Editing Squirrel monkey (section)

== 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"/>