The squirrels are rodents belonging to the family Sciuridae. They are characterized by having slim and agile bodies and by their long and dense tails. Commonly, this species is classified as arboreal, ground or flying squirrels, presenting morphologically notable differences.
Ground squirrels typically have thick, strong forelegs, which they use to burrow into the ground. Their tail is shorter than that of the other groups. As for tree squirrels, their limbs are long and muscular, with sharp claws on their fingers. They have a large, densely packed tail.
In relation to flying squirrels, they have a membrane called patagium that allows them to glide. This structure joins, on each side of the body, the forelimb, at the wrist level, with the heel of the hindlimb.
The Sciuridae family includes, in addition to squirrels, marmots and prairie dogs, species that are distributed worldwide, except in Australia, Antarctica , the southern region of South America and some desert areas.
The squirrel has a long and thin body. The vast majority of species have longer hind limbs than the forelimbs. The front legs are used to hold and handle food.
Its hind legs have five toes, while the front legs have four. All fingers have claws, except the thumb, which has a kind of nail.
The legs have pads, which help cushion the impact of the jumps it performs, which can reach up to six meters. Tree squirrels, unlike the vast majority of tree-climbing mammals, can descend from the head plant.
This can be accomplished by rotating the ankles 180 degrees. Thus, the hind legs point backwards, grasping the bark, from the opposite direction.
Their fur is fine and soft, however, some may have it thick. As for the coloration, it can vary between black, red, white or brown.
In some parts of their body, such as in the eyes, wrist, chin, nose, legs, cheeks and the outer area of the extremities, they have vibrissae. These fulfill the function of being tactile sensory organs.
In relation to the eyes, they are large and located high on the head. This could slightly broaden the visual field of the environment surrounding this rodent.
Squirrels are generally small animals. Due to the wide diversity of species, the dimensions vary considerably. Thus, the African pygmy squirrel ( Myosciurus pumilio ) is the smallest, measuring between 7 and 10 centimeters. Its weight is approximately 10 grams.
One of the largest is the Lao giant flying squirrel, which is 1.08 meters long, and the Alpine marmot, which weighs between 5 and 8 kilograms.
Sciurid teeth follow the pattern of rodents, with large incisors that constantly grow, and eight cheek teeth in each jaw, which are used to grind food.
This rodent has four enamel-coated chisel-shaped incisors with roots that extend into the maxilla. These teeth, as they are used for gnawing, are kept sharp and short.
After the incisors there is a space, known as the diastema, and then there are the cheek teeth, which are deeply rooted. On each side of the maxilla there is a small premolar and three molars, which are tuberculous.
The members of the Sciuridae family present 3 basic morphologies: tree squirrel, ground squirrel and flying squirrel.
This group of rodents do not fly like bats or birds, they glide through the trees. For this, they have several morphological adaptations, among which is the patagium.
The patagium is a membrane that joins the extremities on each side of the body, from the ankle to the wrist. Within the glide, this membrane acts as a parachute. They also have small cartilaginous bones in their wrists, which squirrels hold upward during their gliding.
This specialized cartilage is typical of flying squirrels and is not present in other gliding mammals. This structure, together with the manus, forms a wingtip, which is adjusted by the animal to achieve various angles and to control aerodynamic glide.
Speed and direction vary as limb positions change. Another organ that participates in gliding is the tail, which functions as a flight stabilizer, working as a brake before landing.
They have thin bodies and very bushy tails. The coat is dense and of various shades. They can be brown, black, gray, or reddish, with a light-colored belly.
As they move through the trees, hopping from branch to branch and running up and down the trunk, they use their sharp claws to support themselves and to climb. When they descend from the tree, they do it head first
The tail, during the jump, is used as a rudder, while, if it falls to the ground, it functions as a parachute, cushioning the fall. This structure allows the animal to maintain balance and contributes to maneuvering during the fall.
Also, it keeps the squirrel warm in the winter time and could be an element of communication between them.
Ground squirrels spend much of the day on the ground. Within this group, medium-sized squirrels are generally included, since the largest are marmots and prairie dogs.
Their size is highly variable, as are their habitats. A peculiarity of the members of this group is that they have the ability to stand up on their two hind legs and remain in that position for long periods of time.
One aspect that all squirrels have in common is the structure of their skull and jaw, which is relatively primitive.
In relation to the skull it is short, with a small rostrum and an arched profile. This has a wide and sloping zygomatic plate, which is the point of attachment of the lateral branch of the masseter muscle.
In the infraorbital area it has small holes, through which the muscles are introduced. These openings are not enlarged, as they do in mice and guinea pigs.
Sciurids have long jugulars, large blisters that are not distended, and developed post-orbital processes. The palate is wide and short, ending at the same level as the row of molar teeth.
The vast majority of squirrels do not hibernate. To survive during cold winter days they store food and stay in their nests. However, the thirteen-striped ground squirrel ( Ictidomys tridecemlineatus ) hibernates during the months when the environmental temperature drops markedly.
Thus, the organism of this North American species can decrease its heart rate, metabolism and temperature for almost eight months. During that time, the rodent does not eat food or drink water.
To know the factors associated with this, the specialists carried out a research work, where the blood flow was measured in a group of squirrels that were active, others that were in torpor and those that hibernated.
In general, the high serum concentration causes the animals to feel the need to drink water. In the case of squirrels that were in hibernation, these values are low.
These levels are the product of the elimination of some electrolytes, such as sodium, and of other chemicals such as urea and glucose.
– Role in the ecosystem
Squirrels are essential animals in the regeneration of forests, as they are seed dispersal agents. First of all, their feces contain seeds, which are spread throughout various areas of the ecosystems they inhabit.
In addition, their food storage habits, as a nutritional reserve for the winter period, cause the fruits to germinate in the spring, when the environmental conditions are the most suitable.
Considering the information provided by the first fossil records, squirrels originated in the northern hemisphere, in North America, around 36 million years ago.
The oldest fossil corresponds to Douglassciurus jeffersoni , which was located in Wyoming and dates from the Eocene, between 37.5 and 35 million years ago.
This extinct species was characterized by having dental and skeletal structures similar to modern squirrels. However, it lacked the zygomasetheric system, typical of the Sciuridae family.
As for ground squirrels, the oldest ancestor is Palaeosciurus. It lived between the Lower Oligocene and Lower Miocene periods, approximately 33.7 to 23.8 million years ago.
Morphologically it has great similarities with the current species of squirrels. However, it also has some notable differences, especially when it comes to teething.
In relation to the species of the genus Palaeosciurus, one of the first to appear was P. goti , which had quite short legs. In later forms, such as P. feignouxi, which lived in the Lower Miocene, the bones of the tibia and radius were longer.
The variations in the proportions of the legs, where the first species had them short, could indicate that these animals were probably terrestrial. On the other hand, the lengthening of the limbs that occurred later could be associated with an arboreal life.
Squirrels are distributed on all continents, with the exception of Antarctica, Australia, the southern region of South America, Madagascar, Greenland and desert regions such as the Sahara.
In the 19th century, the species Sciurus carolinensis and Funambulus pennantii were introduced to Australia. Only F. pennantii currently inhabits that region. Squirrels are particularly diverse in Southeast Asia and in African forests.
The species that make up the Sciuridae family are found in a wide variety of habitats, from the semi-arid desert to the tropical forest, avoiding only the high polar regions and dry deserts.
Within the ecosystems where it lives are tropical rainforests, forests, grasslands, arctic tundra, scrublands, semi-arid deserts and in populated areas such as suburban areas and cities.
However, the vast majority of squirrels prefer wooded areas, where shelters are available, and where they have an abundance of foods that make up their diet.
In relation to tree squirrels, they live in the forests of the Americas and Eurasia. The terrestrial ones are associated with open spaces, such as grasslands, in temperate latitudes of Eurasia and North America, as well as in arid areas of Africa.
Within their habitat, the squirrels of this group are located from sea level to in the mountains. As for flying squirrels, southern ones are found throughout the eastern United States, spanning from Maine to Florida and from Minnesota to Texas.
The northern terrestrial species is distributed on the west coast of the United States, in Montana and in Idaho. Flying squirrels live in coniferous and deciduous forests.
Squirrels can build their nests or they could use those left by some birds, such as the woodpecker, or those of other mammals, including other squirrels. There are two types of nests, dens and leaf nests.
Lair in a tree cavity
Shelters in tree holes can be those that have been built by some birds or those that have been created naturally. These nests are preferred by squirrels as they offer protection from rain, snow or wind. In addition, it protects the young from predators.
In relation to the leaf nest, it is generally built on a strong branch of the tree, approximately six meters above the ground. They differ from that of birds because they are larger than these.
For their construction, squirrels use leaves, twigs and moss. Initially the small branches intertwine, thus forming the floor of the nest. Then they make it more stable by adding moss and moist leaves to it.
To create the frame around the base, weave branches together. Finally, they place leaves, herbs and crushed pieces of bark, to condition the space.
Squirrels are animals that are constantly on the move. Because of this, it is common for them to build another nest, close to the main nest. These are used to flee from a predator, to store their food or to make a short rest stop.
Generally, the female nests alone. However, during the low temperature seasons, she could share it with another female, to conserve heat and cope with the winter cold.
State of conservation
Many of the populations of the Sciuridae family have decreased, due, among other factors, to the destruction of their environment. Due to this situation, the IUCN has categorized three species as critically endangered. These are Marmota vancouverensis, Urocitellus brunneus, and Biswamoyopterus biswasi.
Another 13 squirrels are seriously threatened and 16 are vulnerable to disappearing from their natural habitat. On the other hand, there are a total of 23 species that, if they do not solve the problems that afflict them, may quickly be at risk of extinction.
The vast majority, 190 in total, are of Least Concern and 36 of these rodents lack data to categorize.
Threats and actions
There are several factors that play a role in the decline of squirrel communities. Among these is the loss of habitat, motivated by the clearing of forests to build urban centers and agricultural developments. Furthermore, landslides and floods wreak severe damage on the terrain.
Also, some of these areas are exploited by various industries, including the oil and gas industry. In other regions, overgrazing and loss of shrub cover is a major problem, affecting the animal’s permanence in its habitat.
On the other hand, in some localities, members of the Sciuridae family are poached, because their meat is used as food for the inhabitants.
In the wide distribution of squirrels, some local governments have enacted laws protecting the species. Also, there are actions related to the protection of the land and the management of species.
Likewise, there are programs where educational campaigns aimed at the protection of species are planned. In addition, numerous reserve areas have been established, where public and private organizations ensure the protection of the squirrels that live there.
The maturity of the species occurs between 10 and 12 months of age. When the female goes into heat, her body secretes certain odors and, together with the vocalizations that she emits, attract the males.
– Reproduction strategies
The mating chase
By the time the female is about to go into heat, the male squirrels huddle close to her territory, waiting for the moment she becomes receptive. When she is ready to join, the female will face the males, while the two chase each other.
Generally, the dominant male will be the one who first reaches the female and can mate with her. If a female stops to mate, another male could violently attack the copulating male, potentially injuring the female during the lunge.
This strategy is used by some squirrels, such as the Idaho ground squirrel. It consists of the dominant male staying close to the female, rejecting any male who tries to approach him.
It is usually sufficient for the male to show physical dominance, however he may choose to emit vocalizations. These are similar to the so-called anti-predators, which causes the other males to move away or remain immobile, to avoid being detected.
Mating tactics such as copulating plugs and mate protection might suggest that the last male to mate with the female has a reproductive advantage. However, female tree squirrels could remove the copulating plug, thus allowing copulation with other males.
– Mating and gestation
Both males and females can copulate with multiple partners. Once the male mates with the female, she often releases a non-seminal, wax-like substance. This plug constitutes a barrier that prevents other males from mating with that female.
This could be the reason why the vast majority of litters are sired by the same male, despite the fact that the female can accept other males.
As for the length of the gestation period, it varies according to the species. Thus, in larger squirrels and flying squirrels, this stage usually lasts between 38 and 46 days. In the smaller species, the young will be born less than 38 days after being spawned.
African and tropical species gestate up to approximately 65 days and terrestrial species last from 29 to 31 days.
The size of the litter varies between 1 and 5 offspring, although they could be up to 9, depending on the species. The delivery occurs in the nest and the newborns have their eyes closed and lack fur.
Squirrels are omnivorous, although their diet is mainly based on a wide range of plant species. Thus, within its diet are fungi, nuts, seeds, fruits, cones of conifers, berries, leaves, shoots and branches
Also, opportunistically, they could eat animals. According to experts, in a population, at least 10% of sciurids eat some type of insect, bird, reptile and other smaller rodents.
Among the species they consume are snakes, insect and bird eggs, small birds, worms, mice and lizards.
On average, squirrels eat an average of 454 grams of food weekly. However, the amount of each type of food is associated with its accessibility and availability. For this reason, the composition of their diet varies between regions, seasons and the time of year.
During the spring, in temperate regions, the diet has some modifications, compared to what these rodents consume on a regular basis. At that time of year, the walnuts that were buried, to be consumed in winter, begin to germinate and are not available for ingestion.
Also, many of the other nutrient sources are not available, this leads squirrels to change their diet to heavily consume tree shoots.
On the other hand, the organism of these rodents is not specialized to efficiently digest cellulose. This is why they tend to consume species rich in carbohydrates, proteins and fats.
In this sense, the shoots, lichens, flowers and the bark of plants are generally low in energy content, per unit of weight. Because of this, they constitute a minor portion of the diet.
During cold months, the availability of food decreases. This causes the squirrel to store food, in order to meet its energy requirements during the winter.
They can be stored in holes they have dug in the ground, in hollow trees, and in abandoned burrows. Also, in urban areas, they can hide them in flowerpots, abandoned cars and even in vehicle exhaust pipes.
Squirrels are very vocal. These rodents can scream, purr, and bark. Plus, they have separate calls for almost any situation. Thus, the young call out to their mothers and the adults vocalize while exhibiting aggressive behaviors.
Also, males make sounds at the time of mating, with the intention of attracting females. To warn conspecifics of danger, some species employ very particular alarm calls.
These could even transmit information that allows distinguishing details of the predator, such as the distance in which it is.
Also, members of the Sciuridae family can communicate through body language. For this they use various postures of their tail or vigorously move their feet, kicking the ground hard.
Ground squirrels tend to be the most social, as they form groups, where they frequently play and groom each other. As for tree squirrels, they are generally solitary. However, they can form groups at the time of nesting.
Flying squirrels are the only ones with nocturnal habits and can form groups during the winter, to keep warm in the nest.
Wikipedia (2019). Squirrel. Recovered from en.wikipedia.org.
Alina Bradford (2014). Squirrels: Diet, Habits & Other Facts. Recovered from livescience.com.
Eva Frederick (2019). Here’s how hibernating squirrels live for months without water. Recovered from sciencemag.org.
Washington Department of Fish & Wildlife (2019). Living with wildlife: Tree squirrels. Recovered from wdfw.wa.gov.
Brown, E., A. Peri and N. Santarosa (2014). Animal Diversity Web. Recovered from animaldiversity.org.
Virginia Hayssen (2008). Reproductive Effort in Squirrels: Ecological, Phylogenetic, Allometric, and Latitudinal Patterns. Recovered from academic.oup.com.
April Sanders (2017). How Does a Squirrel Play ?. Recovered from sciencing.com.
Ari Reid (2018). How Do Squirrels Mate ?. Recovered from sciencing.com.
Ruth Nix (2018). Squirrel Mating and Gestation. Recovered from sciencing.com.
Roach, N. (2017). Marmota vancouverensis. The IUCN Red List of Threatened Species 2017. Retrieved from iucnredlist.org.
Yensen, E. 2000. Urocitellus brunneus. The IUCN Red List of Threatened Species 2000. Retrieved from iucnredlist.org.
Molur, S. 2016. Biswamoyopterus biswasi (errata version published in 2017). The IUCN Red List of Threatened Species 2016. Retrieved from iucnredlist.org.