Sexual dimorphism

Sexual dimorphism is the condition where the two sexes of the same species exhibit different characteristics beyond the differences in their sexual organs. The condition occurs in many animals and some plants. These differences may consist:

  • in the greater size of the male compared to the female: this is true for most mammals (African elephant, etc.) and birds. However, there are animals in which the exact opposite occurs, i.e. the size of the female is greater than that of the male: this happens in many species of insects, arachnids, fish, and also in some birds (falconiformes, where the different sizes of the two members of the couple reduce competition for food, since the size of the prey of each changes) and mammals (spotted hyena).
  • in the different coloration of the two sexes (sexual dichromatism), where it is usually the male to be more colorful than the female (many galliformes, birds of paradise), but there are also cases where the exact opposite happens (inverse dimorphism, present for example in striped woodcocks, phalaropes, etc.).
  • in the presence or absence in one of the two sexes of certain structures such as antlers (deer), tusks (pigs), elongated and/or colored feathers, stingers, etc.
  • in the presence or absence in one of the two sexes of certain behaviors (parental instinct, innate aggression, etc.).

Often, sexual dimorphism counts for more than one of the above-mentioned characteristics: for example, male peacocks are larger than females, are also more colorful and have long ocellated feathers on the rump.

The phenomenon of sexual dimorphism is a direct product of evolution by natural selection, in that the struggle for reproductive success drives many male and female organisms down different evolutionary paths. This can produce forms of dimorphism which, on the face of it, would actually seem to disadvantage organisms. For instance, the bright coloration of male game birds makes them highly visible targets for predators, while the drab females are far better equipped to camouflage themselves. Likewise, the antlers of deer and other forms of natural weaponry are very expensive to grow and carry in terms of the energy consumed by the animal in the process.

The answer to this apparent paradox is that, at a biological level, the reproductive success of an organism is often more important than its long-term survival. This is particularly apparent in the case of game birds: a male Common Pheasant in the wild often lives no more than 10 months, with females living twice as long. However, a male pheasant’s ability to reproduce depends not on how long he lives but whether females will select him to be their mate. His bright coloration demonstrates to the female that he is fit, healthy and a good choice to father her chicks.

In the case of herd animals such as deer, a male deer’s reproductive success is directly proportional to the number of sexually receptive females with which he can mate. The males’ antlers are an example of a sexually dimorphic weapon with which the males fight each other to establish breeding rights. Again, although they are expensive in terms of personal survival, they ensure that the largest and strongest males will be the most successful in reproducing and thereby ensure that those characteristics are passed on to the next generation.

Access to the opposite sex is not the only reason why sexual dimorphism exists. In insects in particular, females are often larger than the males. It is thought that the reason lies in the huge number of eggs that insects lay; a larger body size enables a female insect to lay more eggs. In some cases, sexual dimorphism enables males and females to exploit different food resources, thus increasing their collective ability to find food. Some species of woodpecker have differently-sized and shaped beaks, enabling the sexes to find insects in different layers of a tree’s bark.

It is also common in birds of prey for the female to be larger than the male, an example of reverse sexual dimorphism. The size difference allows the mated pair to hunt a greater variety of prey for themselves and for their chicks.

The usefulness of sexual dimorphism

The function of sexual dimorphism is mainly to attract the other sex: it is in fact typical of polygamous animals, where during the mating season males duel for the conquest of a territory. In monogamous animals, in fact, is sacrificed the possibility to have more numerous progeny in favor of a joint effort for the breeding of offspring: being the partner fixed, the structures for the defense of the harem become useless.

However, it seems that the characteristics developed by males to attract females make them at a disadvantage compared to the latter, since because of the bright colors are easily located by predators and because of the long feathers or spurs are much slower and clumsy in the escape: but then why an animal evolves unnecessary characteristics, or worse harmful? The theory that deals with this question is the disability theory: it seems that for a living organism reproductive success counts more than the survival of the individual, and therefore it is not important that a male common pheasant lives less than half of a female of the same species, if this allows him to leave as many offspring as possible.

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