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. Differences may include secondary sex characteristicssize,weight, color, markings, and may also include behavioral differences.
These differences may be subtle or exaggerated, and may be subjected to sexual selection. The opposite of dimorphism is monomorphism.
Common and easily identified types of dimorphism are ornamentation and coloration, though not always apparent. A difference in coloration of sexes within a given species is called sexual dichromatism, which is commonly seen in many species of birds and reptiles. The increased fitness resulting from ornamentation offsets its cost to produce or maintain suggesting complex evolutionary implications, but the costs and evolutionary implications vary from species to species.
Exaggerated ornamental traits are "Asexual dichromatism" predominantly in the competition over mates, implying sexual selection.
The peafowl constitute conspicuous illustrations of the principle. The ornate plumage of peacocks, as used in the courting display, attracts peahens. At first sight one might mistake peacocks and peahens for completely different species because of the vibrant colours and the sheer size of the male's plumage; the peahen being of a subdued brown coloration.
Another example of sexual dichromatism is that of the nestling blue tits. Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green lepidopteran larvae, which contain large amounts of the carotenoids lutein zeaxanthin. This plumage is thought to be an indicator of male parental abilities.
There is a positive correlation between the chromas of the tail and breast feathers and body condition. Frogs constitute another conspicuous illustration of the principle.
There are two types of dichromatism for frog species: Ontogenetic frogs are more common and have permanent color changes in males or females. Litoria lesueuri is an example of a dynamic frog that has temporary color changes in males during breeding season. At sexual maturity, the males display a bright green with white dorsolateral lines. The bright coloration in the male population serves to attract females and as an aposematic sign to potential predators. Females often show a preference for exaggerated male secondary sexual characteristics in mate selection.
Similar sexual dimorphism and mating choice are also observed in many fish species. Asexual dichromatism example, male guppies have colorful spots and ornamentations while females are generally grey in color. Female guppies prefer brightly colored males to duller males. In redlip Asexual dichromatismonly the male fish develops an organ at the anal-urogenital region that produces antimicrobial substances.
During parental care, males rub their Asexual dichromatism regions over their nests' internal surfaces, thereby protecting their eggs from microbial infections, one of the most common causes for mortality in young fish.
Catasetum orchids are one interesting exception to this rule. Male Catasetum orchids violently attach pollinia to euglossine bee pollinators. The bees will then avoid other male flowers but may visit the female, which looks different from the males.