Editing Mule (section)

== Fertility ==

The speciation of horses and donkeys from their common ancestor happened sometime between 7.7 and 15.4 million years ago.<ref name=":03">{{cite journal |display-authors=3 |last1=Wang |first1=Xisheng |last2=Bou |first2=Gerelchimeg |last3=Zhang |first3=Xinzhuang |last4=Tao |first4=Li |last5=Shen |first5=Yingchao |last6=Na |first6=Riga |last7=Liu |first7=Guiqin |last8=Ren |first8=Hong |last9=Ren |first9=Xiujuan |last10=Song |first10=Lianjie |last11=Su |first11=Shaofeng |last12=Bai |first12=Dongyi |last13=Zhao |first13=Yiping |last14=Li |first14=Bei |last15=Dugarjaviin |first15=Manglai |title=A Fast PCR Test for the Simultaneous Identification of Species and Gender in Horses, Donkeys, Mules and Hinnies |journal=Journal of Equine Veterinary Science |date=July 2021 |volume=102 |pages=103458 |doi=10.1016/j.jevs.2021.103458 |pmid=34119210 }}</ref> Today, they are phenotypically and genetically different, with a horse having 64 chromosomes and a donkey having 62.<ref name=":0" /> A mule has 63 [[Chromosome|chromosomes]], 32 from the horse and 31 from the donkey. Its odd number of chromosomes makes gamete formation difficult, often leaving mules [[infertile]].<ref name=":0" /> This also disqualifies them as a species under the [[:simple:Biological_species_concept|biological species model]].<ref>{{Cite web |date=2018-07-13 |title=18.2A: The Biological Species Concept |url=https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/18:_Evolution_and_the_Origin_of_Species/18.02:_Formation_of_New_Species/18.2A:_The_Biological_Species_Concept |access-date=2025-03-06 |website=Biology LibreTexts |language=en}}</ref>

The conception of a mule is difficult due to the differences in behavior and mating patterns between donkeys and horses. When in close proximity, groups of horses and groups of donkeys do not interact with each other often, and donkeys remain at the bottom of the equine social hierarchy while horses rule the pasture and mules are left in a mid-tier social caste.<ref>{{cite book |doi=10.1016/B978-0-12-812106-1.00005-X |chapter=Equine Social Behavior |title=Equine Behavioral Medicine |date=2019 |last1=Beaver |first1=Bonnie V. |pages=115–150 |isbn=978-0-12-812106-1 }}</ref><ref>{{Cite journal |last=Proops |first=Leanne |last2=Burden |first2=Faith |last3=Osthaus |first3=Britta |date=2012-07-01 |title=Social relations in a mixed group of mules, ponies and donkeys reflect differences in equid type |url=https://www.sciencedirect.com/science/article/abs/pii/S0376635712000654?fr=RR-12&ref=pdf_download&rr=92617f0e7b213958 |journal=Behavioural Processes |volume=90 |issue=3 |pages=337–342 |doi=10.1016/j.beproc.2012.03.012 |issn=0376-6357}}</ref>

Mule pregnancy is rare, but can occasionally occur naturally, as well as through [[embryo transfer]]. A few mare mules have produced offspring when mated with a horse or a jack.<ref>{{cite journal |last1=Savory |first1=Theodore H |year=1970 |title=The Mule |journal=Scientific American |volume=223 |issue=6 |pages=102–109 |doi=10.1038/scientificamerican1270-102 |bibcode=1970SciAm.223f.102S}}</ref><ref name="Kay2002">{{cite news |last=Kay |first=Katty |date=2 October 2002 |title=Morocco's miracle mule |url=http://news.bbc.co.uk/2/hi/science/nature/2290491.stm |work=[[BBC News]] |access-date=5 February 2009 |archive-date=29 July 2018 |archive-url=https://web.archive.org/web/20180729080951/http://news.bbc.co.uk/2/hi/science/nature/2290491.stm |url-status=live}}</ref> [[Herodotus]] gives an account of such an event as an ill omen of [[Second Persian invasion of Greece|Xerxes' invasion of Greece]] in 480 BC: "There happened also a portent of another kind while he was still at Sardis—a mule brought forth young and gave birth to a mule" (Herodotus ''[[Histories (Herodotus)|The Histories]]'' 7:57), and a mule's giving birth was a frequently recorded portent in antiquity, although scientific writers also doubted whether it was really possible (see e.g. [[Aristotle]], ''Historia animalium'', 6.24; [[Varro]], ''De re rustica'', 2.1.28). Between 1527 and 2002, approximately sixty such births were reported.<ref name="Kay2002" /> In [[Morocco]] in early 2002 and [[Colorado]] in 2007, mare mules produced colts.<ref name="Kay2002" /><ref name="Npr2007-07-26">{{cite news |date=26 July 2007 |title=Befuddling Birth: The Case of the Mule's Foal |url=https://www.npr.org/templates/story/story.php?storyId=12260255 |publisher=[[National Public Radio]] |access-date=5 February 2009 |archive-date=6 December 2008 |archive-url=https://web.archive.org/web/20081206052236/http://www.npr.org/templates/story/story.php?storyId=12260255 |url-status=live}}</ref> Blood and hair samples from the Colorado birth verified that the mother was indeed a mule and the foal was indeed her offspring.<ref name="DenverPost2007-09-19">{{cite news |last=Lofholm |first=Nancy |date=19 September 2007 |title=Mule's foal fools genetics with 'impossible' birth |url=http://www.denverpost.com/haley/ci_6464853.html |url-status=live |archive-url=https://web.archive.org/web/20091107032740/http://www.denverpost.com/haley/ci_6464853.html |archive-date=7 November 2009 |access-date=5 February 2009 |newspaper=[[Denver Post]]}}</ref>

A 1939 article in the ''Journal of Heredity'' describes two offspring of a fertile mare mule named "Old Bec," which was owned at the time by [[Texas A&M University]] in the late 1920s. One of the foals was a female, sired by a jack. Unlike her mother, she was sterile. The other, sired by a five-gaited [[American Saddlebred|Saddlebred]] stallion, exhibited no characteristics of any donkey. That horse, a stallion, was bred to several mares, which gave birth to live foals that showed no characteristics of the donkey.<ref name=JHeredity-30-12>{{cite journal |last=Anderson |first=W. S. |year=1939 |title=Fertile Mare Mules |journal=[[Journal of Heredity]] |volume=30 |issue=12 |pages=549–551 |doi=10.1093/oxfordjournals.jhered.a104657 }}</ref> In 1995, a group from the [[Federal University of Minas Gerais]] described a female mule that was pregnant for a seventh time, having previously produced two donkey sires, two foals with the typical 63 chromosomes of mules, and several horse stallions that had produced four foals. The three of the latter available for testing each bore 64 horse-like chromosomes. These foals phenotypically resembled horses, though they bore markings absent from the sire's known lineages, and one had ears noticeably longer than those typical of her sire's breed. The elder two horse-like foals had proved fertile at the time of publication, with their progeny being typical of horses.<ref>{{cite journal |last1=Henry |first1=M. |last2=Gastal |first2=E.L. |last3=Pinheiro |first3=L.E.L. |last4=Guimarmes |first4=S.E.F. |title=Mating Pattern and Chromosome Analysis of a Mule and Her Offspring |journal=Biology of Reproduction |volume=52 |issue=Equine Reproduction VI – Monograph Series 1 |pages=273–279 |year=1995 |doi=10.1093/biolreprod/52.monograph_series1.273 |doi-access=free}}</ref>