Sexual dimorphism

Female (left) and male Common Pheasant, illustrating the big difference in both colour and size.

Female (left) and male Argiope appensa spiders: the male is much smaller than the female

Butterfly Orygia recens: upper is the male; below is the female, which has no wings. Several species in the genus have this arrangement.

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

It is the most common type of polymorphism. It may be the result of sexual selection, that is, competition between members of the same species for reproduction.^[1] More generally, sexual dimorphism is inherited, and that it means the differences between males and females have survival value.

The features that distinguish the two sexes of a species are called secondary sex characteristics. They are not directly part of the reproductive system. They are the product of sexual selection for traits which give an individual an advantage over its rivals in survival and reproduction.

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.
- Sexual dimorphism is extreme in rotifers, with the males (if present) always much smaller than the females. Even more extreme are the sea devils, whose tiny males physically fuse with the females to form a chimaera.
- Hair (males often have more hair, e.g. the Gorilla)
- Teeth: Asian elephants: only the males have tusks. African elephant: both sexes have tusks. pigs and walrusses: the males have very pronounced canines
- Horns or antlers are often carried only by males.
Colouring: sexual dimorphism in butterflies is common. Batesian mimicry is often seen only on females, while the males have the typical colours of their type. This is the usual explanation: females, often carrying the precious cargo of eggs, get the most benefit from mimicry.^[2]

The main functions of sexual dimorphism are to improve the individual's chance of in various ways:

Mate selection. In this case, usually the males display, and the females select.
Territory defence. Males may signal to other males that they 'own' an area.
Fighting. If males carry weapons, they may use them to fight for mates. Weapons and size are multi-functional: they are used for defence against predators as well as against other males of their species.^[3] 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.

References

↑ Andersson, Malte B. 1994. Sexual selection. Princeton University Press. ISBN 978-0-691-00057-2
↑ Ruxton G.D; Sherratt T.N. and Speed M.P. 2004. Avoiding attack: the evolutionary ecology of crypsis, warning signals & mimicry. Oxford. p156 10.5.3 Why is Batesian mimicry often limited to one sex? "Experimental evidence that males and females of the same species may experience different selective pressures with respect to predation, is widespread".
↑ Bonduriansky, Russell 2007. The evolution of condition-dependent sexual dimorphism. The American Naturalist 169 (1): 9–19. [1]

[1] Andersson, Malte B. 1994. Sexual selection. Princeton University Press. ISBN 978-0-691-00057-2

[2] Ruxton G.D; Sherratt T.N. and Speed M.P. 2004. Avoiding attack: the evolutionary ecology of crypsis, warning signals & mimicry. Oxford. p156 10.5.3 Why is Batesian mimicry often limited to one sex? "Experimental evidence that males and females of the same species may experience different selective pressures with respect to predation, is widespread".

[3] Bonduriansky, Russell 2007. The evolution of condition-dependent sexual dimorphism. The American Naturalist 169 (1): 9–19. [1]

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