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Hiromi Kobayashi &
Shiro Kohshima
Biological Laboratory, Faculty
of Bioscience and
Biotechnology, Tokyo Institute
of Technology (c/o Faculty of
Science), 12-1, O-okayama
2-chome, Meguro-ku, Tokyo
152-8551, Japan. E-mail:
hiromi@innocent.com
Received 30 October 1998
Revision received
29 January 2001
and accepted 5 February
2001
Keywords:primates, eye
morphology, sclera colour,
communication, adaptation,
human evolution, theory of
mind.
Unique morphology of the human eye and
its adaptive meaning: comparative studies
on external morphology of the primate eye
In order to clarify the morphological uniqueness of the human eye
and to obtain cues to understanding its adaptive significance, we
compared the external morphology of the primate eye by measuring
nearly half of all extant primate species. The results clearly showed
exceptional features of the human eye: (1) the exposed white sclera is
void of any pigmentation, (2) humans possess the largest ratio of
exposed sclera in the eye outline, and (3) the eye outline is extraor-
dinarily elongated in the horizontal direction. The close correlation of
the parameters reflecting (2) and (3) with habitat type or body size of
the species examined suggested that these two features are adapta-
tions for extending the visual field by eyeball movement, especially in
the horizontal direction. Comparison of eye coloration and facial
coloration around the eye suggested that the dark coloration of
exposed sclera of nonhuman primates is an adaptation to camouflage
the gaze direction against other individuals and/or predators, and that
the white sclera of the human eye is an adaptation to enhance the gaze
signal. The uniqueness of human eye morphology among primates
illustrates the remarkable difference between human and other
primates in the ability to communicate using gaze signals.
2001 Academic Press
Journal of Human Evolution (2001) 40, 419–435
doi:10.1006/jhev.2001.0468
Available online at http://www.idealibrary.com on
Introduction
Recognizing others’ gaze direction is one of
the important cognitive bases for communi-
cation in humans (Gibson & Pick, 1963;
Kendon, 1967). To clarify the biological
basis of this ability, especially in relation to
the evolution of social intelligence,
researchers have experimentally examined
the cognitive ability to detect gaze direction
of others in nonhuman primates (Gomez,
1991;Itakura & Anderson, 1996;Tomasello
et al., 1998). However, little attention has
been given to external morphology of the
eye, although this ability of humans might
be supported by a unique morphology of
the human eye. For example, in humans, the
widely exposed white sclera (the white of the
eye) surrounding the darker coloured iris
makes it easy for others to discern the gaze
direction and has been said to be a charac-
teristic of humans not found in other pri-
mate species (Morris, 1985). However, this
has not been examined in detail, partly
because of the difficulty in measuring the
soft parts of living animals.
In this study, we measured the external
eye morphologies of nearly half of all extant
primate species with video camera and
computer-aided image analysing techniques
to clarify the morphological uniqueness of
the human eye and to understand adaptive
meanings of external eye morphology in
primates. The results clearly showed excep-
tional features of the human eye in both
Address correspondence to: Hiromi Kobayashi,
Ph.D., 6–8, Nakanoshima-cho, Fukakusa, Fushimi-ku,
Kyoto-city, Kyoto, 612-0049, Japan. Tel.: + 81 75 644
1402; Fax: + 81 75 644 1402.
0047–2484/01/050419+ 17$35.00/0 2001 Academic Press
shape and coloration. In our preceding
paper (Kobayashi & Kohshima, 1997), we
briefly reported the morphological unique-
ness of the human eye and discussed its
adaptive meanings. In the present paper we
fully analysed the results and examined the
following hypotheses on adaptive meanings
of primate eye morphology.
We measured width/height ratio of the eye
outline (WHR) and an index of exposed
sclera size in the eye outline (SSI) to analyse
eye shape. These eye-shape parameters
closely correlated with habitat type or body
size of the species examined. To explain the
correlation, we postulated a hypothesis that
these two features are adaptations for
extending the visual field by eyeball move-
ment, especially in the horizontal direction.
This hypothesis was examined and sup-
ported by analysing the eye movement of
video-recorded primates and comparing the
way that gaze direction changes among
species with various body sizes and habitat
types.
To explain the unique coloration of the
human eye with its exposed white sclera void
of any pigmentation, we postulated a
hypothesis that only coloration of the human
eye is adapted to enhance the gaze signal
while eye coloration of other primates is
adapted to camouflage the gaze direction
against other individuals and/or predators.
This hypothesis was examined and sup-
ported by analysing relationships among iris
coloration, sclera coloration and facial col-
oration around the eye. Our results sug-
gested that unique features of the human eye
started to evolve as adaptations to large body
size and terrestrial life and were completed
as a device for communication using gaze
signal.
Method
Eye shape measurements
A total of 874 adult animals (88 species:
Prosimii; 10, Ceboidea; 26, Cercopithe-
coidea; 43, Hominoidea; 9) were studied
(Table 1). Facial images of 80 species were
recorded by video camera at the Japan
Monkey Centre. Facial images of eight
species (Microcebus (1), Loris tardigradus (2),
Perodicticus potto (1), Tarsius (1), Saguinus
imperator (1), Pithecia monachus (1), Cacajao
rubicundus (1), Cercopithecus hamlyni (1))
were collected from books (Itani & Uehara,
1986;Yoshino, 1994). For humans, facial
images of 244 Japanese, 347 Caucasian and
68 Afro-Caribbean adults were studied. 244
Japanese, 280 Caucasian and 2 Afro-
Caribbean images were recorded by video
camera, and 67 Caucasian and 66 Afro-
Caribbean images were collected from
books (Ohara, 1970;Gomi, 1994).
Two parameters were measured for each
species: the width/height ratio of the eye
outline (WHR) and an index of exposed
sclera size in the eye outline (SSI). Frontal
full-face images without obvious facial
expression of subjects were recorded by
video camera. These images were processed
and analysed on a Macintosh Quadra
840AV computer using the public domain
NIH Image program. For each image, (A)
the distance between the corners of the eye,
(B) the longest perpendicular line between
the upper and lower eyelid, (C) width of the
exposed eyeball, and (D) diameter of the iris
were measured (Figure 1). WHR means
(A)/(B) and SSI means (C)/(D). Data of
weight, crown–rump length and habitat type
of primates were collected from books (Itani
& Uehara, 1986;Napier & Napier, 1985)
since we could not get permission for
physical contact with primates. Walking-
height and sitting-height of primates were
measured in the Japan Monkey Centre using
marks on the wall of the cages.
Eye-coloration measurements
Coloration of the exposed sclera (including
the conjunctiva to be precise), iris and face
around the eye was recorded for each of 91
species by direct observation of living
420 . .
animals (82 species) and of eyeball speci-
mens (55 species, 124 animals) kept in the
Japan Monkey Centre (Table 1). The sclera
colour included in the term ‘‘Pale brown’’
was a paler one than yellow ochre: 10YR
6/7·5 of Munsell Colour System (see
Figure 9).
Eye coloration of 82 primate species were
classified into 4 types (see Type 1–4, Figure
11) by the differences of colour or contrast
between sclera and iris/face. This classifica-
tion was carried out by one person observing
living animals. To check reliability of this
classification, another person independently
classified the face pictures of 76 primate
species (see Figure 11). The results agreed
in 70 species (92%). Disagreement was only
observed between Type 1 and Type 2 in 6
species (8%).
Eyeball specimens of the Japanese
macaque (1 subject) and crab-eating
macaque (2 subjects) were supplied from a
co-operative program of the Primate
Research Institute, Kyoto University,
Inuyama, Aichi, Japan. Eyeballs were fixed
with 4% paraformaldehyde in 0·1 M phos-
phate buffer (pH 7·2) at 4C overnight. The
tissue including the conjunctiva and cornea
separated from eyeballs was washed several
times in cold phosphate-buffered saline
(PBS), dehydrated in an ethanolic series
finishing xylene and embedded in paraffin.
Serial sections with a 4 m thickness were
cut with disposable blades, floated on water
and placed on slides. These sections were
deparaffinized in xylene, washed in ethanol
and PBS and studied by light microscopy
(see Figure 10).
Eye movement analysis
To analyse the movement of the eye and the
head when the animals change the direction
of their look, eating food by hand in cages
various primates were video-recorded. As
for humans, persons eating alone in a
hamburger shop with a hamburger in their
hands were video-recorded.
To calculate the ratio of scanning per-
formed only by eyeball movement, move-
ments of the eyeball and the head scanning
were counted (total observation time:
10,037 sec) for 29 individuals of 18 species
(Table 1).
To calculate the ratio of horizontal scan-
ning to vertical scanning frequency and
time duration of horizontal and vertical
scanning were measured (total observation
time=12,579 sec) for 40 individuals of 26
species (Table 1): arboreal species: Lemur
catta,Cebus apella,C. albifrons*, Pithecia
pithecia*, Ateles belzebuth*, A. geoffroyi,A.
paniscus,Cercocebus galeritus,Cercopithecus
cephus*, Colobus angolensis,Presbytis cristata,
P. vetulus,P. francoisi*, Nasalis larvatus,
Hylobates lar and H. pileatus; semi-arboreal
species: Macaca fuscata*, Cercocebus torqua-
tus,Mandrillus sphinx*, M. leucophaeus*,
Cercopithecus ascanius,Presbytis entellus and
Pan troglodytes; terrestrial species: Papio
hamadryas,Erythrocebus patas and Homo
sapiens (*: duration time only) (Table 1).
All these videotape analyses were carried
out by one person. To check reliability of the
analyses, a second person scored every
0·5 sec randomly sampled 20% of the video-
tapes independently. Agreement between
the persons was 82% on average. The
Cohen’s kappa (Bakeman & Gobbman,
1997) was 0·63 on average.
Results and discussion
Unique shape of the human eye
Figure 2(a) shows that human eyes have the
largest exposed sclera area and show
extraordinary horizontal elongation of the
eye outline among primates. SSI increased
in the following order: Prosimii (prim-
itive type) <Ceboidea<Cercopithecoidea<
Hominoidea. WHR increased in the follow-
ing order: Prosimii (primitive type)
<Ceboidea<Cercopithecoidea and Homi-
noidea [the difference between phylogenic
groups was significant with analysis of
421
Table 1A
Eye shape
[Figure 2(a)]
Eye colour
(Figure 9) Eye movement
Living animals
(Figure 11)
Eyeball
specimens (Figure 5)
(Figure 7—
time)
(Figure 7—
frequency)
Prosimii
Microcebus 13
Lemur catta 33 111
L. macaco 222
L. fulvus 22
Varecia variegata 361
Loris tardigradus 25
Perodicticus potto 12
Galago senegalensis 321
Otolemur crassicaudatus 122
Tarsius 12
Ceboidea
Callimico goeldii 121
Callithrix jacchus 332
C. argentata 22
C. humeralifer 22
G. geoffroyi 332
C. penicillata 21
Cebuella pygmaea 346
Saguinus midas 122
S. weddelli 11
S. imperator 111
S. labiatus 113
S. mystax 222
S. oedipus 143
Cebus capucinus 333
C. albifrons 1412
C. apella 23 111
C. nigrivittatus 1
Aotus trivirgatus 342
Callicebus moloch 252
Saimiri sciureus 2102
Pithecia pithecia 2211
P. monachus 1
Cacajao rubicundus 1
Alouatta caraya 116
Ateles paniscus 12 111
A. belzebuth 24 1
A. geoffroyi 352222
Lagothrix lagothricha 24
Cercopithecoidea
Macaca sylvanus 33
M. silenus 231
M. maurus 11
M. nemestrina 332
M. nigra 11
M. fascicularis 335
M. fuscata 671 1
M. fuscata yakui 332
M. mulatta 41817
422 . .
Table 1B
Eye shape
[Figure 2(a)]
Eye colour
(Figure 9) Eye movement
Living animals
(Figure 11)
Eyeball
specimens (Figure 5)
(Figure 7—
time)
(Figure 7—
frequency)
M. cyclopis 510
M. sinica 341
M. assamensis 111
M. radiata 291
M. thibetana 34
M. arctoides 12
Cercocebus torquatus 22 222
C. torquatus lunulatus 23
C. atys 111
C. galeritus agilis 111111
C. galeritus chrysogaster 23
C. albigena 1
Papio papio 332
P. anubis 12 50 2
P. cynocephalus 13
P. hamadryas 391111
Mandrillus sphinx 34 1
M. leucophaeus 241 2
Theropithecus gelada 111
Cercopithecus hamlyni 11
C. neglectus 33
C. mitis 25
C. ascanius schmidti 22
C. cephus 22 2
C. ascanius 23 222
C. mona 221
C. petaurista 111
C. aethiops 25
Miopithecus talapoin 22
Allenopithecus nigroviridis 13
Erythrocebus patas 44 222
Colobus angolensis 56 111
C. guereza 22
Presbytis cristata 287444
P. francoisi 241 1
P. pileata 111
P. vetulus 22 111
P. entellus 472111
P. obscura 2
Nasalis larvatus 22 222
Hominoidea
Hylobates lar 22 111
H. agilis 112
H. pileatus 22 111
H. syndactylus 22
H. klossii 1
Pongo pygmaeus 231
Pan troglodytes 9132333
P. paniscus 11
Gorilla gorilla 46
Homo sapiens 659 247 2 2 2
423
variance (ANOVA) (SSI: F (3,90)=14·68,
P<0·01, WHR: F (3,90)=32·77, P<0·01).
In multiple comparison (LSD), the differ-
ence between phylogenic groups, except
WHR between Cercopithecoidea and
Hominoidea, was significant (SSI: MSe=
0·02, P<0·01, WHR: MSe=0·04, P<0·01)].
Even in Hominoidea species, SSI and WHR
of human were exceptionally high [Figure
2(b): the difference between phylogenic
groups was significant with ANOVA (SSI: F
(4,675)=79·82, P<0·01, WHR: F (4,676)=
33·92, P<0·01). In multiple comparison
(LSD), the SSI difference between species
of Hominoidea, except between orang-utan
and gorilla and between orang-utan and
chimpanzee, was significant (MSe=0·02,
P<0·01). The WHR difference between
human and others was significant (MSe=
0·19, P<0·01). We measured both sexes of
three human races: Mongoloid, Caucasian
and Afro-Caribbean, and sexual and racial
difference were slight in these parameters
relative to interspecies difference.
It is possible that the difference in these
eye shape parameters reflect some difference
in visual function and/or adaptation to
some environmental or physiological factor
such as the habitat and body size of the
species. Thus, we analysed relationships
between the eye shape parameters and these
factors.
Relationship between eye shape and visual
function
Primates are mammals with well developed
visual function. Many primate species have
(1) cone cells for colour vision, (2) the fovea
(a dense concentration of cones in the retina
focused on the centre of gaze) for a high
resolution visual image, (3) forward-facing
eyes for wide stereoscopic vision by both
eyes, and (4) a well developed postorbital
plate behind the eyes. It is possible that the
difference in eye shape parameters among
phylogenic groups has some relationship
with these anatomical structures for well
developed visual functions. However, since
most primate groups except the prosimians
have all these anatomical structures, the
difference in eye shape parameters among
phylogenic groups cannot be explained by
the difference in these visual functions. For
example, some nocturnal prosimian species
are the only primates which lack both cone
cell and a fovea in their eyes (Wolin &
Massopust, 1970;Alfieri et al., 1976;
Webb & Kass, 1976;Debruyn et al., 1980;
Castenholtz, 1984). Figure 3 shows the re-
lationship of eye shape parameters with
orbital axis angle measured from the
cranium (Shigehara, 1996) which reflects
the ability of stereoscopic vision. In this
figure Prosimians with high values of orbital
axis angle showed low values of SSI and
WHR. However, among simians with more
forward-facing eyes and developed post-
orbital closure, SSI and WHR spread over
various values and no significant correlation
is observed. These facts suggest that the
variation in the eye shape parameters
of each phylogenic group does not reflect
evolutionary trends in these visual functions.
Relationship between body size and SSI
SSI correlates well with various body size
parameters (weight: r=0·59, P<0·001,
crown–rump length: r=0·59, P<0·001,
sitting height: r=0·65, P<0·001, walking
height: r=0·72, P<0·001). The best
Figure 1. (A): the distinace between the corners of the
eye, (B): the longest perpendicular line to (A) between
the upper eyelid and lower eyelid, (C): width of
the exposed eyeball, and (D): diameter of the iris
were measured. WHR means (A)/(B) and SSI means
(C)/(D).
424 . .
Figure 2. Variation of WHR (width/height ratio of the eye outline) and SSI (index of exposed sclera size
in the eye outline) among the phylogenic groups of primates (a), and in Hominoidea (b).
425
correlation was observed with walking
height (Figure 4). This means that species
with larger body size have a larger exposed
scleral area.
A larger SSI means a smaller iris relative
to the eye outline and probably a greater
ability for visual field extension by eyeball
movement; in eyes with a large SSI the small
iris has a wider space to move within the
open eye outline. In mammalian animals,
only primates have central foveae necessary
for fine vision. Therefore, eyeball and/or
head movement are important, especially for
primates, to adjust the images to the central
fovea.
If we suppose that a larger exposed sclera
is an adaptation for extending the visual field
by eyeball movement, the correlation
between SSI and body size can be explained
by the theory of scaling. This is because, as
body size becomes larger, visual field exten-
sion by eyeball movement becomes more
effective than that by head or body move-
ment. This is so because as body height
becomes greater, the weight of the head and
body increases proportionally to the cube of
Figure 3. Orbital axis angle and WHR/SSI. Orbital axis angle is the angle formed when right and left
orbital axes meet. Orbital axis is the line between the lowest point of the optic canal and the centre of the
orbital width (Shigehara, 1996). Circles, prosimians; squares, simians.
426 . .
body height. In contrast, the force required
for movement increases only with the square
of body height as it depends on the size of
the muscle cross-section. Moreover, since
the relative growth of the eyes to body height
is smaller than that of head and body size,
comparative eyeball size becomes smaller in
larger animals (Schultz, 1940). Thus, to
save energy when changing the direction of
gaze, a large-sized species would move the
eyeball more often than a small-sized
species, and have a larger exposed scleral
area. Besides, in small species with com-
paratively large eyeballs in a small skull,
space for muscles moving the eyeball may be
seriously limited.
To examine this hypothesis, we video-
recorded various primates (18 species, 29
individuals) eating food by hand in cages,
and counted the movements of head and
eyeball when they changed the direction in
which they were looking. The results
showed that the proportion of scanning per-
formed only by eyeball movement was cor-
related with SSI (Figure 5,r=0·73,
P<0·001). It was exceptionally high in
humans (6128% of horizontal scan, n=5)
compared with other primates (4·3–24·4%,
mean=10·6%). The highest proportion
in nonhuman primates was observed in
chimpanzees (20–35%, n=3), the largest
nonhuman species examined. These results
agreed with our hypothesis.
Relationship between habitat type and WHR
The mean value of WHR is greatest in
terrestrial species, moderate in semi-
arboreal species and lowest in arboreal
Figure 4. SSI and walking height.
427
species [Figure 6:Difference among habitat
types was significant with ANOVA and with
multiple comparison (ANOVA: F (2,127)=
24·63, P<0·01, LSD: MSe=0·052,
P<0·01)]. This result suggests that a hori-
zontally elongated eye outline is adaptive to
terrestrial life in some way. We speculated
that horizontal elongation of the eye outline
is adaptive in extending the visual field hori-
zontally by eyeball movement, and terres-
trial life needs more horizontal scanning
than vertical scanning.
To examine this hypothesis, we observed
various primates eating food by hand in
cages and measured the time and frequency
of horizontal scanning and vertical scanning.
Figure 5. SSI and eye movement. EyeMove/TotalMove= frequency of gaze direction change only by eye
movement/frequency of all gaze direction change by head movement or/and eye movement.
Figure 6. WHR and habitat type.
428 . .
The result shows that the ratio of horizontal
scanning to vertical scanning is higher in
terrestrial species than in arboreal species
[Figure 7(a): the difference among habitat
types was significant with ANOVA (time: F
(2,23)=14·5, P<0·01, frequency: F (2,15)=
4·53, P<0·05). The difference between
terrestrial species and arboreal ones was
significant in multiple comparison (LSD)
(time: MSe=20·76, P<0·01, frequency:
MSe=20·05, P<0·01)]. These ratios were
also correlated with WHR [Figure 7(b),
time: r=0·74, P<0·001, frequency: r=0·88,
P<0·001]. The results support our hypoth-
esis. These investigations suggest that the
shape of the eye outline and relative size of
the exposed scleral area are the result of an
adaptation for visual field extension by
eyeball movement.
Unique coloration of the human eye
The colour of exposed sclera. The following
four colorations of exposed sclera were
observed (Figures 8 and 9); (a) in almost all
nonhuman primates (85 species out of 92
species, or 92%) the exposed part of the
sclera is uniformly brown or dark brown, (b)
Macaca sylvanus and M. nemestrina with a
pale brown body colour had sclera coloured
pale brown, (c) Saguinus midas,S. labiatus,
Callithrix argentata and Callimico goeldii had
brown sclera with a white part in corner of
the eye, (d) humans were the only primates
Figure 7. (a) Habitat type and the ratio of horizontal scanning to vertical scanning. (b) Ratio of horizontal
scanning to vertical scanning and WHR.
429
having white sclera without any pigmenta-
tion. Microscopic analysis of the eyeball
section specimens from the Japanese
macaque and crab-eating macaque revealed
Figure 8. Three types of sclera coloration in nonhuman
primates.
Figure 9. Variation of scleral colour. The shaded portion of the nonhuman primate eyeballs shows the
general area where colour was noted. The solid line surrounding the cornea represents the eye outline.
Figure 10. Eyeball section of crab-eating macaque.
430 . .
that the brown coloration of the exposed
sclera was due to pigmentation in the epi-
thelium cornea, conjunctiva and sclera
(Figure 10). External observations of other
nonhuman primate eyes suggest that their
dark coloration is also due to similar pig-
mentation. Humans have transparent
conjunctiva and white sclera without pig-
mentation. The inner part of the sclera in
nonhuman primates was also white like that
of humans.
Adaptive meaning of dark-coloured sclera.
Nonhuman primates have sclera coloured
brown. As pigmentation costs some energy,
the dark coloration of the exposed sclera
probably has some adaptive function. Brown
coloration of the exposed sclera was
observed in many other mammal species
too, and the following two hypotheses have
been proposed on the adaptive meanings of
sclera colour. (1) Anti-glare theory: it was
pointed out that the pigmentation may be an
anti-glare device because it seemed to be
absent in nocturnal or crepuscular species
(Duke-Elder, 1985). However, our results
on primate species were contrary to this
expectation: nocturnal species (Galago sen-
egalensis,Tarsius syrichta,Perodicticus potto,
Nycticebus coucang and Aotus trivirgatus) also
had coloured sclera and diurnal humans had
no pigmentation. Therefore, our results
cannot be explained by the ‘‘anti-glare
theory’’. (2) Gaze camouflage theory: in
many nonhuman primates, gaze direction is
important in intraspecific communication
(Chance, 1962;van Hooff, 1962;Andrew,
1964). For example, direct eye contact is
associated with gestures predominantly
showing a tendency to attack in many
monkeys. In macaques, it is reported
that sclera pigmentation obscures gaze
direction and may be adaptive for escaping
the attacks of other individuals (Perrett &
Mistlin, 1990). Coloured sclera obscuring
gaze direction may serve to deceive
natural predators too, by making it difficult
for predators to know if the prey has
them in their gaze. If prey animals can
make it known to the predator that
they already know of its presence, their
chances of survival may increase (Sherman,
1977).
To examine the gaze camouflage theory,
we analysed the relationship between sclera
colour, iris colour and face colour around
the eye. If the dark coloration of exposed
sclera is adaptation for gaze camouflage, the
colour of exposed sclera should be similar to
the colour of iris and/or face around the
eyes, to make it difficult to detect the pos-
ition of iris in the eye outline and/or the eye
outline in the face.
Relationship between sclera colour, iris colour
and face colour
Figure 11 shows the relationship between
sclera colour, iris colour and face colour
around the eye. 82 primate species observed
were classified into the following four types
by the difference of colour or contrast
between sclera and iris/face.
Type (1) faceYscleraYiris (43 species):
darkness of sclera colour is similar to that
of face and iris; both eye outline in the face
and iris position in the eye outline are
unclear.
Type (2) face<scleraYiris (37 species):
sclera colour is darker than face colour but
sclera colour is similar to iris colour; eye
outline in the face is clear but iris position in
the eye outline is unclear.
Type (3) faceYsclera>iris (1 species,
ruffed lemur): sclera colour is darker than
iris colour but sclera colour is similar
to face colour; eye outline in the face is
unclear but iris position in the eye outline is
clear.
Type (4) face>sclera<iris (1 species, Homo
sapiens): sclera colour is paler than face
colour and iris colour; both eye outline in
the face and iris position in the eye outline
are clear.
431
Figure 11. Colour patterns of sclera, iris and face skin around the eye.
Figure 12. Difference of gaze stimulus between human and orang-utan.
Almost all nonhuman primate species
observed (80 out of 81 species) belonged to
Type 1 or Type 2 coloration. In these
coloration types, the position of the iris
in the eye outline was unclear because of
similarity between sclera colour and
iris colour (‘‘Gaze camouflage type’’). In
Type 1 coloration (43 species), the position
of the eye outline in the face was also
unclear because of similarity between
sclera colour and face colour around the
eyes. In addition, the ruffed lemur (Varecia
variegata), the only species that had
Type 3 coloration, has a very small exposed
sclera area (SSI=1·08) and almost all the
area of its eye outline is occupied by
the iris. Thus, the ruffed lemur’s eye also
can be seen as a ‘‘gaze camouflage
type’’. The results thus support the ‘‘gaze
camouflage theory’’.
In contrast, humans were the only species
that had Type 4 coloration, in which both
eye outline in the face and iris position in
the eye outline were very clear because the
colour of the exposed sclera is paler than
that of the facial skin and iris (‘‘gaze
signalling type’’). The human was the
only species with sclera much paler than
the facial skin. Because of this coloration,
it is very easy to discern the gaze direction
in human, in contrast to the gaze-
camouflaging eyes of the other primates.
Figure 12 shows the contrast between sclera
colour and face/iris colour of human and
orang-utan (Pongo pygmaeus). In this figure,
darkness of colours was shown by 256 steps
grey scale number (white=0, black=255,
Figure 12). Even great apes that have SSI
and WHR near that of humans [Figure
2(b)] had ‘‘gaze camouflage eye’’ type with
brown sclera, brown facial skin and brown
iris, in which the eye position and iris pos-
ition is unclear. In contrast to the gaze-
camouflaging eyes of orang-utan, the human
sclera is remarkably paler than the facial skin
and the iris, and it is very easy to discern the
gaze direction.
Evolution of unique morphology of the human
eye
Our results suggest that the unique shape of
human eye (largest SSI and WHR in pri-
mates) is a result of adaptations to extend
the visual field by eyeball movement,
especially in the horizontal direction. In
human evolution, the ratio of exposed sclera
in the eye outline might increase because the
visual field extension by eyeball movement
became more effective as body size
increased. And the eye outline might be
horizontally elongated because the terrestrial
life of humans needed more horizontal
scanning than vertical scanning by eyeball
movement.
Why have only humans discarded sclera
pigmentation? It may be because the neces-
sity for gaze camouflage decreased and that
of gaze-signal enhancement increased in
human evolution. The predation risk might
decrease because of the enlarged body size
and the use of tools and fire. Gaze-signal
enhancement might aid the conspecific
communication required for increased
co-operative and mutualistic behaviours to
allow group hunting and scavenging.
Co-operative and mutualistic behaviours
might need refined communication systems,
such as language, to inform one’s intention
to other members of the group. The human
eye, with a large scleral area surrounding the
iris and a great ability of eyeball movement,
would have provided a chance for a drastic
change from ‘‘the gaze-camouflaging eyes’’
into ‘‘the gaze-signalling eyes’’ through a
small change in scleral coloration. The SSI
and WHR of human eyes are even greater
than those of gorillas, the largest primate,
which suggests adaptation for gaze-signal
enhancement.
Eye morphology and gaze-signal
communication
Baron-Cohen (1995) postulated a neural
mechanism (eye direction detector) in
human brains specialized to detect others’
433
eye direction, and discussed the possibility
that such a mechanism might be related with
the evolution of a ‘‘theory of mind’’.In
recent years, many studies have been carried
out to examine the cognitive ability to detect
others’gaze direction in various nonhuman
primates. The studies have suggested the
limited ability to detect others’gaze direc-
tion in nonhuman primates (Itakura &
Anderson, 1996;Tomasello et al., 1998).
However, there seems to be some confusion
in defining ‘‘gaze direction’’ in these studies.
Since gaze direction can be changed by
eyeball movement, by head movement and
by body movement, it should be defined by
considering all those factors: eyeball direc-
tion, head direction and body direction.
Most of these studies, however, defined
‘‘gaze direction’’ mainly by eyeball direction.
Our results (Figure 5) suggest that the con-
tribution of eyeball movement to the change
in gaze direction is extremely high in
humans compared with other primate
species. The gaze signalling eye coloration of
humans also suggests that the contribution
of eyeball direction to the gaze signal is
exceptionally high in humans. Therefore, for
nonhuman primates, head direction and
body direction might be more important to
detect others’gaze direction than eyeball
direction. We should pay more attention to
head direction and body direction in future
analyses of gaze-signals in nonhuman
primates.
Summary
Comparative analysis of the external mor-
phology of the primate eye revealed excep-
tional features of the human eye: (1) the
exposed white sclera is void of any pigmen-
tation, (2) humans possess the largest ratio
of exposed sclera in the eye outline, and (3)
the eye outline is extraordinarily elongated
in the horizontal direction. The close corre-
lation of the parameters reflecting (2) and
(3) with habitat type or body size of the
species examined suggested that these two
features are adaptations for extending
the visual field by eyeball movement,
especially in the horizontal direction.
Comparison of eye coloration and facial
coloration around the eye suggested that
the dark coloration of exposed sclera of
nonhuman primates is an adaptation to
camouflage the gaze direction against other
individuals and/or predators, and that the
white sclera of humans is an adaptation to
enhance the gaze signal.
Acknowledgements
We would like to thank Shigetaka Kodera
for his interest in our study and for
allowing us to observe the animals of the
Japan Monkey Centre. It is a pleasure
to acknowledge the hospitality and encour-
agement of the members of JMC. We
wish to express our gratitude to Tetsuro
Matsuzawa, Takashi Kageyama and
Nobuo Shigehara for stimulating discussion
under the co-operative research programme
in Primate Research Institute, Kyoto
University, and to Manabu Ogiso,
Nobuyuki Saitoh and Teruhiko Hamanaka
for providing eyeball samples. We are
indebted to a number of our colleagues at
the Tokyo Institute of Technology and
the Primate Research Institute, Kyoto
University, especially to Mitsue Nomura,
Michael A. Huffman, Sou Kanazawa,
Masami Yamaguchi and Kazuhide
Hashiya, for their constructive criticism on
this paper.
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