Editing Testicle (section)

==Other animals==
[[File:Hühner-Organ.jpg|thumb|Testicles of a [[rooster]]]]
===External appearance===
In [[seasonal breeder]]s, the weight of the testes often increases during the breeding season.<ref name="Johnson2012">{{cite book|author=A.D. Johnson|title=Development, Anatomy, and Physiology|url=https://books.google.com/books?id=yAJMlA0FkAUC&q=%22seasonal+breeders%22|date=2 December 2012|publisher=Elsevier|isbn=978-0-323-14323-3|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161308/https://books.google.com/books?id=yAJMlA0FkAUC&q=%22seasonal+breeders%22|url-status=live}}</ref> The [[Dromedary#Testicles|testicles of a dromedary camel]] are {{convert|7|-|10|cm|in|abbr=on}} long, {{convert|4.5|cm|in|abbr=on}} deep and {{convert|5|cm|in|abbr=on}} in width. The right testicle is often smaller than the left.<ref name=anatomy>{{cite book|last=Mukasa-Mugerwa|first=E.|title=The Camel (Camelus dromedarius): A Bibliographical Review|pages=11–3}}</ref>

In [[shark]]s, the testicle on the right side is usually larger. In many bird and mammal species, the left may be larger. [[Fish_anatomy#Testes|Fish usually have two testes]] of a similar size. The primitive [[jawless fish]] have only a single testis, located in the midline of the body, although this forms from the fusion of paired structures in the embryo.<ref name=VB/>

===Location===

====Internal====
The basal condition for mammals is to have internal testes.<ref name="Kleisner">{{cite journal |last1= Kleisner |first1= Karel |last2= Ivell |first2= Richard |last3= Flegr |first3= Jaroslav |date= March 2010|title= The evolutionary history of testicular externalization and the origin of the scrotum |url=http://www.ias.ac.in/article/fulltext/jbsc/035/01/0027-0037  |journal= Journal of Biosciences |publisher= Indian Academy of Sciences |volume= 35 |issue= 1 |pages= 27–37 |doi=  10.1007/s12038-010-0005-7 |pmid= 20413907 |s2cid= 11962872 |access-date=8 December 2018 |archive-url=https://web.archive.org/web/20180225094002/http://www.ias.ac.in/article/fulltext/jbsc/035/01/0027-0037 |archive-date=25 February 2018 |url-status=dead |url-access= subscription }}</ref> The testes of [[monotremes]],<ref name="Griffiths2012">{{cite book|author=Mervyn Griffiths|title=The Biology of the Monotremes|url=https://books.google.com/books?id=W7nu16_DkTgC&q=testes+OR+scrotum|date=2 December 2012|publisher=Elsevier Science|isbn=978-0-323-15331-7|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161534/https://books.google.com/books?id=W7nu16_DkTgC&q=testes+OR+scrotum|url-status=live}}</ref><ref name="NowakWalker1999">{{cite book|author1=Ronald M. Nowak|author2=Ernest Pillsbury Walker|title=Walker's Mammals of the World|url=https://archive.org/details/walkersmammalsof0001nowa|url-access=registration|quote=testes.|date=29 July 1999|publisher=JHU Press|isbn=978-0-8018-5789-8}}</ref> [[xenarthra]]ns,<ref name="NowakWalker1999"/> and [[afrotheria]]ns<ref name="FowlerMikota2008">{{cite book|author1=Murray Fowler|author2=Susan K. Mikota|title=Biology, Medicine, and Surgery of Elephants|url=https://books.google.com/books?id=A3GFYySwY1cC&q=testes+OR+testis+OR+testicles|date=9 January 2008|publisher=John Wiley & Sons|isbn=978-0-470-34411-8|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161516/https://books.google.com/books?id=A3GFYySwY1cC&q=testes+OR+testis+OR+testicles|url-status=live}}</ref> remain within the abdomen ([[testicondy]]). There are also some [[marsupial]]s with external testes<ref name="Hunsaker2012">{{cite book|author=Don II Hunsaker|title=The Biology of Marsupials|url=https://books.google.com/books?id=cESCLrRJGm0C&q=testes+OR+testis+OR+testicles|date=2 December 2012|publisher=Elsevier Science|isbn=978-0-323-14620-3|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161542/https://books.google.com/books?id=cESCLrRJGm0C&q=testes+OR+testis+OR+testicles|url-status=live}}</ref><ref name="Tyndale-Biscoe2005">{{cite book|author=C. Hugh Tyndale-Biscoe|title=Life of Marsupials|url=https://books.google.com/books?id=KqtlPZJ9y8EC&q=testes+OR+testis+OR+testicles|year=2005|publisher=Csiro Publishing|isbn=978-0-643-06257-3|access-date=2020-10-19|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161517/https://books.google.com/books?id=KqtlPZJ9y8EC&q=testes+OR+testis+OR+testicles|url-status=live}}</ref><ref name="Tyndale-BiscoeRenfree1987">{{cite book|author1=Hugh Tyndale-Biscoe|author2=Marilyn Renfree|title=Reproductive Physiology of Marsupials|url=https://books.google.com/books?id=HpjovN0vXW4C&q=testes|date=30 January 1987|publisher=Cambridge University Press|isbn=978-0-521-33792-2|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161518/https://books.google.com/books?id=HpjovN0vXW4C&q=testes|url-status=live}}</ref> and [[boreoeutheria]]n mammals with internal testes, such as the rhinoceros.<ref name="rhinoresourcecenter.com">Schaffer, N. E., et al. "[http://www.rhinoresourcecenter.com/pdf_files/125/1258623199.pdf Ultrasonography of the reproductive anatomy in the Sumatran rhinoceros (Dicerorhinus sumatrensis)] {{Webarchive|url=https://web.archive.org/web/20170923194033/http://www.rhinoresourcecenter.com/pdf_files/125/1258623199.pdf |date=2017-09-23 }}." Journal of Zoo and Wildlife Medicine (1994): 337-348.</ref> [[Cetaceans]] such as whales and dolphins also have internal testes.<ref name="WürsigThewissen2017">{{cite book|author1=Bernd Würsig|author2=J.G.M. Thewissen|author3=Kit M. Kovacs|title=Encyclopedia of Marine Mammals|url=https://books.google.com/books?id=T3zGDgAAQBAJ&q=testes+OR+testis+OR+testicles|date=27 November 2017|publisher=Elsevier Science|isbn=978-0-12-804381-3|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161516/https://books.google.com/books?id=T3zGDgAAQBAJ&q=testes+OR+testis+OR+testicles|url-status=live}}</ref><ref name="rommel2007">{{cite book | last1 = Rommel | first1 = S.A. | first2 = D.A. | last2 = Pabst | first3 = W.A. | last3 = McLellan | chapter = Functional anatomy of the cetacean reproductive system, with comparisons to the domestic dog | title = Reproductive Biology and Phylogeny of Cetacea: Whales, Porpoises and Dolphins | date = 2007 | pages = 127–145 | editor-last = Miller | editor-first = D.L. | chapter-url = https://www.researchgate.net/publication/286334625 | doi = 10.1201/b11001| isbn = 9780429063626 }}</ref> As external testes would increase drag in the water, they have internal testes, which are kept cool by special circulatory systems that cool the arterial blood going to the testes by placing the arteries near veins bringing cooled venous blood from the skin.<ref name="rommel1998">{{cite magazine | last1 = Rommel | first1 = S.A. | first2 = D.A. | last2 = Pabst | first3 = W.A. | last3 = McLellan | title = Reproductive Thermoregulation in Marine Mammals | journal = American Scientist | volume = 86 | number = 5 | date = 1998 | pages = 440–448 | jstor = 27857097 | url = https://uncw.edu/mmsp/documents/rommeletal1998reproductivethermoregulation.pdf | url-status = live | archive-date = 22 November 2021 | archive-url = https://web.archive.org/web/20211122202231/https://uncw.edu/mmsp/documents/rommeletal1998reproductivethermoregulation.pdf}}</ref><ref name="Pabst, D. Ann 1998">{{cite book | last1 = Pabst | first1 = D.A. | last2 = Sentiel | first2 = A.R | first3 = W.A. | last3 = McLellan | chapter = Evolution of thermoregulatory function in cetacean reproductive systems | title = The Emergence of Whales | publisher = Springer US | date = 1998 | pages = 379–397 | series = Advances in Vertebrate Paleobiology | doi = 10.1007/978-1-4899-0159-0_13 | isbn = 978-1-4899-0161-3 | editor-last = Thewissen | editor-first = J.G.M.}}</ref> In [[odobenid]]s and [[phocid]]s, the location of the testes is para-abdominal, though [[otariid]]s have scrotal testes.<ref name="GullandDierauf2018">{{cite book|author1=Frances M.D. Gulland|author2=Leslie A. Dierauf|author3=Karyl L. Whitman|title=CRC Handbook of Marine Mammal Medicine|url=https://books.google.com/books?id=b1ZSDwAAQBAJ|date=20 March 2018|publisher=CRC Press|isbn=978-1-351-38416-2|access-date=23 January 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161820/https://books.google.com/books?id=b1ZSDwAAQBAJ|url-status=live}}</ref>

====External====
[[Boreoeutheria]]n land mammals, the large group of mammals that includes humans, have externalized testes.<ref name="MillsMarchant-Forde2010">{{cite book|author1=D. S. Mills|author2=Jeremy N. Marchant-Forde|title=The Encyclopedia of Applied Animal Behaviour and Welfare|url=https://books.google.com/books?id=vrueZDfPUzoC&q=%22boreoeutherian+OR+boreoeutheria%22+%28testes+OR+testicles%29+-wikipedia+-%22There+are+also+some+Boreoeutherian+mammals%22&pg=PA293|year=2010|publisher=CABI|isbn=978-0-85199-724-7|pages=293–|access-date=2020-10-19|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161816/https://books.google.com/books?id=vrueZDfPUzoC&q=%22boreoeutherian+OR+boreoeutheria%22+%28testes+OR+testicles%29+-wikipedia+-%22There+are+also+some+Boreoeutherian+mammals%22&pg=PA293|url-status=live}}</ref> Their testes function best at temperatures lower than their core body temperature. Their testes are located outside of the body and are suspended by the spermatic cord within the scrotum.

There are several hypotheses as to why most boreotherian mammals have external testes that operate best at a temperature that is slightly less than the core body temperature. One view is that it is stuck with enzymes evolved in a colder temperature due to external testes evolving for different reasons. Another view is that the lower temperature of the testes simply is more efficient for sperm production.

The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile spermatogenesis. There are no possible enzymes operating at normal core body temperature that are as efficient as the ones evolved.

Early mammals had lower body temperatures and thus their testes worked efficiently within their body. However, boreotherian mammals may have higher body temperatures than the other mammals and had to develop external testes to keep them cool. One argument is that mammals with internal testes, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testes.{{citation needed|date=July 2017}}

Researchers have wondered why birds, despite having very high core body temperatures, have internal testes and did not evolve external testes.<ref name=bio>[http://www.biolreprod.org/content/56/6/1570.full.pdf BIOLOGY OF REPRODUCTION 56, 1570–1575 (1997)- Determination of Testis Temperature Rhythms and Effects of Constant Light on Testicular Function in the Domestic Fowl (Gallus domesticus)] {{webarchive|url=https://web.archive.org/web/20150923190501/http://www.biolreprod.org/content/56/6/1570.full.pdf |date=2015-09-23 }}</ref> It was once theorized that birds used their [[air sacs]] to cool the testes internally, but later studies revealed that birds' testes are able to function at core body temperature.<ref name="bio"/>

Some mammals with seasonal breeding cycles keep their testes internal until the breeding season. After that, their testes descend and increase in size and become external.<ref>{{cite web |url=http://www.askabiologist.org.uk/answers/viewtopic.php?id=653 |title=Ask a Biologist Q&A / Human sexual physiology – good design? |publisher=Askabiologist.org.uk |date=4 September 2007 |access-date=25 October 2010 |archive-date=27 October 2010 |archive-url=https://web.archive.org/web/20101027155029/http://askabiologist.org.uk/answers/viewtopic.php?id=653 |url-status=live }}</ref>

The ancestor of the boreoeutherian mammals may have been a small mammal that required very large testes for [[sperm competition]] and thus had to place its testes outside the body.<ref>{{cite web |url=http://old.richarddawkins.net/videos/865-the-human-body-as-an-evolutionary-patchwork/comments |title='The Human Body as an Evolutionary Patchwork' by Alan Walker, Princeton.edu |publisher=RichardDawkins.net |date=2007-04-10 |access-date=25 October 2010 |url-status=dead |archive-url=https://web.archive.org/web/20131109120731/http://old.richarddawkins.net/videos/865-the-human-body-as-an-evolutionary-patchwork/comments |archive-date=9 November 2013 }}</ref> This might have led to enzymes involved in spermatogenesis, spermatogenic [[DNA polymerase beta]] and [[recombinase]] activities evolving a unique temperature optimum that is slightly less than core body temperature. When the boreoeutherian mammals diversified into forms that were larger or did not require intense sperm competition, they still produced enzymes that operated best at cooler temperatures and had to keep their testes outside the body. This position is made less parsimonious because the [[kangaroo]], a non-boreoeutherian mammal, has external testicles. Separately from boreotherian mammals, the ancestors of kangaroos might have also been subject to heavy sperm competition and thus developed external testes; however, kangaroo external testes are suggestive of a possible adaptive function for external testes in large animals.

One argument for the [[Evolution of descended testes in mammals|evolution of external testes]] is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping.<ref>{{cite web|url=https://www.newscientist.com/article/mg15120432.000-science--bumpy-lifestyle-led-to-external-testes.html|title=Science : Bumpy lifestyle led to external testes|access-date=2017-09-15|archive-date=2015-02-22|archive-url=https://web.archive.org/web/20150222041925/http://www.newscientist.com/article/mg15120432.000-science--bumpy-lifestyle-led-to-external-testes.html|url-status=live}}</ref>

Mild, transient scrotal heat stress causes DNA damage, [[infertility|reduced fertility]] and abnormal embryonic development in mice.<ref name="pmid18390691">{{cite journal |vauthors=Paul C, Murray AA, Spears N, Saunders PT |title=A single, mild, transient scrotal heat stress causes DNA damage, subfertility and impairs formation of blastocysts in mice |journal=Reproduction |volume=136 |issue=1 |pages=73–84 |year=2008 |pmid=18390691 |doi=10.1530/REP-08-0036 |doi-access=free }}</ref>  DNA strand breaks were found in spermatocytes recovered from testicles subjected to 40&nbsp;°C or 42&nbsp;°C for 30 minutes.<ref name="pmid18390691" /> These findings suggest that the external location of the testicles provides the adaptive benefit of protecting spermatogenic cells from heat-induced DNA damage that could otherwise lead to infertility and [[germline mutation]].

===Size===
[[Image:Rabbit testis.jpg|thumb|Cross section of rabbit testis, photographed in [[bright-field microscopy]] at 40× magnification]]

The [[Monogamy in animals#Testis size|relative size of the testes]] is often influenced by [[mating system]]s.<ref name="Pitcher, Dunn, Whittingham, 2005">{{cite journal | last1 = Pitcher | first1 = T.E. | last2 = Dunn | first2 = P.O. | last3 = Whittingham | first3 = L.A. | year = 2005 | title = Sperm competition and the evolution of testes size in birds | journal = Journal of Evolutionary Biology | volume = 18 | issue = 3| pages = 557–567 | doi=10.1111/j.1420-9101.2004.00874.x| pmid = 15842485 | s2cid = 18331398 | doi-access = free }}</ref> Testicular size as a proportion of body weight varies widely. In the mammalian kingdom, there is a tendency for testicular size to correspond with multiple mates (e.g., harems, [[polygamy]]). Production of testicular output sperm and spermatic fluid is also larger in polygamous animals, possibly a [[Sperm competition|spermatogenic competition]] for survival. The testes of the [[right whale]] are likely to be the largest of any animal, each weighing around 500&nbsp;kg (1,100&nbsp;lb).<ref>{{cite web |url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Eubalaena_glacialis.html |author=Crane, J. |author2=Scott, R. |year=2002 |title=Eubalaena glacialis |work=Animal Diversity Web |access-date=1 May 2009 |archive-date=28 March 2022 |archive-url=https://web.archive.org/web/20220328231412/https://animaldiversity.org/site/accounts/information/Eubalaena_glacialis.html |url-status=live }}</ref>

Among the [[Hominidae]], [[gorilla]]s have little [[female promiscuity]] and sperm competition and the testes are small compared to body weight (0.03%). [[Chimpanzee]]s have high promiscuity and large testes compared to body weight (0.3%). Human testicular size falls between these extremes (0.08%).<ref name=Shackelford>{{Cite journal | last1 = Shackelford | first1 = T. K. | last2 = Goetz | first2 = A. T. | doi = 10.1111/j.1467-8721.2007.00473.x | title = Adaptation to Sperm Competition in Humans | journal = Current Directions in Psychological Science | volume = 16 | pages = 47–50 | year = 2007 | s2cid = 6179167 }}</ref>

Testis weight also varies in seasonal breeders like [[red fox]]es,<ref name="s537">{{Harvard citation no brackets|Heptner|Naumov|1998|p=537}}</ref> [[golden jackal]]s,<ref name="h154">{{Harvard citation no brackets|Heptner|Naumov|1998|pp=154–155}}</ref> and [[coyote]]s.<ref name="Johnson2012"/>

===Internal structure===
[[Amphibian]]s and most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures called ''sperm ampullae''. These are seasonal structures, releasing their contents during the breeding season, and then being reabsorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in [[higher vertebrates]], including the same range of cell types.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood |author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |location=Philadelphia |publisher=Holt-Saunders International |pages= 385–386|isbn= 978-0-03-910284-5}}</ref>