(writing in progress)
Old World monkeys (baboons, geladas, guenons, colobuses, langurs, macaques, mangabeys, talapoins, etc., https://en.wikipedia.org/wiki/Old_World_monkey.) show extreme specialisation of the canine teeth.
The mandrill (https://www.inaturalist.org/taxa/43533-Mandrillus-sphinx) is a particularly spectacular example (https://www.reddit.com/r/natureismetal/comments/lg5mn3/yawning_mandrill_showing_why_hes_not_to_be_fucked/ and https://www.shutterstock.com/video/clip-1024270445-mandrill-mandrillux-sphinx-yawning and https://www.shutterstock.com/video/clip-1024270421-mandrill-mandrillux-sphinx-yawning).
Note the extremely specialised lower premolar ‘whetting-stone’ for the upper canines, on the mandible (https://search.library.wisc.edu/digital/A4LMK3O4YAMOKG8V and https://www.etsy.com/au/listing/934210586/mandrill-baboon-monkey-skull-replica and https://carnefx.com/shop/mandrill-baboon-monkey-skull-replica/).
What are the reasons for this?
There are basically three different categories of function that could explain the evolution of the extreme canine dentition in Old World monkeys. These are
- defence against predators,
- the killing of prey, and
- intraspecific sparring.
The extreme development of the canines in Old World monkeys makes little sense as the result of natural selective pressure for defence against predators. This is because the females lack the defence; the extremely developed canine dentition is restricted to adult males. There is no coherent logic whereby these defences would show such extreme sexual dimorphism if they were mainly an anti-predator adaptation.
This is not to deny that the canines might be used in deterring or fighting off attacks by Carnivora; it is just to point out that this cannot have been the main evolutionary reason, because any pressure strong enough to shape the males’ canines to such an extreme degree would, logically, also have acted on the females’ canines.
At this point, many readers may be thinking ‘everyone knows that in baboons and other Old World monkeys the adult males take on the role of defending the whole group from predators.’ However, does this sexual skew really make sense?
Moving on to the second possible function:
The extreme development of the canines makes little sense as the result of natural selective pressure for the monkeys’ own predatory habits.
Most monkeys do kill other animals for food, and in the case of baboons this extends to infant gazelles. However, most animals caught do not need to be killed immediately, and in fact monkeys usually eat their prey alive rather than attempting to kill it first. And the monkeys’ jaws and incisors are strong enough that the canines are not needed for butchering the carcases.
I have yet to hear, for example, of an adult male baboon butchering an impala neonate’s carcase by means of the shearing mechanism of the canines. Furthermore, females should be as keen to eat meat as the males are, and they lack specialised teeth for butchering, let alone cleanly killing the prey.
Put differently:
Given the predominant omnivory of monkeys, it makes little sense that these animals have canines rivalling or exceeding those of like-size Carnivora. And it makes even less sense that such canines should be restricted to the males, given that males seldom share their kills with females or juveniles.
Moving on to the third possible function:
Nobody doubts that baboons and other Old World monkeys do indeed use their extremely-developed canines in intraspecific rivalry among males. However, it is something different to assume that this has been the main evolutionary reason for the development of these teeth.
The good all-round naturalist Bill Hamilton III, who loved to work in the Okavango and elsewhere in Africa, and who studied the chacma baboon specifically, told me over the phone in 1989 (seven years before his death) that he had come to the conclusion that the evolution of canines in baboons was all about male-male rivalry. However, I did not record his explanation in my handwritten notes and I gather he never published this view with his reasons. So it remains just an opinion, albeit from a credible researcher.
Most mammals that have elaborate or extreme weaponry, restricted to males, use that weaponry not only to hurt rivals but also to fence/spar/practise. Most horn-like structures in ungulates are used to spar rather than to kill. In those cases where the horns really are purpose-built for impaling (as in e.g. rupicaprins and bushbucks), the skin in appropriate parts of the anatomy is correspondingly thickened into a kind of shield.
As far as I know, male baboons ‘spar’ with their teeth in an ambivalent way (deftly avoiding bites rather than clashing teeth). Although serious rivals do puncture, rip and impale each other’s faces and forequarters there is no thickened skin in these locations. Instead baboons seem simply to heal rapidly from their wounds.
But my main point w.r.t. to the evolution of such extremely dangerous weapons for mere rivalry is as follows.
It seems unnecessary for the weapons to be quite so murderous-looking, even if their main purpose is to show off macho. The males could be unarmed except in the sense of wrestling powerfully. There are various arrangements in the animal world for contests of the male hierarchy, and mating rights. The canines and their whetting-stone lower premolars seem like ‘overkill’ for mere rivalry among males, and I cannot see any advantage in having them so extremely designed that they really are among the most dangerous-looking weapons of any large mammal in the mammal communities in which baboons, in particular, reside.
Even if one argues that males somehow ‘need’ such extreme weaponry for their macho posturings, the associated risks for females should ensure some limitation by natural selection. It is serious enough that mature males of baboons are double the body mass of females, and throw their weight about in a hooliganish way. Adding such murderous weapons to the bluster seems excessive.
Could the explanation invoke a combination of all three factors? Possibly.
If readers remain unconvinced that there is anything anomalous about the extreme evolution of canine teeth in Old World monkeys, I can add particular observations in each of the categories I outlined above, namely defence against predators, the killing of prey, and intraspecific sparring:
In the case of defence against predators:
The island of Sulawesi is on the far side of Wallace’s Line, beyond the natural ranges of any felid or canid. So the seven spp. of macaques restricted to this island have relatively few predators, and could be expected to relax any anti-predators defences typical of congeners on the southeast Asian mainland. Instead, the crested macaque (Macaca nigra, https://www.inaturalist.org/taxa/43457-Macaca-nigra) at least, and probably all of the other six spp. as well, have extremely well-developed canines in males, as seen in photos of fang-baring expressions (http://www.shahrogersphotography.com/detail/29383.html and https://www.dreamstime.com/yawning-celebes-crested-macaque-macaca-nigra-tangkoko-national-park-north-sulawesi-indonesia-yawning-laughing-celebes-image103164247 and https://stock.adobe.com/images/the-celebes-crested-macaque-open-mouth-and-shows-his-fangs-crested-black-macaque-sulawesi-crested-macaque-or-the-black-ape-natural-habitat-sulawesi-island-indonesia/272647303).
This suggests that the development of the canines has little to do with anti-predation.
In the case of the killing of prey:
The gelada (Theropithecus gelada, https://www.inaturalist.org/taxa/43530-Theropithecus-gelada) coexists in places with domestic livestock, particularly the domestic goat (Capra hircus, https://www.shepherdsongfarm.com/our-cause/night-shelters-traditional-sheep-housing/ and https://www.alamy.com/white-goats-grazing-on-the-hillside-simien-mountains-national-park-ethiopia-image339228088.html?imageid=3B89A2FC-E557-4FF7-9123-F45D6414155B&p=266810&pn=1&searchId=e8f5d43729ed5f6426bfeb2a65c7a508&searchtype=0).
If the canines were for killing prey, this situation should be conducive to males of the gelada killing the goat at least occasionally. After all, the masculine canines of the gelada are as large as those of the leopard, and kept sharper. It should be possible for males of the gelada to pounce on an adult individual of the domestic goat and kill it outright for food. Given that the gelada eats mainly greens, this species could be expected to be particularly ‘meat-hungry’. However, I have not heard of any such use of the canines for predation in this monkey.
In the case of intraspecific sparring:
As I explained indirectly above, the usual pattern in most mammals and birds is for males of sexually dimorphic species to possess organs somewhere in a blurry zone between armament and ornament. At one end of the spectrum is e.g. the peacock’s tail, which is purely macho-ornamental because it cannot be used as a weapon. At the other end of the spectrum is e.g. the horns of bushbucks, which are purely macho-armamental because they are not ornamental but are designed functionally for stabbing.
In the case of the gelada, the male is certainly ornamented in having a cape of long hair and an emblazonment on its chest (https://www.facebook.com/earthunreal/photos/a.246085523008107/897297337886919/?type=3 and https://www.dreamstime.com/gelada-baboon-male-portrait-simien-mountains-national-park-north-ethiopia-gelada-baboon-male-simien-mountains-ethiopia-image156334611 and https://www.flickr.com/photos/lindadevolder/5400974286 and https://www.rockjumperbirding.com/the-geladas-of-ethiopia-by-adam-riley/).
So, it is easy to visualise a situation in which these adornments would be enough to signify macho, and any physical struggle would involve the sheer strength of the animal and the loudness of its calls, together perhaps with biting by means of the strong incisors. It is hard to see why the gelada ‘bothers’ to have such extreme canines as armaments for male rivalry as well – particularly because females of this species have the same ‘lip-flipping display’ (https://www.reddit.com/r/natureismetal/comments/atyems/male_gelada_baboon_giving_a_threat_display/ and https://pixels.com/featured/threat-display-of-a-male-gelada-baboon-tony-camacho.html?product=iphone-case-cover&phoneCaseType=iphonexs) of the front teeth regardless of the fact that their canines are short and unimpressive (https://imgur.com/gallery/NDKYv).
Perhaps a clue to the real reasons can be found in the show-off yawn of mature males - which is really a way of passively threatening all in view with their long, sharp canines. Many cercopithecid monkeys, including many macaques, perform this display. (There seems to be nothing analogous with the angry fang-baring so familiar in the wolf and the domestic dog in either baboons or the Japanese macaque.)
The Japanese macaque (Macaca fuscata, https://www.inaturalist.org/taxa/43458-Macaca-fuscata) is the only wild primate on this archipelago, and is a species familiar in many photos. What I have noticed is that this species differs to a surprising degree in its facial expressions from the chacma baboon (Papio ursinus, https://www.inaturalist.org/taxa/57556-Papio-ursinus).
The Japanese macaque is smaller than the chacma baboon; the adult female of the former is about half the body mass of the adult female of the latter. However, the two species are broadly similar in their mainly terrestrial habits and their occurrence at the extremes of the environmental range for wild primates (chacma baboon is the most southerly wild primate and Japanese macaque is the most northerly).
The following is recommended reading: http://www.livescience.com/1498-americans-japanese-read-faces-differently.html.
I have yet to find a single photo convincingly showing mature males of the Japanese macaque fang-baring in the way seen in males of baboons. There are a few photos on the Web of mature males of the Japanese macaque showing the canines, but all seem to be innocent cases of yawning in boredom and relaxation.
Even when males of the Japanese macaque do show the canines in an emotional way, this seems to be
- associated with fear rather than anger, and
- accompanied by a social vocalisation.
The main difference I have found between the two spp. of monkeys is that male baboons use the yawn assertively and confidently, as a passive threat, whereas males of the Japanese macaque do not seem to do so. This is consistent, to some degree, with the fact that neither body mass nor canine size is as extremely different from that of the female in this macaque as in baboons.
One possible evolutionary reason for this difference is that the Japanese macaque, being restricted to islands, had relatively little pressure from predators. An extremely odd biogeographical fact about the Japanese archipelago is that there was not a single species of felid on any of the larger islands, and this applies not only to big cats but even to small cats.
Papio ursinus adult male, sinister yawning as display of weaponry:
http://4.bp.blogspot.com/LbccUVbSRd8/SVVdMW3Xp1I/AAAAAAAADxw/tEpjS-f2VCM/s400/Chacma+Baboon+1.JPG
Papio ursinus adult female, innocent yawning in boredom:
http://media.gettyimages.com/photos/chacma-baboon-picture-id159015682
Macaca fuscata, showing that sexual dimorphism is far less than in Papio ursinus:
https://upload.wikimedia.org/wikipedia/commons/f/f8/Macaca_fuscata_male_and_female,_Iwatayama,_20081019.jpg
Macaca fuscata, adult male yawning in boredom/relaxation:
https://c2.staticflickr.com/4/3133/3155084581_ea7907df3a_z.jpg?zz=1
Macaca fuscata, adult male yawning in boredom/relaxation:
https://toraninjapan.files.wordpress.com/2011/01/img_5447.jpg
Macaca fuscata, adult male yawning in boredom/relaxation:
http://cache4.asset-cache.net/gc/sb10069579a-001-japanese-macaque-in-hot-spring-gettyimages.jpg?v=1&c=IWSAsset&k=2&d=n6SjgRHExVfoo4Tnly7wvfaamXikwNLx%2FpyRIEA7xg8jNwzNPJzpPZyvTvZKaTugCHymUWs3C4zSP%2Fl%2BFmFCVg%3D%3D
Macaca fuscata, adult male yawning in boredom/relaxation:
http://www.visualphotos.com/photo/1x6771757/close-up-of-two-japanese-macaques-macaca-fuscata.jpg
Macaca fuscata, adult male opening mouth in what I interpret to be a fear-grimace, accompanied by the appropriate vocalisation (probably of appeasement or distress):
http://c8.alamy.com/comp/EAGCXE/a-male-japanese-macaque-snow-monkey-bares-his-teeth-EAGCXE.jpg
Macaca fuscata, ditto:
http://l7.alamy.com/zooms/d421b60790544e26be99e21f44277a9b/a-male-japanese-macaque-snow-monkey-bares-his-teeth-eagcxc.jpg
Macaca fuscata, ditto but in this case apparently an adolescent rather than mature male:
http://c8.alamy.com/comp/AB84X3/japanese-macaque-snow-monkey-macaca-fuscata-shouting-displaying-his-AB84X3.jpg
Macaca fuscata, ditto:
http://c8.alamy.com/comp/APNE16/japanese-macaque-snow-monkey-macaca-fuscata-shouting-displaying-his-APNE16.jpg
Macaca fuscata, ditto:
http://c8.alamy.com/comp/AB84X6/japanese-macaque-snow-monkey-macaca-fuscata-leaning-forward-shouting-AB84X6.jpg
Macaca fuscata, showing once again the canines of the adult male but once again probably not an angry expression but rather a fearful or appeasing one, i.e. a fear-grimace:
http://farm6.static.flickr.com/5535/10339069904_cfca603e50_m.jpg
Macaca fuscata, adult female yawning in boredom/relaxation:
http://cache.diomedia.com/230h/01/AB/FV/01AB-FVF2.jpg
Macaca fuscata, adult female fear-grimacing accompanied by appropriate vocalisation:
http://medias.photodeck.com/2f24b304-c599-11df-a40a-00270e09af22/JW_013110_2432_xgaplus.jpg
Macaca fuscata, ditto:
https://josephmallozzi.files.wordpress.com/2012/11/1294.jpg
Macaca fuscata, ditto:
http://natureinstock.com/stockphoto/187-92095/Japanese-Macaque-Macaca-fuscata-showing-teeth-in-hot-spring-90318.jpg
Macaca fuscata, ditto:
http://cache4.asset-cache.net/gc/123519839-japanese-macaque-calling-in-rotenburo-gettyimages.jpg?v=1&c=IWSAsset&k=2&d=oxs5zw4iBCbmtiIem9td9w6vqL6oROWbI7JKEBAXyzGKjOw4S1q2hWtz4nhiXVB5hyBm0hb67OJxSzwbGeIr8w%3D%3D
(writing in progress)