Do orangutans (Pongo) possess an ocular semet, with a white sclera restricted to adult but not fully mature males?

Please see:

In various mammals, including primates and canids, a direct stare is an intimidating expression. It is a crude and in some sense 'archetypal' form of 'body language', requiring no particular accentuation by means of adaptive colouration.

However, more subtle facial expressions may, in certain mammals, be accentuated by dark/pale contrasts.

Humans are unique among the Hominidae in displaying the whites of the eyes as part of social communication.

In great apes (https://www.inaturalist.org/observations?place_id=any&taxon_id=43575&view=species) other than humans, the sclera of the eyeball (https://en.wikipedia.org/wiki/Sclera) is usually hidden (https://www.gettyimages.com.au/detail/photo/female-orangutan-portrait-royalty-free-image/101881466?phrase=orangutan+face+close+up&adppopup=true).

This is mainly because its degree of pigmentation matches the facial skin around the eyes (https://www.gettyimages.com.au/detail/photo/monkey-face-close-up-royalty-free-image/1293720869?phrase=orangutan+face+close+up&adppopup=true).

This difference between humans and apes is consistent with the unique development of language in Homo, testimony to the unique complexity of intraspecific communication in humans.

What has been previously overlooked in one group – the orangutans (Pongo, https://www.inaturalist.org/observations?place_id=any&taxon_id=43581&view=species) – is that the pigmentation of the sclera disappears in adult but not fully mature males.

Young adult males thus show the whites of the eyes (https://www.gettyimages.com.au/detail/photo/big-male-orangutan-at-zoo-negara-royalty-free-image/148598206?phrase=orangutan+face+close+up&adppopup=true and https://orangutan-world.com/bornean_orangutan_face_close-up/ and https://www.gettyimages.com.au/detail/photo/sumatran-orangutan-mature-male-halik-royalty-free-image/475005549?phrase=orangutan+face+close+up&adppopup=true and https://www.gettyimages.com.au/detail/photo/orangutan-royalty-free-image/170103740?phrase=orangutan+face+close+up&adppopup=true).

I suggest that this

  • constitutes a subtle version of an ocular semet, and
  • signals the direction of the gaze, thus
  • possibly revealing their intention/attentions/emotions, albeit in a subtle way.

Unlike young adult males, this ocular semet is hardly noticeable in

This ocular semet, if valid, is temporary. This is because, in full masculine maturity, the eyes are shaded, and appear small, among the fleshy folds on the greatly elaborated visage (https://www.gettyimages.com.au/detail/photo/portrait-of-orang-utan-sitting-on-land-royalty-free-image/1194404247?phrase=orangutan+face+close+up&adppopup=true and https://www.freepik.com/free-photo/borneo-orangutan-closeup-face-animal-closeup_21637681.htm and https://www.dreamstime.com/royalty-free-stock-photo-orangutan-image1407015 and https://www.dreamstime.com/stock-photo-orangutan-eye-contact-close-up-facial-shot-who-faced-camera-makes-us-image46223185 and https://www.gettyimages.com.au/detail/photo/orangutan-indonesia-south-east-asia-royalty-free-image/lpibn14842_02?phrase=orangutan+face+close+up&adppopup=true and https://www.gettyimages.com.au/detail/photo/bornean-orang-utan-shows-funny-expression-royalty-free-image/532710487?phrase=orangutan+face+close+up&adppopup=true).

The sclera of males seems to revert to the clear condition during a phase of life in which their political contests are most intense.

Juveniles, in some cases, have a different pattern of pale on the face, more broadly involving the facial skin (https://www.gettyimages.com.au/detail/photo/orangutan-royalty-free-image/WL003253?phrase=orangutan+face+close+up&adppopup=true and https://www.gettyimages.com.au/detail/photo/orangutan-mother-with-baby-royalty-free-image/6591-000817?phrase=orangutan+face+close+up&adppopup=true and https://www.gettyimages.com.au/detail/photo/sweet-moment-between-infant-and-mother-orangutan-royalty-free-image/98892006?phrase=orangutan+face+close+up&adppopup=true).

It is possible that no other species of primate shows similar ontogenetic complexity in the appearance of the whites of the eyes.

This is despite the fact that several species of macaques have minimal pigmentation of the sclera at all stages of development and resemble humans in displaying the whites of their eyes when glancing sideways. I refer to

The functions of eyewhite displays in macaques have yet to be studied.

Several species of macaques use sideways glances to solicit social support in intraspecific quarrels.

This suggests that the use of the eyes in the southern pig-tailed macaque - the species with the eyewhites most resembling those of humans – is less complex than in either humans or orangutans.

In their own way, orangutans seem second only to humans, in being the most subtle of primates in communicating by means of the eyes. This is despite what I assume to be an avoidance of direct stares on their part.

Please note that I have made no attempt to account for

Posted on June 14, 2022 06:00 AM by milewski milewski

Comments

Too many links

Posted by callicladium 3 months ago

Stop commenting the same thing

Posted by callicladium 3 months ago

@callicladium This is @milewski's journal and their post. They can do as they like as long as it doesn't violate iNat's Terms (which I don't see a violation of).
It might be best to edit the journal post to include the links though, instead of posting as comments, since their meaning is less clear.

Posted by cthawley 3 months ago

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