Sexual dimorphism is the condition where the two sexes of the same species exhibit different physical characteristics beyond the differences in their sexual organs. One of the things that is readily apparent in looking at chimpanzees is that the males are significantly larger than the females, and also tend to have enlarged canine teeth which become prominent in making threat displays against other males during the process of establishing social dominance.
With regard to primates, Christopher Seddon, in his book Humans: From the Beginning, reports:
“Among the living primates, a strong correlation between mating strategy and sexual dimorphism has been found. This in turn is related to male-on-male aggression.”
Aggressive primate males often run off the weaker males so that they can dominate and mate with the females. Charles Darwin was one of the first to recognize this pattern which he attributed to sexual selection in which the larger, more aggressive males breed more and, thus, pass on their characteristics to more offspring. In their book From Lucy to Language, Donald Johanson and Blake Edgar write:
“The degree of sexual dimorphism, therefore, would seem to have some connection with mating systems. As a rough generalization, primates that exhibit little or very reduced dimorphism tend to be monogamous.”
On the other hand, this may not be universally true. Donald Johanson and Blake Edgar also point out:
“Some monogamous species are quite dimorphic, and some species with reduced dimorphism are polygynous.”
The relative lack of sexual dimorphism is one of the traits that distinguishes modern humans—i.e. Homo sapiens—from modern apes, such as chimpanzees and bonobos. Is this an ancient anatomical difference, like bipedalism, or is it of more recent origin? Does the relative lack of sexual dimorphism have any implications for the evolution of human behavior, language, and culture?
One of the earliest anatomical traits that distinguishes the human evolutionary lineage from the ape lineage is bipedalism. Yet the earliest bipeds, the australopithecines who were flourishing about 3 million years ago, exhibit marked sexual dimorphism. This suggests that the relative lack of sexual dimorphism evolved later.
While the australopithecines had sexual dimorphism with regard to body size, there is a reduction in the size of the canine teeth in males. In their book The Dawn of Human Culture, Richard Klein with Blake Edgar write:
“This may indicate that dietary change was accompanied by a reduction in male-on-male aggression, or, more generally, by greater social tolerance.”
In his book Before the Dawn: Recovering the Lost History of Our Ancestors, Nicholas Wade writes:
“But around 1.7 million years ago, the size difference between males and females started to diminish, according to the paleontological record. This shift in size is almost certainly a sign that competition between males had diminished because of the transition to the pair bond system.”
Among humans, males tend to be larger than females, but it is not unusual to find some women who are larger than men and some men who are smaller than some women. In other words, the size distinction is not absolute. In addition, men do not have exceptionally large canine teeth. Christopher Seddon writes:
“Yet modern humans are mildly sexually dimorphic and males are about 20 percent larger by body weight. Men possess greater physical strength than women, and on average are taller. This suggests that strict monogamy may not be the natural mating strategy of Homo sapiens, and that the true strategy is based on pair bonding with a slight tendency for male infidelity.”
According to some paleoanthropologists, one of the major implications of the reduction in sexual dimorphism in human evolution is the emergence of cooperation, particularly cooperation among males. This is critical in cultural evolution, particularly in the social organization needed to sustain hunting and gathering strategies and later food production strategies such as agriculture. Some feel that there is also a correlation between the reduction of sexual dimorphism and stone toolmaking, and possibly with the evolution of language.
More Human Origins
Human Origins: Homo rudolfensis
Human Origins: Homo habilis
Human Origins: Eyes
Human Origins: Teeth
Human Origins: Humans as naked apes
Human Origins: Sex
Human Origins: Domesticating Fire
Human Origins: The Mind