Sexual dimorphism

Female and male mallard, illustrating the difference in both colour.
There is not much differences in male and female horses.
Female (left) and male Common Pheasant, illustrating the big difference in both colour and size.

Sexual dimorphism in biology means that the male and the female of a species look noticeably different. The word comes from the Greek: di (two) and morphe (form).

The features that distinguish the two sexes of a species are called secondary sex characteristics. They are not directly part of the reproductive system.

The difference between sexes can include:

  • Size: Males in some species have harems ('ownership' of a group of females). In this case the males are usually larger than the females, e.g. gorillas, lions. In some species the male is smaller.
  • Sometimes, one sex has additional features : antlers of deer, breasts of women,...
  • Colouring: sexual dimorphism in butterflies is common.

Sexual dimorphism may be the result of natural selection or more specifically sexual selection, that is, competition between members of the same species for reproduction: some traits would give an individual an advantage over its rivals in survival and reproduction.[1] More generally, sexual dimorphism is inherited, which means the differences between males and females have survival value.

Some animals show these differences only during mating season.[2] Deer shed their antlers and peacocks shed their tails out of season. This minimises the key disadvantage of sexual dimorphism, which is, it makes the male much easier to be seen by predators.

There is other polymorphisms.

  • Human have a noticeable sexual dimorphism.
  • Female (left) and male Argiope appensa spiders: the male is much smaller than the female
  • Peacock courting peahen
  • Butterfly Orygia recens: upper is the male; below is the female, which has no wings. Several species in the genus have this arrangement.

References

  1. ↑ Andersson, Malte B. 1994. Sexual selection. Princeton University Press. ISBN 978-0-691-00057-2
  2. ↑ Bonduriansky, Russell 2007. The evolution of condition-dependent sexual dimorphism. The American Naturalist 169 (1): 9–19.
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