This year – 2021 – has been a year of progress in overcoming the effects of the COVID-19 pandemic on human evolution…
Here we are again. Another Halloween, another piece about teeth. Why? Because teeth play a critical role in all sorts of Halloween-related activities: eating candy, wearing costumes, carving jack-o-lanters, sucking blood (mwahahaha…er…). Last year we focused on how the consistency of our food (hard, soft, chewy, gooey) influences how our jaws grow. This year, we focus on those made famous by vampires: canines. Luke Fannin from the Department of Anthropology at Dartmouth College shows us that the canine teeth of our primate relative, the gibbon, have a fascinating evolutionary history and ultimate function than canine teeth in many (most even?) other mammals. Enjoy! [Featured image above: A white handed gibbon (Hylobates lar) in Khao Yai National Park, Thailand uses a canine tooth to break into a tough-husked fruit (Genus: Anthocephalus). Photo taken and provided by Kulpat Saralamba.] –JMO
When brushing your teeth at night, chances are you give very little consideration to your canine teeth—the teeth that sit between your incisors (i.e., your front teeth) and your premolars (or bicuspids, as dentists like to call them). By comparison to other mammals, our canines are quite insignificant. In fact, our puny canines are quite miserable for doing either of the things that mammals typically do with their canines: capture prey or fight for access to mates. Conversely, some non-human primates seem to have the opposite dilemma to us; they have really big canine teeth. Take gibbons, for example—small-bodied, Southeast Asian apes that are closely related to humans. Unlike us, both male and female gibbons possess projecting, dagger-like canines. This pattern is quite distinct among primates, which has perplexed scholars for many years. Why do both male and female gibbons have really large canines, while humans are stuck with our tiny ones?
To get at the meat of this question, we should probably first take a look at the namesake for the canine teeth, the carnivores. The term carnivore, in this instance, refers specifically to families of mammals that belong to the Order Carnivora: dogs, cats, bears, hyenas, raccoons, weasels, mongooses, and many others. In carnivores, as expected, the canines are used primarily for hunting prey. Think back to all those nature documentaries you’ve seen: carnivores like lions and cheetas are famous for chasing down large game animals and their canines are well suited for roles in puncturing and killing prey. Variation in canine size within carnivores often reflects prey killing technique. Felids (cats such as lions, leopards, and jaguars) have longer and stronger canines than canids (dogs, wolves, foxes) because felids tend to deliver deeper and stronger bone-crushing bites on moving, often struggling prey. Canids, in contrast, deliver more shallow, quick wounds to their prey and their canines are not subjected to the same kinds of twisting and bending as felid canines.
For primates, however, the canine teeth serve a different, social role. Unlike other mammals, primates typically lack horns, tusks, and antlers—structures that are useful in aggressive, competitive social interactions, such as those involved in territorial disputes or in competitions for mates. Instead, some primates evolved long, weaponized canine teeth that are used to perform similar social roles as horns, tusks, and antlers. Male members of most primate species tend to have larger canines than females. Most primates live in large social groups, often consisting of one or a few males and a larger number of associated females and their offspring. Within groups, males compete fiercely for access to mates and use their large canines as advertisements of their physical strength and, less often, as actual weapons for inflicting wounds on their rivals. Males with the largest canines tend to be the most dominant, resulting in more opportunities to mate than their rival males, and by extension, siring more offspring.
In some primates, however, males and females have canines that are the same size. Often, a pattern of equal-sized canines reflects reduced competition for mating, and therefore a reduction in male canine length relative to female—such as in the titi monkeys of South America, and in humans. Yet in other primate species, like the previously mentioned gibbons, females evolved larger, weaponized canines to match the males; in this case, the large canines in female gibbons allows them to compete effectively in aggressive encounters with neighboring social groups over territory and food resources.
The variable pattern of canine length between male and female primates is of great interest to anthropologists who study human evolution. Humans and their fossil relatives—collectively referred to as hominins—are among the subset of primates where male individuals have evolved smaller canine sizes to bring them back in line with those of females. Compared to chimpanzees, our closest non-human primate relatives, both male and female humans possess small, equal-sized canines. But even among hominins, this pattern is not unique to our species, Homo sapiens; rather, small canines are present in some of the earliest fossil hominins like Ardipithecus ramidus, who at 4.4 million years ago shows the first definitive signs of this canine size pattern. Male canine reduction in hominins, as compared to the relatively larger canines of other male primates, suggests that there were unique and major shifts in social behavior occurring early in human evolution: decreased competition for mates and increased male social bonding.
But what about the gibbons? Like humans, the evolution of equal-sized, yet long and weaponized canines in male and female gibbons have been attributed primarily to social factors. Gibbons live in groups with a single breeding male and a single breeding female (often called ‘monogamous’ by some scholars), a grouping that differs substantially from the large, multi-member social groups of other primates. Because of this unique sociality, gibbon males face reduced competition for mates, and so direct fights for mates are minimal. But then what explains the large canines of the females? Gibbons are major consumers of ripe fruits and both males and females aggressively defend large territories containing many widely-dispersed fruit trees. The prevailing hypothesis is that both sexes evolved weaponized canines to aid in defending these valuable food resources against other neighboring gibbon groups.
Another, mutually compatible hypothesis relates canine size in gibbons to their diet. The ripe fruits that gibbons eat come in a variety of shapes and sizes, but many have thick, tough outer peels that protect the softer, succulent flesh underneath (Fig. 1). To remove these tough husks, gibbons perform an interesting feeding behavior. By first puncturing the husk with one of the canine tips, a gibbon will then proceed to use that same canine to quickly peel off a section of the husk, revealing part of the inner fruit pulp (see video). This is aided by rotation of the fruit with one of the hands giving the canine the appearance of being a gibbon ‘can opener’ of sorts (Fig. 2). The use of canines for feeding is not novel in primates; we already know that sakis, bearded sakis and uakaris—a group of South American monkeys called pitheciines—also similarly use their canines to puncture and peel hard-husked fruits. And just like the gibbons, males and females of these species have nearly equal-sized canines, providing good support for the dietary hypothesis.
Therefore, it is reasonable to suggest that, along with a role in territorial defense, the frequent consumption of fruits with tough outer peels in gibbons provided another selective pressure to maintain large, blade-like canines in both sexes. How diet may have influenced the shape and size of the canines the other apes, like orangutans, gorillas and chimpanzees, is still not clear. But we know from studying South American monkeys that the common challenges of feeding can often shape the evolution of canine differences (or lack thereof) between the sexes, sometimes independent of social behavior. The interesting case of gibbon canines demonstrates wonderfully the utilitarian function that the canine teeth can play in the daily life of a primate. It also highlights the challenges sometimes faced when using living primates to model the evolution of a specific dental feature–sometimes there is more than one way to use a tooth! Although the evolutionary history of gibbon canines may be more multi-layered than initially thought, we can all agree that sometimes you need to peel away the layers of a complex structure to get at the treasured, sweet truth of its origin.
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Edited by Jason Organ, PhD, Indiana University School of Medicine.