Editing Mammal (section)

===Reproductive system===
{{Main|Mammalian reproduction}}
[[File:Goat family.jpg|thumb|right|[[Goat]] kids stay with their mother until they are weaned.]]
Mammals reproduce by [[internal fertilisation]]<ref name="naguib">{{Cite book|last=Naguib|first=Marc|url=https://books.google.com/books?id=KgTeDwAAQBAJ&pg=PA65|title=Advances in the Study of Behavior|date=19 April 2020|publisher=Academic Press|isbn=978-0-12-820726-0}}</ref> and are solely [[Gonochorism|gonochoric]] (an animal is born with either male or female genitalia, as opposed to [[hermaphrodite]]s where there is no such schism).<ref>{{Cite book| vauthors = Kobayashi K, Kitano T, Iwao Y, Kondo M |url=https://books.google.com/books?id=g4teDwAAQBAJ&q=mammal+gonochorism&pg=PA290|title=Reproductive and Developmental Strategies: The Continuity of Life |date=2018|publisher=Springer|isbn=978-4-431-56609-0|pages=290|language=en}}</ref>  Male mammals [[Ejaculation|ejaculate]] [[semen]] during [[Copulation (zoology)|copulation]] through a [[penis]], which may be contained in a [[Penile sheath|prepuce]] when not erect. Male placentals also [[urinate]] through a penis, and some placentals also have a penis bone ([[baculum]]).<ref name="Lombardi1998">{{cite book| vauthors = Lombardi J |title=Comparative Vertebrate Reproduction|url=https://books.google.com/books?id=cqQX9RMPAegC|date= 1998|publisher=Springer Science & Business Media|isbn=978-0-7923-8336-9}}</ref><ref name="Hyman1992">{{cite book |author=Libbie Henrietta Hyman |url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA583 |title=Hyman's Comparative Vertebrate Anatomy |date=15 September 1992 |publisher=University of Chicago Press |isbn=978-0-226-87013-7 |pages=583–}}</ref><ref name="naguib" /> Marsupials typically have forked penises,<ref name="Tyndale-BiscoeRenfree1987">{{cite book| vauthors = Tyndale-Biscoe H, Renfree M |title=Reproductive Physiology of Marsupials|url=https://books.google.com/books?id=HpjovN0vXW4C|date=1987|publisher=Cambridge University Press|isbn=978-0-521-33792-2}}</ref> while the [[echidna]] penis generally has four heads with only two functioning.<ref>{{Cite journal | doi=10.1086/522847| pmid=18171162| title=One-Sided Ejaculation of Echidna Sperm Bundles| journal=The American Naturalist| volume=170| issue=6| pages=E162–E164| year=2007| vauthors = Johnston SD, Smith B, Pyne M, Stenzel D, Holt WV | bibcode=2007ANat..170E.162J| s2cid=40632746| url=https://espace.library.uq.edu.au/view/UQ:130591/UQ130591_OA.pdf}}</ref> Depending on the species, an [[erection]] may be fuelled by blood flow into vascular, spongy tissue or by muscular action.<ref name="Lombardi1998" />  The [[testicles]] of most mammals descend into the [[scrotum]] which is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have a [[vulva]] ([[clitoris]] and [[labia]]) on the outside, while the internal system contains paired [[oviduct]]s, one or two [[uteri]], one or two [[Cervix|cervices]] and a [[vagina]].<ref>{{cite book|last1=Bacha Jr.|first1=William J.|last2=Bacha|first2=Linda M.|title= Color Atlas of Veterinary Histology |publisher = Wiley |year = 2012|page=308|access-date = 28 November 2023 |isbn= 978-1-11824-364-0|url = https://books.google.com/books?id=08BOg2b7zRgC&pg=PA308}}</ref><ref>{{cite book|last1=Cooke|first1=Fred|last2=Bruce|first2=Jenni|title= The Encyclopedia of Animals: A Complete Visual Guide |publisher = University of California Press |year = 2004|page=79| access-date = 28 November 2023 |isbn= 978-0-52024-406-1|url = https://books.google.com/books?id=2V1tHqi4hLEC&pg=PA79}}</ref> Marsupials have two lateral vaginas and a medial vagina. The "vagina" of monotremes is better understood as a "urogenital sinus". The uterine systems of placentals can vary between a duplex, where there are two uteri and cervices which open into the vagina, a bipartite, where two [[uterine horn]]s have a single cervix that connects to the vagina, a bicornuate, which consists where two uterine horns that are connected distally but separate medially creating a Y-shape, and a simplex, which has a single uterus.<ref>{{cite book| vauthors = Maxwell KE |year=2013|title=The Sex Imperative: An Evolutionary Tale of Sexual Survival|publisher=Springer|pages=112–113|isbn=978-1-4899-5988-1|url=https://books.google.com/books?id=dnf1BwAAQBAJ}}</ref><ref>{{cite book| vauthors = Vaughan TA, Ryan JP, Czaplewski NJ |year= 2011 |title= Mammalogy|publisher=Jones & Bartlett Publishers|page=387 |isbn= 978-0-03-025034-7 }}</ref><ref name="hair"/>{{rp|220–221, 247}}
[[File:Dendrolagus matschiei 1.jpg|thumb|left|[[Matschie's tree-kangaroo]] with young in pouch]]

The ancestral condition for mammal reproduction is the birthing of relatively undeveloped young, either through direct [[vivipary]] or a short period as soft-shelled eggs. This is likely due to the fact that the torso could not expand due to the presence of [[epipubic bones]]. The oldest demonstration of this reproductive style is with ''[[Kayentatherium]]'', which produced undeveloped [[perinate]]s, but at much higher litter sizes than any modern mammal, 38 specimens.<ref name="Hoffman&Rowe">{{cite journal | vauthors = Hoffman EA, Rowe TB | title = Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth | journal = Nature | volume = 561 | issue = 7721 | pages = 104–108 | date = September 2018 | pmid = 30158701 | doi = 10.1038/s41586-018-0441-3 | bibcode = 2018Natur.561..104H| s2cid = 205570021 }}</ref> Most modern mammals are [[viviparity|viviparous]], giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have a [[sex-determination system]] different from that of most other mammals.<ref>{{cite journal | vauthors = Wallis MC, Waters PD, Delbridge ML, Kirby PJ, Pask AJ, Grützner F, Rens W, Ferguson-Smith MA, Graves JA | display-authors = 6 | title = Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes | journal = Chromosome Research | volume = 15 | issue = 8 | pages = 949–959 | year = 2007 | pmid = 18185981 | doi = 10.1007/s10577-007-1185-3 | s2cid = 812974 }}</ref> In particular, the [[sex chromosome]]s of a platypus are more like those of a chicken than those of a therian mammal.<ref>{{cite journal | vauthors = Marshall Graves JA | title = Weird animal genomes and the evolution of vertebrate sex and sex chromosomes | journal = Annual Review of Genetics | volume = 42 | pages = 565–586 | year = 2008 | pmid = 18983263 | doi = 10.1146/annurev.genet.42.110807.091714 | url = https://www.mnf.uni-greifswald.de/fileadmin/Zoologisches_Museum/Hildebrandt/Dokumente/graves08.pdf | url-status = dead | archive-url = http://webarchive.nationalarchives.gov.uk/20120904084145/http%3A//www.mnf.uni%2Dgreifswald.de/fileadmin/Zoologisches_Museum/Hildebrandt/Dokumente/graves08.pdf | archive-date = 4 September 2012 | access-date = 25 January 2024 }}</ref>

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short [[gestation]] period, typically shorter than its [[estrous cycle]] and generally giving birth to a number of undeveloped newborns that then undergo further development; in many species, this takes place within a pouch-like sac, the [[Pouch (marsupial)|marsupium]], located in the front of the mother's [[abdomen]]. This is the [[Symplesiomorphy|plesiomorphic]] condition among viviparous mammals; the presence of epipubic bones in all non-placentals prevents the expansion of the torso needed for full pregnancy.<ref name=schulkin>{{cite book| vauthors = Power ML, Schulkin J |year=2013|title=The Evolution Of The Human Placenta|publisher=Johns Hopkins University Press|url={{Google books|plainurl=yes|id=xfffGC3hjPoC|page=1891}}|pages=1890–1891|location=Baltimore|isbn=978-1-4214-0643-5|oclc=940749490}}</ref> Even non-placental eutherians probably reproduced this way.<ref name="Epipubic bones in eutherian mammals"/> The placentals give birth to relatively complete and developed young, usually after long gestation periods.<ref>{{cite book|chapter-url={{Google books|plainurl=yes|id=dK-D6HMSOQIC|page=6}}|location=Chicago| vauthors = Sally M |title=Mammals|chapter=Mammal Behavior and Lifestyle|year=2005|publisher=Raintree|page=6|isbn=978-1-4109-1050-9|oclc=53476660}}</ref> They get their name from the [[placenta]], which connects the developing fetus to the uterine wall to allow nutrient uptake.<ref>{{cite book|url={{Google books|plainurl=yes|id=23s9DAAAQBAJ|page=288}} | vauthors = Verma PS, Pandey BP |year=2013|title=ISC Biology Book I for Class XI|publisher=S. Chand and Company|page=288|location=New Delhi|isbn=978-81-219-2557-0}}</ref> In placentals, the epipubic is either completely lost or converted into the baculum; allowing the torso to be able to expand and thus birth developed offspring.<ref name="Hoffman&Rowe"/>

The [[mammary gland]]s of mammals are specialised to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the [[teat]]s seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.<ref>{{cite journal | vauthors = Oftedal OT | title = The mammary gland and its origin during synapsid evolution | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 225–252 | date = July 2002 | pmid = 12751889 | doi = 10.1023/a:1022896515287 | s2cid = 25806501 }}</ref> Compared to placental mammals, the milk of marsupials changes greatly in both production rate and in nutrient composition, due to the underdeveloped young. In addition, the mammary glands have more autonomy allowing them to supply separate milks to young at different development stages.<ref>{{cite book| vauthors = Krockenberger A |year=2006 |title= Marsupials | chapter = Lactation| veditors = Dickman CR, Armati PJ, Hume ID |page=109|publisher=Cambridge University Press |isbn=978-1-139-45742-2}}</ref> [[Lactose]] is the main sugar in placental milk while monotreme and marsupial milk is dominated by [[oligosaccharide]]s.<ref>{{cite book| vauthors = Schulkin J, Power ML |year=2016|title=Milk: The Biology of Lactation|publisher=Johns Hopkins University Press|page=66|isbn=978-1-4214-2042-4}}</ref> [[Weaning]] is the process in which a mammal becomes less dependent on their mother's milk and more on solid food.<ref>{{cite book | vauthors = Thompson KV, Baker AJ, Baker AM |year=2010|title=Wild Mammals in Captivity Principles and Techniques for Zoo Management|publisher=University of Chicago Press| chapter = Paternal Care and Behavioral Development in Captive Mammals | veditors = Kleiman DG, Thompson KV, Baer CK |page=374|isbn=978-0-226-44011-8|edition=2nd}}</ref>