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 used predominantly in the competition over mates, implying sexual selection. The peafowl
Sexual dimorphism in mammals conspicuous illustrations of the principle. The ornate plumage of peacocks, as used in the 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.
Sexual dimorphism in mammals It is believed that this is obtained by the ingestion of green lepidopteran larvae, which contain large amounts of the carotenoids lutein and zeaxanthin. This plumage is thought to be Sexual dimorphism in mammals indicator of male parental abilities. There is a positive correlation between the chromas of the tail and breast feathers "Sexual dimorphism in mammals" 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. For 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 blenniesonly the male fish develops an organ at the anal-urogenital region that produces antimicrobial substances.
During parental care, males rub their anal-urogenital 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. Various other dioecious exceptions, such as Loxostylis alata have visibly different genders, with the effect of eliciting the most efficient behaviour from pollinators, who then use the most efficient strategy in visiting each gender of flower instead of searching say, for pollen in a Sexual dimorphism in mammals female flower.
Some plants, such as some species of Geranium have what amounts to serial sexual dimorphism. The flowers of such species might for example present their anthers on opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while the pistil matures; specialist pollinators are very much inclined to concentrate on the exact appearance of the flowers they serve, which saves their time and effort and serves the interests of the plant accordingly.
Some such plants go even further and change their "Sexual dimorphism in mammals" again once they have been fertilised, thereby discouraging further visits from pollinators. This is advantageous to both parties because it avoids damage to the developing fruit and avoids wasting the pollinator's effort on unrewarding visits.
In effect the strategy ensures that the pollinators can expect a reward every time they visit an appropriately advertising flower. Females of the aquatic plant Vallisneria americana have floating flowers attached by a long flower stalk that are fertilized if they contact one of the thousands of free floating flowers released by a male. Sexual dimorphism in plants can also be dependent on reproductive development. Sexual dimorphism in mammals can be seen in Cannabis sativaa type of hemp, which have higher photosynthesis rates in males while growing but higher rates in females once the plants become sexually mature.
It also should be borne in mind that every sexually reproducing extant species of vascular plant actually has an alternation of generations; the plants we see about us generally are diploid sporophytesbut their offspring really are not the seeds that people commonly recognise as the new generation. The seed actually is the offspring of the haploid generation of microgametophytes pollen and megagametophytes the embryo sacs in the ovules.
Each pollen grain accordingly may be seen as a male plant in its own right; it produces a sperm cell and is dramatically different from the female plant, the megagametophyte that produces the female gamete. Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration.
In some species, there is evidence of male dimorphism, but it appears to be for the purpose of distinctions of roles. This is seen in the bee species Macrotera portalis in which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males. The selection for larger size in males rather than females in this species may have resulted due to their aggressive territorial behavior and subsequent differential mating success.
Andrena agilissima is a mining bee where the females only have a slightly larger than the males. Weaponry leads to increased fitness by increasing success in male-male competition in many insect species. Copris ochus also has distinct sexual and male dimorphism in head horns. Sexual dimorphism within insects is also displayed by dichromatism.
In butterfly genera Bicyclus and Junoniadimorphic wing patterns evolved due to sex-limited expression, which mediates the intralocus sexual conflict and leads to increased fitness in males. Size dimorphism shows a correlation with sexual cannibalismwhich is prominent in spiders it is also found in insects such as praying mantises.
In the size dimorphic wolf spiderfood-limited females cannibalize more Sexual dimorphism in mammals. All Argiope species, including Argiope bruennichiuse this method. Some males evolved ornamentation [ vague ] including binding the female with silk, having proportionally longer legs, modifying the female's web, mating while the female is feeding, or providing a nuptial gift in response to sexual cannibalism.
Ray finned fish are an ancient and diverse class, with the widest degree of sexual dimorphism of any Animal class. Fairbairn notes that "females are generally larger than males but males are often larger in species with male-male combat or male paternal care There are cases where males are substantially larger than females.
An example is Lamprologus callipterusa type of cichlid fish. In this fish, the males are characterized as being up to 60 times larger than the females.
The male's increased size is believed to be advantageous because males collect and defend empty snail shells in each of which a female breeds. The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells.
If she grows too large, she will not fit in the shells and will be unable to breed.