Biology Reference
In-Depth Information
2.0
Papio
Fig. 2.6
The
degree of sexual
dimorphism
increases with the
number of females
per male in the
breeding group.
Each point is a
different genus,
some of which are
indicated by
name. From
Clutton-Brock and
Harvey (1977).
Gorilla
Colobus
Erythrocebus
1.5
Macaca
Pan
Lemur
Hylobates
1.0
Indri
Aotus
0.9
1
2
3
4
5
6
7
Sex ratio in breeding group (No.
per
)
against predators. Alternatively, males may have larger teeth for competition with other
males over access to females. There is the problem here of body weight as a confounding
variable; males are larger than females, so a difference between the sexes in tooth size
could just reflect a difference in body size.
This can be controlled for by calculating the line of best fit when female tooth size is
plotted against body weight. If the tooth size of a male is now plotted on the same
graph, it can be seen whether its size is greater than expected for a female of the same
body weight. The results show that in monogamous species male tooth size is as
expected for a female of equivalent body weight. However, it is larger than expected in
harem-forming species. These data support the sexual competition hypothesis for the
evolution of larger teeth in males. Nevertheless, we cannot exclude the predator defence
hypothesis because maybe the harem-forming species are the ones most vulnerable to
predation.
The analysis can be taken a step further by considering species where several males
live together in a group (multimale troops). It is found that, within this type of social
organization, the males of terrestrial species have larger teeth for their body size than
arboreal species. Therefore, even within the same mating system there is a difference in
tooth size in different habitats. The terrestrial environment is usually thought to present
greater risks of predation, so predation pressure may have been responsible for the
evolution of larger teeth in terrestrial species.
Our conclusion is that both sexual competition and predation may have influenced
the evolution of sexual dimorphism in tooth size. There is also the further possibility
that differences in tooth size are important in reducing diet overlap between the
sexes, so preventing competition for food. This example shows that, even with careful
analysis, it may be difficult to tease out the effect of several variables on the
evolution of a trait.
Sexual
competition and
defence against
predators may
both be important
