Dear  Marc Verhaegen

Most probably the answer to your question is no. I am working on forensic Odontology for the last 37 years but I have not come across any  comparative study of enamel thickness among Macaca spp and related monkeys. As regards durophagy I corroborate with your views. REGARDS.

:-)  Thanks a lot.

Dear Marc,

I work on macaque dental morphology (mostly tooth size variation). There is the following paper that finds a latitudinal gradient in macaque enamel thickness.

Kato, Akiko, et al. "Intra‐and interspecific variation in macaque molar enamel thickness." American journal of physical anthropology 155.3 (2014): 447-459.

Also, as per your suggestion re: durophagy, you could compare populations of Macaca fascicularis from coastal and non-coastal regions. I personally do not have enough data to tell you anything about differences in absolute size, but the answer may be out there.

Also, it is well-known that many M. fascicularis populations along coastal regions of their distribution use tools. E.g.:

Gumert, Michael D., and Suchinda Malaivijitnond. "Long-tailed macaques select mass of stone tools according to food type." Philosophical Transactions of the Royal Society B: Biological Sciences 368.1630 (2013): 20120413.

Hope this helps!

Yes, Nicole, thank you very much, I'll write the authors of these papers.

Hello Marc,

I agree with Nicole's suggestion of comparing coastal vs. inland populations of Macaca fascicularis.  Another report on long-tailed macaque tool use in a coastal context comes from: Malaivijitnond, S. et al., 2007. "Stone-Tool Usage by Thai Long-Tailed Macaques (Macaca fascicularis)." American Journal of Primatology 69:227–233.

The authors found that long-tailed macaques on Piak Nam Yai Island, southern Thailand, used, "... axe stones to crack not only the oysters attached to the rocks, but also detached gastropods (Thais tissoti, Petit, 1852), bivalves (Gafrarium divaricatum, Gmelin, 1791), and even swimming crabs (Thalamita danae, Stimpson, 1858)." (Malaivijitnond et al., 2007: 231). 

The Brief Report by Malaivijitnond and colleagues (2007) also includes reference to  a paper by A. Carpenter published in 1887 (" Monkeys opening oysters." Nature 36:53) that is likely the earliest report of tool use by nonhuman primates (again, M. fascicularis observed by Carpenter on islands in the Mergui Archipelago, southern Myanmar).  In addition to using extractive foraging as a way of dealing with "protected" food items (i.e., gastropods, bivalves, and crustaceans), which could still introduce hard items (i.e., bits of shell) into the diet of the macaques, in coastal settings they could also potentially end up with some amount of sand mixed in with the gastropods, bivalves, and crustaceans they ingest.  While not exactly hard-object feeding (durophagy), sand/grit in the diet would definitely increase the abrasiveness of the diet and contribute to the possibility of tooth wear -- could be conditions that, over time, would promote selection for increased molar enamel thickness.  As Nicole suggests, "... the answer may be out there."!

Yes, I fully gree, thanks a lot for your most ingeresting answer, Ian.

Hi Marc,

I'm a graduate student under Dr Gumert, and I study the behaviour of two populations of tool-using macaques in Thailand. I can confirm that there haven't been any studies to measure the tooth enamel specifically of tool-using macaques. However, I can share observations relevant to durophagy. 

1) The majority of the tool-processed foods these macaques eat are rock oysters, that grow with one valve cemented onto substrate (e.g. boulders/tree roots). The macaques use tools to crack open the outward facing valve, but often will then use their teeth to prise the broken shell off the oyster (I presume the adductor muscle of the oysters still sometimes hold the broken shell and block access to the flesh).

2) Not all the macaques use tools, and those who do not sometimes physically bite open their prey, especially clams. 

3) Crabs are a favourite food of these macaques, and upon catching a crab, one of the first things they do is bite down through the carapace to disable it. Again, some will use tools on the crabs, and some just bite through the shells, and in larger crabs the pincers can be quite large with thick shells. 

4) I've also observed them feed on tiny snails (Echinolittorina malaccana), and these they just crunch on like peanuts, often without using tools. 

5) They also feed on the seeds of shore plants like coconuts and sea almonds. For coconuts, they use their teeth to rip off the outer husks. I think this is not unique to tool-using macaque populations though. 

You can find the list of foods eaten by one population in Laem Son National Park in the paper already cited in one of the other answers, and we are preparing a manuscript that will have the foods for another population in Khao Sam Roi Yot National Park. 

Cheers. 

Thank you very much, Amanda. I"ve send your answer to our AAT discussion group.

Some possibly relevant thoughts on durophagy. Alan Shabel found resemblances of the dentition of the robust australopiths with those of Carnivora that eat hard-shelled invertebrates (HSI). Lloyd du Brul OTOH saw convergences between the robust dentition & the giant panda's, e.g. flat broad face, enormous molars, premolar molarisation. Pierre-François Puech (who first studied afarensis' enamel micro-wear electron-microscopically) said the glossy polished appearance of afarensis' enamel suggst they fed on wetland plants (e.g. parts of papyrus sedges according to the paleo-milieu & to isotopic studies); the thick enamel might have been for calorie-poor diets, but possibly also for protecting the enamel against small snails or so living on certain wetland plants? Macaques are omnivores: fruits, insects, crops, young leaves, small animals (incl.HSI). Capuchin monkeys & clawless otters also eat HSI; sometimes they lay the bivalves in the sun, so they can be opened more easily after some time.