Editing Squirrel monkey

{{Short description|Genus of mammals belonging to the capuchin and squirrel monkey family of primates}}
{{Automatic taxobox 
| name = Squirrel monkeys<ref name=msw3>{{MSW3 Groves|pages=138–139|id=12100284}}</ref>
| fossil_range = [[Miocene]] ([[Laventan]] to recent<br/>{{fossil range|13.8|0}}
| image = Saimiri sciureus-1 Luc Viatour.jpg
| image_caption = [[Common squirrel monkey]]
| parent_authority = [[Gerrit Smith Miller Jr.|Miller]], 1912 (1900)
| taxon = Saimiri
| authority = [[Friedrich Sigmund Voigt|Voigt]], 1831
| subdivision_ranks = Species
| subdivision = 
*''[[Saimiri boliviensis]]''
*''[[Saimiri cassiquiarensis]]''
*''[[Saimiri collinsi]]''
*''[[Saimiri oerstedii]]''
*''[[Saimiri sciureus]]''
*''[[Saimiri ustus]]''
*''[[Saimiri vanzolinii]]''
* † ''[[Saimiri annectens]]''
* † ''[[Saimiri fieldsi]]''
| type_species = ''[[Common squirrel monkey|Simia sciurea]]''
| type_species_authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]
| synonyms = 
* ''Laventiana'' {{small|Rosenberger, 1991<ref name=PDBSaimiri>[https://paleobiodb.org/classic/basicTaxonInfo?taxon_no=40847 ''Saimiri''] – Paleobiology Database</ref>}}
* ''Neosaimiri'' {{small|Stirton, 1951<ref name=PDBSaimiri/>}}
}}
[[File:Wild-57.jpg|thumb|Squirrel monkey in Yacuma Park, Bolivia]]
'''Squirrel monkeys''' are [[New World monkey]]s of the genus '''''Saimiri'''''. ''Saimiri'' is the only genus in the subfamily '''Saimiriinae'''. The name of the genus is of [[tupi languages|Tupi]] origin (''sai-mirím'' or ''çai-mbirín'', with ''sai'' meaning 'monkey' and ''mirím'' meaning 'small')<ref>{{cite journal|doi=10.2307/1374677|jstor=1374677|title=Vernacular Names of South American Mammals|year=1941|last1=Simpson|first1=George Gaylord|journal=Journal of Mammalogy|volume=22|issue=1|pages=1–17}}</ref> and was also used as an English name by early researchers.<ref>{{cite journal|author=Palmer, T. S. |year=1897|title=Notes on the Nomenclature of Four Genera of Tropical American Mammals|journal=Proceedings of the Biological Society of Washington|volume= 11|pages= 173–174}}</ref><ref>Leclerc, Georges-Louis, Comte de Buffon (1797). [https://www.gutenberg.org/files/45820/45820-h/45820-h.htm "The Saimiri."] In: ''Barr's Buffon. Buffon's Natural History''. pp. 251–252. London: J. S. Barr.</ref>

Squirrel monkeys live in the tropical forests of [[Central America|Central]] and [[South America]] in the canopy layer. Most species have [[parapatric]] or [[allopatric]] ranges in the [[Amazon Rainforest|Amazon]], while ''S. oerstedii'' is found [[Disjunct distribution|disjunctly]] in [[Costa Rica]] and [[Panama]].

There are two main groups of squirrel monkeys recognized. They are differentiated based on the shape of the white coloration above the eyes. In total there are five recognized species.<ref name=":0" /><ref name=":1" /> Squirrel monkeys have short and close fur colored black at the shoulders, yellow or orange fur along the back and extremities, and white on the face.<ref name=":2" />

Squirrel monkeys have determined breeding seasons which involve large fluctuations in [[hormone]]s and there is evidence of [[sexual dimorphism]] between males and females.<ref name=":3" />

Squirrel monkeys can only sweat through the palms of their hands and feet. This can have the effect of making their hands and feet feel damp to the touch. Squirrel monkeys must make use of other [[thermoregulation]] techniques such as behavioral changes and urine washing. <ref name=":4" /> These monkeys live in habitats of high temperatures and high humidity, making it essential for them to maintain proper [[osmoregulation]] if conditions pass certain thresholds.<ref name=":5" /> Color vision studies have also been performed on squirrel monkeys for the purpose of better understanding vision ailments in humans.<ref name="Neitz" />

The [[common squirrel monkey]] is commonly captured for the pet trade and for medical research,<ref>Rhines, C. (2000). "[http://animaldiversity.ummz.umich.edu/site/accounts/information/Saimiri_sciureus.html Saimiri sciureus]", [[Animal Diversity Web]]. Accessed November 26, 2007</ref> but it is not [[Threatened species|threatened]]. Two squirrel monkey species are endangered: the [[Central American squirrel monkey]] and the [[black squirrel monkey]] are listed as [[vulnerable species|vulnerable]] by the [[IUCN]].<ref name=":0">{{Cite iucn | author = Boubli, J.-P. | author2 = Rylands, A.B. | name-list-style = amp | title = Saimiri vanzolinii | volume = 2008 | page = e.T19839A9023022 | date = 2008 | doi = 10.2305/IUCN.UK.2008.RLTS.T19839A9023022.en }}</ref><ref>{{Cite iucn | author = Wong, G. | author2 = Cuarón, A.D. | author3 = Rodriguez-Luna, E. | author4 = de Grammont, P.C. | name-list-style = amp | title = Saimiri oerstedii | volume = 2008 | page = e.T19836A9022609 | date = 2008 | doi = 10.2305/IUCN.UK.2008.RLTS.T19836A9022609.en }}</ref>

== Evolutionary history ==
[[File:Miss Baker, the squirrel monkey.jpg|thumb|[[Miss Baker]], an 'astronaut' squirrel monkey, rode into space as part of the [[NASA|United States space program]], and returned safely.]]

=== Taxonomy ===
Until 1984, all South American squirrel monkeys were considered part of a single widespread species, and many zoologists considered the Central American squirrel monkey to be a member of that single species as well.<ref name="msw3" /><ref name=":1">{{cite book|title=A Complete Guide to Monkeys, Apes and Other Primates|author=Kavanagh, Michael|year=1983|page=[https://archive.org/details/completeguidetom0000kava/page/84 84]|isbn=978-0-224-02168-5|url=https://archive.org/details/completeguidetom0000kava/page/84}}</ref> The two main groups currently recognized can be separated by the white above the eyes; it is shaped as a [[Gothic architecture|Gothic]] ("pointed") arch in the ''S. sciureus'' group, while it is shaped as a [[Roman architecture|Roman]] ("rounded") arch in the ''S. boliviensis'' group.<ref>{{cite book |author=Rowe, N. |year=1996 |title=The Pictorial Guide to the Living Primates |publisher=Pogonia Press, Charlestown, Rhode Island |isbn=978-0-9648825-0-8 |url-access=registration |url=https://archive.org/details/pictorialguideto0000rowe }}</ref> ''Mammal Species of the World'' (2005) recognized five species.<ref name="msw3"/> 

Subsequent taxonomic research has recognized ''Saimiri sciureus cassiquiarensis'' as a separate species ''[[Saimiri cassiquiarensis]]'', and also recognized an additional species, [[Collins' squirrel monkey]] ''Saimiri collinsi'' that had previously been considered to be within ''S. sciureus''.<ref>{{cite journal|title=Morphology and mitochondrial phylogenetics reveal that the Amazon River separates two eastern squirrel monkey species: Saimiri sciureus and S. collinsi|author=Merces, Michelle P. |author2=Lynch Alfaro, Jessica W. |display-authors=etal |journal=Molecular Phylogenetics and Evolution|volume=82|issue=B|date=January 2015|pages=426–435|doi=10.1016/j.ympev.2014.09.020|pmid=25451802|bibcode=2015MolPE..82..426M }}</ref>  Some more recent taxonomies also recognize ''Saimiri sciureus macrodon'' as a separate species ''Saimiri macrodon'', but others recognize ''S. macrodon'' to be a synonym of ''Saimiri cassiquiarensis''.<ref>{{cite journal|title=Biogeography of squirrel monkeys (genus ''Saimiri''): South-central Amazon origin and rapid pan-Amazonian diversification of a lowland primate|author=Lynch Alfaro, J.W. |display-authors=etal |pages=436–454|journal=Molecular Phylogenetics and Evolution|volume=82|year=2015|doi=10.1016/j.ympev.2014.09.004|pmid=25305518 |doi-access=free|bibcode=2015MolPE..82..436L }}</ref><ref>{{cite web|title=Saimiri|url=https://www.itis.gov/servlet/SingleRpt/SingleRpt|publisher=ITIS|access-date=2019-01-12}}</ref><ref>{{cite web|title=Saimiri|url=https://mammaldiversity.org/#c2FpbWlyaSZnbG9iYWxfc2VhcmNoPXRydWUmbG9vc2U9dHJ1ZQ|publisher=Mammal Diversity Database|access-date=2019-01-12}}</ref><ref>{{cite journal|title=The Taxonomy and Conservation Status of Saimiri sciureus albigena: A Squirrel Monkey Endemic to Colombia |author=Carretero-Pinzón, X. |author2=Ruiz-García, M. |author3-link=Thomas Defler |author3=Defler, T.|s2cid=85881653 |url=https://www.researchgate.net/publication/228369497|journal=Primate Conservation|volume=24|pages=59–64|year=2009|access-date=2019-01-13|doi=10.1896/052.024.0102 |doi-access=free}}</ref>

* '''Genus ''Saimiri'''''
** ''S. sciureus'' group
*** [[Central American squirrel monkey]], ''Saimiri oerstedii''
**** [[Black-crowned Central American squirrel monkey]], ''Saimiri oerstedii oerstedii''
**** [[Grey-crowned Central American squirrel monkey]], ''Saimiri oerstedii citrinellus''
*** [[Guianan squirrel monkey]], ''Saimiri sciureus''
**** ''Saimiri sciureus sciureus''
**** ''Saimiri sciureus albigena''
**** [[Ecuadorian squirrel monkey]], ''Saimiri sciureus macrodon''
*** [[Humboldt's squirrel monkey]], ''Saimiri cassiquiarensis''
*** [[Bare-eared squirrel monkey]], ''Saimiri ustus''
*** [[Collins' squirrel monkey]], ''Saimiri collinsi''
** ''S. boliviensis'' group
*** [[Black-capped squirrel monkey]], ''Saimiri boliviensis''
**** Bolivian squirrel monkey, ''Saimiri boliviensis boliviensis''
**** Peruvian squirrel monkey, ''Saimiri boliviensis peruviensis''
*** [[Black squirrel monkey]], ''Saimiri vanzolinii''
** [[List of fossil primates of South America|Fossil species]]
*** †''[[Saimiri annectens]]'', [[Honda Group, Colombia|Honda Group]], Kay and Meldrum 1997<ref name=PDBSaimiri/>
*** †''[[Saimiri fieldsi]]'', Honda Group, Stirton 1951<ref name=PDBSaimiri/>

=== Evolution ===
The crown group of the extant squirrel monkeys appears to have diverged around 1.5 million years ago.<ref name="Chiou2011">{{cite journal|title= Pleistocene diversification of living squirrel monkeys (''Saimiri'' spp.) inferred from complete mitochondrial genome sequences|year=2011|pmid=21443955|last1=Chiou|first1=K. L.|last2=Pozzi|first2=L.|last3=Lynch Alfaro|first3=J. W.|last4=Di Fiore|first4=A.|journal=Molecular Phylogenetics and Evolution|volume=59|issue=3|pages=736–45|doi=10.1016/j.ympev.2011.03.025|bibcode=2011MolPE..59..736C }}</ref> Diversification of squirrel monkey species appears to have occurred during the [[Pleistocene]] Epoch, likely due to climatic changes associated with interglacial periods in South America at the time. The origin of squirrel monkeys in Central America is unclear, but a possible hypothesis is human transport. More genetic work needs to be done on the subject to reveal a true answer.<ref name="Chiou2011" />''S. boliviensis'' appears to be the first diverging species in the group. ''S. oerstedii'' and ''S. s. sciureus,'' are sister species. ''S. s. macrodon'' is the sister species to the ''S. oerstedii'' / ''S. s. sciureus'' clade.{{clarify|date=February 2016}}<!--By definition, "S. s. sciureus" & "S. s. macrodon" are both subspecies of "S. sciureus"!-->

== Description ==
A squirrel monkey's fur is short and close, coloured black at the shoulders and yellowish orange on its back and extremities. The upper parts of their heads are hairy. This black-and-white face gives them the name "[[Totenkopf|death's head]] monkey" in several Germanic languages (e.g., German {{Lang|de|Totenkopfaffen}}, Swedish {{Lang|sv|dödskalleapor}}, Dutch {{Lang|nl|doodshoofdaapjes}}) and Slovenian ({{Lang|sl|smrtoglavka}}).

Squirrel monkeys grow from {{convert|25|to|35|cm}} long, plus a {{convert|35|to|42|cm}} tail. Male squirrel monkeys weigh {{convert|750|to|1100|g}}. Females weigh {{convert|500|to|750|g}}. Both males and females are equipped with long and hairy tails, flat nails, and pointed claws.<ref name=":2">{{cite book|title=Organic Evolution|year=1921|publisher=The Macmillan Company|location=New York|chapter-url=https://archive.org/stream/organicevolutio01lullgoog#page/n5/mode/2up|chapter=Ch. 77|author=Lull, Richard Swann}}</ref>

Female squirrel monkeys have [[pseudo-penis]]es, which they use to display dominance over smaller monkeys, in much the same way that the male squirrel monkeys display their dominance.{{citation needed|date=December 2013}}

== Behaviour, ecology, and physiology ==
Like most of their New World monkey relatives, squirrel monkeys are [[diurnal animal|diurnal]] and [[arboreal]]. Unlike other New World monkeys, their tail is not used for climbing but as a kind of "balancing pole" and also as a tool. Their movements in the branches can be very rapid.

Squirrel monkeys live together in multi-male/multi-female groups with up to 500 members. These large groups, however, can occasionally break into smaller troupes. The groups have a number of vocal calls, including warning sounds to protect the group from large [[falcon]]s, which are a natural threat. Their small body size also makes them susceptible to predators such as snakes and [[felid]]s. For marking territory, squirrel monkeys rub their tail and their skin with their own urine.

Squirrel monkeys are [[omnivore]]s, eating primarily fruits and insects. Occasionally, they also eat seeds, leaves, flowers, buds, nuts, and eggs.

[[File:Large Male Squirrel Monkey.jpg|thumb|Sexual dimorphism in squirrel monkeys: larger adult male. Spotted in Piedras Blancas National Park, Golfo Dulce, Costa Rica]]
=== Reproduction ===
Squirrel monkey mating is subject to seasonal influences. Squirrel monkeys reach sexual maturity at ages of 2–2.5 years for females and age 3.5–4 years for males. Females give birth to young during the rainy season, after a 150- to 170-day gestation. Only the mothers care for the young. ''Saimiri oerstedti'' are weaned by 4 months of age, while ''S. boliviensis'' are not fully weaned until 18 months old. Squirrel monkeys live to about 15 years old in the wild, and over 20 years in captivity. [[Menopause]] in females probably occurs in the mid-teens.<ref name="MLWalker1">{{cite journal|vauthors=Walker ML, Anderson DC, Herndon JG, Walker LC|year=2009|title=Ovarian aging in squirrel monkeys (Saimiri sciureus)|journal=Reproduction|volume=138|issue=4|pages=793–799|doi=10.1530/REP-08-0449|pmid=19656956|pmc=7723027|doi-access=free}}</ref> Studies show that ''Saimiri collinsi'' time the weaning of their young with the period of time when there will be maximum fruit availability in the environment. This reduces the energetic struggles that newly weaned juveniles will face when transitioning from a milk diet where they are dependent on their mother for food to a more diverse diet where they have to forage for food.<ref>{{Cite journal|last1=Stone|first1=Anita I.|last2=Ruivo|first2=Luana V. P.|date=2020|title=Synchronization of weaning time with peak fruit availability in squirrel monkeys (Saimiri collinsi) living in Amazonian Brazil|journal=American Journal of Primatology|language=en|volume=82|issue=7|pages=e23139|doi=10.1002/ajp.23139|pmid=32333419|issn=1098-2345|doi-access=}}</ref> There is evidence that squirrel monkeys show sexual dimorphism during the breeding season. In the months leading up to breeding and in the months of breeding, sexually mature adult males have been recorded to increase in size by significant amounts relative to females. These size changes are caused by seasonal fluctuations in androgen hormones synthesized in the hypothalamus, pituitary, adrenal and gonadal axes. The fluctuations include increases in the concentrations of [[testosterone]], [[androstenedione]], and [[dehydroepiandrosterone]] levels in sexually mature males during the breeding season, peaking in January. Following the breeding season, these androgen concentrations drop.<ref name=":3">{{Cite journal|last1=Wiebe|first1=R. Herbert|last2=Williams|first2=Lawrence E.|last3=Abee|first3=Christian R.|last4=Yeoman|first4=Richard R.|last5=Diamond|first5=Edward J.|date=1988|title=Seasonal changes in serum dehydroepiandrosterone, androstenedione, and testosterone levels in the squirrel monkey (Saimiri boliviensis boliviensis)|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/ajp.1350140309|journal=American Journal of Primatology|language=en|volume=14|issue=3|pages=285–291|doi=10.1002/ajp.1350140309|pmid=31973444|s2cid=83608106|issn=1098-2345}}</ref> The evolutionary reasoning for these size changes in sexually mature males is suggested to be both intra-sexual selections among males and also female choice selection, as the larger males are more likely to be preferred by females and partake in more copulations. There is not clear evidence yet as to why females choose larger males, but a leading hypothesis is that the larger males are more likely to have better vigilance for their young.<ref>{{Cite journal|last=Boinski|first=S.|date=1987|title=Mating Patterns in Squirrel Monkeys (Saimiri oerstedi): Implications for Seasonal Sexual Dimorphism|url=https://www.jstor.org/stable/4600051|journal=Behavioral Ecology and Sociobiology|volume=21|issue=1|pages=13–21|doi=10.1007/BF00324430|jstor=4600051|bibcode=1987BEcoS..21...13B |s2cid=23241941|issn=0340-5443}}</ref>

=== Thermoregulation ===
Squirrel monkeys can only sweat through the palms of their hands and the soles of their feet. Sweating in these areas alone does not provide enough cooling for the monkeys to survive in the high temperature environments of South and Central America, requiring them to use other methods to thermoregulate. They will use behavioral tactics such as seeking out shaded areas sheltered from the sun and also make use of postural changes to better dissipate heat from their body. They will also make use of a technique to maximize evaporative cooling known as urine washing. The monkeys will urinate on their hands and rub the urine over the soles of their feet. The urine is then evaporated off the body in a cooling process. Studies have shown this behavior to be maximized during times of high temperature, highlighting its importance as a thermoregulatory behavior.<ref name=":4">{{Cite journal|last=Ruiz|first=J.C|date=2005|title=Relative humidity, ambient temperature, and urine washing behavior in Bolivian squirrel monkeys, Saimiri boliviensis boliviensis.|journal=Primate Report|volume=71|pages=57–61|via=webofscience.com:WOS:ZOOREC}}</ref>

=== Osmoregulation ===
Squirrel monkeys are subject to both high temperatures and high humidity in their natural habitat. The humidity can range from 70% saturation in the 'dry' season up to 90% in the 'wet' season. Squirrel monkeys are able to tolerate up to 75% humidity with small adjustments in behavior and physiology that increase in significance as the humidity goes up. When reaching approximately 95% humidity, the monkeys have more drastic changes in osmoregulation in order to maintain homeostasis. As evaporative water loss decreases at these high levels of saturation, the monkeys will take in less water and create a more concentrated urine in order to maintain proper ion and water levels inside the body.<ref name=":5">{{Cite journal|last=Holmes|first=W.N|date=1986|title=The effect of relative humidity on osmoregulation in the squirrel monkey (Saimiri sciureus)|journal=Primates|volume=27|issue=4|pages=465–470|doi=10.1007/BF02381891|s2cid=12458596}}</ref>

=== Cooperation studies ===
[[Cooperation]] is largely evident in human primates. Squirrel monkeys do not often display cooperation in the wild, while many other nonhuman primates do. Studies have been done to suggest that female squirrel monkeys show disadvantageous inequity aversion as it pertains to food rewards. However, the same could not be said for male squirrel monkeys. More studies need to be done on squirrel monkey behavior to determine why squirrel monkeys rarely show cooperation, and whether disadvantageous inequity aversion is a relevant factor.<ref>{{Citation|last=Thorington|first=Richard W.|title=Observations of Squirrel Monkeys in a Colombian Forest|date=1968|url=http://dx.doi.org/10.1016/b978-1-4832-3310-9.50008-4|work=The Squirrel Monkey|pages=69–85|publisher=Elsevier|doi=10.1016/b978-1-4832-3310-9.50008-4|isbn=978-1-4832-3310-9|access-date=2021-04-01}}</ref>

=== Colour vision ===
{{See also|Evolution of colour vision in primates}}
[[Colour vision]] in squirrel monkeys has been extensively studied as a stand-in for human ailments.<ref name="Neitz">{{Cite journal|last1=Mancuso|first1=Katherine|last2=Hauswirth|first2=William W.|last3=Li|first3=Qiuhong|last4=Connor|first4=Thomas B.|last5=Kuchenbecker|first5=James A.|last6=Mauck|first6=Matthew C.|last7=Neitz|first7=Jay|last8=Neitz|first8=Maureen|author8-link= Maureen Neitz |date=2009-09-16|title=Gene therapy for red–green colour blindness in adult primates|journal=Nature|language=En|volume=461|issue=7265|pages=784–787|doi=10.1038/nature08401|pmid=19759534|issn=0028-0836|pmc=2782927|bibcode=2009Natur.461..784M}}</ref> In humans, two [[gene]]s for colour vision are found on the [[X chromosome]]. Typically, one gene ([[OPN1LW]]) produces a pigment that is most sensitive to the 564&nbsp;nm [[wavelength]], while the other gene ([[OPN1MW]]) produces a pigment most sensitive to 534&nbsp;nm. In squirrel monkeys, there is only one gene on the X chromosome but it exists in three varieties: one is most sensitive to 538&nbsp;nm, one to 551&nbsp;nm, and one to 561&nbsp;nm. Since males have only one X chromosome, they are [[dichromacy|dichromatic]], although with different sensitivities. Females have two X chromosomes, so some of them can have copies of two different [[allele]]s. The three alleles seem to be equally common, leading to one-third of females being dichromatic, while two-thirds are [[trichromatic]].<ref>{{cite journal|pmc=304691 | pmid=3470811 | volume=84 | issue=8 | title=Inheritance of color vision in a New World monkey (Saimiri sciureus) |date=April 1987 | journal=Proc. Natl. Acad. Sci. U.S.A. | pages=2545–9 |vauthors=Jacobs GH, Neitz J | doi=10.1073/pnas.84.8.2545| bibcode=1987PNAS...84.2545J | doi-access=free }}</ref> Recently, [[gene therapy]] has given the human OPN1LW gene to adult male squirrel monkeys, producing behaviour consistent with trichromatic colour vision.<ref name="Neitz"/>

== Gallery ==
<gallery>
File:Nice squirrel monkey.jpg|At the [[Phoenix Zoo]]
File:Monkey toes.jpg|At the Phoenix Zoo
File:Squirrel monkey- fuji.jpg|At the Fuji Safari Park, Japan
File:ZooSquirrelMonkey.jpg|At the Phoenix Zoo
File:Squirrel monkey in Ubon Zoo,Thailand.jpg|At Ubon Zoo, Thailand
</gallery>

== References ==
{{Reflist}}

== External links ==
* [http://pin.primate.wisc.edu/factsheets/entry/squirrel_monkey Primate Info Net ''Saimiri'' Factsheet] {{Webarchive|url=https://web.archive.org/web/20190327090607/http://pin.primate.wisc.edu/factsheets/entry/squirrel_monkey |date=2019-03-27 }}
* [https://web.archive.org/web/20080507163428/http://www.saimiri.org/en/ Reforestation for the endemic squirrel monkey of Panama and Costa Rica]
*View the [http://pre.ensembl.org/Saimiri_boliviensis/Info/Index squirrel monkey genome] in [[Ensembl]]

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