Collared peccary with open mouth. Check out those canines.

ResearchBlogging.org

You’ll be familiar with peccaries, of course. They’re pig-shaped, shaggy-coated artiodactyls endemic to southern North America, Central America and South America. Fossils indicate that members of the group once occurred throughout the Old World too. Three extant species are conventionally recognised (Collared peccary Pecari tajacu, White-lipped peccary Tayassu pecari and Chacoan peccary Catagonus wagneri), but a fourth (the Giant peccary P. maximus) was named in 2007 (van Roosmalen et al. 2007) and remains the topic of debate. I wrote a reasonable amount about peccaries back on Tet Zoo ver 1 in 2006, and for reasons of time and such I’m recycling some of that text here. What you read below has not been updated since then (in other words, it’s about five years old), so please keep this in mind.

Like pigs, peccaries use a specialised rhinarial disk for rooting in soil. The disk itself is an unusual novelty, the snout is proportionally lengthened compared to that of other artiodactyls, and the nuchal muscles (which help support the head) are hypertrophied and with enlarged attachment areas. An interesting difference between peccaries and pigs is that the muscles that operate the rhinarial disk originate from somewhat different points on the skull.

White-lipped peccary mother and juvenile, image by Chrumps, from wikipedia.

All of the skull sutures in adult peccaries are completely closed, and in fact even juveniles exhibit closure of certain of the sutures ordinarily open in young mammals. This obviously rules out the possibility of any sort of cranial kinesis. A research team led by Katherine Rafferty and Susan Herring have been looking at strain patterns in pig skulls. They’ve found that - during occlusion of the teeth - the snout bones are deformed and pull apart slightly at their sutures (Rafferty et al. 2003). I’m guessing that this is relevant to the sutural fusions seen in peccaries (maybe peccaries have evolved a novel solution to coping with strains built up during tooth occlusion), but I don’t know if this area has been studied. It’s surely relevant to stuff we’ll get to in a minute.

Self-sharpening, vertically implanted, interlocking canines

Collared peccary dentition, as illustrated by Charles Knight in 1849. The upper dentition is shown at upper left (note the straight canine) and the mandibular dentition (with the longer, curved canine) at upper right.

Peccaries are well known for having big, scary canines and they differ from pigs in that both the upper and lower canines are used in biting. Peccaries also differ from pigs in that both their upper and lower canines are vertically implanted: in pigs each upper canine exits the maxilla anterolaterally, and then curves dorsally. In all placental mammals the lower canine bites ‘ahead’ of the upper canine (sloths appear to be an exception, almost certainly because one of their canine pairs doesn’t actually consist of true canines). In peccaries the almost total absence of enamel on the posterior surfaces of the lower canines means that these teeth are constantly sharpened as they move against the enamelled anterior faces of the upper canines. Because the upper and lower canines tightly interlock, peccaries are virtually incapable of moving the lower jaw from side-to-side when the jaws are closed. Special mucosal pockets, bordered by raised bosses on the maxillae termed the canine buttresses, house the lower canines when the jaws are closed (Herring 1972).

Collared peccary skull in oblique anterolateral view, courtesy of Steve Bodio.

The tight interlocking of the canines prevents jaw movement during full occlusion, but there’s more: bony stops around the jaw joint further prevent anteroposterior movement of the jaws when they’re closed. Consequently the jaws can only open and close in a simple hinge-like arrangement. Herring (1972, p. 502) suggested that “this action probably helps to guide the lower canine into its correct occlusal relationship, thus preventing injury to soft tissues”.

But exactly why do the canines interlock? Inspired by Herring’s study, Kiltie (1981) studied peccary teeth in detail and noted a correlation between tooth morphology and a diet that includes incredibly hard nuts and seeds. Kiltie didn’t propose that the canines were used to break open the food items, but rather that - like the bony stops around the jaw joint itself - their interlocking helped prevent dislocation of the joint when tremendous force was applied across the molars. Several other features of the peccary dentition seem to support the idea that peccaries are specialised for breaking open rock-hard objects, as are behavioural studies. I wonder if anyone has ever measured the bite strength of a peccary: the species aren’t all the same in terms of bite strength, as White-lipped peccaries seem to have a bite strength about 1.3 times greater than that of the Collared peccary (Kiltie 1982) and hence can deal with harder food items. Whatever, all the more reason not to put your hand in a peccary’s mouth.

Collared peccary skull in right lateral view, courtesy of Steve Bodio.

If Kiltie is right about the bracing function of the canines, this would explain why – unlike many other mammals that use their canines as offensive weapons – peccaries aren’t sexually dimorphic in canine size. They aren’t dimorphic in body size, nor in head shape, either. But this isn’t the whole story, as fossil peccaries ordinarily are dimorphic, with many forms exhibiting significant (i.e., distinctly bimodal) differences in canine size (Wright 1998). In fact what’s almost bizarre is that fossil populations of the living species exhibit sexual dimorphism in canine size, meaning that the living populations lost dimorphism somewhere along the way.

So what gives? We don’t know why sexual dimorphism was lost in the group (decreasing need to avoid niche overlap, perhaps due to declining diversity in contemporaneous megafauna?), but comparison with related groups, and examination of the peccary fossil record, indicates that sexual dimorphism in canine size is primitive for the group: that is, it’s the condition they inherited from their ancestors. Use of the canines as bracing structures therefore looks like it might be an exaptation: a new use for a set of structures that were previously used for something else.

For previous Tet Zoo articles on peccaries, pigs and their relatives (the suiforms), and on similar artiodactyls conventionally regarded as suiformes, see...

Refs - -

Herring, S. (1972). The role of canine morphology in the evolutionary divergence of pigs and peccaries Journal of Mammalogy, 53 (3), 500-512 DOI: 10.2307/1379040

Kiltie, R. A. 1981. The function of interlocking canines in rain forest peccaries (Tayassuidae). Journal of Mammalogy 62, 459-469.

- . 1982. Bite force as a basis for niche differentiation between rain forest peccaries (Tayassu tajacu and T. pecari). Biotropica 14, 188-195.

Rafferty, K. L., Herring, S. W. & Marshall, C. D. 2003. Biomechanics of the rostrum and the role of facial sutures. Journal of Morphology 257, 33-44.

van Roosmalen, M. G. M., Frenz, L., van Hooft, P. de Iongh, H. H. & Leirs, H. 2007. A new species of living peccary (Mammalia: Tayassuidae) from the Brazilian Amazon. Bonner Zoologische Beiträge 55, 105-112.

Wright, D. B. 1998. Tayassuidae. In Janis, C. M., Scott, K. M. & Jacobs, L. L. (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, pp. 389-401.