Why do baboons have long muzzles?

@tonyrebelo @dejong @jeremygilmore @ludwig_muller @alexanderr @beartracker

When one thinks of baboons in even the most cursory or superficial way, something that stands out is their long muzzles (https://www.alamy.com/profile-portrait-of-baboon-papio-on-the-green-background-image150562437.html?imageid=BB6F778D-228C-44C4-945F-145C2737138D&p=100776&pn=1&searchId=7dd3a4a6119b57395b529ee64a66712f&searchtype=0 and https://www.alamy.com/chacma-baboon-papio-ursinus-adult-kruger-national-park-south-africa-image65346120.html?imageid=FD6E9040-BD80-4357-B80B-39B76FBE0943&p=642197&pn=1&searchId=7dd3a4a6119b57395b529ee64a66712f&searchtype=0 and https://www.alamy.com/stock-photo-female-yellow-baboon-papio-cynocephalus-and-young-on-a-fallen-tree-93105945.html?imageid=B2B71921-BD0B-4141-B4CF-78E6D269A6C7&p=277722&pn=1&searchId=7dd3a4a6119b57395b529ee64a66712f&searchtype=0).

Baboons (in the loose sense, including Mandrillus, https://en.wikipedia.org/wiki/Mandrillus and https://es.123rf.com/photo_5612584_an-african-male-mandrill-baboon-in-profile.html) are the only 'higher primates' with long muzzles.
 
What is the adaptive value of long muzzles in baboons?
 
One possible answer is the deployment of the extremely-developed canines, for self-defence against predators. This hardly makes sense, because

Another possible answer is for foraging.

Canids have long muzzles to facilitate biting prey, and ungulates have long muzzles to reach into foliage and to compensate for the length of the limbs, which tends to distance the mouth from the herbaceous stratum. Bears have long muzzles for both reasons.

However, there is nothing about the foraging methods of baboons that makes it adaptive to use the muzzle to reach into anything. Instead it is the hands that are used to bring food to the mouth.
 
A third possible answer is for chewing.

It is noteworthy that, unlike most other mammals with long muzzles, baboons lack a diastema (https://en.wikipedia.org/wiki/Diastema and https://www.reddit.com/r/natureismetal/comments/de8u59/the_skull_of_a_baboon_i_like_to_this_this_is_how/), the toothrows instead being continuous from front to back. So, it is possible that baboons eat food so hard or fibrous that a particularly long row of cheek-teeth is needed to chew this food sufficiently.

Baboons do indeed eat grass - a fibrous and abrasive food - to a greater extent than any other primates, and their dentition is also tough and massive enough rapidly to crack hard objects such as acacia seeds.

Perhaps a fairly satisfactory explanation emerges if one combines the chewing by both sexes with the deployment of canines by males.

However, detracting from this is the fact that the gelada (Theropithecus, https://www.flickr.com/photos/helenehoffman/45441924365) – which is the species most specialised for grazing – has a shorter, not longer, muzzle than those of baboons (including Mandrillus). Also, grazing and seed-cracking would hardly explain why the muzzle is sexually dimorphic, being noticeably longer in males than in females in baboons and Mandrillus. The two sexes, after all, have similar diets.
 
To summarise the explanation so far:
Baboons (including Mandrillus and Theropithecus) have long muzzles mainly because they rely on tough, fibrous foods, and this length is further extended in males because of the use of the canines - which is mainly intraspecific, for masculine rivalry, and includes ‘fencing’ with the canines as a mode of fighting.
 
However:
 
I have pointed out, in previous Posts (https://www.inaturalist.org/journal/milewski/67780-how-do-baboons-use-their-eyes-in-social-communication#), that baboons have hidden eyes, particularly compared with humans.

Whereas humans display the eyes in various ways (eyebrow hair, extensive exposure of the sclera, paleness of the sclera in contrast to either iris colour or skin colour, or both), baboons obscure the gaze by means of pigmentation of the sclera and shading of the orbits by superciliary ridges. Instead of displaying the eyeballs themselves, baboons display the pale upper eyelids by blinking (this can be either friendly or antagonistic depending on context).
 
The crucial observation is that, the hiding of the eyes notwithstanding, the direction of gaze is a powerful factor in social interactions in baboons.

See Fagot and Deruelle (2002, https://pubmed.ncbi.nlm.nih.gov/12136705/ and https://www.researchgate.net/publication/11244626_Perception_of_pictorial_gaze_by_baboons_Papio_papio and https://www.semanticscholar.org/paper/Perception-of-pictorial-eye-gaze-by-baboons-(Papio-Fagot-Deruelle/4d76d913856a9c7504a07213d246e335e0eca6d1).

Baboons are extremely sensitive to being gazed at directly, and take this as a sign of aggression. The eyes seem to be adapted to be inconspicuous (apart from the displaying of the closed eyelids). And a feature of baboons is that they remain extremely observant of each other as individuals, without looking at each other directly; they have extremely attentive peripheral vision and manage to observe each other directly by means of furtive and fleeting glances rather than outright staring.

For baboons it is crucial to keep an eye on each other socially while not looking at each other directly as we humans normally do. Baboons want the information but direct gazing is taken as offensive.
 
Hans Kummer (https://de.wikipedia.org/wiki/Hans_Kummer) relates his own experience, which is fascinating in the insight it gives into the psychology of baboons.

Kummer was studying the hamadryas baboon (Papio hamadryas, https://en.wikipedia.org/wiki/Hamadryas_baboon) in remote Ethiopia, by sitting right in among the habituated animals. He was sometimes at risk of being attacked by mature males as the result of some misunderstanding in which he became accidentally implicated in some social faux pas. What he observed was that if one pretended not to notice the angry stare of the mature male individual in question, and its fang-baring, eyelid-flashing yawns, and just looked somewhere else in a relaxed way while busying oneself normally, this would invariably ‘switch off’ the aggression. The masculine anger of the hamadryas baboon would be thus appeased.

The psychology of baboons seems odd to us, because honesty is punished. The fact that Kummer’s ignorance/naivety was completely a pretence was obviously far less provocative to the angry baboon that it would have been for Kummer simply to look back in a questioning and friendly way in a peacemaking attempt - which would have provoked attack. Baboons value feigned innocence more than they value honest and empathetic interaction, and the biology of the eyes reflects this amoral value-system (see https://www.inaturalist.org/journal/milewski/67769-is-it-baboons-rather-than-chimps-that-really-epitomise-machiavellian-intelligence#).
 
This is where the long muzzle may matter.

It occurs to me that the sheer length of the muzzle gives this part of the face considerable value as a pointer to the direction of gaze. All that one individual has to do to see where the other is looking is to glance quickly sideways at how its whole face is oriented, something that is so obvious from its long muzzle that the information can be gleaned even by peripheral vision. So, in a sense we can see that the long muzzle of baboons goes with, i.e. is consistent with, the hiding of the eyeballs by means of dark sclera and shading by the ridge between forehead and orbits.

No baboon wants to look much at the eyes of its kind, because this is usually a fearful experience. However, every baboon wants to know where important individuals are looking, because the mind of baboons is acutely and continually tuned in to keeping track of everyone’s status, via watching and analysing the interactions of every individual in its group.
 
So, is it possible that, in a strange way, the social complexity of baboons goes with their facial shape in profile? The muzzles are, at least in part and at least as a corollary function, pointers to gaze. This is part of the sly indirectness with which baboons juggle/balance the use of gaze as a clue to attention, and the use of gaze as a ‘weapon’.
 
It may be hard for a human to consider this, because our minds work differently.

We do have an element of ‘what are you staring at, asshole’ about us when it comes to strangers. However, by and large – at least among those we know and more or less trust - we value and respect an honest and direct gaze according to our basic value of exchange of technical information, explanation, and emotional candour as part of an empathetic mindset in which honesty is rewarded rather than punished. In us, the ambivalence of the direct gaze is heavily weighted towards valuing this as ‘honest’, ‘good’, and ‘moral’, with the aggressive aspect as a minor part, whereas it is the other way around in baboons.

Hence, I suggest that the extremely projecting muzzle of baboons and flatter-than-flat face of the modern human go together with the differences in the eyes (as well as the specialisation of the dentition, explained above).
 
Supporting the idea presented above is the dual difference between macaques (Macaca, https://en.wikipedia.org/wiki/Macaque) and baboons.

Macaques have showier eyeballs than do baboons (https://www.agefotostock.com/age/en/details-photo/crab-eating-macaque-macaca-fascicularis-adult-close-up-of-head-bong-rong-thailand/FHR-10780-00090-225 and https://primarilyprimates.org/product/bojangles/ and https://www.alamy.com/northern-pig-tailed-macaque-macaca-leolina-portrait-thailand-image184170264.html?imageid=FC800E71-9171-4CFF-B152-62096081E12F&p=704031&pn=1&searchId=2700b3bec091286c405053774f940af6&searchtype=0 and https://www.alamy.com/pigtail-macaque-macaca-nemestrina-portrait-image255394909.html?imageid=7CD77AF8-BA2A-49A9-9F7E-77E3785A9C7D&p=851480&pn=1&searchId=2700b3bec091286c405053774f940af6&searchtype=0 and https://www.alamy.com/pigtail-macaque-macaca-nemestrina-male-thailand-image255233008.html?imageid=41FEBB2C-40FA-4579-B4BC-B10FD1EE212E&p=853403&pn=1&searchId=2700b3bec091286c405053774f940af6&searchtype=0 and https://www.alamy.com/southern-or-sunda-pig-tailed-macaque-macaca-nemestrina-female-portrait-wild-but-used-to-being-fed-by-local-people-gunung-leuser-national-park-sumatra-indonesia-image263193550.html?imageid=76257C35-D2A5-45F0-806B-20CB5E97C633&p=215387&pn=1&searchId=2700b3bec091286c405053774f940af6&searchtype=0), depending on the species.

Furthermore, the eyes are not as shaded by the superciliary ridges as is the case in baboons.

Accordingly, no macaque has a muzzle as long as that of any baboon. Even the superficially baboon-like macaques of Sulawesi have ventrally elongated faces rather than horizontally elongated faces (https://www.dreamstime.com/beautiful-celebes-crested-macaque-macaca-nigra-aka-black-ape-old-world-monkey-tangkoko-nature-reserve-indonesian-image171038437 and https://www.jungledragon.com/specie/1489/celebes_crested_macaque.html and https://www.alamy.com/stock-photo-close-up-side-profile-of-a-captive-sulawesi-crested-macaque-macaca-74752550.html).

So, if one looks at the whole group of baboons and macaques, it does seem that long horizontally-oriented muzzles correlate with ‘hidden eyes’ (except for the pale upper eyelids, which occur in both baboons https://www.alamy.com/stock-photo-yawning-olive-baboon-in-akagera-national-park-rwanda-africa-82189781.html?imageid=E228F186-753D-41B7-ACA8-E7E3260EC65B&p=12354&pn=1&searchId=a8dcfe1380fa4b839f6fc3ec2e5d9cee&searchtype=0 and macaques https://www.alamy.com/a-macaque-monkey-licking-its-fingers-whilst-feeding-his-closed-eyes-show-the-white-eyelids-which-look-as-though-the-monkey-is-wearing-eye-shadow-image369197643.html).

Posted on Ιούλιος 02, 2022 0344 ΠΜ by milewski milewski

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The following facts are uncontested but tend to be underplayed in accounts of the biology of baboons. Both are testimony to the remarkable terrestriality of baboons.
 
Firstly, although baboons are certainly omnivorous with a staple of herbaceous plants, they tend not to take dicotyledonous foliage. ‘Leaves’ are not a common part of their diet (except of course for grass leaves, which are seldom called leaves), which is significant considering how broad the diet is and how versatile baboons are trophically, depending on area and season.
 
I interpret this as supporting a view of terrestrial emphasis, which leaves the leaves to arboreal monkeys such as the samango monkey and colobines, and also to e.g. giraffes.
 
Secondly, while baboons always climb trees to evade the leopard, they always spring down from the trees to evade humans. Since baboons and humans evolved together, the fact that baboons get out of trees as fast as they can when approached by humans indicates that they have been shaped by the selective pressure that humans, as technological hunters, can easily ‘shoot’ them down from even tall trees. When approached by humans, baboons rely on fleeing terrestrially.
 
The omission of ‘leaves’ from the diets of baboons is too easy to overlook owing to the omnivory; and the terrestrial flight from humans is too easy to overlook owing to the dependence of baboons on climbing for safety (not only when fleeing from the leopard but routinely each evening, whether on trees or cliffs, to rest nocturnally).
The following facts are uncontested but tend to be underplayed in accounts of the biology of baboons. Both are testimony to the remarkable terrestriality of baboons.
 
Firstly, although baboons are certainly omnivorous with a staple of herbaceous plants, they tend not to take dicotyledonous foliage. ‘Leaves’ are not a common part of their diet (except of course for grass leaves, which are seldom called leaves), which is significant considering how broad the diet is and how versatile baboons are trophically, depending on area and season.
 
I interpret this as supporting a view of terrestrial emphasis, which leaves the leaves to arboreal monkeys such as the samango monkey and colobines, and also to e.g. giraffes.
 
Secondly, while baboons always climb trees to evade the leopard, they always spring down from the trees to evade humans. Since baboons and humans evolved together, the fact that baboons get out of trees as fast as they can when approached by humans indicates that they have been shaped by the selective pressure that humans, as technological hunters, can easily ‘shoot’ them down from even tall trees. When approached by humans, baboons rely on fleeing terrestrially.
 
The omission of ‘leaves’ from the diets of baboons is too easy to overlook owing to the omnivory; and the terrestrial flight from humans is too easy to overlook owing to the dependence of baboons on climbing for safety (not only when fleeing from the leopard but routinely each evening, whether on trees or cliffs, to rest nocturnally).

Αναρτήθηκε από milewski 11 μήνες πριν (Αναφορά)

I have before me the book ‘Baboon metaphysics: the evolution of a social mind’ by Cheney and Seyfarth (https://press.uchicago.edu/Misc/Chicago/102436.html).
 
On page 165, an incident is described in which a baboon warned a researcher of a spitting cobra nearby. “Chloe sat down several feet in front of him [Conrad Brain] and gazed into his eyes. Next, she looked down at the crevice below the rock where Conrad was sitting and gave a soft alarm bark. She then gazed into his eyes again and repeated the alarm call. Conrad followed Chloe’s direction of gaze and saw a spitting cobra lying just below his feet. Once he had moved away, and taken time to think about the incident, what struck Conrad most forcefully was his strong impression that Chloe had deliberately made eye contact with him before she gave her alarm call. Many studies have shown that monkeys and apes attend to other individuals’ eyes and the direction of gaze...[refs given]. They use gaze to target opponents and to recruit support in aggressive alliances...Monkeys also seem to recognise that displays and facial expressions are ineffective without some degree of eye contact...Monkeys and apes also seem to recognise the relationship between an individual’s gaze and the target of his attention.”
 
What Cheney and Seyfarth fail to distinguish is direct gazes from sideways glances/gazes. My working hypothesis is that baboons can infer the direction of another baboon’s direct gaze, but not its sideways glances/gazes. In the anecdote above, I assume that the baboon Chloe looked directly at both the man and the cobra, i.e. she moved her whole head when looking from one to the other. If this story involved two humans, the information could have been transmitted even if the informer kept her face steady, and just swivelled her eyeballs to view the snake. The change in direction of glance/gaze might have to be repeated several times to get the message across, but just the human eye-whites would tell the human recipient to look down and where to look.

Αναρτήθηκε από milewski 11 μήνες πριν (Αναφορά)

If you and I face each other and my eyes glance sideways, it comes effortlessly to our species to read this directional gaze, i.e. to realise that the other person is looking in a certain direction despite their face remaining orientated towards the observer. This is aided by the shifting patterns of eye-whites to left and right of each iris. Such reading is second nature to humans, but is it possible in baboons? The guinea baboon (Papio papio) shows a poor ability to read sideways glances in the way humans read them (Fagot and Deruelle, 2002). In its eyes, a sideways glance, on a face facing the observer directly, gives little information on where the other is looking, and/or the mind of the baboon has no demand for such information.

This reading failure by baboons is doubly significant. Firstly, it suggests that baboons lack a ‘theory of mind’, i.e. they cannot infer the thoughts of another from his/her eye movements. Even the simple thought, ‘I’m looking there (e.g. left)’ vs ‘I’m looking there (e.g. right)’, cannot be read by baboons on the basis of sideways glances. By contrast, the thought behind a direct, full-on stare certainly can be perceived without a theory of mind. Secondly, it indicates that the shifting chiaroscuro of the pale eye-whites relative to the darker iris and facial skin does not amount to a functional semaphore/code for baboons.

This need not mean that baboons cannot read the direction of gaze at all. When the whole head squarely faces the object being looked at, then the direction of gaze can be perceived. I.e. if one individual faces another head-on, with its eyes trained directly on the second, then the latter will perceive that it is being looked at (potentially stared at), and often take evasive action by looking away to avoid the discomfort of confrontation. Similarly, if the whole head of the gazer turns to view an object, with the eyes looking forward instead of swivelling in their sockets, then it seems that another baboon can indeed infer where the gazer is gazing. It seems that the orientation of the head/face is meaningful in the perception of baboons, but the orientation of the eyeballs within the face is not meaningful if it deviates from the square-one position.

I would not use the word ‘read’ for the perception of a direct gaze/look/stare/glance, because the visual clue is so crude and simple. However, I would use the word ‘read’ for the perception of sideways glances. On that basis, humans can ‘read eyes’ but baboons cannot. The domestic dog is well-known to read human gaze-direction, but I wonder if this species can read sideways glances. I think probably it can, although its wild ancestor cannot. This ability has been bred selectively (if inadvertently) in the domestic dog, making it superior to the far larger-brained baboons in this way.

I infer something not pointed out by Fagot and Deruelle (2002): that this failure to read sideways glances gives baboons an ability to deceive other baboons by means of the sly ploy of making sideways glances. If a baboon glances sideways at an object of interest, any observing baboon will not be able to read where it really is gazing. Thus an object of interest, and an intent, can be hidden. This would apply whether the face of the sly baboon, or merely the back of its head, were visible.

It is one thing to realise that baboons fail to read each other’s (or humans’) sideways or off-centre glances. (That realisation is present in the literature, exemplified by Fagot and Deruelle, 2002). It is another thing to realise that baboons might deliberately glance sideways at objects of interest - as opposed to turning their head to look at these objects - as a way of hiding their interests/intentions from other individuals.

My interpretation thus differs from that of Fagot and Deruelle (2002) These authors do not discuss the pigmentation of the sclera of baboons. If they did, they might conclude that baboons lack eye-whites because eye-whites are useless to a species unable to read sideways glances. Trouble is, that would not explain why the scleras of baboons are actually pigmented, as opposed to being pale but simply unread by others (as in pigs, rhinos, etc., which have a visible white sclera). My suggestion: humans have an actual adaptation for reading sideways glances, whereas baboons have an actual adaptation against reading sideways glances. These are 'opposite' in a sense.

Αναρτήθηκε από milewski 11 μήνες πριν (Αναφορά)

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