How different is a Human basic life different from these species?

They we all accomplish the same states, except that we have a brain that has adapted and come up with life stories, life style, life identities, and so on…… in deed creating every one unique to blabber the story of these  cells.

Read from the original source:


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“Animalia” redirects here. For other uses, see Animalia (disambiguation).
For other uses, see Animal (disambiguation).
Fossil range: Ediacaran – Recent,

Scientific classification
Domain: Eukarya
(unranked): Opisthokonta
Kingdom: Animalia
Linnaeus, 1758

Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. Most animals are also heterotrophs, meaning they must ingest other organisms for sustenance.

Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.




[edit] Characteristics

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and are multicellular[1] (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic,[2] generally digesting food in an internal chamber, which separates them from plants and algae (some sponges are capable of photosynthesis and nitrogen fixation though).[3] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.[4] All animals are motile,[5] if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals.

[edit] Structure

With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissue, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.

All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.

[edit] Reproduction and development

A newt lung cell stained with fluorescent dyes undergoing mitosis, specifically early anaphase.

Nearly all animals undergo some form of sexual reproduction. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, which develop into new individuals.

Many animals are also capable of asexual reproduction. This may take place through parthenogenesis, where fertile eggs are produced without mating, or in some cases through fragmentation.

A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers – an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.

[edit] Food and energy sourcing

A juvenile Red-tailed Hawk eating a California Vole

All animals are heterotrophs, meaning that they feed directly or indirectly on other living things. They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.

Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked). Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey. The other main category of consumption is detritivory, the consumption of dead organic matter. It can at times be difficult to separate the two feeding behaviours, for example where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.

Most animals feed indirectly from the energy of sunlight. Plants use this energy to convert sunlight into simple sugars using a process known as photosynthesis. Starting with the molecules carbon dioxide (CO2) and water (H2O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of glucose (C6H12O6) and releases oxygen (O2). These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion. This process is known as glycolysis.

Animals who live close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight. Instead chemosynthetic archaea and bacteria form the base of the food chain.

[edit] Origin and fossil record

Further information: Urmetazoon

Dunkleosteus was a gigantic, 10 meter (33 ft) long prehistoric fish.[6]

Vernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria.

Animals are generally considered to have evolved from a flagellated eukaryote. Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.

The first fossils that might represent animals appear towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago. It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible. However some paleontologists and geologists would suggest that animals appeared much earlier than previously thought, possibly even as early as 1 billion years ago. Trace fossils such as tracks and burrows found in Tonian era indicate the presence of triploblastic worm like metazoans roughly as large (about 5 mm wide) and complex as earthworms.[7] In addition during the beginning of the Tonian period around 1 billion years ago (roughly the same time that the trace fossils previously discussed in this article date back to) there was a decrease in Stromatolite diversity which may indicate the appearance of grazing animals during this time as Stromatolites also increased in diversity shortly after the end-Ordovician and end-Permian rendered large amounts of grazing marine animals extinct and decreased shortly after their populations recovered. The discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist Gromia sphaerica casts further doubt on their interpretation as evidence of early animal evolution.[8][9]

[edit] Groups of animals

[edit] Porifera

Orange elephant ear sponge, Agelas clathrodes, in foreground. Two corals in the background: a sea fan, Iciligorgia schrammi, and a sea rod, Plexaurella nutans.

The sponges (Porifera) were long thought to have diverged from other animals early. As mentioned above, they lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges are sessile and typically feed by drawing in water through pores. Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum. However, a phylogenomic study in 2008 of 150 genes in 21 genera[10] revealed that it is the Ctenophora or comb jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.

Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. Both have distinct tissues, but they are not organized into organs. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic. The tiny Placozoans are similar, but they do not have a permanent digestive chamber.

The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however – for instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures.

Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: the Deuterostomes and Protostomes, which includes the Ecdysozoa, Platyzoa, and Lophotrochozoa. In addition, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, and Orthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Medusozoa as well.

[edit] Deuterostomes

Superb Fairy-wren, Malurus cyaneus

Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes the initial opening (the archenteron) develops into the mouth, and an anus forms separately. In deuterostomes this is reversed. In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes it forms through invagination of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.

All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata. The former are radially symmetric and exclusively marine, such as starfish, sea urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and mammals.

In addition to these, the deuterostomes also include the Hemichordata or acorn worms. Although they are not especially prominent today, the important fossil graptolites may belong to this group.

The Chaetognatha or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.

[edit] Ecdysozoa

Yellow-winged darter, Sympetrum flaveolum

The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis. The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.

The ecdysozoans also include the Nematoda or roundworms, the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water. A number are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.

The remaining two groups of protostomes are sometimes grouped together as the Spiralia, since in both embryos develop with spiral cleavage.

[edit] Platyzoa

Bedford’s flatworm, Pseudobiceros bedfordi

The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.[11]

A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.[12]

The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora.[13] These groups share the presence of complex jaws, from which they are called the Gnathifera.

[edit] Lophotrochozoa

Roman snail, Helix pomatia

The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida.[14][15] The former, which is the second-largest animal phylum, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods,[16] because they are both segmented. Now this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.[17]

The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a fan of cilia around the mouth, called a lophophore.[18] These were traditionally grouped together as the lophophorates.[19] but it now appears they are paraphyletic,[20] some closer to the Nemertea and some to the Mollusca and Annelida.[21][22] They include the Brachiopoda or lamp shells, which are prominent in the fossil record, the Entoprocta, the Phoronida, and possibly the Bryozoa or moss animals.[23]

[edit] Model organisms

Main articles: Model organism and Animal testing

Because of the great diversity found in animals, it is more economical for scientists to study a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how animals function in general. Because they are easy to keep and breed, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most intensively studied metazoan model organisms, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their genomes, but the double-edged sword here is that with many genes, introns and linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, and molluscan genome projects currently in progress. Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.[24]

An analysis of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.[25]

Other model organisms belonging to the animal kingdom include the mouse (Mus musculus) and zebrafish (Danio rerio).

Carolus Linnaeus known as the father of modern taxonomy

[edit] History of classification

Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus (Carl von Linné), in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately.

In Linnaeus‘s original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. The above lists represent our current understanding of the group, though there is some variation from source to source.

[edit] See also

Wikipedia Books Wikipedia:Books has a book on: Animal
Search Wikispecies Wikispecies has information related to: Animalia


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Please help improve this article by expanding it. Further information might be found on the talk page. (February 2007)
Fossil range: NeoproterozoicRecent

A human holding a mushroom; both are opisthokonts, unlike the trees behind them
Scientific classification
Domain: Eukaryota

The opisthokonts (Greek: ὀπίσθω- (opisthō-) = “rear, posterior” + κοντός (kontos) = “pole” i.e. flagellum) are a broad group of eukaryotes, including both the animal and fungus kingdoms,[1] together with the phylum Choanozoa and Mesomycetozoa of the protist “kingdom”. Both genetic and ultrastructural studies strongly support that opisthokonts form a monophyletic group.

Many protists are now considered opisthokonts.[2]

“Opisthokonta” and “Fungi/Metazoa group” are sometimes considered synonymous.[3]




[edit] Flagella

One common characteristic is that flagellate cells, such as most animal sperm and chytrid spores, propel themselves with a single posterior flagellum. This gives the groups its name.

In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagella.

[edit] History

The close relationship between animals and fungi was suggested by Cavalier-Smith in 1987,[4] who used the informal name opisthokonta (the formal name has been used for the chytrids), and was confirmed by later genetic studies.[5]

Early phylogenies placed them near the plants and other groups that have mitochondria with flat cristae, but this character varies.

Cavalier-Smith and Stechmann[6] argue that the uniciliate eukaryotes such as opisthokonts and Amoebozoa, collectively called unikonts, split off from the other biciliate eukaryotes, called bikonts, shortly after they evolved.

[edit] Taxonomy

Eukaryota tree


Animal mating behaviour

In biology, mating is the pairing of opposite-sex or hermaphroditic internal fertilization animals for copulation and, in social animals, also to raise their offspring. Mating methods include random mating, disassortative mating, assortative mating, or a mating pool.

In some birds, for example, it includes nest-building and feeding offspring. The human practice of making domesticated animals mate and of artificially inseminating them is part of animal husbandry.

Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization. The two individuals may be of opposite sexes or hermaphroditic, as is the case with, for example, snails.

Newly evolved prehistoric animals initially lived only in water and reproduced by external fertilization in the water. About 450 million years ago, during the Late Ordovician, some animals [citation needed] started migrating from oceans to the land, necessitating internal fertilization to maintain gametes in a liquid medium.

In some terrestrial arthropods, including insects representing basal (primitive) phylogenetic clades, the male deposits spermatozoa on the substrate, sometimes stored within a special structure, and courtship involves inducing the female to take up the sperm package into her genital opening; there is no actual copulation. In groups such as dragonflies and many spiders, males extrude sperm into secondary copulatory structures removed from their genital opening, which are then used to inseminate the female (in dragonflies, it is a set of modified sternites on the second abdominal segment; in spiders, it is the male pedipalps). In advanced groups of insects, the male uses its aedeagus, a structure formed from the terminal segments of the abdomen, to deposit sperm directly (though sometimes in a capsule called a “spermatophore“) into the female’s reproductive tract.

Many other animals reproduce sexually with external fertilization, including many basal vertebrates. Many vertebrates (such as reptiles, some fish, and most birds) reproduce with internal fertilization through cloacal copulation (see also hemipenis), while mammals copulate vaginally.

Animal sexuality

Animal sexual behavior takes many different forms, even within the same species. Researchers have observed monogamy, promiscuity, sex between species, sexual arousal from objects or places, rape, necrophilia, sexual orientation (heterosexuality, homosexuality, bisexuality and situational sexual behaviour) and a range of other practices among animals other than humans. Related studies have noted diversity in sexed bodies and gendered behaviour, such as intersex and transgender animals.

The study of animal sexuality (and primate sexuality especially) is a rapidly developing field. It used to be believed that only humans and a handful of species performed sexual acts other than for procreation, and that animals sexuality was instinctive and a simple response to the “right” stimulation (sight, scent). Current understanding is that many species believed monogamous have now been proven to be promiscuous or opportunistic in nature, a wide range of species appear to both masturbate and to use objects as tools to help them do so, in many species animals try to give and get sexual stimulation with others where procreation is not the aim, and homosexual behavior has now been observed among 1,500 species, and in 500 of those it is well documented




edit Types of activity

[edit] Masturbation

It appears that many animals, both male and female, masturbate, both when partners are available and otherwise.

For example, comments in its guide on assessing potential breeding stock purchases: “Masturbation is a normal behavior in all stallions that does not reduce semen production or performance in the breeding shed, and thus the use of devices to prevent such behavior is strongly discouraged and can be harmful to the stallion.” [4] Likewise the paper “Sexual Behavior – Current Topics in Applied Ethology and Clinical Methods” by Sue McDonnell of the Equine Behavior Laboratory, University of Pennsylvania School of Veterinary Medicine states:

“One example is the behavior known within the horse breeding industry as masturbation. This involves normal periodic erections and penile movements. This behavior, both from the descriptive field studies cited above and in extensive study of domestic horses, is now understood as normal, frequent behavior of male equids.[1]

Attempting to inhibit or punish masturbation, which is still a common practice of horse managers regionally around the world, often leads to increased masturbation and disturbances of normal breeding behavior (McDonnell and Hinze, in preparation).” [5]

Sexual release seeking is common in both domestic and non-domestic species. For example, a video (non-explicit) showing a kangaroo masturbating, inadvertently caught during a TV broadcast, can be found here. The female porcupine will use a stick as a vibrator, holding one end of a stick between her paws and walk around, straddling the stick as it bumps against the ground and vibrates against her genitalia. [6]

Sexologist Havelock Ellis in his 1927 “Studies in the Psychology of Sex” identified bulls, goats, sheep, camels and elephants as species known to practice masturbation, adding of some other species:

I am informed by a gentleman who is a recognized authority on goats, that they sometimes take the penis into the mouth and produce actual orgasm, thus practicing auto-fellatio. As regards ferrets … “if the bitch, when in heat, cannot obtain a dog [ie, male ferret] she pines and becomes ill. If a smooth pebble is introduced into the hutch, she will masturbate upon it, thus preserving her normal health for one season. But if this artificial substitute is given to her a second season, she will not, as formerly, be content with it.” […] Blumenbach observed a bear act somewhat similarly on seeing other bears coupling, and hyenas, according to Ploss and Bartels, have been seen practicing mutual masturbation by licking each other’s genitals.

In his 1999 book, Biological exuberance, Bruce Bagemihl PhD documents (p.71, 209-210) that:

Masturbation also occurs widely among animals, both male and female. A variety of creative techniques are used, including genital stimulation using the hand or front paw (primates, Lions), foot (Vampire Bats, primates), flipper (Walruses), or tail (Savanna Baboons), sometimes accompanied by stimulation of the nipples (Rhesus Macaques, Bonobos); auto-fellating or licking, sucking and/or nuzzling by a male of his own penis (Common Chimpanzees, Savanna Bonobos, Vervet Monkeys, Squirrel Monkeys, Thinhorn Sheep, Bharal, Aovdad, Dwarf Cavies); stimulation of the penis by flipping or rubbing it against the belly or in its own sheath (White-tailed and Mule Deer, Zebras and Takhi); spontaneous ejaculations (Mountain Sheep, Warthogs, Spotted Hyenas); and stimulation of the genitals using inanimate objects (found in several primates and cetaceans). [2]
Many birds masturbate by mounting and copulating with tufts of grass, leaves or mounds of earth, and some mammals such as primates and Dolphins also rub their genitals against the ground or other surfaces to stimulate themselves. [2]
Masturbation in female mammals, as well as heterosexual and homosexual intercourse (especially in primates), often involves direct or indirect stimulation of the clitoris […]. This organ is present in the females of all mammalian species and several other animal groups. [2]

and that:

Apes and Monkeys use a variety of objects to masturbate with and even deliberately create implements for sexual stimulation […] often in highly creative ways. [2]

[edit] Oral sex

Animals of several species are documented as engaging in both auto-fellatio and oral sex. Although easily confused by lay-people, this is a separate and sexually oriented behavior, distinct from non-sexual grooming or scent-investigation.

Auto-fellatio or oral sex in animals is documented in goats, primates, hyaenas and sheep at a minimum (see section #Masturbation for details).

[edit] Homosexual behavior

Main article: List of animals displaying homosexual behavior

The presence of same-sex sexual behavior was not scientifically observed on a large scale until recent times, possibly due to observer bias caused by social attitudes to same-sex sexual behavior. Homosexual behaviour does occur in the animal kingdom, especially in social species, particularly in marine birds and mammals, monkeys and the great apes. Homosexual behaviour has been observed among 1,500 species, and in 500 of those it is well documented[3] (there are approximately 1,250,000 species of animal in the world). Georgetown University professor Janet Mann has specifically theorised that homosexual behaviour, at least in dolphins, is an evolutionary advantage that minimises intraspecies aggression, especially among males.

  • Male penguin couples have been documented to mate for life, build nests together, and to use a stone as a surrogate egg in nesting and brooding. In 2004, the Central Park Zoo in the United States replaced one male couple’s stone with a fertile egg, which the couple then raised as their own offspring.[4] German and Japanese zoos have also reported homosexual behaviour among their penguins. This phenomenon has also been reported at Kelly Tarlton’s Aquarium in Auckland, New Zealand.
  • Courtship, mounting, and full anal penetration between bulls has been noted to occur among American Bison. The Mandan nation Okipa festival concludes with a ceremonial enactment of this behaviour, to “ensure the return of the buffalo in the coming season.” [citation needed] Also, mounting of one female by another is common among cattle. (See also, Freemartin. Freemartins occur because of clearly causal hormonal factors at work during gestation.)
  • Homosexual behaviour in male sheep (found in 6-10% of rams) is associated with variations in cerebral mass distribution and chemical activity. A study reported in Endocrinology concluded that biological and physiological factors are in effect.[5] These findings are similar to human findings studied by Simon LeVay.
“Approximately eight percent of [male] rams exhibit sexual preferences [that is, even when given a choice] for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes…”
  • Male bighorn sheep are divisible into two kinds, the typical males among whom homosexual behavior is common and “effiminate sheep” or “behavioral transvestites” which are not known to engage in homosexual behavior. [6] [7]

Same-sex sexual behavior should only be identified as a sexual orientation with caution. In humans the behavior is considered distinct from the orientation – many heterosexuals engage in same-sex behavior at times, and many homosexuals have heterosexual lifestyles. In animals this distinction is still being explored.

[edit] Cross species sex

Although many people believe animal sexuality is instinctive and therefore (it is implied) almost mechanistic, in fact research regularly records that many animals are sexual opportunists, and may show an interest in partners other than their own or related species. [citation needed] This is more visible in domesticated species, as domestication commonly selects for increased breeding rate (and so an accelerated breeding cycle has commonly arisen in domesticated species over the centuries), and also because these species are easier to witness by humans. Nevertheless, non-domesticated animals have been observed to attempt sexual activity with other species or indeed inanimate objects. Such cross-species sex has been observed more with animals in captivity than in the wild, probably due to ease of observation, although attempts by wild moose to obtain sex from domestic horses are apparently well known by wildlife specialists.[8]

In the wild, where observation is harder, genetic studies have shown a “large number” of inter-species hybrids, and other investigations describe productive and non-productive inter-species mating as a “natural occurrence”.[9] Recent genetic evidence strongly suggesting this has occurred even within the history of the human species, and that early humans often had sexual activity with other primate species, is considered below.

Hybrid offspring can result if the two species are related. However, this offspring may not necessarily be fertile itself. The mule, for example (a horse/donkey cross) is normally sterile, whilst the liger (lion/tiger cross) is sometimes fertile. Novosibirsk zoo director Rostislav Shilo says on the liger (born in its zoo); “It’s just that the lion and the tiger live in neighboring caves in the Novosibirsk zoo, and got used to each other. It’s practically impossible in the wild.”

Due to the difficulties of observation, interspecies sex of this kind between two top-level predators, occurring in the wild, was only conclusively documented with the finding of a grizzly-polar bear hybrid in April 2006. Again, as with lions and tigers, the two species would normally not share enough common territory to provide adequate opportunity for much cross-species sexual activity. In other words, whether both species were 0% or 100% promiscuous and sought sex with the same species only or the nearest bear of any species, the overwhelming number of matings would still of necessity be with the same species.

Animal sexual advances on, and attempted interactions with, humans and other species, have been documented by ethologists such as Kohler, Gerald Durrell and Desmond Morris, as well as authoritative researchers such as Birute Galdikas who studied orangutans in Borneo. Peter Singer, the philosopher and animal rights activist reports: “While walking through the camp with Galdikas, my informant was suddenly seized by a large male orangutan, his intentions made obvious by his erect penis. Fighting off so powerful an animal was not an option, but Galdikas called to her companion not to be concerned, because the orangutan would not harm her, and adding, as further reassurance, that ‘they have a very small penis.'” (though “the orangutan lost interest before penetration took place”) [7]

[edit] Sexual fetishes

Although not often reported, it appears animals, or primates at the least, are able to sexualize inanimate objects in a manner similar to human sexual fetishes.

Not only will an animal that has a habitual object for masturbation sometimes appear to sexualize that object, but in some cases primates have generalized that to sexualize kinds of objects in a class where no prior sexual connection exists — similar to human fetishes.

Thus Gabriel, a chimpanzee at the Southwest National Primate Research Center, is said to have a shoe fetish (or possibly a leather fetish) according to caretaker Bert Barrera, and it is reported (probably referring to the same individual) that:

“A male chimpanzee raised in captivity developed a bit of a shoe fetish, masturbating obsessively by rubbing his caretaker’s leather boot.”

The sexualization of objects or locations is also well recognized in the breeding world. So for example, stallions may often ‘drop’ (become sexually aroused) upon visiting a location where they have been allowed to have sex before, or upon seeing a stimulus previously associated with sexual activity such as an artificial vagina. [citation needed]

In this case however, the primary structure is Pavlovian conditioning, and the fetishistic association is due to a conditioned response (or association) formed with a distinctive ‘reward‘. Human fetishism can very also be traced back to similar or near-identical conditioning: likewise based upon the Pavlovian association between an erotic sensation or anticipation, and objects which become immediately associated with that activity. (See also: operant conditioning)

Washoe, a chimpanzee who has been taught American Sign Language, has been reported to frequently make the sign for “tickle me” to researchers. Although not a sexual act per se, tickling is none the less recognized as a fetish in some contexts. (See: Tickling fetish)

[edit] Sexual imagery viewing

A study by Platt, Khera and Deaner at Duke University North Carolina (reported in Current Biology and online here), showed that monkeys will give up privileges (in this case, juice which was highly valued), to be allowed to see a female monkey’s hindquarters.[10]

Deaner and his team reported that monkeys would take a juice cut to look at powerful males’ faces or the perineum of a female, but to persuade the monkeys to stare at subordinate males, the researchers had to bribe them with larger drinks. “Virtually all [male] monkeys will give up juice to see female hindquarters … they really value the images.”

The researchers stress that in monkey society, such sights have great social utility, and therefore it should not be considered simply “monkeys enjoy pornographic pictures”. There is no evidence at this point that viewable pictures or movies of sexual activity are valued for their sexual enjoyment, although as noted above (#Masturbation) there are reports that watching sex in real life, may have such an effect. The subject of animals and sexual imagery is not yet well researched.

[edit] Rape and apparently coercive sex

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Controversial interpretations and implications aside (see Sociobiological theories of rape), sex in a forceful or apparently coercive context has also been documented in a variety of species. A notable example is bottlenose dolphins, where at times, gangs of bachelor males ‘corner’ females. The behavior is also common in some arachnids (spiders), notably those whose females eat the males during sex if not tricked with food and tied down with threads, and in some herbivorous herd species or species where males and females are very different in size, where the male dominates sexually by sheer force and size. Other animals which appear to combine sexual intercourse with apparent violent assault, also include some species of bird such as ducks and geese.

[edit] Faked orgasm

Female brown trout fake orgasm in around 60% (69 of 117) of sexual encounters, according to a researcher at the Swedish National Board of Fisheries. A similar phenomenon has been known in Atlantic salmon since 1954.[11] It is speculated that the female fakes completion of orgasm if she is unsure as to the male being correctly positioned for fertilization of eggs, and that the male is fooled by this into completing his shedding of sperm. It is believed that the female, who has refrained from shedding her eggs, saves them for the next time, shortly after.

A second source reports a slightly different theory, that she does this to encourage a less than satisfactory male to finish the sex act quicker and believe it has been successful, whilst the female withholds her eggs to seek a better partner afterwards. [8]

[edit] Sex between adults and juveniles

It has also been recorded that certain species of mole will impregnate newborns of their own species. [citation needed] It is not clear if this is forceful or not. Similarly, the male stoat (Mustela erminea) will mate with infant females of their species. [citation needed] This apparently is a natural part of their reproductive biology – there is a delayed gestation period, so these females give birth the following year when they are fully grown.

[edit] Necrophilia

A living mouse having sex with a dead mouse.

Necrophilia in animals is where a living animal engages in a sexual act with a dead animal. In one case, it is stated [12] that following a bird impact with a window, Kees Moeliker of Rotterdam, Netherlands, observed sexual activities[13] outside between a live duck and a dead one. He states that the living drake picked at the corpse of the dead one (also male) for a few minutes and then, without provocation, it mounted the corpse and began copulating with it. The act of necrophilia lasted for about 75 minutes, in which time, according to Moeliker, the living drake took two short breaks before resuming with copulating behavior. There is currently no scientific research able to verify certain other assumptions as to motive, which were made by Mr. Moeliker.

edit Mating systems

In sociobiology and behavioural ecology, the term mating system is used to describe the ways in which animal societies are structured in relation to sexual behaviour. The mating system specifies what males mate with what females under what circumstances.

The following are some of the mating systems generally recognised in humans and other animals:

  • Monogamy: One male and one female have an exclusive mating relationship.
  • Polygamy: One or more males have an exclusive relationship with one or more females. Three types are recognised:
    • Polygyny (the most common polygamous mating system in vertebrates so far studied): One male has an exclusive relationship with two or more females
    • Polyandry: One female has an exclusive relationship with two or more males
    • Polygynandry: Two or more males have an exclusive relationship with two or more females; the numbers of males and females need not be equal, and in vertebrate species studied so far, the number of males is usually less.
  • Promiscuity: Any male and female will mate within the social group.

[edit] Monogamy

In past research, many zoologists and writers succumbed to fantasy myths and observer bias, and extolled the virtues of many species said to be faithful, for a season or a lifetime, and which were held up as role models of monogamy.

Zoologists and biologists now have solid evidence that monogamous pairs of animals are not always sexually exclusive. Many animals that form pairs to mate and raise offspring regularly engage in sexual activities with extra-pair partners [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] This includes previous exemplars such as swans and (depending upon circumstances) wolves. Sometimes these extra-pair sexual activities lead to offspring. Genetic tests frequently show that some of the offspring raised by a monogamous pair come from the female having sex with an extra-pair male partner. [16] [17] [29] [30] These discoveries have led biologists to adopt new ways of talking about monogamy:

“Social monogamy refers to a male and female’s social living arrangement (e.g., shared use of a territory, behaviour indicative of a social pair, and/or proximity between a male and female) without inferring any sexual interactions or reproductive patterns. In humans, social monogamy equals monogamous marriage. Sexual monogamy is defined as an exclusive sexual relationship between a female and a male based on observations of sexual interactions. Finally, the term genetic monogamy is used when DNA analyses can confirm that a female-male pair reproduce exclusively with each other. A combination of terms indicates examples where levels of relationships coincide, e.g., sociosexual and sociogenetic monogamy describe corresponding social and sexual, and social and genetic monogamous relationships, respectively.” (Reichard, 2003, page 4)[31]

Whatever makes a pair of animals socially monogamous does not necessarily make them sexually or genetically monogamous. Social monogamy, sexual monogamy, and genetic monogamy can occur in different combinations.

Social monogamy is relatively rare in the animal kingdom. The actual incidence of social monogamy varies greatly across different branches of the evolutionary tree. Over 90 percent of avian species are socially monogamous. [25] [32] This stands in contrast to mammals. Only 3 percent of mammalian species are socially monogamous, although up to 15 percent of primate species are socially monogamous. [25] [32] Social monogamy has also been observed in reptiles, fish, and insects.

Sexual monogamy is very rare among animals. The great majority of socially monogamous species engage in extra-pair copulations, making them sexually non-monogamous. Gowaty has estimated that, out of 180 different species of socially monogomous songbirds, only 10 percent are sexually monogamous.[33]

The incidence of genetic monogamy, determined by DNA fingerprinting, varies widely across species. For a few rare species, the incidence of genetic monogamy is 100 percent, with all offspring genetically related to the socially monogamous pair. But genetic monogamy is strikingly low in other species. Barash and Lipton note:

“The highest known frequency of extra-pair copulations are found among the fairy-wrens, lovely tropical creatures technically known as Malurus spendens and Malurus cyaneus. More than 65 percent of all fairy-wren chicks are fathered by males outside the supposed breeding group.” (Barash & Lipton, 2001, page 12)[32]

Such low levels of genetic monogamy has surprised biologists and zoologists, forcing them to rethink the role of social monogamy in evolution. They can no longer assume social monogamy determines how genes are distributed in a species. The lower the rates of genetic monogamy among socially monogamous pairs, the less of a role social monogamy plays in determining how genes are distributed among offspring.

Readers interested in the evolution of monogamy may wish to visit the Wikipedia article on the Evolution of Monogamy.

[edit] Polygyny

In some species, notably those with harem male structures, only one of few males in a group of females will mate. This is also known as polygyny in sociobiology.[34] Should the active male be driven out, then a number of species, the new male will ensure that breeding resources are not wasted on another males’ young.[35] These can include:

  • Competitive infanticide – in lions and some monkeys the new male will kill other young, and the females, no longer nursing, rapidly become receptive again.
  • Harassment to miscarriage – amongst wild horses and baboons, the male will “systematically harass” pregnant females until they miscarry.
  • Pheromone based spontaneous abortion – in some rodents such as mice, a new male with a different scent will cause females who are pregnant to spontaneously fail to implant recently fertilized eggs. This does not require contact; it is mediated by scent alone. It is known as the Bruce-Parkes effect.

[edit] Promiscuity

Two examples of promiscuous mating systems in primates are chimpanzees and bonobos. These species live in social groups consisting of several males and several females. Each male copulates with many females, and vice versa. In bonobos, the amount of promiscuity is particularly striking because bonobos use sex to alleviate social conflict as well as to reproduce.

[edit] Seasonal nature of animal sexuality

Many animal species have specific mating (or breeding) seasons. These are often associated with changes to herd or group structure, and behavioral changes, including territorialism amongst individuals. These may be annual (eg wolves), biannual (eg dogs) or more frequently (eg horses). During these periods, females of most species are more mentally and physically receptive to sexual advances, a period often described as being “in season” or “in heat”, but outside them animals still engage in sexual behaviors,[36] and such acts as do occur are not necessarily harmful.[37]

edit Sex for pleasure

It is a common urban myth that animals do not (as a rule) have sex for pleasure, or alternatively that humans (and perhaps dolphins and one or two species of primate) are the only species which do. This is sometimes formulated “animals mate only for reproduction”.

Science cannot say at present conclusively what animals do or do not find “pleasurable”, a question considered in more depth under Emotion in animals. The urban myth site considers this particular view in depth. Its conclusions are broadly that the statement is true, but only using a very specific definition of “sex for pleasure” [italics in original], in which sexual acts tied to a reproductive cycle or for which an alternative explanation can be asserted, are ignored, as is all sexual activity that does not involve penetration. Animals put themselves at risk to engage in sex, and as a result, most species have evolved sexual signals (usually scent and behavior) to indicate the presence of receptive periods. During these, sex is sought, and outside these it is usually not sought (or is sought but not permitted). Snopes comments that this is not in fact a reflection of whether sex is pleasurable or not, but rather a reflection of whether individuals have sex at arbitrary times. They conclude [9]:

“Of course, we have to make many seemingly artificial distinctions to arrive at our conclusion. Animals other than humans have no awareness that their sexual activities are connected with reproduction: They engage in sex because they’re biologically driven to do so, and if the fulfillment of their urges produces a physical sensation we might appropriately call ‘pleasure,’ it isn’t the least bit affected by the possibility (or impossibility) of producing offspring. We are also discounting cases in which animals do engage in sex even though reproduction is an impossibility because we claim there are other ‘purposes’ (of which the animals themselves are unaware) at play. (For example, the females of some species of birds will invite males to mate with them even after they have laid their eggs, but we ascribe a purpose to this behavior: this is a biological “trick” to fool males into caring for hatchlings they didn’t father.) We also employ subjective terms such as ‘willingly’ and ‘regularly’ in claiming that bonobos and dolphins are the only other animals who “willingly (and regularly) engage in sex with each other” … and even then it may be the case that these species have some other ‘purpose’ for doing so that we haven’t yet discovered…”

A 2006 Danish Animal Ethics Council report[38] which examined current knowledge of animal sexuality in the context of legal queries concerning sexual acts by humans, has the following comments, primarily related to domestically common animals:

“Even though the evolution-related purpose of mating can be said to be reproduction, it is not actually the creating of offspring which originally causes them to mate. It is probable that they mate because they are motivated for the actual copulation, and because this is connected with a positive experience. It is therefore reasonable to assume that there is some form of pleasure or satisfaction connected with the act. This assumption is confirmed by the behavior of males, who in the case of many species are prepared to work to get access to female animals, especially if the female animal is in oestrus, and males who for breeding purposes are used to having sperm collected become very eager, when the equipment they associate with the collection is taken out.”[39]
“There is nothing in female mammals’ anatomy or physiology, that contradicts that stimulation of the sexual organs and mating is able to be a positive experience. For instance, the clitoris acts in the same way as with women, and scientific studies have shown that the success of reproduction is improved by stimulation of clitoris on (among other species) cows and mares in connection with insemination, because it improves the transportation of the sperm due to contractions of the inner genitalia. This probably also concerns female animals of other animal species, and contractions in the inner genitals are seen e.g. also during orgasm for women. It is therefore reasonable to assume that sexual intercourse may be linked with a positive experience for female animals.”[40]

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