When brushing your teeth at night, chances
are you do not give extensive thought to your
canine teeth: the teeth that sit between your
incisors (your front teeth) and your premolars
5(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:
10capture 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.
Gibbons are small-bodied, Southeast Asian apes
15that 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
20male and female gibbons have such 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
25canine 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, the
30canines are used primarily for hunting prey.
Think back to all those nature documentaries
you've seen: carnivores like lions and cheetahs
are famous for chasing down large game animals
and their canines are well suited for roles in
35puncturing and killing prey.

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,
40competitive 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,
45tusks, 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
50their 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
55largest 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
60females 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
65America, 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
70effectively 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
75anthropologists who study human evolution.
Humans and their fossil relatives (collectively
referred to as hominins) are among the subset of
primates in which male individuals have evolved
smaller canine sizes to bring them back in line
80with 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
85species, 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
90hominins, 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
95male 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.
100Gibbons live in groups with a single breeding
male and a single breeding female, a grouping
that differs substantially from the large, multi-
member social groups of other primates. Because
of this unique sociality, gibbon males face
105reduced 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
110territories 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.

115

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
120primates to model the evolution of a specific
dental feature: sometimes there is more than one
way to use a tooth!

According to the passage, gibbons have large canines because:
A
all primates have large canines.
B
they face competition for food resources.
C
their social system is similar to other primates.
D
they use these teeth to hunt and kill prey.