Members of the genus Sciurus comprise the largest genus in the family Sciuridae with 28 species currently recognized throughout the old and new world. This genus was originally described by Linnaeus in 1758 and are colloquially referred to as the tree squirrels. These species vary across elevations and temperature but are all arboreal, spending a large portion of their lives in trees, making use of large tails to aid in balance and thermoregulation. The squirrels descend from trees mainly to scavenge. They are diurnal and typically solitary where territories between members are not guarded and are prone to overlapping (Armstrong, Fitzgerald, and Meaney, 2011; Diggins, 2021; Magris, 2005; Thorington Jr. et al., 2012). (Armstrong, et al., 2011; Diggins, 2021; Magris, 2005; Thorington Jr., et al., 2012)
Of the 28 species that make up the genus Sciurus 25 are found in the new world, and the remaining 3 are found in the old world. Sciurus can be found all throughout the Continent of North America, at a variety of elevations, aside from the most northern portions of the continent. In South America, these species reside in the central and northern areas, absent from the southern areas. The three species that exist in the old world reside in the northern portions of the Middle East, northern Eurasia, and Japan. Sciurus are not found in the Ethiopian, Oriental, and Australian regions, they are also absent from Antarctica (Armstrong, Fitzgerald, and Meaney, 2011; Moncrief, Lack, and Van Den Bussche, 20120; Oshida and Masuda, 2000; Thorington Jr. et al., 2012). (Armstrong, et al., 2011; Moncrief, et al., 2010; Oshida and Masuda, 2000; Thorington Jr., et al., 2012)
Sciurus live in a variety of habitats such as temperate forests, tropical rainforests, montane, and swamp environments, provided that trees are present for them to seek refuge from predation and getting food from their seeds. Members of the genus have shown a high tolerance for humans, living in urban settings and agricultural settings such as parks and gardens with high human traffic. They are found across elevations, some species are found exclusive to montane environments, others are found over a wide range from coastal to montane (Koprowski and Corse, 2001; Oshida and Masuda, 2000; Thorington Jr. et al., 2012). (Koprowski and Corse, 2001; Oshida and Masuda, 2000; Thorington Jr., et al., 2012)
Family Sciuridae that the genus Sciurus is part of likely first appeared in the late Eocene period in North America roughly 36 million years ago. This family expanded into Asia and South America roughly 3.4 million years ago and diverged into five different subfamilies across every continent except Antarctica and Australia (though they were introduced to Australia by humans). The subfamilies Xerinae and Sciurinae further diversified into five tribes, Sciurus falls under the tribe Sciurini, the Holarctic tree squirrels which is sister taxa to tribe Pteromyini, the flying squirrels (Mendes, Koprowski and Galleti, 2019; Thorington Jr. et al., 2012).
The family Sciuridae is divided into three main types of squirrels; ground squirrels, tree squirrels, and flying squirrels. There are currently 58 genera comprised of 285 species in the family Sciuridae. Sciurus is the most speciose genus with 28 recognized species. The genus Sciurus was originally described by Karl Linnaeus in 1758 (Mendes, Koprowski and Galleti, 2019; Thorington Jr. et al., 2012). (Mendes, et al., 2019; Thorington Jr., et al., 2012)
Tree squirrels in the genus Sciurus are small rodents that have long and bushy tails with long legs that have physiological adaptations in their ankles that allow 180 degree rotation. These traits work in tandem to aid their arboreal lifestyles. There is little sexual dimorphism across Sciurus, most averages in mass and length are close. Females range in size across this genus in the smallest species averaging 200.0g (Sciurus aestuans) to the largest species averaging 764.3g (Sciurus niger). The males range from 176.0g (Sciurus lis) to 767.5g (Sciurus niger). Body length, excluding the tail, in females ranges from 160.1mm (Sciurus pucheranii) to 289.8mm (Sciurus griseus) and 165.1mm (Sciurus pucheranii) to 308.3mm in males. Tail length also differs among the species, frequently longer than their body length with the longest average in females at 313.3mm (Sciurus flammifer) and shortest at 148.8mm (Sciurus anomalus). In males the longest average tail length is in Sciurus nayaritensis at 280.0mm, the shortest is in Sciurus lis species averaging 152.3mm (Hayssen, 2008; Koprowski, 1998; Thorington Jr. et al., 2012). (Hayssen, 2008; Koprowski, 1998; Thorington Jr., et al., 2012)
Female tree squirrels have a preference for mating in low risk areas that are secluded and hidden from potential predators and other threats. They rarely pursue the males for this reason, maintaining their own safety. While generally accepted to be polygynous, there are records of females trying to maximize their own success, limiting the success of individual males by mating with multiple males and removing copulatory plugs created by semen of the males (Cudworth and Koprowski, 2013; Koprowski, 1998; Wood, Koprowski, and Lurz, 2007). (Cudworth and Koprowski, 2013; Koprowski, 1998; Wood, et al., 2007)
Females in the genus Sciurus undergo estrus for a single day, leading to intense male competition for access to reproductive rights. The mating season can occur anywhere from January to July. In preparation for mating season, the male tree squirrels’ testes descend and can remain descended for many months. In order to avoid inbreeding the male squirrels leave their birthplace to seek mating opportunities elsewhere while females may stay in the same locale. The mating competition among the males often leads to the dominant males trying to control access to dens and food for the females. The males also increase their range and move between locations more frequently around this time to increase likelihood of mating. Due to the high energy costs associated with reproduction (feeding during pregnancy and lactation) female Sciurus species undergo estrus during times when food availability is high, meaning reproduction can occur earlier if food becomes available earlier. Mating tends to occur in trees (Cudworth and Koprowski, 2013; Hayssen, 2008; Koprowski, 1998; Wood, Koprowski, and Lurz, 2007). (Cudworth and Koprowski, 2013; Hayssen, 2008; Koprowski, 1998; Wood, et al., 2007)
Sciurus species have a polygynous mating structure where parental care is mostly handled by the females, while the males mate with multiple different females. The genus Sciurus reproduces an average of two times per year with an average litter size of two to four kits (Cudworth and Koprowski, 2013; Koprowski, 1998; Wood, Koprowski, and Lurz, 2007). (Koprowski, 1998; Wood, et al., 2007)
Sciurus species have an average lifespan of 5-10 years, although in captivity this can double to nearly 20 years of age (Thorington Jr. et al., 2012). (Thorington Jr., et al., 2012)
Tree squirrel species in the Sciurus genus are diurnal and spend a large portion of their active day foraging and dispersing seeds. Due to their arboreal nature and physiological adaptations for climbing they spend the majority of their time in the trees and are excellent at making simple calculations regarding jumping between trees based on branch sway and weight. These species will move seeds to micro caches for later retrieval. Some Sciurus tree squirrel species exhibit dominance hierarchies, tend to have overlapping undefended territories, and will den with other members of the same species. (Diggins, 2021; Eason, Nason and Alexander Jr., 2019; Steele and Yi, 2020). (Diggins, 2021; Eason, et al., 2019; Steele and Yi, 2020)
Vocal communication in these tree squirrels serves a multitude of functions in reproduction, predator defense, recognition, and general group unity. Tree squirrels in general are lacking in research regarding vocalization, but several species of Sciurus are known to produce alarm calls, neonatal calls, and agonistic deterrent calls. Different sequences of tail twitches have also been documented as signals and there is some range of specificity in the types of tail twitches given to signal types of predators and whether they are approaching aerially versus terrestrially. Some Sciurus species have been seen to perceive these vocalizations and tail twitches in tandem to make decisions about retreat and they tend to respond quicker to aerial threats than terrestrial and, when foraging, tend to retreat to and climb trees on the side opposite of the threat. There is also evidence to suggest some of these tree squirrels may not be able to differentiate between types of alarm calls (Diggins, 2021; McRae and Green, 2014; McRae and Green, 2017). (Diggins, 2021; McRae and Green, 2014; McRae and Green, 2017)
Sciurus species are granivores that rely on mast-producing trees for their diet that consists mainly of acorns and other tree nuts. Due to this reliance on trees for food their body mass is correlated with rainfall and overall tree health. In preparation for the decrease in food abundance over winter months these tree squirrels will cache these nuts underground. While the majority of the tree squirrels diet consists of nuts, they are known to eat some fungi, berries, bark, and some insects. This is driven in part by food availability since consumption of berries increases in the winter months when access to seeds and nuts is limited (Cudworth and Koprowski, 2013; Koprowski and Corse, 2001). (Cudworth and Koprowski, 2013; Koprowski and Corse, 2001)
Sciurus tree squirrels face risk of predation from a variety of avian and mammalian terrestrial predators. These predators include multiple species of hawk, owl, and marten. Tree squirrels are more exposed to predation while scavenging on the ground and show a stronger response to potential predation during these periods. Young tree squirrels under one year of age are more vulnerable to predation than adults and rely on the protection of trees (Randler, 2006). (Randler, 2006)
Tree squirrels play an important role in seed dispersal and predation, making their role as mutualists versus antagonists difficult to define. Oak species of trees show evidence of mutualism with Sciurus species through adaptations in seeds to encourage their dispersal, some oak are preferentially cached by these tree squirrels in response to the evolutionary relationship they evolved. These tree squirrel species move seeds into micro caches for later retrieval, burying them. This storage of the seeds also serves to allow germination of the seeds and growth of new trees. Their role as a food source for many species of animals also gives them an important role in maintaining healthy populations of birds and meat eating mammals (Randler, 2006; Steele, 2008; Steele and Yi, 2020). (Randler, 2006; Steele, 2008; Steele and Yi, 2020)
These tree squirrels benefit humans as use for food and their pelts, as well as the pet trade. Sciurus are also desirable inhabitants of parks to humans due to their human tolerance and diurnal activity (Mendes, Koprowski and Galleti, 2019). (Mendes, et al., 2019)
Sciurus species can negatively impact humans because they are known to transfer some zoonotic diseases that pose a health risk to humans. Tree squirrels are also known to damage human agriculture and infrastructure (Mendes, Koprowski and Galleti, 2019; Stenger et al., 2015). (Mendes, et al., 2019; )
Sciurus are relatively resistant to habitat loss due to their high tolerance for human proximity (Mendes, Koprowski and Galleti, 2019). There is also a large data deficiency in population trend for many of these tree squirrels, three species are decreasing in population, eight are stable, one is increasing, and the other 16 are data deficient ("IUCN Red List of Threatened Species", 2022; Mendes, Koprowski, and Galleti, 2019). ("IUCN Red List of Threatened Species", 2022; Mendes, et al., 2019)
Brennen Troyer (author), Colorado State University, Audrey Bowman (editor), Colorado State University.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
uses sound to communicate
Referring to an animal that lives in trees; tree-climbing.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
union of egg and spermatozoan
A substance that provides both nutrients and energy to a living thing.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
an animal that mainly eats seeds
An animal that eats mainly plants or parts of plants.
a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.
Found in northern North America and northern Europe or Asia.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
the area in which the animal is naturally found, the region in which it is endemic.
the business of buying and selling animals for people to keep in their homes as pets.
having more than one female as a mate at one time
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
places a food item in a special place to be eaten later. Also called "hoarding"
living in residential areas on the outskirts of large cities or towns.
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
living in cities and large towns, landscapes dominated by human structures and activity.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
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