Early hominid Paranthropus robustus

Sexual dimorphism

Are males always bigger than females? Sometimes, nothing could be further from the truth…

Humans show a degree of sexual dimorphism – males, on average, are larger than females. This is true for most mammalian species, but the situation is often reversed in other orders – in spiders, for example, the female is usually the larger sex.

The origins of sexual dimorphism are not completely understood, but theoretically there is a link between low levels of dimorphism – species where the male and female are similar – and greater input into child rearing by the male.

Sexual selection may also play a role. In many species males compete with males for the attention of females, who choose their mates. This can lead to the evolution of parts of the body that are of no selective advantage but signal fitness to females. Ornamentation is used by males to display their quality, such as when the male peacock proudly parades his large tail feathers. Weaponry enables males to establish authority in combat; a stag’s antlers are a signal of power.

Some forms of sexual dimorphism are extreme. In anglerfish the tiny male attaches to the side of a female semi-parasitically, doing little more than provide the female with sperm. In green spoon worms the female actually engulfs the male, who resides in the female’s digestive tract, churning out sperm.

In early hominids, though, sexual dimorphism may have been more pronounced. Studies of Paranthropus robustus (pictured), which appeared around two million years ago, suggest that males were significantly bigger than females. This may have reflected polygamy – harem building, as seen in today’s silverback gorillas.

The extra size may have helped adult males beat off challenges from other males. The downside of this emphasis on bodybuilding may have been a vulnerability to predation; Paranthropus bones have been found in caves used by large carnivores. Low levels of sexual dimorphism in humans may therefore be a fairly recent evolutionary innovation, arising within the past couple of million years.

Lead image:

Paranthropus robustus, which lived around 2 million years ago, showed greater sexual dimorphism than modern humans. males were around 20 per cent bigger than females. Levels of sexual dimorphism seem to depend on how much males contribute to child rearing.

Sergio Perez/Flickr CC BY NC ND


Further reading

About this resource

This resource was first published in ‘How We Look’ in June 2008 and reviewed and updated in November 2014.

Physiology, Ecology and environment, Genetics and genomics, History
How We Look
Education levels:
16–19, Continuing professional development