Sexual dimorphism
Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species.
Examples of sexual dimorphism
In some species, including many mammals, the male is larger than the female. In others, such as some spiders and birds, and many insect species, the female is larger than the male. Other sex-specific differences include color (sexual dichromatism, many birds), size or presence of parts of the body used in struggles for dominance, such as horns, antlers, and tusks; size of the eyes (bees); possession of stings (various kinds of bees), and different thresholds for certain behaviors (aggression, infant care, etc.).
Sexual dimorphism is particularly apparent in game bird species such as the pheasants. Male pheasants are notably larger than females and possess bright plumage; females are usually a drab brown. In some birds, most of which are waders (such as the phalaropes or painted snipes), females are larger and have brighter colors than males. This is termed reverse sexual dimorphism. 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, a dual functionality of enhancing his grazing forage and attracting the female with this athletic display.
The Huia (Heteralocha acutirostris), a New Zealand bird species (now extinct), was another striking example of 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 adapt at probing into fresh wood for grubs.
One of the most extreme examples of sexual dimorphism is found in small polychaete worms of the genus Osedax, which live on whale falls. The females feed on whale bones. The males live inside the females and do not develop past their larval stage except to produce large amounts of sperm.
Some species of anglerfish also display extreme 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.
Sexual dimorphism in humans
Sexual dimorphism in humans is the subject of much controversy, especially relating to mental ability and psychological gender. (For a discussion, see sex and intelligence, gender, and transgender.)
There is also sexual dimorphism in the amount and distribution of body hair, with males having more terminal hair, especially on the face, chest, abdomen and back, and females having more vellus hair, which is less visible. This may also be linked to neoteny in humans, as vellus hair is a juvenile characteristic.
Human male and female appearances are perceived as different, although Homo sapiens has a low level of sexual dimorphism compared with other species. For instance, the relative similarity in the sizes of many male and female human beings suggests that divisions for homo sapiens are less clear than other species as the overlap of sizes between the two sexes is slightly less than 1 standard deviation. The Centers for Disease Control and Prevention published new American curves in 2000. [1] See also intersex.
The body masses of both male and female humans are approximately normally distributed. In the United States, the mean mass of an adult male is 78.5 kg (173.1 lb), that of the adult female mean is 62.0 kg (136.7 lb). The standard deviation of male body mass is 12.6 kg (27.8 lb), so 10% of adult males are lighter than the average female.
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). Conversely, the low level of sexual dimorphism in humans is said to correlate to the human species' high degree of paternal investment (fathers make a high investment in childrearing).
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, and 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.
Biological aspects of sexual dimorphism
Initially, many instances of sexual dimorphism would seem to place the organism at a disadvantage. 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.
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. His bright colouration demonstrates to the female that he is fit, healthy and a good choice to father her chicks. (See also Handicap principle)
Comparison of sexual dimorphism in birds and their mating habits shows that another major - possibly the original - purpose of drab color in one sex of sexually dimorphic species is to provide a better camouflage during breeding. In birds that display reverse sexual dimorphism, males do most of incubation and care for the young while females stake out territories and court for males. For birds 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. These two traits - sexual attractiveness in one sex and protection during breeding in the other, and limitation for offspring care to one sex only coupled with polygamous mating habits - evolve cooperatively: as soon as one sex has started to evolve in the direction of brighter plumage, it is at a disadvantage when sharing in incubation or chick rearing. This encourages polygamous mating without lasting pair bonds, which automatically places those members of the other sex at a fitness advantage which happen to have duller coloration than the others. It works just as well from the standpoint of the other sex, because the more the brighter-colored sex assist in care for offspring, the higher the risk of them falling victim to predation. Thus, dull-bright sexually dimorphic bird species are the consequence of a strategy for reproductive - ultimately evolutionary - success where the tasks associated with reproduction are not shared but divided between the sexes. Note that this does not hold true for the Eclectus Parrot and other species where sexes are dissimilar, but both conspicuous (in the parrot's case mainly due to their voalizations).
In the case of herd or lekking animals such as red 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, although especially in deer, the fights are usually not deadly, but rather tests of strength and stamina preceded by a phase where they present each other to their best effort, parading in front of the herd of females. Again, although they are expensive in terms of personal survival, large antlers ensure ultimately, that the largest and strongest males will be the most successful in reproducing, thereby passing on their good qualities to the next generation. It is interesting to note that, as opposed to common wisdom, the size of a stag's antlers are not an indication of his place in the present social hierarchy, but result from the situation in the preceding season. Thus, no matter what chance events have influenced the stag's current social standing, large antlers indicate that this particular individual was able to maintain a high social position for considerable time during the last year and survive the winter, suggesting it has well-developed physical and mental attributes.
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, a less extreme version of the situation in the huia.
Sexual dimorphism is sometimes quantified by biologists through the dimorphism index, which is usually the ratio between the average adult male mass and average adult female mass (see for a thorough review sexual dimorphism measures). For some species mass is inconvenient to measure, so a similar parameter such as volume is used instead. This index is commonly written as the abbreviation "SSDI", for "sexual size dimorphism index". Species that are typically polygynous tend to have high SSDI ratios, while species that are typically polyandrous tend to have low ratios.
Sexual dimorphism is regarded as a classic example of genetic polymorphism, though its underlying mechanism varies in different organisms. It is often controlled by genes on the sexual chromosomes.