Bee: Difference between revisions

Bees
	Osmia ribifloris
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Suborder:	Apocrita
Superfamily:	Apoidea
(unranked):	Anthophila
Families
	Andrenidae; Apidae; Colletidae; Dasypodaidae; Halictidae; Megachilidae; Meganomiidae; Melittidae; Stenotritidae
Synonyms
	Apiformes

Browse history interactively

← Previous edit Next edit →

Content deleted Content added

VisualWikitext

Inline

Revision as of 11:23, 14 June 2008

Template:Otheruses6

Bees are flying insects closely related to wasps and ants. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila. There are slightly fewer than 20,000 known species of bee, in nine recognized families,^[1] though many are undescribed and the actual number is probably higher. They are found on every continent except Antarctica, in every habitat on the planet that contains flowering dicotyledons.

Introduction

Bees are adapted for feeding on nectar and pollen, the former primarily as an energy source, and the latter primarily for protein and other nutrients. Most pollen is used as food for larvae.

Bees have a long proboscis (a complex "tongue") that enables them to obtain the nectar from flowers. They have antennae almost universally made up of thirteen segments in males and twelve in females, as is typical for the superfamily. Bees all have two pairs of wings, the hind pair being the smaller of the two; in a very few species, one sex or caste has relatively short wings that make flight difficult or impossible, but none are wingless.

The smallest bee is the dwarf bee (Trigona minima), about 2.1 mm (5/64") long. The largest bee in the world is Megachile pluto, which can grow to a size of 39 mm (1.5"). Member of the family Halictidae, or sweat bees, are the most common type of bee in the Northern Hemisphere, though they are small and often mistaken for wasps or flies.

The best-known bee species is the Western honey bee, which, as its name suggests, produces honey, as do a few other types of bee. Human management of this species is known as beekeeping or apiculture.

Bees are the favorite meal of Merops apiaster, the bee-eater bird. Other common predators are kingbirds, mockingbirds, bee wolves, and dragonflies.

Pollination

Bees play an important role in pollinating flowering plants, and are the major type of pollinator in ecosystems that contain flowering plants. Bees either focus on gathering nectar or on gathering pollen depending on demand, especially in social species. Bees gathering nectar may accomplish pollination, but bees that are deliberately gathering pollen are more efficient pollinators.

It is estimated that one third of the human food supply depends on insect pollination, most of which is accomplished by bees, especially the domesticated Western honey bee. Contract pollination has overtaken the role of honey production for beekeepers in many countries. Monoculture and pollinator decline (of many bee species) have increasingly caused honey bee keepers to become migratory so that bees can be concentrated in seasonally-varying high-demand areas of pollination. Recently, many such migratory beekeepers have experienced substantial losses, prompting the announcement of investigation into the phenomenon, dubbed "Colony Collapse Disorder," amidst great concern over the nature and extent of the losses. Many other species of bees such as mason bees are increasingly cultured and used to meet the agricultural pollination need. Most native pollinators are solitary bees, which often survive in refuge in wild areas away from agricultural spraying, but may still be poisoned in massive spray programs for mosquitoes, gypsy moths, or other insect pests.

Most bees are fuzzy and carry an electrostatic charge, which aids in the adherence of pollen. Female bees periodically stop foraging and groom themselves to pack the pollen into the scopa, which is on the legs in most bees, and on the ventral abdomen on others, and modified into specialized pollen baskets on the legs of honey bees and their relatives. Many bees are opportunistic foragers, and will gather pollen from a variety of plants, while others are oligolectic, gathering pollen from only one or a few types of plant. A small number of plants produce nutritious floral oils rather than pollen, which are gathered and used by oligolectic bees. One small subgroup of stingless bees (called "vulture bees") is specialized to feed on carrion, and these are the only bees that do not use plant products as food. Pollen and nectar are usually combined together to form a "provision mass", which is often soupy, but can be firm. It is formed into various shapes (typically spheroid), and stored in a small chamber (a "cell"), with the egg deposited on the mass. The cell is typically sealed after the egg is laid, and the adult and larva never interact directly (a system called "mass provisioning").

Visiting flowers can be a dangerous occupation. Many assassin bugs and crab spiders hide in flowers to capture unwary bees. Other bees are lost to birds in flight. Insecticides used on blooming plants kills many bees, both by direct poisoning and by contamination of their food supply. A honey bee queen may lay 2000 eggs per day during spring buildup, but she also must lay 1000 to 1500 eggs per day during the foraging season, mostly to replace daily casualties, most of which are workers dying of old age. Among solitary and primitively social bees, however, lifetime reproduction is among the lowest of all insects, as it is common for females of such species to produce fewer than 25 offspring.

The population value of bees depends partly on the individual efficiency of the bees, but also on the population itself. Thus, while bumblebees have been found to be about ten times more efficient pollinators on cucurbits, the total efficiency of a colony of honey bees is much greater, due to greater numbers. Likewise, during early spring orchard blossoms, bumblebee populations are limited to only a few queens, and thus are not significant pollinators of early fruit.

Evolution

Bees, like ants, are a specialized form of wasp. The ancestors of bees were wasps in the family Crabronidae, and therefore predators of other insects. The switch from insect prey to pollen may have resulted from the consumption of prey insects that were flower visitors and were partially covered with pollen when they were fed to the wasp larvae. This same evolutionary scenario has also occurred within the vespoid wasps, where the group known as "pollen wasps" also evolved from predatory ancestors. Up until recently the oldest non-compression bee fossil had been Cretotrigona prisca in New Jersey amber and of Cretaceous age, a meliponine. A recently reported bee fossil, of the genus Melittosphex, is considered "an extinct lineage of pollen-collecting Apoidea sister to the modern bees", and dates from the early Cretaceous (~100 mya).^[2] Derived features of its morphology ("apomorphies") place it clearly within the bees, but it retains two unmodified ancestral traits ("plesiomorphies") of the legs (two mid-tibial spurs, and a slender hind basitarsus), indicative of its transitional status.

The earliest animal-pollinated flowers were pollinated by insects such as beetles, so the syndrome of insect pollination was well established before bees first appeared. The novelty is that bees are specialized as pollination agents, with behavioral and physical modifications that specifically enhance pollination, and are much more efficient at the task than beetles, flies, butterflies, pollen wasps, or any other pollinating insect. The appearance of such floral specialists is believed to have driven the adaptive radiation of the angiosperms, and, in turn, the bees themselves.

Among living bee groups, the Dasypodaidae are now considered to be the most "primitive", and sister taxon to the remainder of the bees, contrary to earlier hypotheses that the "short-tongued" bee family Colletidae was the basal group of bees; the short, wasp-like mouthparts of colletids are the result of convergent evolution, rather than indicative of a plesiomorphic condition.^[1]

Eusocial and semisocial bees

Bees may be solitary or may live in various types of communities. The most advanced of these are eusocial colonies found among the honey bees, bumblebees, and stingless bees. Sociality, of several different types, is believed to have evolved separately many times within the bees.

In some species, groups of cohabiting females may be sisters, and if there is a division of labor within the group, then they are considered semisocial.

If, in addition to a division of labor, the group consists of a mother and her daughters, then the group is called eusocial. The mother is considered the "queen" and the daughters are "workers". These castes may be purely behavioral alternatives, in which case the system is considered "primitively eusocial" (similar to many paper wasps), and if the castes are morphologically discrete, then the system is "highly eusocial".

There are many more species of primitively eusocial bees than highly eusocial bees, but they have rarely been studied. The biology of most such species is almost completely unknown. The vast majority are in the family Halictidae, or "sweat bees". Colonies are typically small, with a dozen or fewer workers, on average. The only physical difference between queens and workers is average size, if they differ at all. Most species have a single season colony cycle, even in the tropics, and only mated females (future queens, or "gynes") hibernate (called diapause). A few species have long active seasons and attain colony sizes in the hundreds. The orchid bees include a number of primitively eusocial species with similar biology. Certain species of allodapine bees (relatives of carpenter bees) also have primitively eusocial colonies, with unusual levels of interaction between the adult bees and the developing brood. This is "progressive provisioning"; a larva's food is supplied gradually as it develops. This system is also seen in honey bees and some bumblebees.

Highly eusocial bees live in colonies. Each colony has a single queen, many workers and, at certain stages in the colony cycle, drones. When humans provide the nest, it is called a hive. A honey bee hive can contain up to 40,000 bees at their annual peak, which occurs in the spring, but usually have fewer.

Bumblebees

Bumblebees (Bombus terrestris, B. pratorum, et al.) are eusocial in a manner quite similar to the eusocial Vespidae such as hornets. The queen initiates a nest on her own (unlike queens of honey bees and stingless bees which start nests via swarms in the company of a large worker force). Bumblebee colonies typically have from 50 to 200 bees at peak population, which occurs in mid to late summer. Nest architecture is simple, limited by the size of the nest cavity (pre-existing), and colonies are rarely perennial. Bumblebee queens sometimes seek winter safety in honey bee hives, where they are sometimes found dead in the spring by beekeepers, presumably stung to death by the honey bees. It is unknown whether any survive winter in such an environment.

Stingless bees

Stingless bees are very diverse in behavior, but all are highly eusocial. They practice mass provisioning, complex nest architecture, and perennial colonies.

Honey bees

The true honey bees (genus Apis) have arguably the most complex social behavior among the bees. The Western (or European) honey bee, Apis mellifera, is the best known bee species and one of the best known of all insects.

Africanized honey bee

Africanized bees, also called killer bees, are a hybrid strain of Apis mellifera derived from experiments to cross European and African honey bees by Warwick Estevam Kerr. Several queen bees escaped his laboratory in South America and have spread throughout the Americas. Africanized honey bees are more defensive than European honey bees.

Solitary and communal bees

Most other bees, including familiar species of bee such as the Eastern carpenter bee (Xylocopa virginica), alfalfa leafcutter bee (Megachile rotundata), orchard mason bee (Osmia lignaria) and the hornfaced bee (Osmia cornifrons) are solitary in the sense that every female is fertile, and typically inhabits a nest she constructs herself. There are no worker bees for these species. Solitary bees typically produce neither honey nor beeswax. They are immune from acarine and Varroa mites (see diseases of the honey bee), but have their own unique parasites, pests and diseases.

Solitary bees are important pollinators, and pollen is gathered for provisioning the nest with food for their brood. Often it is mixed with nectar to form a paste-like consistency. Some solitary bees have very advanced types of pollen carrying structures on their bodies. A very few species of solitary bees are being increasingly cultured for commercial pollination.

Solitary bees are often oligoleges, in that they only gather pollen from one or a few species/genera of plants (unlike honey bees and bumblebees which are generalists). No known bees are nectar specialists; many oligolectic bees will visit multiple plants for nectar, but there are no bees which visit only one plant for nectar while also gathering pollen from many different sources. Specialist pollinators also include bee species that gather floral oils instead of pollen, and male orchid bees, which gather aromatic compounds from orchids (one of the only cases where male bees are effective pollinators). In a very few cases only one species of bee can effectively pollinate a plant species, and some plants are endangered at least in part because their pollinator is dying off. There is, however, a pronounced tendency for oligolectic bees to be associated with common, widespread plants which are visited by multiple pollinators (e.g., there are some 40 oligoleges associated with creosotebush in the US desert southwest^[3], and a similar pattern is seen in sunflowers, asters, mesquite, etc.)

Solitary bees create nests in hollow reeds or twigs, holes in wood, or, most commonly, in tunnels in the ground. The female typically creates a compartment (a "cell") with an egg and some provisions for the resulting larva, then seals it off. A nest may consist of numerous cells. When the nest is in wood, usually the last (those closer to the entrance) contain eggs that will become males. The adult does not provide care for the brood once the egg is laid, and usually dies after making one or more nests. The males typically emerge first and are ready for mating when the females emerge. Providing nest boxes for solitary bees is increasingly popular for gardeners. Solitary bees are either stingless or very unlikely to sting (only in self defense, if ever).

While solitary females each make individual nests, some species are gregarious, preferring to make nests near others of the same species, giving the appearance to the casual observer that they are social. Large groups of solitary bee nests are called aggregations, to distinguish them from colonies.

In some species, multiple females share a common nest, but each makes and provisions her own cells independently. This type of group is called "communal" and is not uncommon. The primary advantage appears to be that a nest entrance is easier to defend from predators and parasites when there are multiple females using that same entrance on a regular basis.

Cleptoparasitic bees

Cleptoparasitic bees, commonly called "cuckoo bees" because their behavior is similar to cuckoo birds, occur in several bee families, though the name is technically best applied to the apid subfamily Nomadinae. Females of these bees lack pollen collecting structures (the scopa) and do not construct their own nests. They typically enter the nests of pollen collecting species, and lay their eggs in cells provisioned by the host bee. When the cuckoo bee larva hatches it consumes the host larva's pollen ball, and if the female cleptoparasite has not already done so, kills and eats the host larva. In a few cases where the hosts are social species, the cleptoparasite remains in the host nest and lays many eggs, sometimes even killing the host queen and replacing her.

Many cleptoparasitic bees are closely related to, and resemble, their hosts in looks and size, (i.e., the Bombus subgenus Psithyrus, which are parasitic bumblebees that infiltrate nests of species in other subgenera of Bombus). This common pattern gave rise to the ecological principle known as "Emery's Rule". Others parasitize bees in different families, like Townsendiella, a nomadine apid, one species of which is a cleptoparasite of the dasypodaid genus Hesperapis, while the other species in the same genus attack halictid bees.

Nocturnal bees

Four bee families (Andrenidae, Colletidae, Halictidae, and Apidae) contain some species that are crepuscular (these may be either the vespertine or matinal type). These bees have greatly enlarged ocelli, which are extremely sensitive to light and dark, though incapable of forming images. Many are pollinators of flowers that themselves are crepuscular, such as evening primroses, and some live in desert habitats where daytime temperatures are extremely high.

Bee flight

In his 1934 French book Le vol des insectes, M. Magnan wrote that he and a Mr. Saint-Lague had applied the equations of air resistance to bumblebees and found that their flight was impossible, but that "One shouldn't be surprised that the results of the calculations don't square with reality".^[4]

In 1996 Charlie Ellington at Cambridge University showed that vortices created by many insects’ wings and non-linear effects were a vital source of lift;^[5] vortices and non-linear phenomena are notoriously difficult areas of hydrodynamics, which has made for slow progress in theoretical understanding of insect flight.

In 2005 Michael Dickinson and his Caltech colleagues studied honey bee flight with the assistance of high-speed cinematography [1] and a giant robotic mock-up of a bee wing^[6]. Their analysis revealed sufficient lift was generated by "the unconventional combination of short, choppy wing strokes, a rapid rotation of the wing as it flops over and reverses direction, and a very fast wing-beat frequency". Wing beat frequency normally increases as size decreases, but as the bee's wing beat covers such a small arc, it flaps approximately 230 times per second, faster than a fruitfly (200 times per second) which is 80 times smaller.^[7]

Bees and humans

Bees figure prominently in mythology (See Bee (mythology)) and have been used by political theorists as a model for human society. Journalist Bee Wilson states that the image of a community of honey bees "occurs from ancient to modern times, in Aristotle and Plato; in Virgil and Seneca; in Erasmus and Shakespeare; Tolstoy, as well as by social theorists Bernard Mandeville and Karl Marx."^[8]

Despite the honey bee's painful sting and the stereotype of insects as pests, bees are generally held in high regard. This is most likely due to their usefulness as pollinators and as producers of honey, their social nature and their reputation for diligence. Bees are one of the few insects used on advertisements, being used to illustrate honey and foods made with honey (e.g. Honey Nut Cheerios), and appearing in the 2007 Bee Movie.

In North America, yellowjackets and hornets, especially when encountered as flying pests, are often misidentified as bees, despite numerous differences between them. Although a bee sting can be deadly to those with allergies, virtually all bee species are non-aggressive if undisturbed and many cannot sting at all. In fact humans will often be a greater danger to the bees, as bees are often affected or even harmed by encounters with toxic chemicals in the environment (see Bees and toxic chemicals).

Gallery

two species together
Western honey bee, Poland
Western honey bee on a Sphaeralcea flower. Mesa, Az
Western honey bee in a Sphaeralcea flower. Mesa, Az
Sweat bee, Agapostemon virescens (female) on a Coreopsis flower. Madison, Wi
Bumblebee, Bombus sp. startles Agapostemon virescens. Madison, Wi
Western honey bee
Western honey bee, Kaunakakai, HI
Western honey bees, Lebanon.
Western honey bee, Lebanon.
Western honey bee collecting pollen from a rose.
Western honey bee collecting nectar from small flowers. McKinney, Texas.
Hovering bumblebee at lupine
Western honey bee on Apple blossom
Bumblebee, Montreal
Euglossine bees on orchid Mormodes buccinator (Suriname)

References

^ ^a ^b Danforth, B.N., Sipes, S., Fang, J., Brady, S.G. (2006) The history of early bee diversification based on five genes plus morphology. Proceedings of the National Academy of Sciences 103: 15118-15123.
^ Poinar, G.O. Jr., Danforth, B.N. 2006. A fossil bee from early Cretaceous Burmese amber. Science 314: 614.
^ Hurd, P.D. Jr., Linsley, E.G. 1975. The principal Larrea bees of the southwestern United States. Smithsonian Contributions to Zoology 193: 1-74.
^ Ingram, Jay The Barmaid's Brain, Aurum Press, 2001, pp.91-92.
^ Secrets of bee flight revealed, Phillips, Helen. 28 November 2005. Retrieved 2007-12-28
^ Deciphering the Mystery of Bee Flight Caltech Media Relations. Nov. 29, 2005. Retrieved 2007, 4-7.
^ Douglas L. Altshuler, William B. Dickson, Jason T. Vance, Stephen P. Roberts, and Michael H. Dickinson (2005). "Short-amplitude high-frequency wing strokes determine the aerodynamics of honeybee flight". Proc Natl Acad Sci U S A. 102: 18213–18218. doi:10.1073/pnas.0506590102. PMID 16330767.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Wilson, Bee (2004). The Hive: The Story Of The Honeybee. London, Great Britain: John Murray (publisher). ISBN 0 7195 6598 7.

External links

All Living Things Images, identification guides, and maps of bees
Bee Genera of the World
Carl Hayden Bee Research Center
Rescuing Australian stingless bees
The first bee of spring
Solitary Bees & Things Solitary Bees in British gardens
Scientists identify the oldest known bee, a 100 million-year-old specimen preserved in amber
Search for North American species at Bugguide here
For Hymenoptera: Bees and other related Insects Natural History of Bees, Wasps, and Insects
Bee images on Morphbank, biological image database
Dickinson Lab
Video: Life Cycle of a Honey Bee
Video: Orchid Bees

[Danforthetal2006-1] Danforth, B.N., Sipes, S., Fang, J., Brady, S.G. (2006) The history of early bee diversification based on five genes plus morphology. Proceedings of the National Academy of Sciences 103: 15118-15123.

[2] Poinar, G.O. Jr., Danforth, B.N. 2006. A fossil bee from early Cretaceous Burmese amber. Science 314: 614.

[3] Hurd, P.D. Jr., Linsley, E.G. 1975. The principal Larrea bees of the southwestern United States. Smithsonian Contributions to Zoology 193: 1-74.

[4] Ingram, Jay The Barmaid's Brain, Aurum Press, 2001, pp.91-92.

[5] Secrets of bee flight revealed, Phillips, Helen. 28 November 2005. Retrieved 2007-12-28

[caltech-6] Deciphering the Mystery of Bee Flight Caltech Media Relations. Nov. 29, 2005. Retrieved 2007, 4-7.

[7] Douglas L. Altshuler, William B. Dickson, Jason T. Vance, Stephen P. Roberts, and Michael H. Dickinson (2005). "Short-amplitude high-frequency wing strokes determine the aerodynamics of honeybee flight". Proc Natl Acad Sci U S A. 102: 18213–18218. doi:10.1073/pnas.0506590102. PMID 16330767.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[8] Wilson, Bee (2004). The Hive: The Story Of The Honeybee. London, Great Britain: John Murray (publisher). ISBN 0 7195 6598 7.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

@@ Line 43: / Line 43: @@
 Bees are the favorite meal of ''Merops apiaster'', the [[bee-eater]] bird. Other common predators are [[kingbird]]s, [[mockingbird]]s, [[Philanthus|bee wolves]], and [[dragonfly|dragonflies]].
-Sex
-From Wikipedia, the free encyclopedia
-Jump to: navigation, search
-This article is about biological sex. For other uses, such as sexual intercourse and the social concept of gender, see Sex (disambiguation).
-Sex in animals involves the fusion of a sperm and an egg cell.In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into male and female types (or sexes). Sexual reproduction involves combining specialized cells (gametes) to form offspring that inherit traits from both parents. Gametes can be identical in form and function (known as isogametes), but in many cases an asymmetry has evolved such that two sex-specific types of gametes (heterogametes) exist: male gametes are small, motile, and optimized to transport their genetic information over a distance, while female gametes are large, non-motile and contain the nutrients necessary for the early development of the young organism.
-An organism's sex is defined by the gametes it produces: males produce male gametes (spermatozoa, or sperm) while females produce female gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic. Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience. In some cases male or (more commonly) female organisms also have the role of caring for offspring through the first part of development.
-Contents [hide]
-Sexual reproduction
-.1 Animals
-.2 Plants
-.3 Fungi
-.4 Evolution
-Sex determination
-.1 Genetic
-.2 Nongenetic
-Sexual dimorphism
-See also
-References
-Notes
-External links and further reading
-Sexual reproduction
-Main article: sexual reproduction
-The life cycle of sexually reproducing organisms cycles through haploid and diploid stages.Sexual reproduction is a process where organisms form offspring that combine genetic traits from both parents.[1] Genetic traits are contained within the dioxyribonucleic acid (DNA) of chromosomes — by combining one of each type of chromosomes from each parent, an organism is formed containing a doubled set of chromosomes. This double-chromosome stage is called "diploid", while the single chromosome stage is "haploid". Diploid organisms can, in turn, form haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via a process called meiosis.[2] Meiosis also involves a stage of chromosomal crossover, in which regions of DNA are exchanged between matched types of chromosomes to form a new pair of mixed chromosomes. Crossing over and fertilization (the recombining of single sets of chromosomes to make a new diploid) result in the new organism containing a different set of genetic traits from either parent.
-In many organisms the haploid stage has been reduced to just gametes specialized to recombine and form a new diploid organism; in others, the gametes are capable of undergoing cell division to produce multicellular haploid organisms. In either case, gametes may be externally similar, particularly in size (isogamy), or may have evolved an asymmetry such that the gametes are different in size and other aspects (anisogamy).[3] By convention, the larger gamete (called an ovum, or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male. An individual that produces both types of gametes is a hermaphrodite; in some cases hermaphrodites are able to self-fertilize and produce offspring on their own, without a second organism.[4]
-Hoverflies engaging in sexual intercourse
-Animals
-Most sexually reproducing animals spend their lives as diploid organisms, with the haploid stage reduced to single cell gametes.[5] The gametes of animals have male and female forms—spermatozoa and egg cells. These gametes combine to form embryos which develop into a new organism.
-The male gamete, a spermatozoan (produced within a testicle), is a small cell containing a single long flagellum which propels it.[6] Spermatozoa are extremely reduced cells, lacking many cellular components that would be necessary for embryonic development. They are specialized for motility, seeking out an egg cell and fusing with it in a process called fertilization.
-Female gametes are egg cells (produced within ovaries), large immobile cells that contain the nutrients and cellular components necessary for a developing embryo.[7] Egg cells are often associated with other cells which support the development of the embryo, forming an egg. In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother.
-Animals are usually mobile and seek out a partner of the opposite sex for mating. Animals which live in the water can mate using external fertilization, where the eggs and sperm are released into and combine within the surrounding water.[8] Most animals that live outside of water, however, must use transfer sperm from male to female to achieve internal fertilization.
-In most birds, both excretion and reproduction is done through a single posterior opening, called the cloaca—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing".[9] In many other terrestrial animals, males use specialized sex organs to assist the transport of sperm—these male sex organs are called intromittent organs. In humans and other mammals this male organ is the penis, which enters the female reproductive tract (called the vagina) to achieve insemination—a process called sexual intercourse. The penis contains a tube through which semen (a fluid containing sperm) travels. In female mammals the vagina connects with the uterus, an organ which directly supports the development of a fertilized embryo within (a process called gestation).
-Flowers are the sexual organs of flowering plants, containing both male and female parts.
-Plants
-Main article: Plant sexuality
-Like animals, plants have developed specialized male and female gametes.[10] Within most familiar plants, male gametes are contained within hard coats, forming pollen. The female gametes of plants are contained within ovules; once fertilized by pollen these form seeds which, like eggs, contain the nutrients necessary for the development of the embryonic plant.
-Female (left) and male (right) cones are the sex organs of pines and other conifers.Many plants have flowers and these are the sexual organs of those plants. Flowers are usually hermaphroditic, producing both male and female gametes. The female parts, in the center of a flower, are the carpels—one or more of these may be merged to form a single pistil. Within carpels are ovules which develop into seeds after fertilization. The male parts of the flower are the stamens: these long filamentous organs are arranged between the pistil and the petals and produce pollen at their tips. When a pollen grain lands upon the top of a carpel, the tissues of the plant react to transport the grain down into the carpel to merge with an ovule, eventually forming seeds.
-In pines and other conifers the sex organs are cones and have male and female forms. The more familiar female cones are typically more durable, containing ovules within them. Male cones are smaller and produce pollen which is transported by wind to land in female cones. As with flowers, seeds form within the female cone after pollination.
-Because plants are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many plants, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Other plants have heavier, sticky pollen that is specialized for transportation by insects. The plants attract these insects with nectar-containing flowers. Insects transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination.
-Mushrooms are produced as part of fungal sexual reproduction.
-Fungi
-Main article: Mating in fungi
-Most fungi reproduce sexually, having both a haploid and diploid stage in their life cycles. These fungi are typically isogamous, lacking male and female specialization: haploid fungi grow into contact with each other and then fuse their cells. In some of these cases the fusion is asymmetric, and the cell which donates only a nucleus (and not accompanying cellular material) could arguably be considered "male".[11]
-Some fungi, including baker's yeast, have mating types that create a duality similar to male and female roles. Yeast with the same mating type will not fuse with each other to form diploid cells, only with yeast carrying the other mating type.[12]
-Fungi produce mushrooms as part of their sexual reproduction. Within the mushroom diploid cells are formed, later dividing into haploid spores—the height of the mushroom aids the dispersal of these sexually produced offspring.
-Sex helps the spread of advantageous traits through recombination. The diagrams compare evolution of allele frequency in a sexual population (a) and an asexual population (b). The verticle axis shows frequency and the horizontal axis shows time. The alleles a/A and b/B occur at random. The advantageous combination AB arises rapidly with recombination (a), but must arise independently in (b).
-Evolution
-Main article: Evolution of sex
-Sexual reproduction first appeared about a billion years ago, evolved within ancestral single-celled eukaryotes.[13] The reason for the initial evolution of sex, and the reason it has survived to the present are still matters of debate. Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, and that sex helps in the removal of disadvantageous traits.
-Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria. In addition to animals, plants, and fungi, other eukaryotes (eg. the malaria parasite) also engage in sexual reproduction. Some bacteria use conjugation to transfer genetic material between bacteria; while not the same as sexual reproduction, this also results in the mixture of genetic traits.
-What is considered defining of sexual reproduction is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, as far as we know no third gamete has appeared in multicellular animals.[14][15][16]
-Sex determination
-Main article: sex determination
-The most basic sexual system is one in which all organisms are hermaphrodites, producing both male and female gametes—this is true of some animals (eg. snails) and the majority of flowering plants.[17] In many cases, however, specialization of sex has evolved such that some organisms produce only male or only female gametes. The biological cause for an organism developing into one sex or the other is called sex determination.
-In the majority of species with sex specialization organisms are either male (producing only male gametes) or female (producing only female gametes). A few exceptions exist—for example, in the roundworm C. elegans the two sexes are hermaphrodite and male (a system called androdioecy).
-Sometimes an organism's development is intermediate between male and female, a condition called intersex. Sometimes intersex individuals are called "hermaphrodite" but, unlike biological hermaphrodites, intersex individuals are unusual cases and are not typically fertile in both male and female aspects.
-Genetic
-Like humans and other mammals, the common fruit fly has an XY sex determination system.In genetic sex determination systems, an organism's sex is determined by the genome it inherits. Genetic sex determines usually depends on asymmetrically inherited sex chromosomes which carry genetic features that influence development; sex may be determined either by the presence of a sex chromosome or by how many the organism has. Genetic sex determination, because it is determined by chromosome assortment, usually results in a 1:1 ratio of male and female offspring.
-Humans and other mammals have an XY sex determination system: the Y chromosome carries factors responsible for triggering male development. The default sex, in the absence of a Y chromosome, is female. Thus, XX mammals are female and XY are male. XY sex determination is found in other organisms, including the common fruit fly and some plants.[17] In some cases, including in the fruit fly, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome.
-In birds, which have a ZW sex-determination system, the opposite is true: the W chromosome carries factors responsible for female development, and default development is male.[18] In this case ZZ individuals are male and ZW are female. The majority of butterflies and moths also have a ZW sex-determination system. In both XY and ZW sex determination systems the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.[19]
-Many insects use a sex determination system based on the number of sex chromosomes. This is called XX/XO sex determination—the O indicates the absence of the sex chromosome. All other chromosomes in these organisms are diploid, but organisms may inherit one or two X chromosomes. In field crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female.[20] In the nematode C. elegans most worms are self-fertilizing XX hermaphrodites, but occasionally abnormalities in chromosome inheritance regularly give rise to individuals with only one X chromosome—these XO individuals are fertile males (and half their offspring are male).[21]
-Other insects, including honey bees and ants, use a haplodiploid sex-determination system.[22] In this case diploid individuals are generally female, and haploid individuals (which develop from unfertilized eggs) are male. This sex-determination system results in highly biased sex ratios, as the sex of offspring is determined by fertilization rather than the assortment of chromosomes during meiosis.
-Clownfish are initially male; the largest fish in a group becomes female.
-Nongenetic
-For many species sex is not determined by inherited traits, but instead by environmental factors experienced during development or later in life. Many reptiles have temperature-dependent sex determination: the temperature embryos experience during their development determines the sex of the organism. In some turtles, for example, males are produced at lower incubation temperatures than females; this difference in critical temperatures can be as little as 1-2°C.
-Many fish change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism. In clownfish, smaller fish are male, and the dominant and largest fish in a group becomes female. In many wrasses the opposite is true—most fish are initially female and become male when they reach a certain size. Sequential hermaphrodites may produce both types of gametes over the course of their lifetime, but at any given point they are either female or male.
-In some ferns the default sex is hermaphrodite, but ferns which grow in soil that has previously supported hermaphrodites are influenced by residual hormones to instead develop as male.[23]
-Sexual dimorphism
-Common pheasants are sexually dimorphic in both size and appearance.Main article: sexual dimorphism
-Many animals have differences between the male and female sexes in size and appearance, a phenomenon called sexual dimorphism. Sexual dimorphisms are often associated with sexual selection - the competition between individuals of one sex to mate with the opposite sex.[24] Antlers in male deer, for example, are used in combat between males to win reproductive access to female deer. In many cases the male of a species is larger in size; in mammals species with high sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males.
-Other animals, including most insects and many fish, have larger females. This may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm—larger females are able to produce more eggs.[25] Occasionally this dimorphism is extreme, with males reduced to living as parasites dependent on the female.
-In birds, males often have a more colorful appearance and may have features (like the long tail of male peacocks) that would seem to put the organism at a disadvantage (eg. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the handicap principle.[26] This hypothesis says that, by demonstrating he can survive with such handicaps, the male is advertising his genetic fitness to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps.
-Sex differences in humans include a larger size and more body hair in men; women have breasts, wider hips, and a higher body fat percentage.
 == Pollination ==