Ocular displays (starting with humans) and what we should call them, part 1

@paradoxornithidae @tonyrebelo @jeremygilmore @dinofelis @botswanabugs @maxallen

Everyone knows that, in the human species, the movements of the eyes can be expressive enough to outweigh the words spoken, e.g. when lies are being told.

And that the sclera (https://en.wikipedia.org/wiki/Sclera), i.e. the white of the eye, accentuates the slight movements of the eyeballs, as if to spell out the unspoken messages in a rapid series of triangular flickers.

And many may have noticed that the selective breeding of the domestic dog has inadvertently made canine eyes more human-like in their expressions of emotion, partly by exposing the sclera (https://abc13.com/dog-study-can-dogs-communicate-with-humans-eyes-intelligence/5351460/ and https://www.jsonline.com/story/news/2019/08/19/puppy-dog-eyes-dogs-evolved-eyebrow-muscle-help-bond-humans/1867592001/ and https://www.wildculture.com/article/urban-rehab-humans-animals/1434).

What is less-known is that our closest relatives among the primates are not only unlike us in this way, they are in some sense the antithesis.

Apes have a sclera so pigmented that it seems adapted actually to hide intentions and emotions, keeping the eyes inscrutable (scroll in https://www.americanscientist.org/article/do-the-eyes-have-it and https://www.researchgate.net/figure/Human-eyes-vs-chimpanzee-eyes_fig1_284308905 and https://www.nbcnews.com/id/wbna15625720 and https://www.facebook.com/CenterForGreatApes/posts/natsu-has-her-eyes-on-the-long-weekend-friyay-weekendvibes/10155916830732331/ and https://www.rgbstock.com/photo/r1OJLPk/The+Chimpanzee and https://www.sciencedirect.com/science/article/abs/pii/S1090513815000641).

What this means is:

  • There is a 'default setting', in which the eyes of mammals have a somewhat visible pale sclera, mainly covered by the eyelids, and with little function in communication.
  • This has been adaptively emphasised in humans and a few other species, to elucidate expression and promote communication.
  • However, it has been adaptively modified in the opposite direction in other, closely related species (including all apes), to obfuscate expression and to withhold communication.

Those large mammals that we humans tend to regard as rather expressionless may have analogous systems operating about their ears, which are far more mobile than human ears, and more relevant to the sensory priorities of the species involved (https://i.kym-cdn.com/photos/images/original/001/363/236/ba2.jpg).

In this initial Post, I illustrate some of these points, helping to put our human eye-white displays into a broader biological context.

In later Posts, I will propose new technical terms for the features of anatomy and adaptive colouration involved in communication by means of 'eye language'.

And, after these digressions, I will eventually return to the subjects of my most recent Posts, namely the felids, to listen to their ear-language with new eyes.

The following show how pigmented the sclera is in chimpanzees and gorillas, as if to achieve the opposite of the facilitation seen in humans: https://www.smithsonianmag.com/science-nature/bonobos-teach-humans-about-nature-language-180975191/ and https://www.naturalworldsafaris.com/holidays/africa/congo/the-ultimate-gorilla-safari.

The following show the difference between the wolf, in which the sclera is tightly covered by the eyelids, and the domestic dog, in which the sclera is exposed:

The following show how much more of the sclera is visible in hyenas than in the wolf or most wild canids: https://www.flickr.com/photos/tambako/12796302043 and https://www.istockphoto.com/video/brown-hyena-being-harassed-by-jackal-gm1138822949-304180773.

The following show that the cheetah, unusually for felids, displays the sclera in fear: https://www.photosincolor.com/wildlife-photographer-captures-amazing-photos-of-deadly-cheetah/.

to be continued in https://www.inaturalist.org/posts/54893-ocular-displays-and-what-we-should-call-them-part-2#...

Posted on August 02, 2021 03:09 AM by milewski milewski


Posted by milewski almost 3 years ago

The movements of the eyeballs of Homo sapiens are 'scrutable' (https://www.merriam-webster.com/dictionary/scrutable and https://dictionary.cambridge.org/dictionary/english/scrutable), whereas those of other hominids tend to be 'inscrutable'.

This is mainly because the sclera is white in H. sapiens, but pigmented in other hominids. The main element in the ocular semet is the paleness of the sclera.

Posted by milewski 10 days ago

Ocular display in Canis lupus, consisting of dark/pale contrast between eyelids (dark) and iris (pale):




This too may constitute an ocular semet, communicating threat via staring.

However, a) the effect is baleful (https://www.merriam-webster.com/dictionary/baleful), rather than communicative in a complex way, and b) the semet is not scleral, because in C. lupus the sclera is largely hidden by the tightness of the eyelids.

The following shows how little of the sclera is visible in the wolf (please bear in mind that virtually all populations have been inadvertently hybridised with the domestic dog): https://www.inaturalist.org/observations/48955.

Posted by milewski 10 days ago

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