Male

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The symbol of the Roman god Mars (god of war) is often used to represent the male sex. It also stands for the planet Mars and is the alchemical symbol for iron. Mars symbol.svg
The symbol of the Roman god Mars (god of war) is often used to represent the male sex. It also stands for the planet Mars and is the alchemical symbol for iron.

Male (symbol: ) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, [1] [2] [3] or ovum, in the process of fertilization.

Contents

A male organism cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually and asexually. [4] Most male mammals, including male humans, have a Y chromosome, [5] [6] which codes for the production of larger amounts of testosterone to develop male reproductive organs. Not all species share a common sex-determination system. In most animals, including humans, sex is determined genetically; however, species such as Cymothoa exigua change sex depending on the number of females present in the vicinity. [7] [ better source needed ]

In humans, the word male can also be used to refer to gender in the social sense of gender role or gender identity.

Overview

The existence of separate sexes has evolved independently at different times and in different lineages, an example of convergent evolution. [8] [9] The repeated pattern is sexual reproduction in isogamous species with two or more mating types with gametes of identical form and behavior (but different at the molecular level) to anisogamous species with gametes of male and female types to oogamous species in which the female gamete is very much larger than the male and has no ability to move. There is a good argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction. [10] [ page needed ]

Accordingly, sex is defined across species by the type of gametes produced (i.e.: spermatozoa vs. ova) and differences between males and females in one lineage are not always predictive of differences in another. [9] [11] [12]

Male/female dimorphism between organisms or reproductive organs of different sexes is not limited to animals; male gametes are produced by chytrids, diatoms and land plants, among others. In land plants, female and male designate not only the female and male gamete-producing organisms and structures but also the structures of the sporophytes that give rise to male and female plants.[ citation needed ]

Evolution

The evolution of anisogamy led to the evolution of male and female function. [13] Before the evolution of anisogamy, mating types in a species were isogamous: the same size and both could move, catalogued only as "+" or "-" types. [14] In anisogamy, the mating type is called a gamete. The male gamete is smaller than the female gamete, and usually mobile. [15] Anisogamy remains poorly understood, as there is no fossil record of its emergence. Numerous theories exist as to why anisogamy emerged. Many share a common thread, in that larger female gametes are more likely to survive, and that smaller male gametes are more likely to find other gametes because they can travel faster. Current models often fail to account for why isogamy remains in a few species. [16] Anisogamy appears to have evolved multiple times from isogamy; for example female Volvocales (a type of green algae) evolved from the plus mating type. [16] [17] Although sexual evolution emerged at least 1.2 billion years ago, the lack of anisogamous fossil records make it hard to pinpoint when males evolved. [18] One theory suggests male evolved from the dominant mating type (called mating type minus). [19]

Symbol and usage

Symbol

A common symbol used to represent the male sex is the Mars symbol ♂, a circle with an arrow pointing northeast. The Unicode code-point is:

U+2642MALE SIGN (♂)

The symbol is identical to the planetary symbol of Mars. It was first used to denote sex by Carl Linnaeus in 1751. The symbol is sometimes seen as a stylized representation of the shield and spear of the Roman god Mars. According to Stearn, however, this derivation is "fanciful" and all the historical evidence favours "the conclusion of the French classical scholar Claude de Saumaise (Salmasius, 15881683)" that it is derived from θρ, the contraction of a Greek name for the planet Mars, which is Thouros. [20]

Usage

In addition to its meaning in the context of biology, male can also refer to gender [21] or a shape of connectors. [22] [23]

Males across species

Species that are divided into females and males are classified as gonochoric in animals, as dioecious in seed plants [24] and as dioicous in cryptogams. [25] :82

Males can coexist with hermaphrodites, a sexual system called androdioecy. They can also coexist with females and hermaphrodites, a sexual system called trioecy. [26]

Sex determination

Photograph of an adult male human, with an adult female for comparison. Note that both models have partially shaved body hair; e.g. clean-shaven pubic regions. Anterior view of human female and male, with labels 2.png
Photograph of an adult male human, with an adult female for comparison. Note that both models have partially shaved body hair; e.g. clean-shaven pubic regions.

The sex of a particular organism may be determined by a number of factors. These may be genetic or environmental, or may naturally change during the course of an organism's life. Although most species have only two sexes (either male or female), [8] [9] [27] hermaphroditic animals, such as worms, have both male and female reproductive organs. [28]

Genetic determination

Most mammals, including humans, are genetically determined as such by the XY sex-determination system where males have an XY (as opposed to XX) sex chromosome. It is also possible in a variety of species, including humans, to be XX male or have other karyotypes. During reproduction, a male can give either an X sperm or a Y sperm, while a female can only give an X egg. A Y sperm and an X egg produce a male, while an X sperm and an X egg produce a female. [29]

The part of the Y-chromosome which is responsible for maleness is the sex-determining region of the Y-chromosome, the SRY. [30] The SRY activates Sox9, which forms feedforward loops with FGF9 and PGD2 in the gonads, allowing the levels of these genes to stay high enough in order to cause male development; [31] for example, Fgf9 is responsible for development of the spermatic cords and the multiplication of Sertoli cells, both of which are crucial to male sexual development. [32]

The ZW sex-determination system, where males have a ZZ (as opposed to ZW) sex chromosome may be found in birds and some insects (mostly butterflies and moths) and other organisms. Members of the insect order Hymenoptera, such as ants and bees, are often determined by haplodiploidy, [33] where most males are haploid and females and some sterile males are diploid. However, fertile diploid males may still appear in some species such as Cataglyphis cursor . [34]

Environmental determination

In some species of reptiles, such as alligators, sex is determined by the temperature at which the egg is incubated. Other species, such as some snails, practice sex change: adults start out male, then become female. [35] In tropical clown fish, the dominant individual in a group becomes female while the other ones are male. [36]

In many arthropods, sex is determined by infection with parasitic, endosymbiotic bacteria of the genus Wolbachia . The bacterium can only be transmitted via infected ova, and the presence of the obligate endoparasite may be required for female sexual viability. [37]

Secondary sex characteristics

Male animals evolved to utilize secondary sex characteristics as a method of displaying traits which display fitness, which is believed to be the product of sexual selection. [38] Differences in physical size and fulfilling the needs of sexual selection have contributed significantly to the outcome of secondary sex characteristics of each species. [38]

In those species with two sexes, males may differ from females in ways other than the production of spermatozoa.

In many insects and fish, the male is smaller than the female.

In seed plants, which exhibit alternation of generations, the female and male parts are both included within the sporophyte sex organ of a single organism.

In mammals, including humans, males are typically larger than females. [39] [ page needed ]

In humans, males have more body hair and muscle mass. [40] [ page needed ]

In birds, the male often exhibits a colorful plumage that attracts females. [41] [ page needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Asexual reproduction</span> Reproduction without a sexual process

Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria. Many eukaryotic organisms including plants, animals, and fungi can also reproduce asexually. In vertebrates, the most common form of asexual reproduction is parthenogenesis, which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited. Komodo dragons and some monitor lizards can also reproduce asexually.

<span class="mw-page-title-main">Gamete</span> Cell that fuses during fertilisation, such as a sperm or egg cell

A gamete is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual that produces the larger type of gamete—called an ovum— and a male produces the smaller type—called a sperm. Sperm cells or spermatozoa are small and motile due to the flagellum, a tail-shaped structure that allows the cell to propel and move. In contrast, each egg cell or ovum is relatively large and non-motile. In short a gamete is an egg cell or a sperm. In animals, ova mature in the ovaries of females and sperm develop in the testes of males. During fertilization, a spermatozoon and ovum unite to form a new diploid organism. Gametes carry half the genetic information of an individual, one ploidy of each type, and are created through meiosis, in which a germ cell undergoes two fissions, resulting in the production of four gametes. In biology, the type of gamete an organism produces determines the classification of its sex.

<span class="mw-page-title-main">Reproduction</span> Biological process by which new organisms are generated from one or more parent organisms

Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. There are two forms of reproduction: asexual and sexual.

<span class="mw-page-title-main">Sex</span> Trait that determines an individuals sexually reproductive function

Sex is the trait that determines whether a sexually reproducing animal or plant produces male or female gametes. Male plants and animals produce smaller mobile gametes, while females produce larger ones. Organisms that produce both types of gametes are called hermaphrodites. During sexual reproduction, male and female gametes fuse to form zygotes, which develop into offspring that inherit traits from each parent.

<span class="mw-page-title-main">XY sex-determination system</span> Method of determining sex in all animals

The XY sex-determination system is a sex-determination system used to classify many mammals, including humans, some insects (Drosophila), some snakes, some fish (guppies), and some plants. In this system, the sex of an individual is determined by a pair of sex chromosomes. Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two different kinds of sex chromosomes (XY), and are called the heterogametic sex.

<span class="mw-page-title-main">Sex-determination system</span> Biological system that determines the development of organism’s sex

A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes.

<span class="mw-page-title-main">Sexual differentiation</span> Embryonic development of sex differences

Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. Sex determination is often distinct from sex differentiation; sex determination is the designation for the development stage towards either male or female, while sex differentiation is the pathway towards the development of the phenotype.

<span class="mw-page-title-main">Evolution of sexual reproduction</span> How sexually reproducing multicellular organisms could have evolved from a common ancestor species

Sexual reproduction is an adaptive feature which is common to almost all multicellular organisms and various unicellular organisms, with some organisms being incapable of asexual reproduction. Currently the adaptive advantage of sexual reproduction is widely regarded as a major unsolved problem in biology. As discussed below, one prominent theory is that sex evolved as an efficient mechanism for producing variation, and this had the advantage of enabling organisms to adapt to changing environments. Another prominent theory, also discussed below, is that a primary advantage of outcrossing sex is the masking of the expression of deleterious mutations. Additional theories concerning the adaptive advantage of sex are also discussed below. Sex does, however, come with a cost. In reproducing asexually, no time nor energy needs to be expended in choosing a mate. And if the environment has not changed, then there may be little reason for variation, as the organism may already be well adapted. Sex also halves the amount of offspring a given population is able to produce. Sex, however, has evolved as the most prolific means of species branching into the tree of life. Diversification into the phylogenetic tree happens much more rapidly via sexual reproduction than it does by way of asexual reproduction.

In biology, gonochorism is a sexual system where there are only two sexes and each individual organism is either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric.

<span class="mw-page-title-main">Anisogamy</span> Sexual reproduction involving a large, female gamete and a small, male gamete

Anisogamy is a form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form. The smaller gamete is male, a sperm cell, whereas the larger gamete is female, typically an egg cell. Anisogamy is predominant among multicellular organisms. In both plants and animals gamete size difference is the fundamental difference between females and males.

<span class="mw-page-title-main">Isogamy</span>

Isogamy is a form of sexual reproduction that involves gametes of the same morphology, found in most unicellular eukaryotes. Because both gametes look alike, they generally cannot be classified as male or female. Instead, organisms undergoing isogamy are said to have different mating types, most commonly noted as "+" and "−" strains.

<span class="mw-page-title-main">Oogamy</span> Sexual reproduction involving a large, non-motile female gamete and a small, motile male gamete

Oogamy is an extreme form of anisogamy where the gametes differ in both size and form. In oogamy the large female gamete is immobile, while the small male gamete is mobile. Oogamy is a common form of anisogamy, with almost all animals and land plants being oogamous.

<span class="mw-page-title-main">XO sex-determination system</span> Biological system that determines the sex of offspring

The XO sex-determination system is a system that some species of insects, arachnids, and mammals use to determine the sex of offspring. In this system, there is only one sex chromosome, referred to as X. Males only have one X chromosome (XO), while females have two (XX). The letter O signifies the lack of a second X. Maternal gametes always contain an X chromosome, so the sex of the animals' offspring depends on whether a sex chromosome is present in the male gamete. Its sperm normally contains either one X chromosome or no sex chromosomes at all.

Monogamous pairing in animals refers to the natural history of mating systems in which species pair bond to raise offspring. This is associated, usually implicitly, with sexual monogamy.

Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct sexes. They also occur in macro-organisms such as fungi.

<span class="mw-page-title-main">Hermaphrodite</span> Organism that has complete or partial male and female reproductive organs

In reproductive biology, a hermaphrodite is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.

<span class="mw-page-title-main">Female</span> Sex of an organism that produces ova

Female is the sex of an organism that produces the large non-motile ova, the type of gamete that fuses with the male gamete during sexual reproduction.

<span class="mw-page-title-main">Sexual reproduction</span> Reproduction process that creates a new organism by combining the genetic material of two organisms

Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete with a single set of chromosomes combines with another gamete to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid). This is typical in animals, though the number of chromosome sets and how that number changes in sexual reproduction varies, especially among plants, fungi, and other eukaryotes.

Social selection is a term used with varying meanings in biology.

Gametogamy is sexual fusion – copulation or fertlization – of two single-celled gametes of different sex and the union of their gamete nuclei giving the zygote nucleus, as well as whole zygotic content.

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Further reading