Sexual dimorphism
Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction.[1] The condition occurs in most dioecious species, which consist of most animals and some plants. Differences may include secondary sex characteristics, size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection.[2] These differences may be subtle or exaggerated and may be subjected to sexual selection and natural selection. The opposite of dimorphism is monomorphism, when both biological sexes are phenotypically indistinguishable from each other.[3]
"Sex differences" redirects here. For sexual dimorphism in humans, see Sex differences in humans.Plants[edit]
Most flowering plants are hermaphroditic but approximately 6% of species have separate males and females (dioecy).[21] Sexual dimorphism is common in dioecious plants[22]: 403 and dioicous species.[23]: 71
Males and females in insect-pollinated species generally look similar to one another because plants provide rewards (e.g. nectar) that encourage pollinators to visit another similar flower, completing pollination. 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.[24]
Various other dioecious exceptions, such as Loxostylis alata have visibly different sexes, with the effect of eliciting the most efficient behavior from pollinators, who then use the most efficient strategy in visiting each gender of flower instead of searching, say, for pollen in a nectar-bearing 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 appearance once fertilized, thereby discouraging further visits from pollinators. This is advantageous to both parties because it avoids damaging the developing fruit and wasting the pollinator's effort on unrewarding visits. In effect, the strategy ensures that 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.[25] Sexual dimorphism is most often associated with wind-pollination in plants due to selection for efficient pollen dispersal in males vs pollen capture in females, e.g. Leucadendron rubrum.[26]
Sexual dimorphism in plants can also be dependent on reproductive development. This can be seen in Cannabis sativa, a type of hemp, which have higher photosynthesis rates in males while growing but higher rates in females once the plants become sexually mature.[27]
Every sexually reproducing extant species of the vascular plant has an alternation of generations; the plants we see about us generally are diploid sporophytes, but their offspring are not the seeds that people commonly recognize 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.
Fish[edit]
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 ... [sizes range] from dwarf males to males more than 12 times heavier than females."[52]
There are cases where males are substantially larger than females. An example is Lamprologus callipterus, a 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.[53] Males must be larger and more powerful in order to collect the largest shells. 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. The female's small body size is also likely beneficial to her chances of finding an unoccupied shell. Larger shells, although preferred by females, are often limited in availability.[54] Hence, the female is limited to the growth of the size of the shell and may actually change her growth rate according to shell size availability.[55] In other words, the male's ability to collect large shells depends on his size. The larger the male, the larger the shells he is able to collect. This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial. Male–male competition in this fish species also selects for large size in males. There is aggressive competition by males over territory and access to larger shells. Large males win fights and steal shells from competitors. Another example is the dragonet, in which males are considerably larger than females and possess longer fins.
Sexual dimorphism also occurs in hermaphroditic fish. These species are known as sequential hermaphrodites. In fish, reproductive histories often include the sex-change from female to male where there is a strong connection between growth, the sex of an individual, and the mating system within which it operates.[56] In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success.[57] Males have a propensity to be larger than females of a comparable age but it is unclear whether the size increase is due to a growth spurt at the time of the sexual transition or due to the history of faster growth in sex changing individuals.[58] Larger males are able to stifle the growth of females and control environmental resources.
Social organization plays a large role in the changing of sex by the fish. It is often seen that a fish will change its sex when there is a lack of a dominant male within the social hierarchy. The females that change sex are often those who attain and preserve an initial size advantage early in life. In either case, females which change sex to males are larger and often prove to be a good example of dimorphism.
In other cases with fish, males will go through noticeable changes in body size, and females will go through morphological changes that can only be seen inside of the body. For example, in sockeye salmon, males develop larger body size at maturity, including an increase in body depth, hump height, and snout length. Females experience minor changes in snout length, but the most noticeable difference is the huge increase in gonad size, which accounts for about 25% of body mass.[59]
Sexual selection was observed for female ornamentation in Gobiusculus flavescens, known as two-spotted gobies.[60] Traditional hypotheses suggest that male–male competition drives selection. However, selection for ornamentation within this species suggests that showy female traits can be selected through either female–female competition or male mate choice.[60] Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during breeding season are considered favorable to males.[61] The males invest heavily in offspring during incubation, which leads to the sexual preference in colorful females due to higher egg quality.[61]
Immune function[edit]
Sexual dimorphism in immune function is a common pattern in vertebrates and also in a number of invertebrates. Most often, females are more 'immunocompetent' than males. This trait is not consistent among all animals, but differs depending on taxonomy, with the most female-biased immune systems being found in insects.[127] In mammals this results in more frequent and severe infections in males and higher rates of autoimmune disorders in females. One potential cause may be differences in gene expression of immune cells between the sexes.[128] Another explanation is that endocrinological differences between the sexes impact the immune system – for example, testosterone acts as an immunosuppressive agent.[129]
Cells[edit]
Phenotypic differences between sexes are evident even in cultured cells from tissues.[130] For example, female muscle-derived stem cells have a better muscle regeneration efficiency than male ones.[131] There are reports of several metabolic differences between male and female cells[132] and they also respond to stress differently.[133]