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Para conocer las diferencias de sexo en humanos, consulte Diferencias de sexo en humanos .
Patos mandarines , machos (izquierda) y hembras (derecha), ilustrando la dramática diferencia entre sexos
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El dimorfismo sexual es la condición en la que los dos sexos de la misma especie exhiben características diferentes más allá de las diferencias en sus órganos sexuales. [1] [2] La afección se presenta en muchos animales y algunas plantas. Las diferencias pueden incluir características sexuales secundarias , tamaño, peso, color, marcas y también pueden incluir diferencias cognitivas y de comportamiento. Estas diferencias pueden ser sutiles o exageradas y pueden estar sujetas a selección sexual y selección natural . Lo opuesto al dimorfismo es el monomorfismo . [3]

Resumen [ editar ]

El pavo real , a la derecha, está cortejando al pavo real , a la izquierda.
Ánades reales machos (abajo) y hembras . El ánade real macho tiene una inconfundible cabeza verde botella cuando su plumaje nupcial está presente.
Orgyia antiqua masculino (izquierda) y femenino (derecha).

Ornamentación y coloración [ editar ]

Los tipos de dimorfismo comunes y fáciles de identificar consisten en ornamentación y coloración, aunque no siempre son evidentes. Una diferencia en la coloración de los sexos dentro de una especie determinada se denomina dicromatismo sexual, que se observa comúnmente en muchas especies de aves y reptiles. [4] La selección sexual conduce a los rasgos dimórficos exagerados que se utilizan predominantemente en la competencia por parejas. La mayor aptitud resultante de la ornamentación compensa su costo de producción o mantenimiento, lo que sugiere implicaciones evolutivas complejas, pero los costos y las implicaciones evolutivas varían de una especie a otra. [5] [6] Los costos y las implicaciones difieren según la naturaleza de la ornamentación (como el mecanismo de color involucrado).

Los pavos reales constituyen ilustraciones conspicuas del principio. El ornamentado plumaje de los pavos reales, como se usa en la exhibición de cortejo, atrae a las pavas . A primera vista, uno podría confundir pavos reales y pavos reales con especies completamente diferentes debido a los colores vibrantes y al tamaño del plumaje del macho; siendo la pava de una coloración marrón tenue. [7] El plumaje del pavo real aumenta su vulnerabilidad a los depredadores porque es un obstáculo en el vuelo y hace que el ave sea visible en general. [7] Hay muchos ejemplos similares, como en las aves del paraíso y los faisanes argus .

Otro ejemplo de dicromatismo sexual es el del pichón de herrerillos azules . Los machos son cromáticamente más amarillos que las hembras. Se cree que esto se obtiene mediante la ingestión de larvas de lepidópteros verdes , que contienen grandes cantidades de los carotenoides luteína y zeaxantina . [8] Esta dieta también afecta los colores sexualmente dimórficos en el espectro ultravioleta invisible para los humanos . [9] [10] Por lo tanto, los pájaros machos, aunque parecen amarillos para los humanos, en realidad tienen un plumaje teñido de violeta que ven las hembras. Se cree que este plumaje es un indicador de las capacidades parentales masculinas. [11] Perhaps this is a good indicator for females because it shows that they are good at obtaining a food supply from which the carotenoid is obtained. There is a positive correlation between the chromas of the tail and breast feathers and body condition.[12] Carotenoids play an important role in immune function for many animals, so carotenoid dependent signals might indicate health.[13]

Las ranas constituyen otra ilustración conspicua del principio. Hay dos tipos de dicromatismo para las especies de ranas: ontogenético y dinámico. Las ranas ontogenéticas son más comunes y tienen cambios de color permanentes en machos o hembras. Ranoidea lesueuri es un ejemplo de una rana dinámica que tiene cambios temporales de color en los machos durante la temporada de reproducción. [14] Hyperolius ocellatus es una rana ontogenética con diferencias dramáticas tanto en el color como en el patrón entre los sexos. En la madurez sexual, los machos muestran un verde brillante con líneas dorsolaterales blancas. [15] En contraste, las hembras son de color rojo oxidado a plateado con pequeñas manchas. La coloración brillante en la población masculina sirve para atraer a las hembras y como aposemático. señal a los depredadores potenciales.

Las hembras a menudo muestran una preferencia por características sexuales secundarias masculinas exageradas en la selección de pareja. [16] La hipótesis del hijo sexy explica que las hembras prefieren machos más elaborados y seleccionan a machos que son de color apagado, independientemente de la visión de la especie. [17]

También se observan dimorfismo sexual y elección de apareamiento similares en muchas especies de peces. Por ejemplo, los guppies machos tienen manchas y ornamentaciones de colores, mientras que las hembras son generalmente de color gris. Las hembras guppies prefieren los machos de colores brillantes a los machos más apagados. [18]

Diferenciación fisiológica [ editar ]

En los blenios de labios rojos , solo el pez macho desarrolla un órgano en la región anal-urogenital que produce sustancias antimicrobianas. Durante el cuidado de los padres, los machos frotan sus regiones urogenitales anales sobre las superficies internas de sus nidos, protegiendo así sus huevos de infecciones microbianas, una de las causas más comunes de mortalidad en peces jóvenes. [19]

Plantas [ editar ]

La mayoría de las plantas con flores son hermafroditas, pero aproximadamente el 6% de las especies tienen machos y hembras separados ( dioecia ). [20] Los machos y las hembras de las especies polinizadas por insectos generalmente se parecen entre sí porque las plantas proporcionan recompensas (por ejemplo, néctar ) que animan a los polinizadores a visitar otra flor similar , completando la polinización . Las orquídeas Catasetum son una excepción interesante a esta regla. Los machos de las orquídeas Catasetum adhieren violentamente pollinia a la euglosinapolinizadores de abejas. Las abejas evitarán otras flores masculinas, pero pueden visitar a la hembra, que se ve diferente a los machos. [21]

Varias otras excepciones dioicas, como Loxostylis alata, tienen géneros visiblemente diferentes, con el efecto de provocar el comportamiento más eficiente de los polinizadores, que luego utilizan la estrategia más eficiente para visitar cada género de flor en lugar de buscar, por ejemplo, polen en un néctar. teniendo flor femenina.

Algunas plantas, como algunas especies de geranio, tienen lo que equivale a un dimorfismo sexual en serie. Las flores de tales especies pueden, por ejemplo, presentar sus anteras al abrirse, luego arrojar las anteras agotadas después de uno o dos días y tal vez cambiar sus colores también mientras el pistilo 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 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.[22] 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.[23]

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.[24]

Every sexually reproducing extant species of vascular plant actually has an alternation of generations; the plants we see about us generally are diploid sporophytes, but 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[edit]

Dimorfismo sexual: Anthocharis cardamines hembra

Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration.[25] The female-biased sexual size dimorphism observed in many taxa evolved despite intense male-male competition for mates.[26] In Osmia rufa, for example, the female is larger/broader than males, with males being 8–10 mm in size and females being 10–12 mm in size.[27] In the hackberry emperor females are similarly larger than males.[28] The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males.[29]

En algunas especies, hay evidencia de dimorfismo masculino, pero parece ser con el propósito de distinciones de roles. Esto se ve en la especie de abeja Macrotera portalis en la que hay una morfología de cabeza pequeña, capaz de volar, y una morfología de cabeza grande, incapaz de volar, para los machos. [30] Anthidium manicatum también muestra dimorfismo sexual sesgado por los machos. La selección de machos de mayor tamaño en lugar de hembras en esta especie puede deberse a su comportamiento territorial agresivo y al posterior éxito de apareamiento diferencial. [31] Otro ejemplo es Lasioglossum hemichalceum , que es una especie de abeja del sudor que muestra drásticos dimorfismos físicos entre los descendientes masculinos. [32]No todo dimorfismo tiene que tener una diferencia drástica entre los sexos. Andrena agilissima es una abeja minera donde las hembras solo tienen una cabeza un poco más grande que los machos. [33]

El armamento conduce a una mayor aptitud al aumentar el éxito en la competencia macho-macho en muchas especies de insectos. [34] Los cuernos de escarabajo en Onthophagus taurus son crecimientos agrandados de la cabeza o el tórax expresados ​​solo en los machos. Copris ochus también tiene un dimorfismo masculino y sexual distinto en los cuernos de la cabeza. [35] Estas estructuras son impresionantes debido a los tamaños exagerados. [36] Existe una correlación directa entre la longitud de los cuernos masculinos y el tamaño del cuerpo y un mayor acceso a las parejas y la aptitud. [36] En otras especies de escarabajos, tanto los machos como las hembras pueden tener ornamentación como cuernos. [35] Generalmente, el dimorfismo del tamaño sexual de los insectos (SSD) dentro de las especies aumenta con el tamaño corporal.[37]

El dimorfismo sexual dentro de los insectos también se manifiesta por el dicromatismo. En los géneros de mariposas Bicyclus y Junonia , los patrones de alas dimórficos evolucionaron debido a la expresión limitada por el sexo, que media el conflicto sexual intralocus y conduce a una mayor aptitud en los machos. [38] La naturaleza dicromática sexual de Bicyclus anynana se refleja en la selección femenina sobre la base de las pupilas dorsales de las manchas oculares que reflejan los rayos ultravioleta. [39] El azufre común también muestra dicromatismo sexual; los machos tienen alas amarillas e iridiscentes, mientras que las hembras son blancas y no iridiscentes. [40]La desviación seleccionada naturalmente en la coloración femenina protectora se muestra en las mariposas miméticas. [41]

Arañas y canibalismo sexual [ editar ]

Argiope appensa hembra (izquierda) y macho (derecha) , que muestra las diferencias sexuales típicas de las arañas, con machos dramáticamente más pequeños

Muchos grupos de arácnidos exhiben dimorfismo sexual, [42] pero se estudia más ampliamente en las arañas. En la araña tejedora de orbes Zygiella x-notata , por ejemplo, las hembras adultas tienen un tamaño corporal mayor que los machos adultos. [43] El dimorfismo del tamaño muestra una correlación con el canibalismo sexual , [44] que es prominente en las arañas (también se encuentra en insectos como las mantis religiosas ). En el tamaño de la araña lobo dimórfica Tigrosa Helluo , las hembras con alimentos limitados canibalizan con mayor frecuencia. [45]Por lo tanto, existe un alto riesgo de baja aptitud para los machos debido al canibalismo pre-copulatorio, que llevó a la selección masculina de hembras más grandes por dos razones: mayor fecundidad y menores tasas de canibalismo. [45] Además, la fecundidad de las hembras se correlaciona positivamente con el tamaño corporal de la hembra y se selecciona el tamaño corporal grande de la hembra, que se ve en la familia Araneidae . Todas las especies de Argiope , incluido Argiope bruennichi , utilizan este método. Algunos machos desarrollaron ornamentación [ vaga ]incluyendo atar a la hembra con seda, tener piernas proporcionalmente más largas, modificar la red de la hembra, aparearse mientras la hembra se alimenta o proporcionar un regalo nupcial en respuesta al canibalismo sexual. [45] El tamaño del cuerpo de los machos no está bajo selección debido al canibalismo en todas las especies de arañas como Nephila pilipes , pero se selecciona de manera más prominente en especies de arañas menos dimórficas, que a menudo se seleccionan para machos de mayor tamaño. [46] En la especie Maratus volans , los machos son conocidos por su característico abanico de colores que atrae a las hembras durante el apareamiento. [47]

Pescado [ editar ]

Los peces con aletas radiadas son una clase antigua y diversa, con el grado más amplio de dimorfismo sexual de cualquier clase animal. Fairbairn señala que "las hembras son generalmente más grandes que los machos, pero los machos suelen ser más grandes en especies con combate macho-macho o cuidado paterno macho ... [los tamaños varían] desde machos enanos a machos más de 12 veces más pesados ​​que las hembras". [48]

Hay casos en los que los machos son sustancialmente más grandes que las hembras. Un ejemplo es Lamprologus callipterus , un tipo de pez cíclido. En este pez, los machos se caracterizan por ser hasta 60 veces más grandes que las hembras. Se cree que el aumento de tamaño del macho es ventajoso porque los machos recolectan y defienden las conchas vacías de los caracoles en cada una de las cuales se reproduce una hembra. [49]Los machos deben ser más grandes y poderosos para poder recolectar los caparazones más grandes. El tamaño del cuerpo de la hembra debe permanecer pequeño porque para que pueda reproducirse, debe poner sus huevos dentro de las cáscaras vacías. Si crece demasiado, no cabrá en las conchas y no podrá reproducirse. Es probable que el pequeño tamaño corporal de la hembra también sea beneficioso para sus posibilidades de encontrar un caparazón desocupado. Las conchas más grandes, aunque las preferidas por las hembras, a menudo tienen una disponibilidad limitada. [50] Por lo tanto, la hembra está limitada al crecimiento del tamaño del caparazón y en realidad puede cambiar su tasa de crecimiento de acuerdo con la disponibilidad del tamaño del caparazón. [51]En otras palabras, la capacidad del macho para recolectar conchas grandes depende de su tamaño. Cuanto más grande es el macho, más grandes son las conchas que puede recolectar. Esto permite que las hembras sean más grandes en su nido de cría, lo que hace que la diferencia entre los tamaños de los sexos sea menos sustancial. La competencia macho-macho en esta especie de pez también selecciona el tamaño grande en los machos. Existe una competencia agresiva de los machos por el territorio y el acceso a caparazones más grandes. Los machos grandes ganan peleas y roban conchas a los competidores. Otro ejemplo es el dragonet , en el que los machos son considerablemente más grandes que las hembras y poseen aletas más largas.

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.[52] In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success.[53] 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.[54] Los machos más grandes pueden sofocar el crecimiento de las hembras y controlar los recursos ambientales.

La organización social juega un papel importante en el cambio de sexo de los peces. A menudo se ve que un pez cambia de sexo cuando falta un macho dominante dentro de la jerarquía social. Las hembras que cambian de sexo suelen ser las que alcanzan y conservan una ventaja de tamaño inicial en una etapa temprana de la vida. En cualquier caso, las hembras que cambian de sexo a machos son más grandes y, a menudo, resultan ser un buen ejemplo de dimorfismo.

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.[55]

Sexual selection was observed for female ornamentation in Gobiusculus flavescens, known as two-spotted gobies.[56] 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.[56] Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during the breeding season are considered favorable to males.[57] The males invest heavily in offspring during the incubation, which leads to the sexual preference in colorful females due to higher egg quality.[57]

Amphibians and non-avian reptiles[edit]

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 amphibians, snakes, and 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 larger than females. For instance, the average male Anolis sagrei was 53.4 mm vs. 40 mm in females.[58] Different sizes of the heads in anoles have been explained by differences in the estrogen pathway.[59] The sexual dimorphism in lizards is generally attributed to the effects of sexual selection, but other mechanisms including ecological divergence and fecundity selection provide alternative explanations.[60] The development of color dimorphism in lizards is induced by hormonal changes at the onset of sexual maturity, as seen in Psamodromus algirus, Sceloporus gadoviae, and S. undulates erythrocheilus.[60]

Male painted dragon lizards, Ctenophorus pictus. are brightly conspicuous in their breeding coloration, but male colour declines with aging. Male coloration appears to reflect innate anti-oxidation capacity that protects against oxidative DNA damage.[61] Male breeding coloration is likely an indicator to females of the underlying level of oxidative DNA damage (a significant component of aging) in potential mates.[61]

Birds[edit]

Female (left) and male (right) common pheasant, showing that the male is much larger and more colorful than the female
Some bird species, such as this Mute swan, do not display sexual dimorphism through their plumage, and instead can be distinguished by other physiological or behavioural characteristics. Generally, male Mute swans, or cobs, are taller and larger than females, or pens, and have thicker necks and a more pronounced 'knob' above their bill.

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.g. birds of prey, hummingbirds, and some species of flightless birds.[62][63] Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex.[64] Such differences have been attributed to the unequal reproductive contributions of the sexes.[65] This difference produces a stronger female choice since they have more risk in producing offspring. 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[66] or survival.[67] The male phenotype sends signals to females who then choose the 'fittest' available male.

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.[66] These differences arise in response to the bird's body condition: if they are healthy they will produce more androgens thus becoming black breeders, while less healthy birds produce less androgens and become brown auxiliaries.[66] The reproductive success of the male is thus determined by his success during each year's non-breeding season, causing reproductive success to vary with each year's environmental conditions.

Migratory patterns and behaviors also influence sexual dimorphisms. This aspect also stems back to the size dimorphism in species. It has been shown that the larger males are better at coping with the difficulties of migration and thus are more successful in reproducing when reaching the breeding destination.[68] When viewing this in an evolutionary standpoint many theories and explanations come to consideration. If these are the result for every migration and breeding season the expected results should be a shift towards a larger male population through sexual selection. Sexual selection is strong when the factor of environmental selection is also introduced. The environmental selection may support a smaller chick size if those chicks were born in an area that allowed them to grow to a larger size, even though under normal conditions they would not be able to reach this optimal size for migration. When the environment gives advantages and disadvantages of this sort, the strength of selection is weakened and the environmental forces are given greater morphological weight. The sexual dimorphism could also produce a change in timing of migration leading to differences in mating success within the bird population.[69] When the dimorphism produces that large of a variation between the sexes and between the members of the sexes multiple evolutionary effects can take place. This timing could even lead to a speciation phenomenon if the variation becomes strongly drastic and favorable towards two different outcomes.

Skeletons of female (left) and Male (right) black-casqued hornbills (Ceratogymna atrata). The difference between the sexes is apparent in the casque on the top of their bill. This pair is on display at the Museum of Osteology.

Sexual dimorphism is maintained by the counteracting pressures of natural selection and sexual selection. For example, sexual dimorphism in coloration increases the vulnerability of bird species to predation by European sparrowhawks in Denmark.[70] Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation.[70] Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function.[66] So long as the reproductive benefits of the trait due to sexual selection are greater than the costs imposed by natural selection, then the trait will propagate throughout the population. Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival. This means that even if the trait causes males to die earlier, the trait is still beneficial so long as males with the trait produce more offspring than males lacking the trait. This balance keeps the dimorphism alive in these species and ensures that the next generation of successful males will also display these traits that are attractive to the females.

Such differences in form and reproductive roles often cause differences in behavior. As previously stated, males and females often have different roles in reproduction. The courtship and mating behavior of males and females are regulated largely by hormones throughout a bird's lifetime.[71] Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season.[71] Organizational hormones occur only during a critical period early in development, either just before or just after hatching in most birds, and determine patterns of behavior for the rest of the bird's life.[71] Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures.[72] Females of the whinchat in Switzerland breed in intensely managed grasslands.[72] Earlier harvesting of the grasses during the breeding season lead to more female deaths.[72] Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate.[72] Also not all male dimorphic traits are due to hormones like testosterone, instead they are a naturally occurring part of development, for example plumage.[73] In addition, the strong hormonal influence on phenotypic differences suggest that the genetic mechanism and genetic basis of these sexually dimorphic traits may involve transcription factors or cofactors rather than regulatory sequences. [74]

Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the blue-footed booby, the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. This then results in the maximization of parental lifetime reproductive success.[75] In Black-tailed Godwits Limosa limosa limosa females are also the larger sex, and the growth rates of female chicks are more susceptible to limited environmental conditions.[76]

Sexual dimorphism may also only appear during mating season, some species of birds only show dimorphic traits in seasonal variation. The males of these species will molt into a less bright or less exaggerated color during the off breeding season.[74] This occurs because the species is more focused on survival than reproduction, causing a shift into a less ornate state.[dubious – discuss]

Consequently, sexual dimorphism has important ramifications for conservation. However, sexual dimorphism is not only found in birds and is thus important to the conservation of many animals. Such differences in form and behavior can lead to sexual segregation, defined as sex differences in space and resource use.[77] Most sexual segregation research has been done on ungulates,[77] but such research extends to bats,[78] kangaroos,[79] and birds.[80] Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation.[78]

The term sesquimorphism (the Latin numeral prefix sesqui- means one-and-one-half, so halfway between mono- (one) and di- (two)) has been proposed for bird species in which "both sexes have basically the same plumage pattern, though the female is clearly distinguishable by reason of her paler or washed-out colour".[81]:14 Examples include Cape sparrow (Passer melanurus),[81]:67 rufous sparrow (subspecies P. motinensis motinensis),[81]:80 and saxaul sparrow (P. ammodendri).[81]:245

Mammals[edit]

In a large proportion of mammal species, males are larger than females.[82] Both genes and hormones affect the formation of many animal brains before "birth" (or hatching), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. A 2004 review in Nature Reviews Neuroscience observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." It concluded that while "the differentiating effects of gonadal secretions seem to be dominant," the existing body of research "support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease."[83]

Male and female northern elephant seal, the male being larger with a big proboscis

Pinnipeds[edit]

Marine mammals show some of the greatest sexual size differences of mammals, because of sexual selection and environmental factors like breeding location.[84][85] The mating system of pinnipeds varies from polygamy to serial monogamy. Pinnipeds are known for early differential growth and maternal investment since the only nutrients for newborn pups is the milk provided by the mother.[86] For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups.[87] The pattern of differential investment can be varied principally prenatally and post-natally.[88] Mirounga leonina, the southern elephant seal, is one of the most dimorphic mammals.[89]

Sexual dimorphism in elephant seals is associated with the ability of a male to defend territories and control large groups of females, which correlates with polygynic behavior.[90] The large sexual size dimorphism is partially due to sexual selection, but also because females reach reproductive age much earlier than males. In addition the males do not provide parental care for the young and allocate more energy to growth.[91] This is supported by the secondary growth spurt in males during adolescent years.[91]

Primates[edit]

Main article: Sexual dimorphism in non-human primates

Humans[edit]

Main articles: Sex differences in humans and Sex differences in human psychology

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

Sexual dimorphism among humans includes differentiation among gonads, internal genitals, external genitals, breasts, muscle mass, height, the endocrine (hormonal) systems and their physiological and behavioral effects. Human sexual differentiation is effected primarily at the gene level, by the presence or absence of a Y-chromosome, which encodes biochemical modifiers for sexual development in males.[92] According to Clark Spencer Larsen, modern day Homo sapiens show a range of sexual dimorphism, with average body mass difference between the sexes being roughly equal to 15%.[93]

The average basal metabolic rate is about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Females tend to convert more food into fat, while males convert more into muscle and expendable circulating energy reserves. Aggregated data of absolute strength indicates that females have, on average, 40–60% the upper body strength of males, and 70–75% the lower body strength.[94] The difference in strength relative to body mass is less pronounced in trained individuals. In Olympic weightlifting, male records vary from 5.5× body mass in the lowest weight category to 4.2× in the highest weight category, while female records vary from 4.4× to 3.8×, a weight adjusted difference of only 10–20%, and an absolute difference of about 30% (i.e. 472 kg vs 333 kg for unlimited weight classes)(see Olympic weightlifting records). A study, carried about by analyzing annual world rankings from 1980 to 1996, found that males' running times were, on average, 11% faster than females'.[95]

Females are taller, on average, than males in early adolescence, but males, on average, surpass them in height in later adolescence and adulthood. In the United States, adult males are, on average, 9% taller[96] and 16.5% heavier[97] than adult females. There is no comparative evidence of differing levels of sexual selection having produced sexual size dimorphism between human populations.[98]

Males typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. On average, males have larger hearts, 10 percent higher red blood cell count, higher hemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance.[99]

Females typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males. Hence they develop fewer infectious diseases and succumb for shorter periods.[99] Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage.[100][101][102][103][104]

Considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies.[105] According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."[106]

In the human brain, a difference between sexes was observed in the transcription of the PCDH11X/Y gene pair unique to Homo sapiens.[107] Sexual differentiation in the human brain from the undifferentiated state is triggered by testosterone from the fetal testis. Testosterone is converted to estrogen in the brain through the action of the enzyme aromatase. Testosterone acts on many brain areas, including the SDN-POA, to create the masculinized brain pattern.[108] Brains of pregnant females carrying male fetuses may be shielded from the masculinizing effects of androgen through the action of sex hormone-binding globulin.[109]

The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large.[110][111] Many females tend to have a higher ratio of gray matter in the left hemisphere of the brain in comparison to males.[112][113] Males on average have larger brains than females; however, when adjusted for total brain volume the gray matter differences between sexes is almost nonexistent. Thus, the percentage of gray matter appears to be more related to brain size than it is to sex.[114][115] Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier et al. found in a 2004 study that "men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance".[116] (See the sex and intelligence article for more on this subject.) Strict graph-theoretical analysis of the human brain connections revealed[117] that in numerous graph-theoretical parameters (e.g., minimum bipartition width, edge number, the expander graph property, minimum vertex cover), the structural connectome of women are significantly "better" connected than the connectome of men. It was shown[118] that the graph-theoretical differences are due to the sex and not to the differences in the cerebral volume, by analyzing the data of 36 females and 36 males, where the brain volume of each man in the group was smaller than the brain volume of each woman in the group.

Sexual dimorphism was also described in the gene level and shown to extend from the sex chromosomes. Overall, about 6500 genes have been found to have sex-differential expression in at least one tissue. Many of these genes are not directly associated with reproduction, but rather linked to more general biological features. In addition, it has been shown that genes with sex specific expression undergo reduced selection efficiency, which lead to higher population frequencies of deleterious mutations and contributing to the prevalence of several human diseases.[119][120]

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. The underlying causes are explained by either the role of immunosuppressive substances, such as testosterone, or by fundamental differences in male and female life histories. It has been shown that female mammals tend to have higher white blood cell counts (WBC), with further associations between cell counts and longevity in females. There is also a positive covariance between sexual dimorphism in immunity, as measured by a subset of WBC, and dimorphism in the duration of effective breeding. This is consistent with the application of ‘Bateman’s principle’ to immunity, with females maximizing fitness by lengthening lifespan through greater investment in immune defences.[121]

Cells[edit]

Phenotypic differences between sexes are evident even in cultured cells from tissues.[122] For example, female muscle-derived stem cells have a better muscle regeneration efficiency than male ones.[123] There are reports of several metabolic differences between male and female cells[124] and they also respond to stress differently.[125]

Reproductively advantageous[edit]

In theory, larger females are favored by competition for mates, especially in polygamous species. Larger females offer an advantage in fertility, since the physiological demands of reproduction are limiting in females. Hence there is a theoretical expectation that females tend to be larger in species that are monogamous. Females are larger in many species of insects, many spiders, many fish, many reptiles, owls, birds of prey and certain mammals such as the spotted hyena, and baleen whales such as blue whale. As an example, in some species, females are sedentary, and so males must search for them. Fritz Vollrath and Geoff Parker argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males.[126] Cases where the male is larger than the female have been studied as well,[126] and require alternative explanations.

One example of this type of sexual size dimorphism is the bat Myotis nigricans, (black myotis bat) where females are substantially larger than males in terms of body weight, skull measurement, and forearm length.[127] The interaction between the sexes and the energy needed to produce viable offspring make it favorable for females to be larger in this species. Females bear the energetic cost of producing eggs, which is much greater than the cost of making sperm by the males. The fecundity advantage hypothesis states that a larger female is able to produce more offspring and give them more favorable conditions to ensure their survival; this is true for most ectotherms. A larger female can provide parental care for a longer time while the offspring matures. The gestation and lactation periods are fairly long in M. nigricans, the females suckling their offspring until they reach nearly adult size.[128] They would not be able to fly and catch prey if they did not compensate for the additional mass of the offspring during this time. Smaller male size may be an adaptation to increase maneuverability and agility, allowing males to compete better with females for food and other resources.

Female triplewart seadevil, an anglerfish, with male attached near vent (arrow)

Some species of anglerfish also display extreme sexual dimorphism. Females are more typical in appearance to other fish, whereas the males are tiny rudimentary creatures with stunted digestive systems. A male must find a female and fuse with her: he then lives parasitically, becoming little more than a sperm-producing body in what amounts to an effectively hermaphrodite composite organism. A similar situation is found in the Zeus water bug Phoreticovelia disparata where the female has a glandular area on her back that can serve to feed a male, which clings to her (note that although males can survive away from females, they generally are not free-living).[129] This is taken to the logical extreme in the Rhizocephala crustaceans, like the Sacculina, where the male injects itself into the female's body and becomes nothing more than sperm producing cells, to the point that the superorder used to be mistaken for hermaphroditic.[130]

Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum[131] and the liverwort Sphaerocarpos.[132] There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome,[132][133] or to chemical signalling from females.[134]

Another complicated example of sexual dimorphism is in Vespula squamosa, the southern yellowjacket. In this wasp species, the female workers are the smallest, the male workers are slightly larger, and the female queens are significantly larger than her female worker and male counterparts.[citation needed]

Evolution[edit]

See also: Sexual selection and Mate choice
Sexual dimorphism in Cambrian trilobites.[135]

Sexual dimorphism by size is evident in some extinct species such as the velociraptor. In the case of velociraptors the sexual size dimorphism may have been caused by two factors: male competition for hunting ground to attract mates, and/or female competition for nesting locations and mates, males being a scarce breeding resource.[136]

In 1871, Charles Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection.

It has been proposed that the earliest sexual dimorphism is the size differentiation of sperm and eggs (anisogamy), but the evolutionary significance of sexual dimorphism is more complex than that would suggest.[137] Anisogamy and the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strong sperm competition,[138][139] because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites[140]) more redundant. This intensifies male competition for mates and promotes the evolution of other sexual dimorphism in many species, especially in vertebrates including mammals. However, in some species, the females can be larger than males, irrespective of gametes, and in some species females (usually of species in which males invest a lot in rearing offspring and thus no longer considered as so redundant) compete for mates in ways more usually associated with males.

In many non-monogamous species, the benefit to a male's reproductive fitness of mating with multiple females is large, whereas the benefit to a female's reproductive fitness of mating with multiple males is small or nonexistent.[141] In these species, there is a selection pressure for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.

Male (left), offspring, and female (right) Sumatran orangutans.

These traits could be ones that allow him to fight off other males for control of territory or a harem, such as large size or weapons;[142] or they could be traits that females, for whatever reason, prefer in mates.[143] Male-male competition poses no deep theoretical questions[144] but mate choice does.

Females may choose males that appear strong and healthy, thus likely to possess "good alleles" and give rise to healthy offspring.[145] In some species, however, females seem to choose males with traits that do not improve offspring survival rates, and even traits that reduce it (potentially leading to traits like the peacock's tail).[144] Two hypotheses for explaining this fact are the sexy son hypothesis and the handicap principle.

The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings.[146]

The handicap principle states that a male who survives despite possessing some sort of handicap thus proves that the rest of his genes are "good alleles". If males with "bad alleles" could not survive the handicap, females may evolve to choose males with this sort of handicap; the trait is acting as a hard-to-fake signal of fitness.[147]

See also[edit]

  • Bateman's principle
  • List of homologues of the human reproductive system      
  • Sex differences in humans
  • Sex differences in human psychology
  • Sexual differentiation
  • Sexual dimorphism in dinosaurs
  • Sexual dimorphism in non-human primates
  • Sexual dimorphism measures
  • Sexually dimorphic nucleus
  • Gynandromorphism

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Sources[edit]

  • Andersson MB (1994). Sexual Selection. Princeton University Press. ISBN 978-0-691-00057-2.
  • Futuyma D (2005). Evolution (1st ed.). Sunderland, Massachusetts: Sinauer Associates. ISBN 978-0-87893-187-3.
  • Ridley M (2004). Evolution (3rd ed.). Malden, Massachusetts: Blackwell Publishing. ISBN 978-1-4051-0345-9.

Further reading[edit]

  • Bonduriansky R (January 2007). "The evolution of condition-dependent sexual dimorphism". The American Naturalist. 169 (1): 9–19. doi:10.1086/510214. PMID 17206580. S2CID 17439073.
  • Figuerola J (1999). "A comparative study on the evolution of reversed size dimorphism in monogamous waders". Biological Journal of the Linnean Society. 67 (1): 1–18. doi:10.1111/j.1095-8312.1999.tb01926.x. hdl:10261/44557.
  • Székely T, Lislevand T, Figuerola J, Fairbairn D, Blanckenhorn W (2007). Sex, Size, and Gender Roles: Evolutionary Studies of Sexual Size Dimorphism. pp. 16–26.

External links[edit]

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