The natural history of a wildlife species provides critical information for wildlife rehabilitators providing care for young and adult animals. Yet many wildlife rehabilitation training programs focus on other rehabilitation fundamentals, such as first aid, generic diets, caging, and zoonoses and only give limited attention to natural history. Natural history information is used to make basic rehabilitation decisions, such as whether rescue is needed or release is possible, and the selection of medical treatment, cage design, diet, and release habitat. This paper briefly describes differences between species of squirrels and how that affects rehabilitation decisions of animals commonly admitted for rehabilitation.
Natural history is the study of natural objects and organisms and their origin, evolution, interrelationships, and description. It includes species distribution, description and physiology, annual cycle, food habits, reproduction, growth and development, behavior, ecology (including habitat, home range), social hierarchy, mortality rate and factors, and more.
Squirrel Natural History
Squirrels are members of the order Rodentia and comprise 50 genera and 273 species worldwide. North America has 8 genera and 66 species of squirrels. There are three main categories of squirrels are tree (Sciurus and Tamiasciurus), ground (Spermophilus and Ammospermphilus), and flying squirrels. Some squirrel species, such as the eastern gray squirrel (Scurius carolinensis) are common and conspicuous, while others are rare and unfamiliar (e.g. the Mount Graham red squirrel, Tamiasciurus hudsonicus grahamensis). Species such as the fox squirrel (Sciurus niger) is adaptable to non-native environments while other species are not (e.g. Abert’s squirrel, Sciurus aberti).
Size and color vary widely between North American squirrels. Most squirrel species have soft, short, thick body pelage that molts twice a year. The spring molt moves from head to tail while the autumn molt proceeds in the opposite direction. Squirrels have excellent senses of smell, vision and hearing. Their powerful jaws and teeth include four very sharp incisors that grow continually. The teeth of tree squirrels turn an orange color at approximately 4-5 months of age.
Squirrels have double-jointed feet with four toes on the front feet and five on the back. The claws are strong and sharp. Tail lengths vary with species, and the tails can easily break or be pulled off thus facilitating escape from predators. The teats of lactating females are 0.06 to 0.33 inches (0.15 to .085 cm) long, and become longer and more swollen as lactation progresses.
Habitat and home range
Some squirrels require very specific habitat while other species use a wider range of habitats. Some squirrels require very specific habitat such as Ponderosa pines or dry, rocky hillsides; other species use a more wide range of habitats, such as grasslands, or general hardwoods. Different species have different water needs and sources.
The area over which a squirrel moves during its daily activity (home range) varies with species, food availability, density, and gender. Males tend to have larger home ranges than females. Tree squirrels tend to be independent and very territorial. Flying squirrels are more social and several may share a nest during winter. Ground squirrels are territorial and live in loose colonies.
Sleeping and nursery quarters
Tree squirrels often nest in hollow tree trunks and leaf nests (dreys) although more unusual nest sites, including buildings and vehicles are sometimes used. Dreys may be 25 to 45 cm in diameter. Pine squirrels (Tamiasciurus hudsonicus), called red squirrels or chickarees in some areas, also nest in middens in the ground. Ground squirrels create tunnel systems with multiple entrances and chambers that can be over 9 m long. Some ground squirrels use rocks and shrubs to hide burrow entrances.
Squirrels use a variety of nesting materials. Tree squirrels use green leaves or pine needles for the outside of dreys. Shredded grasses, fresh leaves, moss, and lichens are used to line the inside of the nest. Ground squirrels use similar materials to line the subsoil sleeping chambers connected to tunnels. Occasionally both natural and man-made (string, cloth) nesting materials can cause problems by entangling limbs, tails, or heads.
Squirrels often build multiple nests and may change the primary nest for many reasons. Damage to a nest, reduction in food supply, increase in predator activity, increase in noise or activity levels, the presence of toxins, or an overabundance of ectoparasites may prompt a squirrel to move to a new nest.
The daily and annual activity cycle varies with species. Tree and ground squirrels are diurnal, with most of their activity being early in the morning or late afternoon. Flying squirrels are nocturnal. Tree squirrels tend to be active the year round, although they stay close to their nest during severe winter weather. Tree squirrels are active and consume more food in autumn. Ground squirrels eat more in late summer and hibernate during fall and winter. Body weights are highest in fall and the beginning of winter. Some ground squirrels estivate during hot and dry months.
Tree squirrels spend time in trees and on the ground, but seek trees when stressed or eluding predators. Tree and flying squirrels are very effective in climbing in and between trees. Their escape behavior includes climbing to the opposite side of the tree, leaping from tree to tree, or remaining motionless. Ground squirrels escape predators by going into tunnels or under rocks.
Different squirrel species demonstrate a variety of vocalizations and tendencies to vocalize (Wilson and Ruff 1999). Squirrels regularly spend time grooming.
Diets vary with species and time of year. Most squirrel species are primarily herbivores and eat bark and buds, nuts, pinecones, seeds, fungi, lichen, and roots. However, they are opportunistic feeders and will eat easily available protein sources such as eggs. Squirrels cache food by burying nuts in the ground, stashing pinecones in middens or placing seeds and mushrooms in burrows. A squirrel’s water needs are usually met through its food, although water from other sources may be necessary in winter or during the summer’s heat.
Most squirrels mature sexually at approximately 11 months of age. Their reproductive anatomy is similar to that of other mammals. The number of litters per year varies by species and environmental conditions. Many species of squirrels have one litter annually each spring (pine, northern flying (Glaucomys sabrinus), thirteen-line ground, and rock squirrels). Several squirrel species (fox and eastern gray) have two litters annually (mid-January through April and July through September). Female squirrels in estrus may be chased by multiple males over several hours.
Gestation of the larger tree squirrels ranges from 40 to 45 days. Pine and flying squirrels gestation lasts approximately 35 days and ground squirrels from 27 to 30 days. Litter size varies with species and environmental conditions. Litters can range from one to seven with a typical size of three. Female squirrels raise the young alone. Adult male squirrels will occasionally kill juvenile squirrels.
Squirrels are born hairless, blind, deaf, and defenseless. Female squirrels nurse the neonates often throughout the day and night. The female only leaves for very short periods. The skin of the infant squirrel turns from pink to gray, and then fur begins to appear on the head. The juvenile squirrel ages develop more fur and become more active in the nest. The mother squirrels use their tongue to stimulate urination and defecation of the young until they are starting to eat solids. The age at which they open their eyes varies with species, but can range from 4 to 7 weeks.
Juvenile squirrels normally do not leave the natal nest until their eyes have been open for 1 to 3 weeks. At this time they can walk and climb adeptly and begin to chew on solids. Female squirrels nurse their offspring until the juveniles are able to find and consume enough food to survive on their own. Weaning may occur as early as 7 weeks for some of the smallest ground squirrel species and around 3 months for larger species.
Squirrel mortality like other small mammal prey species is high. The squirrel mortality rate in the first year can be 75 percent, and 50 percent annually in subsequent years. Mortality factors include severe winters, inadequate food, hunting, parasites, disease, collisions with vehicles, habitat loss, poisons, trapping, and predation by wild canines, raptors, and domestic dogs and cats.
Making Rehabilitation Decisions
The following section provides some examples of how the natural history of a squirrel species can influence a wide range of rehabilitation decisions.
A wildlife rehabilitator needs to be familiar with natural history long before a squirrel is admitted for care. This prior knowledge can prevent a variety of problems and help the curious public develop more appreciation for and tolerance of squirrels. The public commonly contacts rehabilitators with questions ranging from request for general natural history information, suggestions on how to reduce human-wildlife conflicts, and help deciding whether or not to rescue a squirrel believed to be in distress.
Below are listed examples of some general questions the public poses and how they can be answered with knowledge of natural history of specific squirrel species.
“Why are the squirrels chasing each other?” (Territoriality or a mating chase.)
“When do the squirrels raise their babies?” (This depends on the species…)
“Why did the ground squirrels disappear in September? Did they all die?” (Ground squirrels hibernate September – April.)
“What can I feed squirrels?” (It’s best to let them eat their natural foods, such as bark and buds. It is especially important to not feed sunflower seeds and peanuts which can result in serious nutritional problems.)
Many people call rehabilitators looking for humane solutions to human-wildlife conflicts, such as squirrels raiding bird feeders, nesting in attics, and eating fruit in trees. Familiarity with natural history can provide valuable suggestions.
The public often calls wildlife rehabilitators about wildlife in distress, such as possible orphans, squirrels hit by vehicles, dog or cat attacks, poisonings, injuries and diseases. It is important for a rehabilitator to be able to assess whether any action is needed, and if so, the action necessary and by whom, especially in the case of possible orphan situations. It is necessary to know the condition of the young; the availability of the mother; whether there is time and a safe way for the mother to retrieve her young; if and for how long the rescuer has provided care; and other relevant information. The more knowledgeable the rehabilitator is about the natural history of wildlife and common problems, the more effective is the decision that can be made.
Capture and handling methods
People handling squirrels need to exercise caution since squirrels will use their teeth and claws to escape and defend themselves. While most of the squirrels arriving for rehabilitation are juveniles with smaller teeth and claws that may appear less dangerous, it is still prudent to be careful. Wearing gloves and wrapping the animal in a towel for handling gives the animal a place to hide away from humans, makes the animal easier to restrain, and reduces the risk of injury, parasites and disease for the handler. Towels should not be left in the cage as bedding since claws and toes can be snagged and fractured in terrycloth.
Considerable caution needs to be exercised with adult squirrels due to risk of serious injury from the teeth and claws. Do not grab the squirrel’s tail since it can be injured, or break off easily and allow the squirrel to escape. Since squirrels are agile, fast and strong, it is helpful to conduct any examination in a small, closed room. Nets provide some help, but heavy leather gloves and terrycloth towels are necessary. Live traps may be helpful in some situations. Any traps should be covered with a towel, and the animal should only be kept in them for a very limited time.
Parasites and parasite treatments
It is common for squirrels to have ectoparasites, especially fleas and mites. Since such parasites may transmit diseases to other animals and humans, it is necessary to remove them mechanically or chemically. Flea powders that are safe for young kittens may be used with healthy squirrels according to instructions in rehabilitation literature.
Soft, ravel-free nesting material that does not have tears or small holes should be used for bedding (e.g., knit t-shirts, fleece). Terrycloth towels should not be used for bedding since they may snag and injure claws and toes. Supplemental heat is needed for squirrels young enough to still be in a warm, cozy nest, or injured adults in shock (specifics are noted in rehabilitation manuals). Cedar chips can cause respiratory problems and should not be used in cages.
The selection of a cage depends on the condition and age of the squirrel. Neonate squirrels need much smaller containers than juveniles just about to open their eyes. After a juvenile’ s eyes are open for more than a few days they start chewing solids. At that time they may be able to chew out of plastic and should be moved to a more secure wire cage. A juvenile tree squirrel that is learning to climb and jump accurately should be held in a smaller cage than an older juvenile that can crawl upside down and drop down accurately to wrestle a sibling. An adult with a head trauma or recently fractured leg needs a smaller cage to restrict movement than the recovered adult squirrel regaining stamina in a pre-release cage.
Understanding how a species reacts to stress or predators is valuable. Bedding in the cage, such as a pile of t-shirts, provides a place to hide and as a source of warmth. Since tree squirrels climb trees in order to escape, keeping a mobile, recovered tree squirrel in a dog kennel or similar cage that prevents climbing can increase the squirrel’s stress. Squirrels try to escape by gnawing through wood. Wood cage supports should be on the outside of the cage.
Wire cages must be sturdy enough to prevent the squirrel from escape and/or being taken by predators. Light gauge wire, like chicken wire, is inadequate to contain squirrels. Commonly used wire sizes, such as 1 x 2 in. wire (14 gauge), 1/2 x 1/2 in. or 1/4 x 1/4 in. wire mesh have been safely used with older juvenile and adult eastern gray or fox squirrels. The commonly available 1/2 x 1 in. or 1 x 1 in. wire has resulted in injuries for both eastern gray and fox squirrels. Pine, flying, and ground squirrels are considerably smaller than larger tree squirrels and require smaller openings in the wire, such as 1/2 x1 in., 1/2 x 1/2 in., or 1/4 x 1/4 in. The Minimum Standards for Wildlife Rehabilitation provides information on cage sizes and activity levels.
Tree and flying squirrels require branches in their cage that are appropriate for the species. For example, fox squirrels use both deciduous and coniferous trees; pine squirrels use various conifers; and Abert’s squirrels eat and nest in Ponderosa pine or similar long needled pines. Wood used in cage construction or used within the cage should not be treated with any chemicals since treated wood can be ingested and cause health problems. Branches in pre-release cages of tree squirrels should include a mix of sturdy and flexible branches to prepare facilitate climbing and jumping.
Ground squirrels escape predators by seeking cover in tunnels and underground nests. Cages should have small tunnels and hiding places. Different sized cardboard tubes or PVC pipe can be fashioned into temporary tunnels depending on the size of the animal. Ground squirrels do considerable digging and take dust baths regularly. A small amount of clean, dry dirt in the cage is needed in cages of older juvenile or adult ground squirrels.
The nest boxes should be constructed to be secure, have adequate space for the squirrel, and easily for the rehabilitator to open in case of emergencies. Wooden nest boxes are preferred over other materials for tree squirrel nest boxes since the squirrel’s natural behavior is to climb around in and on the nest, and chew. Although easier for some rehabilitators to acquire, cardboard nest boxes are less sturdy when climbed upon, louder, and may cause digestion problems. Pine squirrels use nests at the tops of trees and in ground middens, and should be provided with nest boxes at the top and bottom of tall cages. The use of nest boxes is not recommended with adult squirrels until the animal’s recovery is more certain, and the squirrel does not need to be frequently checked or handled. An adult squirrel with compromised health can nest and hide in a pile of t-shirts until it recovers enough to have a nest box in larger caging. Bedding materials should be kept dry and reasonably clean.
Since squirrels are fast, agile and eager to escape, it is important to have cages that reduce the risk of escape (e.g. use small doors to provide food). Cages should be easy to clean and disinfect. Full spectrum light should be available over an appropriate day length. Cages should be placed in a quiet area that is protected and hidden from predator species, including humans and pets.
It is well known that squirrels find buried nuts with their extremely sensitive sense of smell. Sensitivity to scents means that a squirrel may be easily irritated and stressed by exposure to strong scents, such as cleaning agents, detergents, perfumes, hair spray, and cigarette smoke. The use of strong scents should be avoided.
Natural history provides guidance when and how squirrels can be housed with conspecifics. Many species of juvenile squirrels may be placed with others of the same species that are of comparable age and size that are still nursing. However, there are exceptions such as juvenile thirteen-lined ground squirrels that may cause mortal wounds when placed in a cage with non-related juveniles.
Squirrels are very territorial after they are weaned. Weaned juveniles and adults may be extremely aggressive when placed in a cage with other squirrels and may cause serious injury or death. When a rehabilitator places non-related conspecifics together he/she should be very familiar with the species’ behavior, place the juveniles in a cage new to all of them at the same time, and closely monitor the squirrels for aggression for a minimum of 2 hours. Situations may arise when it is necessary to house squirrels separately but where they can see each other due to health conditions, wide variances in sizes or age, or aggression.
Many squirrels are admitted to rehabilitation with health problems. Some are relatively minor, some life threatening. One of the first things a rehabilitator must do is to conduct a physical examination in order to determine whether first aid is needed, if veterinary care is necessary, and what general care is required (type cage or food).
In some cases, the rehabilitator and veterinarian will decide that the squirrel either has such severe injuries that it cannot recover or it would be unable to survive when released in the wild. Conditions preventing survival in the wild include severed or crushed spine, permanent neurological damage, punctured organs, blindness in both eyes, a fractured jaw, or permanent malocclusion. Some squirrels may be able to recover from serious conditions such as shock, dehydration, poisoning, aspiration pneumonia, diarrhea, fractures, and abscesses with timely and effective treatment.
It is critical to know what is normal for a species in order for a rehabilitator to effectively evaluate a squirrel’s behavioral development, health, care needs, and possibility of release. For example, it is normal for infant squirrels to wag their tails when eating and to gently twitch while sleeping. Many neonate squirrels move their mouths in fish-like movements when they eat. It is common for juveniles to open one eye to before the other. Juvenile squirrels have white teeth, while pre-adult or adult squirrels have orange teeth. A squirrel’s broken teeth will grow back, but need to be monitored to ensure that they attain proper length and align correctly.
Familiarity with normal stool, urine, and elimination can help the rehabilitator recognize and treat health problems. Juvenile squirrels with their eyes closed need to have their genitals stimulated often in order to urinate and defecate. The stool of an adult squirrel and a young squirrel nursing on its mother’s milk should be dark, round, and solid. A young squirrel fed a squirrel milk replacer should have a firm, round, golden-colored stool. The urine stream should be steady and the genitals should be dry unless being stimulated. If the urine is very slow (a drop at a time) the squirrel may have a urinary tract infection. The same is true if the young squirrel is not urinating on its own (starts around the time the eyes open), and its abdomen and bedding are consistently damp. Juvenile squirrels tend to be rather quiet unless startled. A painful condition is possible if a young squirrel vocalizes when sleeping or when touched.
Feeding frequency, timing, and supplies
Female squirrels stay with their young and feed them most often when they are newborns. The number of feedings a day decreases as the juvenile develops. In the wild, the young squirrel is not weaned until is able to sustain itself on solids such as buds. leaves, and bark. The female squirrel continues nursing her young until they are able to find food on their own, eat an adequate amount, and otherwise act independently (such as watching for predators). Juvenile squirrels may begin chewing on solids shortly after they open their eyes. However, the ability to chew and shred solids does not mean that the juveniles are self-feeding.
Small feeding syringes should approximate the size of the female squirrel’s teat, which is smaller when the squirrels are newborn. Rehabilitators have found the 1 and 3 cc Monoject Luer style syringes to be similar to the squirrel teat. Some rehabilitators have effectively used very short nipples on the end of the syringe. Longer nipples can cause aspiration problems. Tube feeding should not be used with squirrels since it is often fatal.
Juvenile squirrels that are still nursing require effective squirrel milk replacers (e.g., Esbilac and Multimilk, and Zoologic; manufactured by PetAg, Hampshire, IL). Product selection, formula mix, and feeding protocol should closely match the nutrient content and caloric energy provided by the mother animal during a 24 hour period. Caution should be used when using milk replacer formulas since a number of factors can effect the nutrient and caloric content of mother’s milk. Additionally, preparation of a milk replacer formula can effect the nutrient content. The nutritional content of commercial diets may change over time. Rehabilitators should check product labels or obtain the current nutritional analysis from the manufacturer, and consult with professional animal nutritionists when trying to match the nutrient composition of mother’s milk.
Squirrels do not feed exclusively on one food item in the wild. Diets consist of a variety of foods and change seasonally with food availability. Excess feeding of nuts, fruits, or seeds in captivity can cause serious gastrointestinal problems and nutritional disorders. Sunflower seeds, peanuts, high fat foods, and non-native nuts should not be offered to squirrels since they can prevent the effective absorption of calcium and cause nutritional problems. Squirrels seek a balanced diet in the wild and chew and gnaw on items that would keep their ever-growing teeth at an appropriate length.
Juvenile squirrels whose eyes have opened and adults should be provided with an assortment of branches, buds, and other native vegetation that they eat in the wild. However, it is impossible to effectively meet the full dietary needs of the squirrels in captivity by just providing such vegetation. Older juvenile and adult squirrels should be provided with a solid, balanced, commercially prepared high quality rodent diet.
Growth and development
Familiarity with normal development stages helps the rehabilitator anticipate the squirrel’s growth and to be prepared with appropriate diets, food, caging etc. Recognition of normal physical, mental, and behavioral stages can help identify and treat potential problems.
Release criteria begins with the animal being physically fit enough to survive in the wild. The squirrel must be able to identify and secure its own food, create shelter, and adjust to the external environment. The squirrel needs to be able to recognize and elude predators, including humans. It needs to have an adequate pelage, be acclimated to external temperatures, be able to groom itself, and survive without support from humans.
Squirrels need adequate cover, food, and materials to create shelter, living space and water. Tree squirrels need enough trees spaced close enough together to allow for “tree top travel”. Ground squirrels need to have the appropriate type of soil and vegetation. Squirrels need to have proximity to others of their species, but not be so close as to cause density problems or result in fatal territoriality clashes. Population density varies with species and habitat quality. Squirrels should be released in areas that minimize future problems with humans and are in compliance with state wildlife rehabilitation regulations.
All diurnal squirrels should be released very early in the day. This allows the animal time to survey the area and locate/prepare shelter for the night. Some squirrel species, such fox and eastern gray, build spring/summer nests with green leaves, and need leaves available on trees for back-up nests. Preferably, tree squirrels should be released before the onset of winter when they can construct nests using pliable green leaves. Although rehabilitators have provided squirrels with several nest boxes when green leaves are unavailable, problems can occur if the squirrels leave the release area or cannot use the nest boxes for some reason. Ground squirrels should be released in adequate time for preparation for hibernation (creating burrows and putting on adequate body weight).
Squirrels are one of the more common animals admitted for wildlife rehabilitation. The many differences between squirrel species demand that the rehabilitator become familiar with a species’ specific needs so that effective rehabilitation decisions can be made. A good knowledge base is beneficial for the squirrel and the rehabilitator striving to provide the highest quality care. Rehabilitators are encouraged to expand their understanding of squirrel natural history beyond the scope of this paper and the limited examples of how it affects rehabilitation decisions. References such as Squirrels of the West, (Harston 1999), Squirrels A Wildlife Handbook (Long 1995), Mammals of North America (Chapman and Feldhamer 1982), Smithsonian Book of Mammals of North America (Wilson and Ruff 1999), Natural History of Squirrels (Gurnell 1987), Hand-rearing Infant Tree Squirrels (Hanes 1998), and Squirrel Rehabilitation Manual (Casey and Casey 2001) are particularly useful to the rehabilitator.
Written by Shirley J. Casey.