Information about Sexual Dimorphism

Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. Examples include size, color, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks.

Examples

In most species, including many mammals, the male is larger than the female. In a few, such as some spiders and birds, many insect species, and certain mammals such as the Spotted Hyena, the female is larger than the male. Other sex-specific differences include differences in colouration (sexual dichromatism), presence vs. absence of certain body parts such as horns, antlers, tusks or display feathers; size of the eyes (some insects); possession of stings (various kinds of Hymenoptera), and different thresholds for certain behaviors (aggression, infant care, etc).



Sexual dimorphism is particularly apparent in most fowl such as ducks, pheasants, and perhaps most dramatically, peafowl. Male pheasants are notably larger than females and possess bright plumage; females are usually a drab brown irrespective of the particular species. In some birds, most of which are waders (such as the phalaropes or painted snipes), females are larger and have brighter colors than males. Also, females are larger than males - often considerably so - in almost all falconiformes and owls. This is termed reverse sexual dimorphism. In the case of the predatory birds, it seems to reduce competition between members of a pair, as they have different optimal prey sizes. Some cases of sexual dimorphism in birds are so striking that males and females of the same species were originally taken to be members of entirely different species, as in the case of the Eclectus Parrot (Eclectus roratus), where the male is predominantly green with an orange beak and the female scarlet and deep blue with a black beak.

Certain sexual dimorphisms have obvious utility beyond mate attraction, such as the Blue Wildebeest. The horns of the male are much larger, allowing the male to engage in combat more effectively as he competes with other bucks for mating privileges. He also uses the horns aggressively to mutilate trees, giving him the dual advantage of enhancing his grazing forage and displaying his strength and stamina to females.

The Huia (Heteralocha acutirostris), a New Zealand bird species (now extinct), was another striking example of reverse sexual dimorphism. The male's bill was short, sharp and stout while the female's was long, thin and crescent shaped. This beak dimorphism allowed mated pairs of Huia to avoid competing for the same food source, with males chiseling into and breaking apart rotting logs, while females were adept at probing into fresher wood for grubs.



Extreme examples of reverse sexual dimorphism are found for example in polychaete worms, for example the genus Osedax, which lives on whale falls where the females feed on the bones. The males live inside the females and do not develop past their larval stage except to produce large amounts of sperm. In the echiuran Bonellia viridis, females force larvae which encounter them to develop into the tiny, semi-parasitic males. The argonauts also have males which are tiny compared to the female. In the parasitic barnacles Sacculina, the males are tiny, free-ranging animals, whereas the females only exist as a web-like tissue inside their hosts.

Some species of anglerfish also display extreme reverse sexual dimorphism. Females are the more recognized "representatives" of the species with illicium for bait, while males are small larvae fish with no digestive systems. The males must find a female and fuse with it and live off her body while producing sperm. A similar situation is found in the Zeus water bug Phoreticovelia disparata where the female has a cavity on her back where males live permanently attached.^[1]

Psychological and behavioral differentiation

Sex steroid-induced differentiation of adult reproductive and other behavior has been demonstrated experimentally in many animals. In some mammals, adult sex-dimorphic reproductive behavior (e.g., mounting or receptive lordosis) can be shifted to that of the other sex by supplementation or deprivation of androgens in fetal life or early infancy, even if adult levels are normal.

Evolution of sexual dimorphism

Most sexual dimorphisms can be explained by evolution. However, there is still uncertainty regarding others.

Handicap principle

Main article: Handicap principle

The handicap principle is the evolutional force that gives males of some species traits that by the first glance seem to place the organism at a disadvantage.

Examples

For instance, the bright colouration of male game birds makes them highly visible targets for predators, while the drably coloured 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.

Other examples are decorations and bright coloration of many bird males, long tail feathers in bird of paradise or lyrebird males which inhibit their flight. Strong smells, loud cries and singing can also attract predators.

Explanation

The answer to this apparent paradox is that, at a biological level, the reproductive success of an organism is often more important than duration of life. 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.

One explanation for why females select more brightly coloured males is that it demonstrates to the female that he is fit in spite of the impediments and therefore a healthy and a good choice to father her chicks. This explanation was first proposed by Amotz Zahavi.

Development of such characters could not be explained in terms of natural selection. For their explanation in 1871 Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection. It was a matter of controversy even then. Many authors thought it to be the weakest point of Darwin's theory.

Polygamy

Comparison of sexual dimorphism in birds and their mating habits shows that the time spent in search for mates, staking territories and mating competes with the demands of taking care of young. For birds and in general, it can be stated that the stronger the dimorphism in a species, the more likely is it to be polygamous and the less is the task of caring for offspring shared among the sexes. This theory is developed by R. L. Trivers' in the parental investment theory. It applies to all ecology.

Uncertainty

Treating the general phenomenon of sexual dimorphism as a consequence of narrow mechanism of sexual selection created many problems. It was hard to explain sexual dimorphism for characters, which with great difficulty can be related to sexual selection (e.g., leaf number and shape, branching pattern in plants). Interpretation of the same phenomenon needed different logics. For example, in birds larger size of males is explained by preference in the struggle for female and larger size of females—by advantage of laying large eggs. But it is unclear why in the first case no large eggs and in the second no struggle for female are needed. It is still difficult for the theory to explain large size of females in some mammals (bats, rabbits, flying squirrels, spotted hyenas, dwarf mongooses, some whales and seals), existence of marked sexual dimorphism in monogamic species with sex ratio 1:1, and the dependence of sexual dimorphism on the reproductive structure of the population.^[2]

Sexual dimorphism in humans

Pioneer plaque
Top: Stylised illustration of humans on the Pioneer plaque, showing both male and female. Above: Comparison between a male (left) and a female pelvis (right).

Main article: Sex differences in humans

Sexual dimorphism in humans is the subject of much controversy, especially relating to mental ability and psychological gender. (For a discussion, see biology of gender, sex and intelligence, gender, and transgender.) Obvious differences between men and women include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physical, psychological and behavioural effects.

Such undisputed sexual dimorphism include gonadal differentiation, internal genital differentiation, external genital differentiation, breast differentiation and hair differentiation.

Some biologists theorise that a species' degree of sexual dimorphism is inversely related to the degree of paternal investment in parenting. Species with the highest sexual dimorphism, such as the pheasant, tend to be those species in which the care and raising of offspring is done only by the mother, with no involvement of the father (low degree of paternal investment). This would also explain the low degree of sexual dimorphism in humans, who have a high degree of paternal investment compared to most other mammals. Older texts sometimes claim that humans have a high degree of sexual dimorphism, but closer study has shown that this is not the case (Brin, 2004).

Comparative and social psychologists have observed that males and females, in general, differ in the way they carry books while walking. Upon using a classification system of the five common methods of carrying books, a high percentage of females will partially cover their body with the books they are carrying, such as by holding them in front of the chest. Most males carry their books at the side of body, leaving the front uncovered (Jenni, M.A. 1976). The most common explanation of this observation is that women typically have less upper body strength than men, making it difficult to balance, and resulting in the need to rest the objects they are carrying on their bodies. Some psychologists hypothesize that it is a maternal instinct in many women causing them to carry inanimate objects in a protective manner. It is also possible that this difference is due to expectations and roles of men and women in society.

Non-nervous system

The basal metabolic rate is about 6 percent higher in adolecent boys than girls and increases to about 10 per cent higher after puberty. During metabolism, girls convert more energy into stored fat, while boys convert more energy to muscle and expendable circulating reserves. At age eighteen, boys have about 50 percent more muscle mass than girls, particularly in the upper body. Men, on average, have denser, stronger bones, tendons, and ligaments.^{[3] [4] [5] [6] [7] [8]}

See also

• Bateman's principle
• Digit ratio
• Sexual selection
• Sexual differentiation
• Sexually dimorphic nucleus
• Sexual dimorphism measures
• Sexual reproduction
• Sex-limited genes
• Gender differences

References

• Bonduriansky, R. (2007) The evolution of condition-dependent sexual dimorphism. The American Naturalist, 169:1 pp9-19.
• Biological Journal of the Linnean Society (1999), 67: 1–18.

External links

• MeSH Sex+dimorphism

•  • [ e] Physiology, endocrinology, sex: Reproductive physiology and endocrinology
Menstrual cycle/Estrous cycle	Menstruation - Follicular phase - Ovulation - Luteal phase
Gametogenesis	Spermatogenesis -Oogenesis
Sexuality	Human sexual behavior - Sexual intercourse - Erection - Ejaculation - Orgasm - Insemination - Fertilisation/Fertility - Masturbation - Pregnancy - Postpartum period
Lifespan	Prenatal development - Sexual dimorphism - Sexual differentiation - Puberty (Menarche, Adrenarche) - Maternal age/Paternal age - Climacteric (Menopause, Andropause)
Eggs	Oviposition - Oviparity - Ovoviviparity - Viviparity