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In zoology, copulation is animal sexual behavior in which a male introduces sperm into the female's body, especially directly into her reproductive tract.^[1]^[2] This is an aspect of mating. Many aquatic animals use external fertilization, whereas internal fertilization may have developed from a need to maintain gametes in a liquid medium in the Late Ordovician epoch.^{[citation needed]} Internal fertilization with many vertebrates (such as all reptiles, some fish, and most birds) occurs via cloacal copulation, known as cloacal kiss (see also hemipenis), while most mammals copulate vaginally, and many basal vertebrates reproduce sexually with external fertilization.^[3]^[4]

In spiders and insects

Spiders are often confused with insects, but they are not insects; instead, they are arachnids.^[5]^[6] Spiders have separate male and female sexes. Before mating and copulation, the male spider spins a small web and ejaculates on to it. He then stores the sperm in reservoirs on his large pedipalps, from which he transfers sperm to the female's genitals. The females can store sperm indefinitely.^[7]

For primitive insects, the male deposits spermatozoa on the substrate, sometimes stored within a special structure; courtship involves inducing the female to take up the sperm package into her genital opening, but there is no actual copulation.^[8]^[9] In groups that have reproduction similar to spiders, such as dragonflies, 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.^[10] 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.^[11]

In mammals

Mating postures of mammals

Sexual behavior can be classified into behavioral states associated with reward motivation ("wanting"), reward consummation also known as pleasure ("liking"), and satiety ("inhibition");^[12] these behavioral states are regulated in mammals by reward-based sexual learning, fluctuations in various neurochemicals (i.e., dopamine − sexual desire also known as "wanting"; norepinephrine − sexual arousal; oxytocin and melanocortins − sexual attraction), and gonadal hormone cycles and further influenced by sex pheromones and motor reflexes (i.e., lordosis behaviour) in some mammals.^[12]^[13]

These behavioral states correlate with the phases of the human sexual response cycle: motivation − excitement; consummation − plateau and orgasm; satiety − refraction.^[12] Sexual learning (a form of associative learning) occurs when an animal starts to associate bodily features, personality, contextual cues, and other stimuli with genitally-induced sexual pleasure.^[12]^[13] Once formed, these associations in turn impinge upon both sexual wanting and sexual liking.

In most female mammals, the act of copulation is controlled by several innate neurobiological processes, including the motor sexual reflex of lordosis.^[14] In males, the act of copulation is more complex, because some learning is necessary, but the innate processes (retrocontrol of penis intromission in the vagina, rhythmic movement of the pelvis, detection of female pheromones) are specific to copulation. These innate processes direct heterosexual copulation.^[15] Female lordosis behaviour became secondary in Hominidae and is non-functional in humans.^[16] Mammals usually copulate in a dorso-ventral posture, although some primate species copulate in a ventro-vental posture.^[17]^[18]

Most mammals possess a vomeronasal organ that is involved in pheromone detection, including sex pheromones.^[19] Despite the fact that humans do not possess this organ, adult humans appear to be sensitive to certain mammalian pheromones that putative pheromone receptor proteins in the olfactory epithelium are capable of detecting.^{[note 1]}^[19] While sex pheromones clearly play a role in modifying sexual behavior in some mammals, the capacity for general pheromone detection and the involvement of pheromones in human sexual behavior has not yet been determined.^[12]

The duration of copulation varies significantly between mammal species,^[23] and may be correlated with body mass, lasting longer in large mammals than in small mammals.^[24] The duration of copulation may also be correlated with the length of the baculum in mammals.^[25]

Male mammals ejaculate semen through the penis into the female reproductive tract during copulation.^[26]^[27] Ejaculation usually occurs after only one intromission in humans, canids, and ungulates, but occurs after multiple intromissions in most mammal species.^[28]^[29]

Copulation can induce ovulation in mammal species that do not ovulate spontaneously.^[30]

Notes

^ In humans and other animals, trace amine-associated receptors (TAARs) that are expressed in the olfactory epithelium function as olfactory receptors that detect volatile amine odorants, including certain pheromones;^[20]^[21] these TAARs putatively function as a class of pheromone receptors involved in the olfactive detection of social cues.^[20]^[21]

A review of studies involving non-human animals indicated that TAARs in the olfactory epithelium can mediate attractive or aversive behavioral responses to an agonist.^[20] This review also noted that the behavioral response evoked by a TAAR can vary across species.^[20] For example, TAAR5 mediates attraction to trimethylamine in mice and aversion to trimethylamine in rats.^[20] In humans, hTAAR5 presumably mediates aversion to trimethylamine, which is known to act as an hTAAR5 agonist and to possess a foul, fishy odor that is aversive to humans;^[20]^[22] however, hTAAR5 is not the only olfactory receptor that is responsible for trimethylamine olfaction in humans.^[20]^[22] As of December 2015,^[update] hTAAR5-mediated trimethylamine aversion has not been examined in published research.^[22]

References

^ Kent, Michael (2000). Advanced Biology. Oxford University Press. pp. 250–253. ISBN 0199141959. Retrieved 2015-10-21.
^ "Copulation". The Free Dictionary. Archived from the original on November 19, 2022. Retrieved 2012-09-06.
^ Cecie Starr; Christine Evers; Lisa Starr (2010). Cengage Advantage Books: Biology: A Human Emphasis. Cengage Learning. pp. 630–631. ISBN 978-1133170051. Retrieved December 9, 2010.
^ Edward J. Denecke Jr. (2006). New York State Grade 8 Intermediate Level Science Test. Barron's Educational Series. p. 105. ISBN 0764134337. Retrieved December 9, 2014.
^ Donna M. Jackson (2004). The Bug Scientists. Houghton Mifflin Harcourt. p. 13. ISBN 0618432329. Retrieved March 7, 2017.
^ Fred F. Ferri (2016). Ferri's Clinical Advisor 2017: 5 Books in 1. Elsevier Health Sciences. p. 178. ISBN 978-0323448383. Retrieved March 7, 2017.
^ Ruppert, E.E.; Fox, R.S. & Barnes, R.D. (2004). "Chelicerata: Araneae". Invertebrate Zoology (7th ed.). Brooks/Cole. pp. 571–584. ISBN 0-03-025982-7.
^ M. Yadav (2003). Breeding in Insects. Discovery Publishing House. p. 59. ISBN 817141737X. Retrieved December 9, 2014.
^ Franz Engelmann (2013). The Physiology of Insect Reproduction: International Series of Monographs in Pure and Applied Biology: Zoology. Elsevier. pp. 58–59. ISBN 978-1483186535. Retrieved December 9, 2014.
^ Janet Leonard; Alex Cordoba-Aguilar (2010). The Evolution of Primary Sexual Characters in Animals. Oxford University Press. p. 334. ISBN 978-0199717033. Retrieved December 9, 2014.
^ P. J. Gullan; P. S. Cranston (2009). The Insects: An Outline of Entomology. John Wiley & Sons. p. 124. ISBN 978-1405144575. Retrieved December 9, 2014.
^ ^a ^b ^c ^d ^e Georgiadis JR, Kringelbach ML, Pfaus JG (September 2012). "Sex for fun: a synthesis of human and animal neurobiology". Nature Reviews. Urology. 9 (9): 486–98. doi:10.1038/nrurol.2012.151. PMID 22926422. S2CID 13813765. The sexual pleasure cycle adheres to the basic structure of pleasure cycles related to other rewards (such as food), and can therefore also be expressed in terms of motivation–consummation–satiety or wanting–liking–inhibition (Figure 1; Box 2).6,11,1 ... Similar to other forms of learning, sexual behaviour develops over time as people learn to associate stimuli such as bodily features, personality, and contextual cues with genitally-induced sexual pleasure.7 Adolescence is arguably the most critical phase in sexual development ... Popular belief holds that humans also respond to some distal sexual incentive stimuli (breasts, pheromones) in an unconditioned manner, but this has been difficult to evaluate empirically (Box 1) ... Sexual wanting in both rats and humans involves interaction between gonadal hormones and external stimuli that become sexual incentives through association with genitally-induced sexual reward; pleasurable genital stimulation is thus a major factor in sexual learning ...
• Unconditioned sexual stimuli (that is, those for which the pleasurable effect requires no learning) include proximal genital tactile stimulation in humans and distal stimuli such as pheromones, odours, and certain auditory vocalizations in rats.7,16
• Sexual inhibition involves similar brain mechanisms in rats and humans
• Rats show a similar pattern of brain activation to humans in response to cues related to sexual reward
• Cortical, limbic, hypothalamic, and cerebellar regions are activated by sex-related stimuli in both humans and rats
^ ^a ^b Schultz W (2015). "Neuronal reward and decision signals: from theories to data". Physiological Reviews. 95 (3): 853–951. doi:10.1152/physrev.00023.2014. PMC 4491543. PMID 26109341. Sexual behavior follows hormonal imbalances, at least in men, but is also strongly based on pleasure. To acquire and follow these primary alimentary and mating rewards is the main reason why the brain's reward system has evolved in the first place. Note that "primary" reward does not refer to the distinction between unconditioned versus conditioned reward; indeed, most primary rewards are learned and thus conditioned (foods are primary rewards that are typically learnt). ... Pleasure is not only one of the three main reward functions but also provides a definition of reward. As homeostasis explains the functions of only a limited number of rewards, the prevailing reason why particular stimuli, objects, events, situations, and activities are rewarding may be pleasure. This applies first of all to sex (who would engage in the ridiculous gymnastics of reproductive activity if it were not for the pleasure) and to the primary homeostatic rewards of food and liquid, and extends to money, taste, beauty, social encounters and nonmaterial, internally set, and intrinsic rewards. ... Desire makes behavior purposeful and directs it towards identifiable goals. Thus desire is the emotion that helps to actively direct behavior towards known rewards, whereas pleasure is the passive experience that derives from a received or anticipated reward. Desire has multiple relations to pleasure; it may be pleasant in itself (I feel a pleasant desire), and it may lead to pleasure (I desire to obtain a pleasant object). Thus pleasure and desire have distinctive characteristics but are closely intertwined. They constitute the most important positive emotions induced by rewards. They prioritize our conscious processing and thus constitute important components of behavioral control. These emotions are also called liking (for pleasure) and wanting (for desire) in addiction research (471) and strongly support the learning and approach generating functions of reward. ... Some of the stimuli and events that are pleasurable in humans may not even evoke pleasure in animals but act instead through innate mechanisms. We simply do not know. Nevertheless, the invention of pleasure and desire by evolution had the huge advantage of allowing a large number of stimuli, objects, events, situations, and activities to be attractive. This mechanism importantly supports the primary reward functions in obtaining essential substances and mating partners.
^ Pfaff Donald W., Schwartz-Giblin Susan, Maccarthy Margareth M., Kow Lee-Ming : Cellular and molecular mechanisms of female reproductive behaviors, in Knobil Ernest, Neill Jimmy D. : The physiology of reproduction, Raven Press, 2nd edition, 1994
^ Meisel Robert L., Sachs Benjamin D. : The physiology of male sexual behavior. In Knobil Ernest, Neill Jimmy D. The physiology of reproduction, Raven Press, 2nd edition, 1994
^ Dixson A.F. Primate sexuality: Comparative studies of the Prosimians, Monkeys, Apes, and Human Beings. Oxford University Press, 2nd edition, 2012.
^ Smith, Robert L. (2012-12-02). Sperm Competition and the Evolution of Animal Mating systems. Elsevier. ISBN 978-0-323-14313-4.
^ Dixson, Alan F. (2009-05-15). Sexual Selection and the Origins of Human Mating Systems. OUP Oxford. ISBN 978-0-19-156973-9.
^ ^a ^b Nei M, Niimura Y, Nozawa M (December 2008). "The evolution of animal chemosensory receptor gene repertoires: roles of chance and necessity". Nature Reviews. Genetics. 9 (12): 951–63. doi:10.1038/nrg2480. PMID 19002141. S2CID 11135361. OR genes are predominantly expressed in sensory neurons of the main olfactory epithelium (MOE) in the nasal cavity. Mammals detect many types of chemicals in the air and some in the water as odorants, whereas fishes recognize water-soluble molecules, such as amino acids, bile acids, sex steroids and prostaglandins. Some mammalian OR genes are known to be expressed in other tissues, including the testis, tongue, brain and placenta17. However, the functional significance of such 'ectopic expression' of OR genes is not definitively known. TAARs are also expressed in the MOE. These receptors were first identified as brain receptors for the trace amines, a collection of amines that are present at low concentrations in the central nervous system18. TAARs were originally suspected to be involved in psychiatric disorders19 but are now known to function as a second class of olfactory receptors10. Some mouse TAARs recognize volatile amines that are present in urine, and it seems that the TAARs function to detect ligands associated with social cues10. ... Most mammals possess an additional olfactory organ called the vomeronasal organ (VnO). ... The VnO was previously thought to be a specialized organ for pheromone detection, but it is now known that the VnO and MOE share some overlapping functions22. ... However, at least one of the five V1R genes is expressed in the human olfactory mucosa72. Furthermore, a recent study suggests that these five genes can activate an OR-like signal transduction pathway in a heterologous expression system73. It is therefore possible that the products of these genes function as pheromone or olfactory receptors. Adult humans do not have a VnO but seem to be sensitive to pheromones74. Another interesting observation is that chickens have no functional or non-functional V1R and V2R genes or a VnO75, although birds use pheromones for mate choice and other behaviours76. It is possible that some OR genes in the MOE are able to detect pheromones, as in humans74,77.
^ ^a ^b ^c ^d ^e ^f ^g Liberles SD (October 2015). "Trace amine-associated receptors: ligands, neural circuits, and behaviors". Curr. Opin. Neurobiol. 34: 1–7. doi:10.1016/j.conb.2015.01.001. PMC 4508243. PMID 25616211. Roles for another receptor are supported by TAAR5-independent trimethylamine anosmias in humans [32]. ... Several TAARs detect volatile and aversive amines, but the olfactory system is capable of discarding ligand-based or function-based constraints on TAAR evolution. Particular TAARs have mutated to recognize new ligands, with almost an entire teleost clade losing the canonical amine-recognition motif. Furthermore, while some TAARs detect aversive odors, TAAR-mediated behaviors can vary across species. ... The ability of particular TAARs to mediate aversion and attraction behavior provides an exciting opportunity for mechanistic unraveling of odor valence encoding.
Figure 2: Table of ligands, expression patterns, and species-specific behavioral responses for each TAAR
^ ^a ^b "Trace amine receptor: Introduction". International Union of Basic and Clinical Pharmacology. Archived from the original on 23 February 2014. Retrieved 15 February 2014. Importantly, three ligands identified activating mouse Taars are natural components of mouse urine, a major source of social cues in rodents. Mouse Taar4 recognizes β-phenylethylamine, a compound whose elevation in urine is correlated with increases in stress and stress responses in both rodents and humans. Both mouse Taar3 and Taar5 detect compounds (isoamylamine and trimethylamine, respectively) that are enriched in male versus female mouse urine. Isoamylamine in male urine is reported to act as a pheromone, accelerating puberty onset in female mice [34]. The authors suggest the Taar family has a chemosensory function that is distinct from odorant receptors with a role associated with the detection of social cues. ... The evolutionary pattern of the TAAR gene family is characterized by lineage-specific phylogenetic clustering [26,30,35]. These characteristics are very similar to those observed in the olfactory GPCRs and vomeronasal (V1R, V2R) GPCR gene families.
^ ^a ^b ^c Wallrabenstein I, Singer M, Panten J, Hatt H, Gisselmann G (2015). "Timberol® Inhibits TAAR5-Mediated Responses to Trimethylamine and Influences the Olfactory Threshold in Humans". PLOS ONE. 10 (12): e0144704. Bibcode:2015PLoSO..1044704W. doi:10.1371/journal.pone.0144704. PMC 4684214. PMID 26684881. While mice produce gender-specific amounts of urinary TMA levels and were attracted by TMA, this odor is repellent to rats and aversive to humans [19], indicating that there must be species-specific functions. ... Furthermore, a homozygous knockout of murine TAAR5 abolished the attraction behavior to TMA [19]. Thus, it is concluded that TAAR5 itself is sufficient to mediate a behavioral response at least in mice. ... Whether the TAAR5 activation by TMA elicits specific behavioral output like avoidance behavior in humans still needs to be examined.
^ Naguib, Marc (2020-04-19). Advances in the Study of Behavior. Academic Press. ISBN 978-0-12-820726-0.
^ Stallmann, Robert R., and A. H. Harcourt. "Size matters: the (negative) allometry of copulatory duration in mammals Archived 2022-04-20 at the Wayback Machine." Biological Journal of the Linnean Society 87.2 (2006): 185-193. doi:10.1111/j.1095-8312.2006.00566.x
^ DIXSON33, Alan, N. YHOL T. Jenna, and Matt Anderson. "A positive relationship between baculum length and prolonged intromission patterns in mammals." 动物学报 50.4 (2004): 490-503.
^ Norris, David O.; Lopez, Kristin H. (2024-08-08). Hormones and Reproduction of Vertebrates, Volume 5: Mammals. Elsevier. ISBN 978-0-443-15985-5.
^ Lombardi, Julian (2012-12-06). Comparative Vertebrate Reproduction. Springer Science & Business Media. ISBN 978-1-4615-4937-6.
^ Dixson, Alan F. (2012-01-26). Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes, and Humans. OUP Oxford. ISBN 978-0-19-954464-6.
^ Encyclopedia of Behavioral Neuroscience. Elsevier. 2010-06-03. ISBN 978-0-08-045396-5.
^ Jöchle, Wolfgang (1973). "Coitus-induced ovulation". Contraception. 7 (6): 523–564. doi:10.1016/0010-7824(73)90023-1. ISSN 0010-7824.

Bibliography

Peter J Chenoweth; Steven Lorton (30 April 2014). Animal Andrology: Theories and Applications. CABI. ISBN 978-1-78064-316-8.
Møller, A. P., and T. R. Birkhead. "Copulation behaviour in mammals: evidence that sperm competition is widespread." Biological Journal of the Linnean Society 38.2 (1989): 119–131.
Birkhead, Timothy R., L. Atkin, and A. P. Møller. "Copulation behaviour of birds." Behaviour 101.1 (1987): 101–138.
Richard Sadleir (2 December 2012). The Reproduction of Vertebrates. Elsevier Science. ISBN 978-0-323-15935-7.
Broom, D.M.; Fraser, A.F. (2015). Domestic Animal Behaviou... CABI. pp. 155–. ISBN 978-1-78064-053-2.
P. Bateson (29 June 2013). Perspectives in Ethology. Springer Science & Business Media. pp. 12–. ISBN 978-1-4615-7572-6.
Roger L. Gentry (14 July 2014). Behavior and Ecology of the Northern Fur Seal. Princeton University Press. pp. 172–. ISBN 978-1-4008-6472-0.
Richard Estes (1991). The Behavior Guide to African Mammals: Including Hoofed Mammals, Carnivores, Primates. University of California Press. ISBN 978-0-520-08085-0.
Carlson, Debra A. Reproductive biology of the coyote (Canis latrans): integration of behavior and physiology. Utah State University, 2008.
Castro, Ana Mafalda Lopes Sardica Velez. Mexican gray wolf courtship and mating: behavior & basic endocrinology during breeding season. Diss. Universidade de Lisboa. Faculdade de Medicina Veterinária, 2016.
Szykman, Micaela, et al. "Courtship and mating in free-living spotted hyenas." Behaviour 144.7 (2007): 815–846.
Dixson, Alan F. "Baculum length and copulatory behavior in primates." American Journal of Primatology 13.1 (1987): 51–60.

[pheromone-23] In humans and other animals, trace amine-associated receptors (TAARs) that are expressed in the olfactory epithelium function as olfactory receptors that detect volatile amine odorants, including certain pheromones;^[20]^[21] these TAARs putatively function as a class of pheromone receptors involved in the olfactive detection of social cues.^[20]^[21]

A review of studies involving non-human animals indicated that TAARs in the olfactory epithelium can mediate attractive or aversive behavioral responses to an agonist.^[20] This review also noted that the behavioral response evoked by a TAAR can vary across species.^[20] For example, TAAR5 mediates attraction to trimethylamine in mice and aversion to trimethylamine in rats.^[20] In humans, hTAAR5 presumably mediates aversion to trimethylamine, which is known to act as an hTAAR5 agonist and to possess a foul, fishy odor that is aversive to humans;^[20]^[22] however, hTAAR5 is not the only olfactory receptor that is responsible for trimethylamine olfaction in humans.^[20]^[22] As of December 2015,^[update] hTAAR5-mediated trimethylamine aversion has not been examined in published research.^[22]

[Kent-1] Kent, Michael (2000). Advanced Biology. Oxford University Press. pp. 250–253. ISBN 0199141959. Retrieved 2015-10-21.

[Copulation1-2] "Copulation". The Free Dictionary. Archived from the original on November 19, 2022. Retrieved 2012-09-06.

[Starr-3] Cecie Starr; Christine Evers; Lisa Starr (2010). Cengage Advantage Books: Biology: A Human Emphasis. Cengage Learning. pp. 630–631. ISBN 978-1133170051. Retrieved December 9, 2010.

[Denecke-4] Edward J. Denecke Jr. (2006). New York State Grade 8 Intermediate Level Science Test. Barron's Educational Series. p. 105. ISBN 0764134337. Retrieved December 9, 2014.

[Jackson-5] Donna M. Jackson (2004). The Bug Scientists. Houghton Mifflin Harcourt. p. 13. ISBN 0618432329. Retrieved March 7, 2017.

[Ferri-6] Fred F. Ferri (2016). Ferri's Clinical Advisor 2017: 5 Books in 1. Elsevier Health Sciences. p. 178. ISBN 978-0323448383. Retrieved March 7, 2017.

[RFB2004Spiders-7] Ruppert, E.E.; Fox, R.S. & Barnes, R.D. (2004). "Chelicerata: Araneae". Invertebrate Zoology (7th ed.). Brooks/Cole. pp. 571–584. ISBN 0-03-025982-7.

[Yadav-8] M. Yadav (2003). Breeding in Insects. Discovery Publishing House. p. 59. ISBN 817141737X. Retrieved December 9, 2014.

[Engelmann-9] Franz Engelmann (2013). The Physiology of Insect Reproduction: International Series of Monographs in Pure and Applied Biology: Zoology. Elsevier. pp. 58–59. ISBN 978-1483186535. Retrieved December 9, 2014.

[Leonard-10] Janet Leonard; Alex Cordoba-Aguilar (2010). The Evolution of Primary Sexual Characters in Animals. Oxford University Press. p. 334. ISBN 978-0199717033. Retrieved December 9, 2014.

[Gullan-11] P. J. Gullan; P. S. Cranston (2009). The Insects: An Outline of Entomology. John Wiley & Sons. p. 124. ISBN 978-1405144575. Retrieved December 9, 2014.

[Neuropsychology_of_sex-12] Georgiadis JR, Kringelbach ML, Pfaus JG (September 2012). "Sex for fun: a synthesis of human and animal neurobiology". Nature Reviews. Urology. 9 (9): 486–98. doi:10.1038/nrurol.2012.151. PMID 22926422. S2CID 13813765. The sexual pleasure cycle adheres to the basic structure of pleasure cycles related to other rewards (such as food), and can therefore also be expressed in terms of motivation–consummation–satiety or wanting–liking–inhibition (Figure 1; Box 2).6,11,1 ... Similar to other forms of learning, sexual behaviour develops over time as people learn to associate stimuli such as bodily features, personality, and contextual cues with genitally-induced sexual pleasure.7 Adolescence is arguably the most critical phase in sexual development ... Popular belief holds that humans also respond to some distal sexual incentive stimuli (breasts, pheromones) in an unconditioned manner, but this has been difficult to evaluate empirically (Box 1) ... Sexual wanting in both rats and humans involves interaction between gonadal hormones and external stimuli that become sexual incentives through association with genitally-induced sexual reward; pleasurable genital stimulation is thus a major factor in sexual learning ...
• Unconditioned sexual stimuli (that is, those for which the pleasurable effect requires no learning) include proximal genital tactile stimulation in humans and distal stimuli such as pheromones, odours, and certain auditory vocalizations in rats.7,16
• Sexual inhibition involves similar brain mechanisms in rats and humans
• Rats show a similar pattern of brain activation to humans in response to cues related to sexual reward
• Cortical, limbic, hypothalamic, and cerebellar regions are activated by sex-related stimuli in both humans and rats

[Schultz_-_sexual_behavior-13] Schultz W (2015). "Neuronal reward and decision signals: from theories to data". Physiological Reviews. 95 (3): 853–951. doi:10.1152/physrev.00023.2014. PMC 4491543. PMID 26109341. Sexual behavior follows hormonal imbalances, at least in men, but is also strongly based on pleasure. To acquire and follow these primary alimentary and mating rewards is the main reason why the brain's reward system has evolved in the first place. Note that "primary" reward does not refer to the distinction between unconditioned versus conditioned reward; indeed, most primary rewards are learned and thus conditioned (foods are primary rewards that are typically learnt). ... Pleasure is not only one of the three main reward functions but also provides a definition of reward. As homeostasis explains the functions of only a limited number of rewards, the prevailing reason why particular stimuli, objects, events, situations, and activities are rewarding may be pleasure. This applies first of all to sex (who would engage in the ridiculous gymnastics of reproductive activity if it were not for the pleasure) and to the primary homeostatic rewards of food and liquid, and extends to money, taste, beauty, social encounters and nonmaterial, internally set, and intrinsic rewards. ... Desire makes behavior purposeful and directs it towards identifiable goals. Thus desire is the emotion that helps to actively direct behavior towards known rewards, whereas pleasure is the passive experience that derives from a received or anticipated reward. Desire has multiple relations to pleasure; it may be pleasant in itself (I feel a pleasant desire), and it may lead to pleasure (I desire to obtain a pleasant object). Thus pleasure and desire have distinctive characteristics but are closely intertwined. They constitute the most important positive emotions induced by rewards. They prioritize our conscious processing and thus constitute important components of behavioral control. These emotions are also called liking (for pleasure) and wanting (for desire) in addiction research (471) and strongly support the learning and approach generating functions of reward. ... Some of the stimuli and events that are pleasurable in humans may not even evoke pleasure in animals but act instead through innate mechanisms. We simply do not know. Nevertheless, the invention of pleasure and desire by evolution had the huge advantage of allowing a large number of stimuli, objects, events, situations, and activities to be attractive. This mechanism importantly supports the primary reward functions in obtaining essential substances and mating partners.

[pfaff1994-14] Pfaff Donald W., Schwartz-Giblin Susan, Maccarthy Margareth M., Kow Lee-Ming : Cellular and molecular mechanisms of female reproductive behaviors, in Knobil Ernest, Neill Jimmy D. : The physiology of reproduction, Raven Press, 2nd edition, 1994

[meisel-15] Meisel Robert L., Sachs Benjamin D. : The physiology of male sexual behavior. In Knobil Ernest, Neill Jimmy D. The physiology of reproduction, Raven Press, 2nd edition, 1994

[Dixson2012-16] Dixson A.F. Primate sexuality: Comparative studies of the Prosimians, Monkeys, Apes, and Human Beings. Oxford University Press, 2nd edition, 2012.

[17] Smith, Robert L. (2012-12-02). Sperm Competition and the Evolution of Animal Mating systems. Elsevier. ISBN 978-0-323-14313-4.

[18] Dixson, Alan F. (2009-05-15). Sexual Selection and the Origins of Human Mating Systems. OUP Oxford. ISBN 978-0-19-156973-9.

[Evolution_of_odorant_and_pheromone_receptors-19] Nei M, Niimura Y, Nozawa M (December 2008). "The evolution of animal chemosensory receptor gene repertoires: roles of chance and necessity". Nature Reviews. Genetics. 9 (12): 951–63. doi:10.1038/nrg2480. PMID 19002141. S2CID 11135361. OR genes are predominantly expressed in sensory neurons of the main olfactory epithelium (MOE) in the nasal cavity. Mammals detect many types of chemicals in the air and some in the water as odorants, whereas fishes recognize water-soluble molecules, such as amino acids, bile acids, sex steroids and prostaglandins. Some mammalian OR genes are known to be expressed in other tissues, including the testis, tongue, brain and placenta17. However, the functional significance of such 'ectopic expression' of OR genes is not definitively known. TAARs are also expressed in the MOE. These receptors were first identified as brain receptors for the trace amines, a collection of amines that are present at low concentrations in the central nervous system18. TAARs were originally suspected to be involved in psychiatric disorders19 but are now known to function as a second class of olfactory receptors10. Some mouse TAARs recognize volatile amines that are present in urine, and it seems that the TAARs function to detect ligands associated with social cues10. ... Most mammals possess an additional olfactory organ called the vomeronasal organ (VnO). ... The VnO was previously thought to be a specialized organ for pheromone detection, but it is now known that the VnO and MOE share some overlapping functions22. ... However, at least one of the five V1R genes is expressed in the human olfactory mucosa72. Furthermore, a recent study suggests that these five genes can activate an OR-like signal transduction pathway in a heterologous expression system73. It is therefore possible that the products of these genes function as pheromone or olfactory receptors. Adult humans do not have a VnO but seem to be sensitive to pheromones74. Another interesting observation is that chickens have no functional or non-functional V1R and V2R genes or a VnO75, although birds use pheromones for mate choice and other behaviours76. It is possible that some OR genes in the MOE are able to detect pheromones, as in humans74,77.

[TAAR_2015_review_-_olfactory_TAARs-20] ^ ^a ^b ^c ^d ^e ^f ^g Liberles SD (October 2015). "Trace amine-associated receptors: ligands, neural circuits, and behaviors". Curr. Opin. Neurobiol. 34: 1–7. doi:10.1016/j.conb.2015.01.001. PMC 4508243. PMID 25616211. Roles for another receptor are supported by TAAR5-independent trimethylamine anosmias in humans [32]. ... Several TAARs detect volatile and aversive amines, but the olfactory system is capable of discarding ligand-based or function-based constraints on TAAR evolution. Particular TAARs have mutated to recognize new ligands, with almost an entire teleost clade losing the canonical amine-recognition motif. Furthermore, while some TAARs detect aversive odors, TAAR-mediated behaviors can vary across species. ... The ability of particular TAARs to mediate aversion and attraction behavior provides an exciting opportunity for mechanistic unraveling of odor valence encoding.
Figure 2: Table of ligands, expression patterns, and species-specific behavioral responses for each TAAR

[IUPHAR_TAAR-21] "Trace amine receptor: Introduction". International Union of Basic and Clinical Pharmacology. Archived from the original on 23 February 2014. Retrieved 15 February 2014. Importantly, three ligands identified activating mouse Taars are natural components of mouse urine, a major source of social cues in rodents. Mouse Taar4 recognizes β-phenylethylamine, a compound whose elevation in urine is correlated with increases in stress and stress responses in both rodents and humans. Both mouse Taar3 and Taar5 detect compounds (isoamylamine and trimethylamine, respectively) that are enriched in male versus female mouse urine. Isoamylamine in male urine is reported to act as a pheromone, accelerating puberty onset in female mice [34]. The authors suggest the Taar family has a chemosensory function that is distinct from odorant receptors with a role associated with the detection of social cues. ... The evolutionary pattern of the TAAR gene family is characterized by lineage-specific phylogenetic clustering [26,30,35]. These characteristics are very similar to those observed in the olfactory GPCRs and vomeronasal (V1R, V2R) GPCR gene families.

[hTAAR5_and_trimethylamine_aversion-22] Wallrabenstein I, Singer M, Panten J, Hatt H, Gisselmann G (2015). "Timberol® Inhibits TAAR5-Mediated Responses to Trimethylamine and Influences the Olfactory Threshold in Humans". PLOS ONE. 10 (12): e0144704. Bibcode:2015PLoSO..1044704W. doi:10.1371/journal.pone.0144704. PMC 4684214. PMID 26684881. While mice produce gender-specific amounts of urinary TMA levels and were attracted by TMA, this odor is repellent to rats and aversive to humans [19], indicating that there must be species-specific functions. ... Furthermore, a homozygous knockout of murine TAAR5 abolished the attraction behavior to TMA [19]. Thus, it is concluded that TAAR5 itself is sufficient to mediate a behavioral response at least in mice. ... Whether the TAAR5 activation by TMA elicits specific behavioral output like avoidance behavior in humans still needs to be examined.

[24] Naguib, Marc (2020-04-19). Advances in the Study of Behavior. Academic Press. ISBN 978-0-12-820726-0.

[25] Stallmann, Robert R., and A. H. Harcourt. "Size matters: the (negative) allometry of copulatory duration in mammals Archived 2022-04-20 at the Wayback Machine." Biological Journal of the Linnean Society 87.2 (2006): 185-193. doi:10.1111/j.1095-8312.2006.00566.x

[26] DIXSON33, Alan, N. YHOL T. Jenna, and Matt Anderson. "A positive relationship between baculum length and prolonged intromission patterns in mammals." 动物学报 50.4 (2004): 490-503.

[27] Norris, David O.; Lopez, Kristin H. (2024-08-08). Hormones and Reproduction of Vertebrates, Volume 5: Mammals. Elsevier. ISBN 978-0-443-15985-5.

[28] Lombardi, Julian (2012-12-06). Comparative Vertebrate Reproduction. Springer Science & Business Media. ISBN 978-1-4615-4937-6.

[29] Dixson, Alan F. (2012-01-26). Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes, and Humans. OUP Oxford. ISBN 978-0-19-954464-6.

[30] Encyclopedia of Behavioral Neuroscience. Elsevier. 2010-06-03. ISBN 978-0-08-045396-5.

[:2-31] Jöchle, Wolfgang (1973). "Coitus-induced ovulation". Contraception. 7 (6): 523–564. doi:10.1016/0010-7824(73)90023-1. ISSN 0010-7824.

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