social setting in the usual

enclosure and offers enrichment opportunities (Cronin et al., 2017). It is suitable for tests in various

primate species and potentially in other species capable of maneuvering a small mirror, as the size and

weight of the mirrors can be adapted to species-specific characteristics.

However, we are aware that the method proposed here is not generally applicable across

different taxa, because being able to hold and maneuver the mirror – either by hand, trunk or other means

– is a predisposition, which is not or not easily fulfilled in many species of interest, e.g., in marine

mammals, canines, ungulates without a trunk or birds (Vonk, 2020). While acknowledging this limitation,

we suggest that the proposed method has the potential to inform comparative research and to draw more

representative data of the capacity to recognize oneself in the mirror than large-mirror setups, not only

in chimpanzees, but across the primate order and potentially other species able to maneuver a hand 

mirror. Adequate cross-species methods are needed to aid our understanding of the evolutionary origins

of human self-awareness.