Squirrel Monkey

Squirrel monkeys (the genus Saimiri) are the most abundant primate of riverside forests in the Amazon River basin and Guianas, distinguished by the circle of black hairless skin around the mouth set including nose against an expressive white face. Their short and soft fur is gray to olive green, having whitish underparts.

Squirrel monkeys grow from 25–40 cm (10–16 inches) long, not including the heavy nonprehensile tail that is at least as long as the body. The arms, hands, and feet are yellow to orange. The common squirrel monkeys (called Saimiri sciureus) have either olive or grayish crowns and they are found only in South America, whereas the endangered Central American squirrel monkeys (called S. Oerstedii) hold reddish backs and black crowns. Unlike the bare-eared squirrel monkey (S. Ustus) of central Brazil, both the common and Central American species have hair on their ears.

About Squirrel Monkeys

Squirrel monkeys form larger groups compared to any other New World monkey, where one group of 300 was counted in the pristine Amazonian rainforest. Also, they are among the most vocal of primates, communicating with a minimum of 26 different calls, including purrs, barks, peeps, screams and squawks. These monkeys, who are exceptional runners and jumpers, prefer to live in the trees, however they may occasionally descend to the ground.

They are active in the daytime and move the single file after a leader along frequently used pathways in the treetops. Whereas, at night, they sleep huddled together on the branches, tails wrapped around their bodies. Sometimes, the capuchin monkeys and squirrel monkeys forage together, eating leaves, fruits, tree gum, buds, insects, spiders, and small vertebrates. Females give birth to a single baby squirrel monkey after a gestation of around 6 months. The baby squirrel monkey rides the back jockey-style of the mother for the first few weeks of life and remains dependent on the mother for a complete year.

[Image will be uploaded soon]

Gentle, attractive, clean and affectionate squirrel monkeys were popular pets in the United States until the capture and importation of the wild primates as pets was outlawed in 1975. They seem to fare best when kept with the others, which are their kind and may live around 20 years under some favorable conditions. Squirrel monkeys are the primates of the Cebidae family.

Evolution

The crown group of the extant squirrel monkeys seems to diverged about 1.5 million years ago. Diversification of the squirrel monkey species seems to have taken place during the Pleistocene Epoch, likely because of the climatic changes associated with interglacial periods in South America at the time.

The squirrel monkey's origin in Central America is not clear, but a possible hypothesis is human transport. More genetic work needs to be done on this subject to reveal a true answer. S. Boliviensis appears as the first diverging species in the group. S. S. Sciureus and S. Oerstedii are the sister species. S. S. Macrodon is the sister species to either the S. S. Sciureus clade or S. Oerstedii.

Description

The fur of a squirrel monkey is close and short, colored black at the shoulders and yellowish orange on its extremities and back. The upper parts of the heads are hairy. Many Germanic languages (for example, Swedish dödskalleapor, German Totenkopfaffen, Dutch doodshoofdaapjes) and Slovenian call them "death's head monkeys'' because of their black-and-white visage (smrtoglavka).

Squirrel monkeys normally grow from 25 - 35 cm (9.8 - 13.8 in) long, plus a 35 - 42 centimetres (14 - 17 in) tail. Squirrel monkey males weigh between 750 and 1,100 grams (26 to 39 oz). Females weigh about 500 to 750 grams (18 to 26 oz). Both the male and female squirrel monkeys are equipped with hairy and long tails, pointed claws and flat nails.

Female squirrel monkeys have pseudo-penises that they utilize to demonstrate dominance over lesser monkeys, just like male squirrel monkeys do.

Reproduction

Squirrel monkey mating is subjected to seasonal influences. They reach sexual maturity at ages 3.5 - 4 years for males and 2 - 2.5 years for females. Females give birth to the young ones during the rainy season, after a 150 to 170-day gestation. Only the mothers care for the younger ones. The S. Oerstedti is fully weaned at 4 months, but the S. Boliviensis is not entirely weaned until 18 months.

Squirrel monkeys generally live up to 15 years old in the wild and about 20 years in captivity. In females, menopause probably takes place in the mid-teens. Studies have shown that the Saimiri collins time the weaning of their young with a period of time when there will be maximum availability of fruit in the environment. This reduces the energetic struggles, which newly weaned juveniles will face when transitioning from the milk diet where they are completely dependent on their mother for food to a more diverse diet at which they have to forage for food. There is also evidence that squirrel monkeys exhibit sexual dimorphism during the breeding season.

[Image will be uploaded soon]

In the months leading up to the breeding and in the breeding months, sexually mature adult males have been recorded to increase in size by significant amounts relative to the female ones. These changes in size are caused by the seasonal fluctuations in androgen hormones synthesized in the hypothalamus, adrenal, gonadal and pituitary axes. The fluctuations include increases in the concentrations of androstenedione, testosterone and dehydroepiandrosterone levels in sexually mature males at the time of the breeding season, peaking in January.

These androgen concentrations drop by following the breeding season. The evolutionary reasoning for these size changes in sexually mature males can be suggested to be both intra-sexual selections among males and females also choice selection, as the larger males are much likely to be preferred by females and partake in copulations. There is no clear piece of evidence yet as to why females choose the larger males, but a leading hypothesis is that the larger males are very likely to have better vigilance for their young ones.

Physiology

Thermoregulation

Squirrel monkeys may only sweat through the soles of their feet and the palms of their hands. Sweating in these areas alone does not provide the required cooling for the monkeys to survive in the environments of high temperatures of Central and South America, requiring them to use the other methods to thermoregulate.

They will use behavioral tactics such as seeking out shaded areas sheltered from the sun and make use of the postural changes to better dissipate heat from their respective bodies. Also, they will make use of a technique to maximize evaporative cooling called urine washing. The monkeys will urinate on their hands and rub that urine over the soles of their feet. Then, the urine is evaporated off the body in a cooling process. Many studies have revealed this behavior to be maximized during times of high temperature, highlighting its importance as a behavior of thermoregulatory.

Osmoregulation

Squirrel monkeys are subjected to both high humidity and high temperatures in their natural habitat. The humidity can range from 70% saturation in the 'dry' season to around 90% in the 'wet' season. These monkeys are able to tolerate up to 75% humidity with a few small adjustments in behavior and physiology, which increase in significance as the humidity goes up. When reaching nearly 95% humidity, the monkeys have more drastic changes in osmoregulation to maintain homeostasis. As evaporative water loss decreases at these high saturation levels, the squirrel monkeys will take in less water and create more concentrated urine to maintain the proper water and ion levels inside the body.

Cooperation Studies

Cooperation is largely evident in the human primates. Often, squirrel monkeys do not exhibit cooperation in the wild, while several other nonhuman primates do. Few studies have been performed to suggest that female squirrel monkeys exhibit disadvantageous inequity aversion as it pertains to food rewards.

However, the same could not be said for the male squirrel monkeys. And, more studies need to be done on the behavior of squirrel monkeys to yield more information regarding why squirrel monkeys rarely exhibit cooperation and if disadvantageous inequity aversion is a relevant factor.

Classification of Squirrel Monkeys

There exist two major groups of squirrel monkeys recognized. They are categorized based on the shape of the white coloration above the eyes. In total, there exist 5 recognized species. Squirrel monkeys contain close and short fur colored black at the shoulders, either orange or yellow fur along the back and extremities, and white on the face.

Squirrel monkeys contain determined breeding seasons that involve large fluctuations in the hormones, and there is also evidence of sexual dimorphism in males.

Squirrel monkeys can sweat only through the palms of their feet and hands and thus must make use of the other thermoregulation techniques such as urine washing and behavioral changes. These monkeys live in the habitats of high humidity and high temperatures, making it important for them to maintain the proper osmoregulation if any conditions pass certain thresh-holds. Also, color vision studies have been performed on squirrel monkeys for the purpose of better understanding vision ailments in humans.

Commonly, the common squirrel monkeys are captured for the pet trade and for medical research, but it is not threatened. Species of two squirrel monkeys are threatened: the Central American squirrel monkey, the black squirrel monkey, are listed as vulnerable by the IUCN.