How do baboons use their eyes in social communication?

(writing in progress)
I have chosen the photos below, to illustrate the patterns in one species, the olive baboon (Papio anubis, and The chacma baboon (P. ursinus), not shown here, is similar.
Baboons differ from humans in that they generally hide their eye movements.

Humans reveal our direction of gaze, and we can read these movements by virtue of the shifting proportions of dark and pale in an eyeball exposing the whitish sclera. The system is different in baboons, which do use their eyes to communicate, but in a crude way and mainly by means of the closed eyelids, something missing in the repertoire of humans.

Humans read another’s intentions with great subtlety by means of eye movements. Baboons are morphologically adapted not to do so. Humans share information by means of their eyes; baboons withhold information by means of their eyes.
Baboons differ from macaques (Macaca spp.), which do accentuate the gaze by means of scleral tone, albeit in a simper way than in humans and only for restricted messages.

Although baboons and macaques are convergent in that both include the most terrestrial of monkeys on their continents, they are not convergent in the use of their eyes for social communication. Baboons have specialised on eye-hiding while macaques have specialised on eye-display (with more variation within the genus Macaca than within Papio/Theropithecus/Mandrillus.
From a human point of view, the eyes of baboons seem inscrutable/obscure/beady/shadowed. In another Post, I show photos of Papio hamadryas (which has flesh-coloured facial skin) and Theropithecus gelada (which has an extraordinary lip-flipping display). Although the various genera and species of baboons differ greatly in various ways, they all seem to have the same approach to the display of the eyes: all hide their eye movements as much as possible, presumably to avoid divulging their intentions.
Please follow the captions below, which use selected photos to convey the limited complexity in the appearance of the eyes in adults and juveniles (but not infants) of the olive baboon.
The following photo is unusual in showing the irides brightly against the dark facial skin. Usually the eyes are harder to see than this. And even in this view, please note the lack of any visible sclera, the part of the eyeball that is so conspicuously whitish in all humans, including black-skinned human races.

The following close-up of the eyes shows that the sclera of the olive baboon is actually dark-pigmented, more or less matching the darkness of the facial skin. This effectively hides eye movements by reducing tonal contrast. The same is true for chimpanzees and gorillas.

The following again shows that the sclera is dark in the olive baboon, where it would be pale in all humans. Although the direction in which this individual is looking is clear in such close-up, well-illuminated view, in normal views it would be hard for one baboon to see exactly where another is looking.
The following photo shows that, although the irides are somewhat brighter-hued than the facial skin, the tone (in terms of dark vs pale) is similar to the facial skin, and in the absence of exposure of a white sclera the whole eye is rather hidden.
The following photo shows that, although the upper eyelids are pale (grey) in the olive baboon, this is not obvious when this surface is shaded, which it tends to be owing to the brow-ridges.
The following photo shows that the upper eyelids are rather pale in the olive baboon, but this is not normally conspicuous (in contrast to e.g. the long-tailed macaque Macaca fascicularis).

The following photo shows that in intraspecific confrontation the olive baboon does not stare, but instead partly closes the eyes, signalling by means of its pale upper eyelids. Although this is common among monkey spp., baboons have reduced the repertoire of eye displays to this display alone, which is a crude system of signalling.

The following photo is revealing because it shows that the olive baboon can do something with its brows beyond what humans can do: it can contract the brows far back above the eyes, stretching the pale upper eyelids to the point that they actually cover the brow-ridges, greatly expanding the pale flash. This was even more spectacularly illustrated in my email on the gelada Theropithecus, which really goes in for this ‘pale eyebrow’ display in conjunction with lip-flipping. Spectacular but still a relatively crude signal.

The following photo shows that the pale of the upper eyelids is not normally displayed when the animal raises its eyes to survey the sky or the treetops.

The following photo shows that the passive-aggressive fang-baring display by males of the olive baboon involves shutting the eyes to show the pale upper eyelids. This is, in a sense, a ‘false-stare’, but the meaning it conveys is simple and crude.
The following photo shows that the pale upper eyelids in the olive baboon are found in all age/sex classes beyond infancy. and and

The following photo of the olive baboon in Uganda shows unusually extensive pale upper eyelids. This is perhaps a geographical variation? However, once again the shading by the brow-ridges ensures that the pale is not conspicuous unless the eyes are closed.

The following photo shows once again that the visible part of the sclera, in a sideways glance, is dark enough in its pigmentation to hide the direction of gaze. This is not true in various spp. of Macaca, in which there is a panel of whitish that accentuates a sideways glance/gaze.
The following photo, and its enlargement, show how even an extreme sideways glance is hidden in the olive baboon where it would be obvious in humans and certain species of macaques. This is because of the pigmentation of the sclera adjacent to the iris. Chimpanzees and gorillas show the same pattern. At any distance, an observer cannot easily see the direction of movement of the eyes.,_2009).jpg
The following photo shows that, in some individuals of the olive baboon, there is a macaque-like exposure of a pale surface of the sclera. This is exceptional in the olive baboon although normal in some spp. of macaques.

The following ‘toddler’ of the olive baboon still has its unpigmented sclera (as at birth), because the scleral pigmentation sets in only when the distinctively infantile black-and-pink colouration of the face is lost.

The eyelid/brow is also used in courtship – in chacmas at least - where males and females use rapid blinking in the direction of a potential mate.

(writing in progress)

Publicado el julio 1, 2022 10:19 MAÑANA por milewski milewski


Publicado por milewski hace alrededor de 2 años

In the chacma baboon, there is a subtle patch of pale fur on the dark muzzle distal from the eyes, which almost gives the animal a ‘four-eyed’ look. I suspect that this is part of the syndrome in which baboons avoid gazing directly at each other. It strikes me as odd that all baboons have especially pale upper eyelids, which are flashed as a way of emphasising the message ‘I’m looking at you’, and this species has additional pale patches of similar size on the muzzle that likewise might emphasise the direction of gaze.

It strikes me that, whereas hamadryas, and to a lesser degree anubis and guinea, baboons use a facial ruff to accentuate the orientation of the head (and thus allow others to establish the direction of gaze by looking at the gazing animal only furtively with peripheral vision), in the chacma baboon the combination is different: instead of a facial ruff there are these pale ‘spots’ on the muzzle.

I suspect that this is an original thought: that the pale patches/’spots’ of fur on the otherwise bare, dark muzzle in the chacma baboon (which has a muzzle not as horizontal in orientation as is true for hamadryas, guinea and anubis baboons) function in a way analogous to the facial ruffs of other species, in facilitating the detection of direction of gaze without direct full-frontal observation.

Publicado por milewski hace alrededor de 2 años

Here I examine the scleral pigmentation in two species of baboons, namely the gelada (Theropithecus gelada) and the hamadryas (Papio hamadryas).
In both species, the colouration of the eyes is somewhat nebulous, because the iris is a medium brown and the sclera tends also to be brown. The facial skin is paler in P. hamadryas than in T. gelada, but in both cases the dark/pale contrast between face and eyes is minor – except when T. gelada produces a conspicuous flash of pale (flesh-coloured skin) above and below the eyes by a stretching of the skin, presumably by facial muscles with no equivalent in humans.

In the gelada, the main display of the face is the flipping of the upper lip to reveal the flesh-coloured gums. Instead of the eyes themselves being accentuated, the skin above and below the eyes is accentuated in parallel with the mouth display.   

The gelada has some pigmentation on the sclera, but this is no darker than brown, not reaching dark brown/black as in the bonnet macaque. In the bonnet macaque, the scleral pigmentation is much darker than the iris, whereas in the gelada the scleral pigmentation is similar to that of the iris.
The following photo similarly shows a pale sclera. The flesh-coloured skin above and below the eyes is inconsistent with the two photos above. I suspect that this is the result of a facial expression in which this skin is stretched, revealing pale skin that is usually largely hidden.
The following shows a rather pale sclera and the pale (flesh-coloured skin) around the eyes.
In the following, the pale skin around the eyes is hidden and there is not enough tonal contrast between iris and sclera to make the eyes conspicuous.
In the following, the expression is such that the eyes (which in this case have a brown sclera of a similar colour to the iris) show no dark/pale contrast.

In the following, once again the eyes virtually disappear into the face owing to a lack of dark/pale contrast.

In the following, we see that the upper eyelids are flesh-coloured.

The following shows that it is not only the upper lip that can be stretched to the point of being turned inside out; the eyebrows show a similar effect.
The following sequence shows how this occurs.
The following shows that the facial display is not restricted to males.

Now we move to the hamadryas baboon.
The following shows that the eyes in Papio hamadryas are similar to those in the gelada. As in the barbary macaque, the iris is darker in the infant than in adults.
In the following, the pale in the corner of the eye is probably reflection of the light from the shiny surface, and not pale sclera.
The following shows how similar the scleral pigmentation is to that in the gelada.
The following shows that in some individuals the sclera is relatively lacking in pigmentation, much as we saw in the case of the gelada.

The following once again shows a relatively pale sclera.

And the following likewise.

Overall, the following is typical of P. hamadryas: there is scleral pigmentation but this is merely brown (not black) and of a similar tone to the iris, so that the eyes are not conspicuous in P. hamadryas.

Publicado por milewski hace alrededor de 2 años

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