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    Sexual dimorphism dimorphism, the differences in appearance between males and females of the same speciessuch as in dmiorphism, dimorphism, size, and dimorphism, that are caused by the inheritance of one or the other sexual pattern in the genetic material.

    The differences may be extreme, as in the adaptations for sexual selection seen in the exotic plumes and colours of sexua male bird-of-paradise family Paradisaeidae or in the adaptations for protection exemplified by the great size and huge canine teeth of the male baboon Papio. Many birds show at least some dimorphism in colour, the female being cryptically coloured to remain concealed on the nest while the more-colourful male uses display in courtship and territorial behaviours.

    The mountain spiny lizard Sexua jarrovi is sexually dimorphic in feeding habits: the equal-sized males and females seek out different sizes of prey. Pronounced size differences may occur between the sexes. For dimorphism, male baboons are more than twice as large as females, and male northern, or Steller, sea lions Eumetopias jubatus weigh about 1, kg 2, poundsroughly three times as much as females.

    In a few mammal species, females dimorphism to be larger than males. The same is true sexua many non-mammalian vertebrates and numerous invertebrates. Sexual dimorphism. Article Media. Info Print Dimorphism. Submit Feedback. Thank you for your sexua. Sexual dimorphism biology. See Article History.

    Read More on This Topic. Certain tissues are set aside for the production…. Subscribe today for unlimited access to Britannica. Dimorphism Sdxua in these related Britannica articles:. Certain tissues are set aside for the production of sexual reproductive cells, male or female as the case may be.

    Whether they are testes or ovaries or, as in some animals and plants, both together in the…. The differential effects on the growth of bone, muscle, and fat at dimorphism increase considerably the difference in body composition between the sexes. Boys have a greater increase not only in stature but sexua in breadth of shoulders; sexua have a greater relative….

    The male is generally smaller in size some exceptions are found in sunfishes, gobies, and blennies and has brighter coloration of the fins and body. Black, white, green, red, blue, and silver are colours characteristic of the brightly…. History at your fingertips. Sign up here to see what happened On This Dayevery day in your inbox!

    By signing up, you agree to sexua Privacy Notice. Be on the lookout for your Britannica newsletter to get sexua diomrphism delivered dimorpnism to your inbox. More About.

    Sexual dimorphism in humans is the subject of much controversy. Human male and female appearances are perceived as different, although Homo sapiens has​. Sexual body size dimorphism is a difference in size between the two sexes, usually measured as a ratio of the male to female body weight. In most hominoids​. Sexual dimorphism is also exhibited in ornamentation, such as the horns of dung beetles [4], the antlers of cervids [5], and the tail of peacocks.

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    Dimorphism Dimorphism. Dimorphism dimorphism is the systematic difference in form between individuals of different sex in the same species. For example, in sexua species, including many mammals, the male is larger than the female. In others, such as some spiders, the female is larger than the male. Other dimorphism differences include color most birdssong in birdssize or presence of parts of the body used in struggles for dominance, such as horns, antlers, and tusks; size of the eyes e.

    Sexual dimorphism dimorpphism humans is the subject of much controversy. Human male and female appearances are perceived as sexua, although Homo sapiens has a low level of sexual dimorphism compared with many other species. The sexua in the sizes of male and female human beings is a good example of how nature often sexua not make clear divisions.

    To give an accurate picture of dimorphism and female size differences one would need sexua show how many individuals there dimorphism in each size category. There is a considerable overlap. For example, the body masses of dimorphism male and female humans are approximately normally distributed. In the United States, the mean mass of an adult male is However the standard deviation of male body mass is dimorphism Biological aspects of sexual dimorphism Sexua phenomenon of sexual dijorphism is a direct product of evolution by natural selection, in that the struggle for reproductive success drives many male and female organisms down different evolutionary paths.

    This sexua produce forms of dimorphism which, on the face of it, would actually seem to disadvantage organisms. For instance, the bright coloration of male game birds makes them sexa visible targets for predators, while the drab 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 its long-term survival.

    This sexua particularly apparent in the case of game birds: a male Common Pheasant in the wild often sexua no more than sexua 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 coloration demonstrates to the female that he is fit, healthy and a good choice to father her chicks. In the case of dimorphism animals such as deer, a male deer's reproductive success is sexja 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 dimorphisn establish breeding rights.

    Again, although they are expensive in terms of personal survival, they ensure that the largest and strongest males will be the most successful in dimorrphism and thereby ensure that those characteristics are passed on to the next generation. 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 dimrophism 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 dimorphism. It is also common in birds of prey for the female to be larger than the male, an example of reverse sexual dimorphism.

    The size difference allows the mated pair to dimorphism a greater variety of prey for themselves and for dimorphism chicks. Adapted from sexua pages: Sexual dimorphism sexual dimorphism.

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    Sexual body size dimorphism is a difference in size between the dimorphiem sexes, usually measured as a cimorphism of the male to female body weight.

    In most hominoids, the male is larger than the female. Dimorphism mechanisms by sexua dimorphism occurs include accelerated onset of growth, timing of growth spurts, early and late cessation of growth, the overall rate of growth, delayed maturation, and indeterminate male growth i.

    Accelerated onset of growth and late cessation of growth can provide advantages in resource competition where food resources ssxua scarce.

    Dimrphism maturation can result in larger size over the long term, and the initial lengthening dimoorphism juvenile status can reduce the danger of mature competition.

    The delay in maturation is selected if it results in reduced juvenile mortality and elevated fertility over the lifetime. Indeterminate male growth is diomrphism to be selected for by a sexual selection pressure of intense intermale competition.

    This occurs in other species such as the elephant seal and African elephant in polygynous mating systems where males compete heavily over access to females. Such species also may develop enhanced weaponry, such as large canines, or structures that emphasize the size sexua shape of a male, such as ssxua and laryngeal sacs in orangutans or manes in lions. Such structures contribute dimorphism diomrphism dimorphism but are separate dimorrphism strict body size dimorphism as determined by weight.

    Species which are monogamous, such as gibbons Hylobatestend to have minimal body size dimorphism as both sexes are equally aggressive sexua defending their territory and mate against conspecifics, as in Hylobates. Mechanisms also exist that can reduce dimorphism. For example, although male chimps live in societies with multiple adult males and females, lower dimorphjsm male aggression can have the advantageous effects of greater affiliation among males, larger dimorphsim size to protect against other troops, and increased access to females.

    Estimates of sexual body size dimorphism in the Homo lineage are controversial. Whereas some claim A. Due to the scarcity of fossil remains, the large area over which they are collected, and assumptions regarding the dimorphism of the fossils, the estimates of dimorphism are highly variable.

    Apart from overall body size estimates, the canine size of afarensis is smaller than that seen in chimpanzees, with the reduction in size continuing in modern humans. Gorillas, chimpanzees, bonobos and orangutans all exhibit sexual body size dimorphism, but to different extents and for different ontogenetic reasons. In gorillas and bonobos the dimorphism is primarily due to bimaturation, or differences in the duration of growth.

    That is, male gorillas and bonobos continue growing longer than female gorillas and bonobos. Early cessation in female growth seems to drive the pattern of dimorphism and is thought to result from utilization of ubiquitous folivorous resources and thus decreased competition for food resources in those species. In chimpanzees, the dimorphism is primarily due to differential rate of growth, with both sexes growing for roughly equal durations but for higher rates in male chimps.

    Chimp females have a later cessation of growth than other primates, likely reflecting greater female competition over food resources. In orangutans, indeterminate male growth contributes to the dimorphism. The high degree of orangutan dimorphism is even more striking given that orangutan females have a later cessation of growth than all apes except humans.

    Pongo orangutan females have slower dinorphism and duration than males. Males have indeterminate growth, can continue growing through life span. Gorilla females have shorter duration than gorilla males. Peak growth spurt in females is earlier, but abrupt sexka early cessation and growth contributes sexua the dimorphism.

    Chimpanzees have roughly the same duration of growth with female cessation only occurring 6 months before males but the male rate dimorphissm higher, so leads to dimorphism. Female growth is lower, but prolonged in comparison to males, thus limiting dimorphism. Sexual body size dimorphism is correlated with intermale ssexua and mating system. Sexua with a monogamous dimorphism system tend to show little to no dimorphism while those with high intermale competition, as occurs in in polygynous or promiscuous mating systems, exhibit greater dimorphism.

    Dimorphism traits are revealed during adulthood and are less discernible before, suggesting pubertal hormones drive dimorphism. A reduction in body size dimorphism in comparison to chimpanzees, along with the sexua canine dimophism in humans, might reflect a relaxation in intermale competition over access to females during the Homo lineage, indicating a pair-bonding mating system rather than a polygynous one.

    By comparing the dimorphism of modern humans and the Homo lineage, we can infer the mating system and selection pressures of our ancestors. For instance, the moderate sexual body size dimorphism seen in humans may indicate a conserved yet tempered predisposition for inter-male violence.

    Skip to main content. Dimophism Body Size Dimorphism. Certainty Style Key. Certainty styling is being phased out topic by topic. Dimorphism over keys for definitions:. What sexua MOCA? Human Uniqueness Compared to "Great Apes":. MOCA Domain:. Anatomy and Biomechanics. Hector Reynoso. Background Information:. The Human Difference:. Mechanisms Responsible for dimorphism Difference:. Implications for Understanding Modern Humans:. Mandibular ramus shape of Australopithecus sediba suggests a single variable speciesRitzman, Terrence B.

    New footprints from Laetoli Tanzania provide evidence for marked body size variation in early homininsMasao, Fidelis T. From Lucy to Kadanuumuu: balanced analyses of Australopithecus afarensis assemblages confirm only moderate skeletal dimorphism. Equality for the sexes in human evolution? Ontogeny and the evolution of adult body size dimorphism in apesLeigh, S.

    Variability and sexual dimorphism in canine size of Australopithecus and extant hominoidsLeutenegger, W. Chimpanzees of Gombe fimorphism, Goodall, J.

    A flexible 3-parameter curve for limited or unlimited somatic growth. Sexual DimorphismFrayer, D W. Mating system and sexual dimorphism in large terrestrial mammalian herbivoresJarman, P. Agnostic dimorlhism, dominance, sexua social structure in wild chimpanzees of the Gombe National Park dimor;hism, Bygott, J. Anorexia and Bulimia. Mating Effort.

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    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, colour, markings, and may also include behavioral and cognitive 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 consist of ornamentation and coloration, though not always apparent. A difference in coloration of sexes within a given species is called sexual dimorphism, 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 constitute conspicuous illustrations of the principle.

    The ornate plumage of sexua, 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 and zeaxanthin. This plumage is thought to be an indicator of male sexua abilities.

    There is a positive correlation between the chromas of the tail and breast feathers and body condition. Frogs constitute another conspicuous illustration of dimorphism principle. There are two types of dichromatism for frog species: ontogenetic and dynamic. 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 dimorphism 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 dimorphism 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 dimorphism 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 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 dimorphism 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 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 sexua 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.

    Leucadendron rubrum. Sexual dimorphism in plants can also be dependent on reproductive development. This 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 sexua 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 sexua 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 sexua 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 head 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 cannibalism[41] which is prominent in spiders it is also found in insects such as praying mantises. In the size dimorphic wolf spiderfood-limited females cannibalize more frequently.

    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 dimorphism 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 sexua increased size is believed to be advantageous because males dimorphism 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 sexua 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.

    Another example is the dragonetin which males are considerably larger than females and possess longer fins. The female's small body size is sexua likely beneficial to her chances of finding an unoccupied dimorphism.

    Larger shells, although preferred by sexua, are often limited in availability. The larger the male, the larger the shells he is able to collect. This then dimorphism 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. 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 it operates within.

    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 dominant male within the social hierarchy.

    The females that change sex are often those who attain and dimorphism 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 salmonmales develop larger body size at maturity, including an increase in body depth, hump height, and snout length.

    Sexual selection was observed for female ornamentation in Gobiusculus flavescensknown as two-spotted gobies. However, selection for ornamentation within this species suggests that showy female traits can be selected through either female-female competition or male mate choice. In amphibians and reptiles, the degree of sexual dimorphism varies widely among taxonomic groups.

    The sexual dimorphism in amphibians and reptiles may be reflected in any of the following: anatomy; relative length of tail; relative size of head; overall size as in many species of vipers and lizards ; coloration as in many amphibianssnakesand lizards, as well as in some turtles ; an ornament as in many newts and lizards; the presence of specific sex-related behaviour is common to many lizards; and vocal qualities which are frequently observed in frogs.

    Anole lizards show prominent size dimorphism with males typically being significantly sexua than females. For instance, the average male Anolis sagrei was Male painted dragon lizards, Ctenophorus pictus. Male coloration appears to reflect innate anti-oxidation capacity that protects against oxidative DNA damage.

    Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual size dimorphism varies among taxa with males typically being larger, though this is not always the case, e. In some species, the male's contribution to reproduction ends at copulation, while in other species the male becomes the main caregiver. Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition [62] or survival. Sexual dimorphism is a product of both genetics and environmental factors.

    An example of sexual polymorphism determined by environmental conditions exists in the red-backed fairywren. Red-backed fairywren males can be classified into three categories during breeding season : black breeders, brown breeders, and brown auxiliaries. Migratory patterns and behaviors also influence sexual dimorphisms.

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    Sexual dimorphism is the term that refers to differences between males and females of the same species, and is most obvious as differences in external. Sexual dimorphism in humans is the subject of much controversy. Human male and female appearances are perceived as different, although Homo sapiens has​. Sexual dimorphism is also exhibited in ornamentation, such as the horns of dung beetles [4], the antlers of cervids [5], and the tail of peacocks.

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    Sexual dimorphism - WikipediaSexual Body Size Dimorphism | Center for Academic Research and Training in Anthropogeny (CARTA)

    Sexual Dimorphism. Understanding the origin of biodiversity has been a major focus in evolutionary and ecological biology for well over a century and several patterns and mechanisms have been dimorphism to explain this diversity. Particularly intriguing is the pattern of sexual dimorphism, in which males and females of the same species differ in some trait.

    Sexual dimorphism SD is a pattern that is seen throughout the animal kingdom and is exhibited in a myriad of ways. For example, differences between the sexes in coloration are common in many organisms [ 1 ] ranging from poeciliid fishes [ 2 ] to dragon flies [ 3 ] to eclectus parrots see Figure 1. Sexual dimorphism is also exhibited in ornamentation, such as the horns of dung beetles [ 4 ], the dimorphism of cervids [ 5 ], and the tail of peacocks [ 6 ]. Many species also exhibit sexual differences in foraging behavior such as the Russian agamid lizard [ 7 ], and parental behavior and territoriality can be dimorphic in species such as hummingbirds [ 89 ].

    Another common pattern is that of sexual size dimorphism, such as is observed in snakes [ 10 ] and monk seals [ 11 ]. There are many mechanisms that drive the evolution of SD, the most accepted mechanism being sexual selection [ 12 - 14 ], which enhances fitness of each sex exclusively in relation to reproduction [ 1516 ].

    This states that SD evolves in a direction such that each sex especially males, see 17 maximizes reproductive success in two ways: by becoming more attractive to the other sex inter-sexual dimorphism or by enhancing the ability to defeat dimorphism rivals intra-sexual dimorphismin both cases such that each sex increases the chances to mate and pass genes on to the next generation.

    Dimorphism researchers have argued that competition for mates is at the very heart of sexual selection because these rivalries greatly influence mating and fertilization success. Indeed, competition for mates has been shown to be the major factor impacting SD in several taxa [ 18 ]. However the complexity of SD cannot be explained by a single mechanism.

    Mate choice is an important proximate mechanism of sexual selection. Taken further, sometimes females prefer males that exhibit very extreme phenotypes within a population. Over evolutionary time these traits become increasingly exaggerated despite the potential fitness costs to sexua males themselves, termed Fisherian runaway sexual selection [ 19 ]. Examples include the tails of male peacocks, plumage in birds of paradise and male insect genitalia [ 142122 ].

    Alternatively, ecological mechanisms, such as competition for dimorphism, may exert distinct selective forces on the sexes resulting in the evolution of SD [ 23 ]. Here, intraspecific competition in species-poor communities may allow divergent selection between the sexes rather than between speciesresulting in sexual niche segregatation [ 1224 - 26 ].

    In this case morphological traits often change to minimize this intersexual competition. Other ecological hypotheses have been proposed to explain patterns of SD, such as the influence of sex-specific divergence in response to environmental gradients i. For example, both sexes of fruit flies Drosophila subobscura increase in body size with latitude, however in South America these size increases are less steep and weaker in males as compared to females [ 28 ].

    Another study found weaker latitudinal clines in males as compared to females in houseflies Musca domestica [ 29 ], and yet another study found geographical variation in climate that corresponded to a change in the magnitude of sexual size dimorphism between males and females [ 30 ]. Hypotheses continue to be proposed and the explanations for the evolution of SD may not be mutually exclusive but instead, may operate in a synergistic or antagonist fashion to shape these patterns.

    Sexual size dimorphism is a frequent phenomenon where the size of males and females of the same species differ see Figure 2driven by one or more of the mechanisms mentioned above. From R. Colwell, Am. One proposes that the combination of genetic correlations between male and female size with directional sexual selection for larger male size will cause the evolution of larger males relative to female body size [ 133233 ].

    Another argues that sexual size dimorphism evolves through intraspecific competition between the sexes when foraging is related to size [ 1526 ]. Finally, many researchers have hypothesized that this pattern is due to female fecundity, where the larger female will have bigger eggs and a greater capacity to reproduce successfully [ 153435 ]. This is due to directional selection for a large body size and individuals with sub-optimal body sizes will have lower fitness [ 4041 ].

    Alternatively, there may be condition-dependence, where the larger sex is under stronger directional selection for a large size and will be more affected by different environmental factors as compared to the smaller sex. This indicates that sexual size dimorphism should change with changing environments.

    These hypotheses and studies have sexua to much understanding of the patterns and processes underlying sexual size dimorphism. In addition to sexual size dimorphism, males and females often differ widely in shape [ 4243 ]. Curiously, although shape can contribute meaningfully to various functions such as feeding, mating, parental care and other life history characteristics, patterns of sexual shape dimorphism have historically received considerably less attention than sexual size differences [ 12444546 ].

    Examining the size and shape of traits together provides a sexua more complete quantification of sexual dimorphism, as the two components are necessarily related to one another. As such, shape analysis allows a deeper understanding of mechanisms underlying SD, because different parts of the body can serve multiple functions and be sexua distinct selective regimes.

    Shape is defined as the specific form of a distinct object that is invariant to changes in position, rotation and scale [ 4647 ], and many methods have been proposed to study shape. For instance, sets of linear distances may be measured on each individual e. Sets of linear distances do sexua always accurately capture shape because of shortcomings that limit their general sexua. For instance, it is possible that for some objects the same set of distance measurements may be obtained from two different shapes, because the location of the measurements is not recorded in the distance measures themselves.

    For example, if the maximum length and width were taken on an oval and teardrop, the linear values might be the same even though the shapes are clearly different see Figure 5.

    Additionally, it is not possible to generate graphical representations of shape using these measurements alone because the geometric distances among variables is not preserved and aspects of shape are lost [ 48 ]. As a result of these shortcomings, other analytical approaches for quantifying shape have been developed.

    A : adapted from Kaliontzopoulou et al. B : adapted from Berns dimorphism Adams, Maximum height and width taken on two different shapes results in the same linear measurement on both.

    A major advance in the study of shape dimorphism landmark-based geometric morphometric sexua, which do not have these difficulties. These methods quantify the shape of anatomical objects using the Sexua coordinates of biologically homologous landmarks whose location is identified on each specimen Figure 6. These landmarks can be digitized in either two- or three-dimensions, and provide a means of shape quantification that enables graphical representations of shape see below.

    Geometric morphometric analyses of shape are accomplished in several sequential steps. First, the landmark coordinates are digitized from each specimen. Next, differences in specimen position, orientation and size are eliminated through a generalized Procrustes analysis.

    This procedure translates all specimens to the origin, scales them to unit centroid size, and optimally rotates them to minimize the total sums-of-squares deviations of the landmark coordinates from all specimens to the average configuration. In terms of sexual shape dimorphism, dimorphism, sets of both linear measurements and geometric morphometric methods have been utilized to identify patterns of shape dimorphism in numerous taxa, dimorphism fish [ 56 ], turtles [ 57 ], birds [ 58 - 61 ] and lizards [ 6263 ].

    In addition to quantifying sexual shape dimorphism, identifying the potential mechanisms that generate these patterns is a current focus of many evolutionary biologists. For instance, one central hypothesis for the evolution of sexual shape dimorphism is that males and females diverge phenotypically due to intersexual competition for similar resources.

    Here, functional morphological traits diverge between the sexes such that the sexes partition resources. Under this scenario, SD is more strongly influenced by natural selection than sexual selection.

    For example, in the cottonmouth Agikistrodon piscivorous, sex-specific prey consumption as a function of prey size is directly correlated with differences in head morphology between males and females [ 64 ].

    Thus natural sexua, and not sexual selection, maintains both foraging and morphological differences between the sexes in this species. By contrast, sexual shape dimorphism can be the result of sexual selection. For example, in the tuatara Sphenodon punctatus, Herrel et al.

    Head shape is much larger in males as compared to females sexua this may be functionally tied to the larger prey of males. The authors suggested that sexual selection for male-male combat may play a role, but that bite force differences between males and females may be impacting the maintenance of these sexual differences. Interestingly, it was found that males do have a greater bite force relative to females, but that these differences sexua their maintenance are the result of sexual selection, as bite force is correlated with good male condition but not with female condition [ 66 ].

    Another study also rejects the hypothesis that differential niches maintain sexual shape dimorphism. Feeding, territory, and mate acquisition have been proposed as functions for dimorphism bill of the Cory shearwater Calonectris diomedea [ 61 ]. The bill morphology is such that sexual differences are related not to feeding ecology, but to sexual selection and antagonistic interactions. On the other hand, the Purple-throated Carib Eulampis sexua hummingbird exhibits the clear link between function and the different food preference of males and females, suggesting that the longer and more curved bill of the female as compared to the male is due to the division of resources [ 67 - 69 ].

    In other species of hummingbirds that exhibit sexual size and shape dimorphism in their bills however, it is unclear whether interspecific competition and niche differentiation, sexual selection, or some other force drives this sex-specific morphology [ 5860 ].

    One study investigated the relative contributions of intersexual resource partitioning and sexual selection in the amagid lizard Japalura swinhonis [ 63 ]. Here, sexual shape dimorphism was not correlated with diet, however limb size and dimorphism were associated with perch habitats. Under this hypothesis, a large mother can produce more offspring than a small mother, and can give her offspring better conditions through directional selection [ 14 ].

    Olsson et al. Results did not uphold one part of this prediction however, as sex divergence in head morphology was genetic and not specifically due to sexual selection. Evidence was presented in favor of the prediction that difference in trunk length is driven by fecundity advantage, and that sexual selection favored males with smaller trunk size. Studies such as these suggest that sexually dimorphic shape traits may be driven by the combination of natural selection for fecundity advantage and by sexual selection.

    Evidence supporting fecundity advantage is weak or not existent in many systems however. For instance, investigators examining the tortoise Testudo horsfieldii hypothesized that the wider shells of the females provided more room for eggs, but were unable to provide conclusive evidence for fecundity advantage.

    Instead, the patterns of sexual shape dimorphism seemed to be due primarily to locomotive constraints of mate seeking and male-male combat [ 74 ]. In two species of crested newt Triturus dimorphism and T. Evidence presented by Willemsen and Haile [ 76 ] outright reject the fecundity advantage hypothesis.

    Three tortoise species Testudo graecaT. In contrast to previous studies, the authors suggest that these results indicate that sexual shape dimorphism is driven not by fecundity advantage and natural selection, but rather by sexual selection. From the inconcordant results of studies such as these, it remains unknown whether patterns of the evolution of sexual shape dimorphism are primarily driven by natural selection for fecundity advantage or by some other mechanism.

    Environmental conditions are also hypothesized to drive the evolution of different shapes between the sexes. Evidence for one environmentally-driven hypothesis is presented in a study looking at environmental gradients underlying SD and parallel evolution of a species of guppy Poecilia reticulata [ 28 ].

    Results indicate that populations experiencing high predation were made up of males with smaller heads and deeper caudal peduncles. Open canopy sites resulted in selection for females with smaller heads and distended abdomens, whereas both sexes in high flow sites had small heads and deeper caudal peduncles. Males and females showed some shared responses to the environmental gradients, thus indicating that environmental variables may be responsible for sexual shape dimorphism more than sexual selection pressures might be.

    More support for the hypothesis that environmental processes drive variation in sexual shape dimorphism is found in the Greater Antillean Anolis lizards that exhibit sexual size and shape dimorphism. Males and females use habitats differently and although sexual size dimorphism is not highly related to habitat use, sexual shape dimorphism is [ 77 ].

    Further study on West Indian Anolis lizards also suggests environment as a major factor driving the patterns of sexual shape dimorphism. Concordant with the Greater Antillean Anolis lizards, the shape dimorphism clearly reflects the different niches occupied by males and females [ 43 ]. Although these and numerous other examples dimorphism the influence of environment on the evolution of sexual shape dimorphism, a recent study examined sexual shape dimorphism in the snapping turtle Chelydra serpentinaand found no evidence that environmental condition was correlated with shape dimorphism.

    Unlike sexual size dimorphism, shape dimorphism was evident at hatching and at When adults however, sexual size dimorphism was present and differed under conditions such that there is increased plasticity of the larger sex as compared to the smaller.