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Sciurus oculatusPeters's squirrel

By Whitney Mowbray

Geographic Range

Sciurus oculatus is a squirrel species endemic to Mexico. It ranges within the Trans-Mexican neovolcanic belt and the Mexican Plateau from Morelos, Distrito Federal, Mexico, Puebla and Veracruz northward into San Luis Potosi. In the summer months, Peter's squirrels are common in Alvarez, inhabiting the low oak forest of the mountainsides. As temperatures cool, they move to nearby valleys by late October. There are three recognized subspecies within this range: S. o. oculatus, S. o. shawi, and S. o. tolucae. (Best, 1995)

Habitat

Sciurus oculatus prefer habitats of oak (Quercus), pine (Pinus), and fir (Abies, Pseudotsuga) forest. Populations tend to occupy different tree species depending on their location and time of year. In the summer months, some populations live in low oak trees on mountainsides in Alvarez. By late October, however, this group migrates to Veracruz and usually occupies pine forests at higher elevations or on the timbered slopes of Cofre de Perote and Mount Orizaba. (Best, 1995; Burt, 1952; Kays and Wilson, 2002)

  • Range elevation
    1500 to 3600 m
    4921.26 to 11811.02 ft
  • Average elevation
    1800 m
    5905.51 ft

Physical Description

Peter's squirrels are large tree squirrels with an average weight of 638 grams. They are mostly gray with white ears and a white underside. Peter’s squirrels have black tails and a 25 to 50 mm wide black stripe that extends from the back of the head to the tail. They have four pairs of mammae (one pectoral, one inguinal and two abdominal). Subspecies S. o. oculatus, S. o. tolucae, and S. o. shawi vary in total, tail, and hind foot lengths. The average total lengths for the subspecies are: S. o. oculatus 543mm, S. o. tolucae 531mm, and S. o. shawi 508mm. Average tail length of S. o. oculatus is 269 mm, S. o. tolucae is 263 mm, and S. o. shawi is 256 mm. Average hind foot length for S. o. oculatus is 73 mm, S. o. tolucae is 69 mm, and S. o. shawi is 68 mm. (Best, 1995; Davis, 1944)

  • Sexual Dimorphism
  • sexes alike
  • male larger
  • Average mass
    638 g
    22.48 oz
  • Range length
    530 to 560 mm
    20.87 to 22.05 in
  • Average length
    543 mm
    21.38 in

Reproduction

No specific studies were found which examined the mating systems of Peter's squirrels. However, detailed information is available for a closely related species, eastern gray squirrels (Sciurus carolinensis). In this species, male and female squirrels communicate reproductive information through scent. Males leave their normal home range to pursue females when the females are near estrus. Males then follow females for about 5 days before she goes into estrus and attract her attention through a unique sneeze-like mating call. On the day of a female's estrus many males aggressively pursue her in a "mating chase." She then chooses a male with which to mate. After copulation, the male's semen forms a plug, in an attempt to prevent further mating. However, in as little as 20 seconds, females may remove the plug by eating or discarding it. Therefore, eastern gray squirrel females remain in estrus for about a day and may breed with several males. It is likely that the mating system of Sciurus oculatus is similar. (Koprowski, 1998; Osborn, 1999)

Specific information on the reproductive behavior of Peter's squirrels is not available. However, their close relative, eastern gray squirrels (Sciurus carolinensis) are seasonal breeders; their mating activities are triggered by changes in day length and their productivity is linked strongly to food supply. A female squirrel usually produces her first litter at about a year old, unless food is scarce. In that case, they may wait until their second year to breed. (Osborn, 1999)

  • Breeding interval
    Yearly, probably starting at age 2.
  • Breeding season
    Female Sciurus oculatus have had enlarged mammae in July and August, indicating litters earlier in the season.
  • Average age at sexual or reproductive maturity (female)
    2 years

Information on parental investment in Peter's squirrels has not been documented. However, eastern grey squirrels (Sciurus carolinensis) depend on their mother's milk for 7 to 10 weeks and are born in strongly-built leaf nest held together by a woven framework of twigs or in tree cavities. (Elbroch, 2003; Koprowski, 1998; Osborn, 1999)

  • Parental Investment
  • altricial
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • male
      • female
  • pre-independence
    • protecting
      • male
      • female

Lifespan/Longevity

No studies have determined the average lifespan for Sciurus oculatus.

Behavior

Little information is available concerning the behavior of this squirrel. Sciurus oculatus is diurnal/crepuscular, most active around sunrise and before sunset. They are expert travelers in trees and can cross gaps greater than 2 m wide.

If this squirrel is similar in behavior to other tree squirrels, social hierarchy may be based on age and body size. Males usually dominate, but females will defend their nests against other females. (Best, 1995; Osborn, 1999; Thorington and Ferrell, 2006)

Home Range

No information was available for home range size of this species. However, in other tree squirrels ranges are larger during mating seasons and usually overlap with other individuals. Female home range sizes are usually smaller than those of males. (Osborn, 1999)

Communication and Perception

No information was found directly relating to Sciurus oculatus. However, tree squirrels rely heavily on vision to perceive their environment. Their retinas contain layers of both rod and cone cells. They can see yellow, red, and green, and can also distinguish blue from green. However, they lack a tapetum which allows some animals to see better in low light. They do have yellow tinted lenses to protect them from damaging ultraviolet rays in the daylight hours. Tree squirrels also process visual information quite rapidly; one study showed they process visual information at twice the speed of an average human. Like other mammals, they also use chemosensation and touch as important modes of perception. Squirrels have whiskers on their faces and wrists to help in navigating their complex environment. (Osborn, 1999; Thorington and Ferrell, 2006)

Food Habits

No specific studies were found that examined food habits of S. oculatus, however Best (1995) noted that some populations of S. oculatus consumed acorns and wild figs, while others rely primarily on pine nuts. Tree squirrels in general prefer foods that can be eaten quickly, are digestible, and provide high amounts of energy. Squirrels often eat animal foods as well as seeds and fruit. They drink surface water when it is available, however, in its absence they can extract enough moisture from their food with the exception of pregnant or nursing females. (Best, 1995; Osborn, 1999; Smith, 1998)

  • Plant Foods
  • seeds, grains, and nuts
  • fruit

Predation

No other information was found directly relating to S. oculatus, but other tree squirrels (Sciurus) are preyed on by a number of predatory bird and mammal species. Large raptors and arboreal carnivores, such as Bassariscus species, are also likely predators of S. oculatus. (Best, 1995; Kays and Wilson, 2002; Sanchez-Cordero, et al., 2005)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Tree squirrels (Sciurus) in general are host to a wide range of parasites such as botflies, fleas, ticks, mites, lice, roundworms, and tapeworms and are also vulnerable to rabies, pox, fibroma, monkeypox, and parapox viruses. The genus Enderleinellus is a parasite that is found on Sciurus oculatus and occurs exclusively on hosts of the family Sciuridae. Squirrels also disperse the seeds of plants through their seed caching behavior and via attachment to their fur. (Best, 1995; Kim, 1966; Osborn, 1999; Thorington and Ferrell, 2006)

  • Ecosystem Impact
  • disperses seeds
Commensal/Parasitic Species

Economic Importance for Humans: Positive

No information was found directly related to the hunting of Sciurus oculatus, but is assumed that they are hunted for food and disperse seeds like other squirrel species. (Osborn, 1999; Thorington and Ferrell, 2006)

  • Positive Impacts
  • food
  • body parts are source of valuable material

Economic Importance for Humans: Negative

There are no known adverse effects of S. oculatus on humans. (Dagnall, et al., 1998; Elbroch, 2003; Thorington and Ferrell, 2006)

Conservation Status

Sciurus oculatus has suffered severe habitat loss through logging, burning, and clearing of forest for agriculture in Mexico. As of 1991, it was considered a "fragile" species, though, as of 1996, IUCN listed it as "Lowest Risk/Least Concern." The IUCN web page recognizes that this listing is out of date. (Sanchez-Cordero, et al., 2005)

Other Comments

This squirrel's name is derived from the Latin Sciurus meaning "squirrel" and oculus referring to the eye. It also goes by names black-backed, Toluca, and spectacled squirrel. (Best, 1995; Osborn, 1999)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Whitney Mowbray (author), Radford University, Karen Powers (editor, instructor), Radford University.

Glossary

Nearctic

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.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

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.

chemical

uses smells or other chemicals to communicate

crepuscular

active at dawn and dusk

cryptic

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.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

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.

food

A substance that provides both nutrients and energy to a living thing.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

frugivore

an animal that mainly eats fruit

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

iteroparous

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).

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

saltatorial

specialized for leaping or bounding locomotion; jumps or hops.

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

temperate

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).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Best, T. 1995. Sciurus oculatus. Mammalian Species, 498: 1-3.

Burt, W. 1952. A Field Guide to the Mammals. Massachusetts: The Riverside Press.

Dagnall, J., J. Gurnell, H. Pepper. 1998. Bark-Stripping Damage by Gray Squirrels in State Forest of the United Kingdom: A Review. Ecology and Evolutionary Biology of Tree Squirrels, 6: 249-261.

Davis, W. 1944. Notes on Mexican Mammals. Journal of Mammalogy, 25: 385.

Elbroch, M. 2003. Mammal Tracks & Sign: A Guide to North American Species. Pennsylvania: Stockpole Books.

Kays, R., D. Wilson. 2002. Mammals of North America. Princeton: Princeton University Press.

Kim, K. 1966. The Species of Enderleinellus (Anoplura, Hoplopleuridae) Parasitic on the Sciurini and Tamiasciurini. The Journal of Parasitology, 52: 988-1024.

Koprowski, J. 1998. Conflict Between the Sexes: A Review of Social and Mating Systems in Tree Squirrels. Ecology and Evolutionary Biology of Tree Squirrels, 6: 33-41.

Osborn, D. 1999. Squirrel Dog Basics: A Guide to Hunting Squirrels with Dogs. Treetop Publications.

Sanchez-Cordero, V., P. Illoldi-Rangel, M. Linaje, S. Sarkar, A. Peterson. 2005. Deforestation and extant distributions of Mexican endemic mammals. Biological Conservation, 126: 465-473.

Smith, C. 1998. The Evolution of Reproduction in Trees: Its Effect on Squirrel Ecology and Behavior. Ecology and Evolutionary Biology of Tree Squirrels, 6: 203-209.

Thorington, R., K. Ferrell. 2006. Squirrels: The Animal Answer Guide. Baltimore: The John Hopkins University Press.

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