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Gaze following in Great Apes (Pan Troglodytes, Pan Panicus, Gorilla Gorilla) and 12 and 18 month old human children

H. Lehmann*, B. Hare, J. Call & M. Tomasello Max-Planck-Institute for Evolutionary Anthropology Leipzig

*now of Artificial models of natural Intelligence, University of Bath

Gaze following is one of the most basic abilities for communication without vocalisation between individuals. In the wild this is important for locating possible predators or food sources.

The question is when does this ability appear in the phylogenetic and ontogenetic development and is it possible to arrive at an understanding of the intention of the other from the occurrence of this ability?

Scientists have become interested in the topic of gaze following in nonhuman primates in the last ten years. In several studies it was shown that nonhuman primates have the ability to follow human gaze under different conditions (e.g. Tomasello, Hare & Agnetta [1999]; Tomasello, Call & Hare [1998]; Povinelli & Eddy [1996]). In human children this research has a long tradition. In their study, Scaife & Bruner (1975) showed that children between 6 and 12 months of age followed the gaze of an experimenter even when there were no target objects visible.Further studies over the last 30 years examined the issue of when the ability to follow gaze first occurs in the ontogenetic development of the child, and what kind of target cues have to be in the children`s field of vision (e.g. Butterworth & Cochran [1980]; Butterworth & Jarett [1991]; Moore C., Angelopoulos M. und Bennett P. [1997]).

PURPOSE: This study replicated and extended the results of previous studies, in which it had been shown that chimpanzees and children from the age of 12 months follow the gaze of others. This study looks systematically what kind of cues the subject uses to follow gaze. It makes possible a direct comparison between the results of the apes and the 12 & 18 month old children due to a similar experimental setting, and extends the ape species to include western-lowland gorillas and bonobos. HYPOTHESES: 1. Apes follow the gaze of others and there will be no difference between the species.2. Children will follow the gaze of others from the age of 12 month.3. Children and apes use both head and eyes as cues to follow the gaze of others.

1. The results in Figures 1 and 2 show that 12 and 18 month old children and all ape species follow the gaze of others. They follow also the gaze in the “back of the

head” condition, which suggests that children and apes do not necessarily need facial features to do so. There are two possible conclusions.

a) the subjects followed the gaze geometrically using the head movement b) the subjects were able to understand the intention of the other2. In Figures 3 and 4 we can see, that children and apes use both head and eye

movement to follow the gaze of others. 3. As shown in Figures 5 and 6, there are no significant differences in the results of 12

and 18 month old children and also between the different ape species.4. The results in Figures 3 and 4 suggest that children tend to use the eyes as a cue to

follow the of others gaze rather than head movement, whereas apes tend to use head movement as cue more then the eyes.

B A C K G R O U N D P U R P O S E AND H Y P O T H E S E S

M E T H O D S

R E S U L T S

C O N C L U S I O N S REFERENCES

yes no yes no

0

10

20

30

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50

% M of reaction during attention

headeyes

gorilla

bonobo

chimpanzee

**

0

5

10

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25

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35

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45

Head Eyes

cue

% mean of reactions

yes

no

*

*

*

0

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60

eg vs. las hgeo vs.hgec bh vs. lod

conditions

% M of react. For attention

exp. cond.

contr. cond.

expcon

expcon

eg vs. las

hgeo vs. hgec

bh vs. lod

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*

*

*

* *

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1

2

2

3

3

4

ln % M reaction

12 month18 month

eg vs. las

hgeo vs. hgec

bh vs. lod

**

0

0.2

0.4

0.6

0.8

1

1.2

1.4

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head eyes

cue

ln % M reactions

yes

no

yes noyes no

12 month

**

* *

0

0.5

1

1.5

2

ln % M reaction

head

eyes

12 month

18 month

Figure 2: % means of reactions for children in the different conditions

- Butterworth G. & Cochran E. (1980) Towards a mechanism of joint visual attention in human infancy. International journal of behavioral development, 3, 253-272.

- Butterworth G., Jarett N. (1991) What minds have in common is space: spatial mechanism serving joint visual attention in infancy, Br J Dev Psychol, 9, 55-72.- Moore C., Angelopoulos M. & Bennett P. (1997) The role of movement in the development of joint visual attention, Infant behavior and development, 20, 83-92.- Povinelli D. J. & Eddy T. J. (1996) Chimpanzees: Joint visual attention. Psychological

science, 7, 129-135.- Scaife & Bruner (1975) 8 – 14 month old infants show gaze following, Nature, 253, 265-

266.- Tomasello M., Call J. & Hare B. (1998) Five primate species follow the visual gaze of

conspecifics. Animal Behavior, 55, 1063-1069.- Tomasello M., Hare B., Agnetta B. (1999) Chimpanzees follow gaze direction geometrically. Animal Behavior, 58, 769-777.

Each individual was tested separately in six conditions, which were designed to systematically examine the phenomenon- the 6 conditions were: - eyes gaze [eg] (only eye movement during the gaze) and look at subject[las] (just starring at subject) as control condition for eg - head gaze eyes open [hgeo] (both head and eye movement during the gaze) and head gaze eyes closed [hgec] (head movement with closed eyes) as control condition for hgeo - back of head [bh] (head movement with your back turned towards the subject) and look opposite direction [lod] (looking straight in the opposite direction of the subject) as control condition for bh- each condition was presented on different days and was counterbalanced within and between days - for the children the lod condition was not used, the other conditions were all presented counterbalanced on one day - each condition was presented 5 times and these trails lasted 10 sec.- the inter-trail-interval lasted 15 sec. and the interval between the conditions lasted 1 min. -the conditions in which eyes or head were involved were combined to create either the dimension head or eyes 

Table 1: dimension for gaze following and conditions involved

  eg las hgeo hgec

head no no yes yes

eyes yes no yes no

The result of an ANOVA for repeated measurement show that children and apes react in every experimental condition significantly more than in the corresponding control condition.

Figure 1: % means of reactions for apes in the different conditions

The results of an ANOVA for repeated measurements show significant differences for apes and for children between the two dimension (head and eyes) and the corresponding dimensions (no-head and no-eyes).

Figure 6: % means for the different conditions for the different ape species.

Figure 5: % means for the different conditions for both ages.

Figure 4: % means of reactions for the children for the factors involved in gazing

The results of a variance analysis show that both groups of children and all ape species follow the gaze of the experimenter significantly more, if the head or the eyes are involved in the gaze.

Figure 3: % means of reactions for the apes for the factors involved in gazing

ChildrenApes

SUBJECTS

- - experiments were done with 4 west-lowland gorillas - (gorilla gorilla), 4 Bonobos (pan paniscus) and 11 - Chimpanzees at the Wolfgang-Köhler Zentrum for Primate - Research and with 40 children at the MPI EVAN