Yes and no. The social systems of the 278 species of squirrels in the world range from completely asocial to highly social, with many permutations of sociality in between.
Some live solitary lives except when mating; others live in complex social groups and have extensive social interactions with other members of the group.
GROUND SQUIRRELS. Among the most social of the squirrels are certain species of ground squirrels, marmots, and prairie dogs. Gail Michener, of the University of Lethbridge in Alberta, Canada, has been studying ground squirrels for 37 years, and has identified five grades of social organization among the ground squirrels: asocial, single-family kin clusters, female kin clusters with male territoriality, polygynous harems with male dominance, and egalitarian polygynous harems. Woodchucks and Franklin’s ground squirrels are classified as asocial, which means they are characterized by solitary living, no amicable interaction with other conspecifics (except mating), and little amicable interaction with offspring. At the other end of the spectrum are the highly social black-tailed prairie dogs and the Olympic marmots.
Yellow-bellied marmots, a frequently studied squirrel, are moderately social, living in polygynous harems with male dominance. These harems are composed of females, yearling young, babies, and a dominant male who defends his territory from potential usurpers. The females in a harem are most often related, and the social relationship among all members of the harem is almost always amicable. The older females in a harem tend to be the only breeding females, with the younger females being reproductively suppressed by the older females’ presence. The younger, nonbreeding females are important, though, in that they assist in the care of babies. The dominant male will act aggressively to yearling males, which encourages them to disperse.
South African ground squirrels in southern Africa have been studied extensively by Jane Waterman, of the University of Central Florida, who has documented their social structure. Female South African ground squirrels live in all-female groups, similar to some other ground squirrels, which are made up of only related females and young. These females feed on communal ranges and live communally in several burrows, except during lactation when the female lives alone in a separate burrow. Females interact amicably and there is no identifiable hierarchy among them.The unique aspect of the South African ground squirrel social system are the males. Male South African ground squirrels do not live with females, but instead they live together in groups of up to 20 unrelated individuals of varying ages. Members of a male group share a common home range and forage together throughout this range. Although there is some hierarchy and dominance in these groups, interactions among males are largely amicable, with males grooming each other and sharing burrows.
Among all ground squirrels, social behavior is similar. Some examples of social behavior among ground squirrels include play, grooming, vocalizations, and greeting (such as nose-to-nose or nose-to-mouth contact upon encountering another squirrel). Other social behavior involves group care of the young, such as communal nursing and communal hibernation.
Because of the variation in the level of ground squirrel social systems, they are good research subjects for comparative studies. Daniel Blumstein, of the University of California, Los Angeles, and Kenneth Armitage, of the University of Kansas, did one such study in 1998, which compared life-history tactics across various species of social and nonsocial ground squirrels. In the study, they examined both the costs and benefits of living in a social group. One of the primary benefits of social living is increased survivorship. In fact, the authors state, “No matter how we examined the relationship, more socially complex species had a higher survival rate to age one.” Increased survivorship results from a decrease in predation due to a larger number of individuals to detect predators. It also results from an increase in cooperation, through allogrooming (which reduces ectoparasites) and cooperative care of young (to include communal nursing, young females as helpers, and communal hibernation).
Social living comes with a cost. With many animals in a small area, the competition for resources, such as food, mates, and burrows, increases.This competition can lead to increased aggression, increased dominance, reproductive suppression of younger animals, and infanticide. John Hoogland, of the University of Maryland Appalachian Environmental Laboratory, reports that infanticide, mostly by other lactating females, is responsible for the loss of 39% of black-tailed prairie dog litters. Social living, according to Kenneth Armitage, also results in later reproduction, fewer females reproducing, and smaller litter sizes. It also increases the potential for the spread of disease and parasites. One extreme example of this is the quick and complete annihilation of prairie dog colonies by the plague.
Two South African ground squirrels (Xerus inauris) greet each other. Glands around the mouth release scents that are used by many ground squirrels to recognize each other.
Photo © Jane Waterman
As mentioned above, not all ground squirrels are social. Numerous species of ground squirrels are asocial, including chipmunks (Tamias spp.), woodchucks (Marmota monax), 13-lined ground squirrels (Spermophilus tridecemlineatus), and Franklin’s ground squirrel (Spermophilus franklinii).
One view of how sociality evolves among ground squirrels is that the animals first band together to reduce predation or to better use resources. Social behavior evolves from these groupings. Another view is that, because most species of ground squirrels live in harsh environments and hibernate, there simply is not enough time for the young to successfully disperse in the first year. Therefore, young are retained in a family group for another year or more until they are able to disperse successfully.This necessary reten tion of young is thought to have led to the evolution of social behavior in the ground squirrels.
Young prairie dogs greet adult. Amicable relationships among family members are strengthened by frequent contact. Photo © Shirley Curtis, www.scarysquirrel.org
TREE SQUIRRELS. Tree squirrels are not considered social animals, although they do exhibit some social behaviors from time to time. One of the main examples of this is communal nesting. Eastern gray squirrel females will occasionally nest communally with close relatives during the winter. Interestingly, female fox squirrels do not do this. Male eastern gray squirrels and male fox squirrels also will nest communally on occasion, but in this case the choice of nesting partners does not depend on relatedness, as it does with female eastern gray squirrels. Eurasian red squirrels, Mexican fox squirrels, Abert’s squirrels, and Arizona gray squirrels also have been documented sharing nests with conspecifics. One decidedly asocial squirrel is the North American red squirrel, who defends an exclusive territory and, with the exception of mating, does not interact with other conspecifics.
FLYING SQUIRRELS. Not much is known about the social systems of flying squirrels, because they are nocturnal and difficult to observe. The most studied of the flying squirrels are the North American ones, the southern and northern flying squirrels. Both these species of flying squirrel, similar to tree squirrels, will nest communally. At times, aggregations of flying squirrels can be as large as fifty squirrels in one nest hole. Unlike the communal nesting in tree squirrels, flying squirrel aggregations are composed of both male and female squirrels, and also squirrels of various ages. What we don’t know for certain is whether these aggregations consist of squirrels that are related. Work is going on at Peter Weigl’s lab at Wake Forest Uni versity to examine this question. The hypothesis is that, at least in southern flying squirrels, there is some form of kin recognition and that this plays a role in the decision of a squirrel about which individuals it will nest with. Advances in molecular biology are assisting researchers in trying to tease apart this problem.
Yes, squirrels do fight. Squirrels may fight in a number of situations, such as when establishing dominance hierarchies, determining order during mating chases, defending a mate or territory, and protecting young. Fights are ordinarily quickly determined and are concluded by chases, which are what we normally observe. The results of fights are easily seen and include lacerated ears, bobbed tails, scars, missing fur, and sometimes even blood from fresh wounds. Fighting is costly in time and energy and can result in serious injury. Therefore, in most instances disputes between squirrels are settled with minimal or no physical interaction. Chasing, aggressive body posturing, and vocalizations are all nonphysical ways squirrels can deter other squirrels.
The North American red squirrel (Tamiasciurus hudsonicus) is renowned for its belligerent territorial behavior. Stories abound of its persistent attempts to protect its territory from all sorts of other animals, including other larger squirrels, birds, humans, and even lawn ornaments. Pregnant and lactating female black-tailed prairie dogs can be extremely aggressive, defending nursery burrows and protecting their babies from other females. In 1980, John Hoogland documented a 6-hour-long dispute between two pregnant prairie dogs. And, during breeding season, Hoogland observed many male black-tailed prairie dogs with facial injuries and scars, and in two cases he witnessed fights between males that resulted in the death of the losing animal.
One should judge animal intelligence not from the perspective of human behavior, but from the perspective of how well the animal is adapted to the demands of its own environment. I see little value in asking how well a crow performs as a human being.
WILLIAM HODOS
Throughout the course of his career Thorington has tagged and monitored eastern gray squirrels in his suburban Maryland neighborhood. One cold rainy morning, he found a small, very wet, young female in his trap.He tagged and released her. Even though she remained in his neighborhood for several more years raising her litters, he never again found “Blackie” in his trap. On the other hand, he had a big male, “Old Yellow Beads” whom he easily trapped more than six times. It seems the squirrel rather enjoyed the peanut butter he used as bait. And when he would release him, “Old Yellow Beads” would not run away, but rather walk calmly to the nearest tree, sometimes pausing to take some more peanut butter before he left. Which squirrel was smarter—the one that avoided the trap, or the one that took the extra food?
Two lactating black-tailed prairie dog females battle with each other. Lactating females are responsible for the majority of infanticides in black-tailed prairie dog communities. Photo © Shirley Curtis, www.scarysquirrel.org
Animal intellect is a thorny subject, but one that has fascinated researchers for decades. How we should first define, measure, and finally compare animal intelligence across species has been and still is hotly debated. Brain size has been considered by some to be a measure of intelligence. Humans and other primates, for example, have relatively large brains compared with body size. In squirrels, it was hypothesized that social ground squirrels, such as prairie dogs, would have a larger brain compared with body size than nonsocial squirrels—based on the assumption that sociality is complex and would require more brain power. This did not turn out to be the case. What was found, instead, was that tree squirrels tend to have larger brains with respect to body size than ground squirrels do. So why would tree squirrel brains be bigger? It might have something to do with living in a complex three-dimensional environment. Tree squirrels need to be able to move their bodies quickly and safely up, down, over, and across branches and wires. Controlling all of this complex movement requires a lot of brain power and so would require a larger brain. But, does this mean tree squirrels are smarter than other squirrels?
An eastern gray squirrel steals a drink from a water fountain on a summer’s day in Chicago. Photo © Jay Shrestha
English novelist and publisher Ford Madox Ford once said, “Genius is memory.” The brightest birds are considered to be the corvids (jays and crows), which have very good memories. Scatter-hoarding squirrels have shown that they remember where they bury food, and other squirrels remember the location of a good food source from day to day or year to year—an essential survival skill. Squirrels also memorize specific routes through the trees, typically the quickest way to and from their nest to a prime feeding location.
Learning and memory go hand in hand. To survive, squirrels must learn—learn where to hide when danger threatens, learn what foods are good to eat, learn how to open a nut quickly, learn where to find the best food, learn how to build a sturdy home, learn how to interact with other squirrels. Beyond survival skills, squirrels have also proven that they are adept at learning novel tasks. The BBC special “Daylight Robbery” chronicled wild squirrels as they mastered an increasingly complicated obstacle course to reach food. And many of us watch as our own backyard squirrels “outsmart” all attempts to keep them out of our bird feeders. Is this intelligence or just dogged persistence?
As for the two squirrels mentioned in the beginning, we would say they both are smart, each remembering and learning from their individual experience. Blackie had a rather unpleasant experience in the trap and conse quently avoided it. Old Yellow Beads, on the other hand, obtained a tasty treat from the trap and as a result did not avoid it. They both adapted their behavior in response to their individual experience.
Youngsters in play practice a different leap. It looks so like an expression of sheer high spirits, “jumping for joy,” that its serious purpose might be overlooked.
BARKALOW AND SHORTEN
Squirrels, like many mammals, engage in play behavior. Play behavior, normally restricted to young squirrels, can be solitary or social and has been documented in both tree and ground squirrels. Solitary play behavior can include running, climbing, jumping/leaping, twisting, tumbling, and play fighting with objects (e.g., twigs, leaves). Social play behavior takes place between two (or more) squirrels and can include play fighting (wrestling, tackling, boxing, chasing) and sexual play (chasing, mounting, play copulation). Social squirrels, such as prairie dogs, sometimes engage in solitary play, and solitary squirrels will engage in social play, usually with littermates. Occasionally in some social, ground-dwelling squirrels, adults, including parents, will engage in play with young.
Play behavior begins at a set stage in a juvenile squirrel’s development, decreases as the squirrel ages, and eventually disappears. The timing of the onset and disappearance of play behavior varies between species. For example, play in young yellow-bellied marmots begins approximately 3 days after they emerge from the burrow and continues throughout the summer. In contrast, young woodchucks, the only solitary marmot, play briefly, only for the first day or two after emergence. Belding’s ground squirrels begin to play when they emerge from the burrow at around 4 weeks of age. They play vigorously for one to two weeks, but by the third week play behavior all but disappears.
The amount of time each day young squirrels engage in play also varies between species and occasionally between sexes. Juvenile Olympic marmots, when not feeding, spend the majority of their time playing in some form with conspecifics. Young yellow-bellied marmots spend about 40% of their time playing, with males engaging in more play than females, whereas young Columbian ground squirrels devote less than 10% of their time to play.
Why do squirrels (or other animals) play? Some propose that play is a way for a squirrel to practice behaviors and actions that it will use as an adult (e.g., play fighting, sexual play, leaping to avoid predators). Many researchers suggest that play behavior is essential to a squirrel’s physiological development, including neuromuscular control, bone growth, lung capacity, and overall physical stamina and coordination. Others suggest that play behavior, specifically social play, enables a squirrel to establish important relationships, including dominance hierarchies, among siblings and other juveniles.
Why do squirrels (or other animals) play? Some propose that play is a way for a squirrel to practice behaviors and actions that it will use as an adult (e.g., play fighting, sexual play, leaping to avoid predators). Many re-Squirrel Behavior researchers suggest that play behavior is essential to a squirrel’s physiological development, including neuromuscular control, bone growth, lung capacity, and overall physical stamina and coordination. Others suggest that play behavior, specifically social play, enables a squirrel to establish important relationships, including dominance hierarchies, among siblings and other juveniles.
Two black-tailed prairie dog pups play with each other. Photo © Shirley Curtis www.scarysquirrelworld.com
Scott Nunes, of the University of San Francisco, and colleagues studied play behavior in Belding’s ground squirrels. They found that juvenile squirrels that engaged in more social play had increased motor skills. This increase in motor skills was correlated with successful male dispersal in the first year. Additionally, females that engaged in more social play as juveniles, weaned more young as yearlings than those females that engaged in less social play. One wonders whether the more successful females play more, or whether the more playful females are more successful.
Nunes and colleagues also found that squirrels that had a higher percentage body fat played more than those that had less body fat. Well-fed young squirrels play more and increase their motor skills at a greater rate, which can help them avoid predators and win in fights. For females, increased play leads to greater reproductive success.
Squirrels communicate, but they do not use language the way we do. Despite the literature, like the Doctor Doolittle books, that suggests that animals speak to one another, humans are the only animals that use speech.
The vocalizations of squirrels are distinctive, and you can learn to recognize them and the contexts in which they are given. The alarm calls will probably be the easiest to learn. In our neighborhoods, the eastern gray squirrels are most concerned about cats, and their alarm calls, “chuck, chuck, kwa,” frequently alert us to the presence of neighbors’ cats before we see them. The intensity and repetition rate probably indicate how upset the calling squirrel is. Social ground squirrels, such as prairie dogs and some marmots, are prolific alarm callers. You only have to approach a colony to hear numerous chirps and trills alerting others to your presence. (More information on alarm calling appears below in “How do squirrels avoid predators?”)
Baby squirrels also vocalize, and their high-pitched calls will quickly summon a mother squirrel. At first, baby eastern gray squirrels express discomfort with a squeak vocalization, as early as 3 days of age. By 3 weeks of age, they can emit growls, and by 4 weeks intense discomfort is signaled by a short scream. At thirty to forty days of age, they develop a “muk muk” call, given in anticipation of nursing or feeding. It is like a search call or a contact call. This has been allied to the vocalization given during mating chases by male eastern gray squirrels, also described as “muk, muk, muk” or sneezelike calls, which appear to have an analogous context to the infant vocalization.
Vocalizations given during aggressive interactions between eastern gray squirrels include the growl and the squawk growl, which are very different from alarm calls and the calls given by male squirrels searching for an estrous female.
Many squirrels, even nestlings, have a continuum of calls, expressing degrees of intensity. For example, eastern gray squirrels emit a “kuk” or moan when mildly distressed, and a buzz sound when more intensely distressed. Under extreme distress they will emit a scream. This pattern is common among many of the squirrels that have been studied.
Squirrels also communicate by body signals, but these can be more diffi-cult for us to decipher. Common body signals used by squirrels are flicking of the tail and stamping of the feet. More subtle signals involve body positions, head positions, and ear movements. The orientation of the eyes may also play a role in communication.
Even more subtle yet, for us but perhaps not for the squirrels, are signals communicated by odors. One can surmise that some information is being communicated when squirrels sniff one another—their cheeks, sides, or perineum—but we do not know what messages are being communicated. Similarly, when tree squirrels rub their perineum on a branch or leave a few drops of urine on a branch, we do not know what they’re communicating to other squirrels, but presumably some information is being exchanged. Black-tailed prairie dogs often use their perianal scent glands during disputes. The secretions from these glands are very strong and skunklike in odor.
Squirrels, because of their size, habits, and abundance around the world are an important prey item to all types of animals. Because of this, over evolutionary time, squirrels have evolved multiple means to detect and elude predators. Described below are eight common tactics squirrels use to avoid predators.
CAMOUFLAGE. Blending in with the environment is one of the main ways squirrels avoid being detected by predators, and so it should be expected that the coloration of a squirrel’s fur would vary depending on the environment in which it lives. It is well documented among many mammalian species that animals living in more humid environments have darker pelage color than animals living in more xeric, or dry, environments. Many tropical squirrels, such as Callosciurus melanogaster, have quite dark, almost black pelage, which certainly keeps them well concealed among leaf shadows and wet bark and soil. Chris Smith, at Kansas State University, documented how the pelage among populations of North American red squirrels (Tamiasciurus hudsonicus) varied in accordance with humidity—with squirrels living in coastal, more humid, British Columbia having darker pelage than those living in the drier interior. Desert-dwelling squirrels, such as North American and African ground squirrels, on the other hand, tend to be light colored, which helps them to “disappear” against the sandy soil.
In many squirrels, most noticeably in tree squirrels, the belly (ventral) color is much lighter than the back (dorsal) fur color. This is called counter-shading and is used by numerous mammals, birds, and fish as camouflage. The principle of counter-shading is that the animal is less visible when its best-lighted surfaces are darker and its shadowed surfaces are paler. Abbott Thayer, an artist and naturalist, developed this principle in an 1896 article in the ornithological journal The Auk. His many ideas—some outlandish— led to great debate, but they also led to the military’s use of counter-shading and camouflage in the First World War.
The types of trees on which a squirrel lives can also influence pelage color. Chris Smith discussed how the grayish dorsal fur of eastern gray squirrels (and some eastern fox squirrels) tends to blend in with the grayish bark of beech, maples, and oaks; whereas the reddish fur of North American red squirrels (Tamiasciurus) tends to blend in with the more reddish bark of the many species of conifers throughout their more northern range.
The seasonal molt of some squirrels also may be adaptive in helping squirrels remain concealed. For example, the summer pelage of eastern gray squirrels is brown on the sides and back. The winter pelage is paler, gray on the sides and brown only near the midline of the back. In the sum mer, these squirrels are well concealed by the shadows and leaves of deciduous trees. In the winter, though, when the leaf cover is gone, the squirrels are more exposed. The lighter winter pelage matches the exposed bark of trees and acts as camouflage for these squirrels.
Some researchers have suggested that squirrels living in areas prone to forest fires have evolved melanism, or dark coloration, as a means of camouflage. Richard Kiltie, at the University of Florida, compared fox squirrels from habitats that burn frequently with those from habitats that do not burn frequently and found that melanistic fox squirrels were more common in frequently burned areas.
SHORT NEST TIMES. Baby squirrels begin life with only a slim chance of survival. Young squirrels lack the defenses and the experience of mature squirrels and therefore are easy targets for savvy predators. Baby squirrels are most vulnerable while they are still in the nest and unable to get away from the snakes, birds, army ants, or even other squirrels that might prey on them. Over the course of time, squirrels in some tropical regions such as Africa and Southeast Asia (where there are significantly more nest predators) have evolved a way to limit nest predation—longer gestation times and shorter nest times. This means babies spend more time inside the mother, are more developed when born, and therefore can leave the nest much earlier than other species.
PHYSICAL AVOIDANCE. Squirrels are gifted athletes in fur coats. Tree squirrels race with ease across branches or rooftops and through labyrinths of vegetation and leap without caution from branch to branch. Equally swift chipmunks disappear before your eyes, flattening themselves out and vanishing through invisible openings. All squirrels also are endowed with sharp claws and ever-growing incisors that encourage predators to make a quick and clean kill. Many mammalogists have scratches and bite marks to evidence the painful results of mishandling a squirrel. The squirrels’ skills mean even the most savvy predator sometimes ends up empty handed. Author and scientist Lawrence Wishner once observed a hawk swoop down on a seemingly oblivious chipmunk, talons outstretched, only to come up with nothing—the chipmunk, at the last moment, having dashed to safety.
Chipmunks can run up to speeds of 19 kilometers per hour (kph) (12 miles per hour [mph]), and some ground squirrels are also quite fast, with the golden-mantled ground squirrel (Spermophilus lateralis) being clocked at 21.7 kph (13.5 mph). The fastest are the tree squirrels that can run almost 26 kph (16 mph) on a flat surface, and once they reach a tree trunk, their claws grip the surface tightly and allow them to move with ease up, down, and around the trunk. Tree squirrels will flatten themselves against a trunk of a tree and maneuver themselves to the side opposite the predator, in particular, human hunters, continuing to move around the trunk, out of sight, if the curious predator persists in looking for them.
VIGILANCE. Squirrels, like all wild animals, have acute senses, which they use in combination to help them identify and avoid potential predators. Because squirrels live in varying habitats—from open plains to dense forests—the environment plays a significant role in how squirrels puts these senses to use.
Most prairie dogs, marmots, and ground squirrels live in relatively open habitats with little to no vegetative cover. Although this means that predators have little trouble seeing the squirrels foraging in the open, it also means the squirrels are able to spot the predators from a reasonable distance (provided they are paying attention). To “pay attention” squirrels often adopt an alert posture—sitting or standing up on their hind legs, scanning the horizon. Even when foraging, ground squirrels remain vigilant, stopping every few seconds to look about while chewing. Dirt mounds, stumps, rocks, and other elevated patches of earth provide a good view of the surroundings and are common spots for the squirrels to adopt alert postures. The amount of time a squirrel spends in the alert posture or looking about while foraging depends on many interacting factors, including group size, vegetative cover, distance from a burrow, the amount of predators in the area, sex, and time of year. For example, yellow-bellied marmots feeding far from a burrow spend more time looking up than those feeding near a burrow. In contrast, the amount of time Olympic marmots spend looking up does not depend on how close they are to a burrow but instead on how many other marmots are close by. Adult Olympic marmots feeding in a group look up about 15 seconds out of every minute, whereas those feeding alone spend 33 seconds looking up per minute.
As we mentioned above in the answer to “Are squirrels social?” increased protection is afforded to squirrels living in social groups. Regarding vigilance, more animals in an area means more eyes watching out for predators. And, in the end, this means less predation and more offspring that survive to the next generation. It is posited that squirrels first congregated into groups because the combined vigilance of the group members reduced predation. And, from these groupings, social behavior such as alarm calling, kin recognition, and nepotism, evolved.
Tree and flying squirrels live in habitats with significantly more vegetative cover. This type of environment provides many hiding spots, but it also can limit the ability for the squirrel to detect an incoming predator (either from the ground or from the air). Many tree and flying squirrels spend significant time on the ground foraging or storing food. Tree squirrels, while scanning for predators, commonly use the same alert posture seen in ground-dwelling squirrels. While vision plays a large role in predator detection in tree squirrels, sound also plays a significant part. Cracking twigs or shuffling leaves may alert a squirrel to a possible danger obscured by tree trunks or high brush. Upon hearing a strange noise, the squirrel will run up the nearest tree and freeze, listening for more cues.
Fox squirrels have a hearing range of approximately 63–56,000 Hz, which allows them to hear higher pitches than humans and is roughly comparable to the hearing of dogs. Prairie dogs, on the other hand, have a hearing range of approximately 16–26,000 Hz, similar to humans. It is possible that this difference in range of hearing in prairie dogs is an adaptation to a more underground environment, since low frequencies carry better underground and, while above ground, vision is their primary means of detecting predators.
All squirrels have excellent vision and their entire field of view remains in focus. This allows a squirrel, frozen in place, to continue to scan for predators without moving its head. In addition, a squirrel’s blind spot (the point where the optic nerve attaches to the back of the eye) is located such that it allows them an uninterrupted view of the sky—extremely important for detecting birds of prey.
NESTS AND BURROWS. If a predator poses an immediate threat, the safest thing for a squirrel to do is hide. Knowing the locations of appropriate hiding spots and the ability to get to them quickly is essential for a squirrel’s survival. The most commonly used hiding spots are burrows (for ground-dwelling squirrels) and nests and tree holes (for tree and flying squirrels), but in a real emergency any cover will do. Squirrels choose the appropriate hiding spot based on the type of predator. For example, when pursued by weasels or badgers, Belding’s ground squirrels dive into burrows with multiple entrances and exits, but if pursued by an aerial predator, a squirrel will use any burrow, even if it only has one opening. This is because terrestrial predators, such as weasels and badgers, can follow squirrels into a burrow, whereas birds of prey cannot.
Squirrels do not always go into hiding at the first hint of a predator. Running to a nest or hiding in a burrow is energetically costly and time consuming. A subterranean marmot, for example, has no way of knowing if a predator is still in the area and so might stay down for hours before reemerging to forage. This results in a significant loss of feeding time, which for a hibernating animal such as a marmot is limited to begin with. Often squirrels submerge only partially into a burrow, peeking out of the opening to assess the danger. University of Washington professor David Barash, while studying Olympic marmots, saw this amusing scenario: “A coyote elicited an alarm call upon approaching an Olympic marmot colony, whereupon all the marmots ran to burrows. When the coyote rushed the nearest one, it [the marmot] entered its burrow, leaving the coyote scratching furiously outside. After a few minutes another marmot about 7 meters away looked out of its burrow, saw the coyote and called, whereupon the coyote ran to this new burrow, but again to no avail. This was repeated four times, after which a frustrated coyote left the area to the marmots and an amused observer.”
(Left) A Belding’s ground squirrel (Spermophilus beldingi) mother and her pup stand at alert. The alert posture enables squirrels to better see potential danger. Photo © Gregg Elovich, www.scarysquirrel.org (Right) A round-tailed ground squirrel (Spermophilus tereticaudus) surveys its surroundings from the safety of its burrow. Photo © Jim Hughes, www.scarysquirrel.org
The location and construction of a nest is important, both as a hiding place for adult squirrels and as protection for baby squirrels. A tree squirrel nest should be strong enough to withstand shaking and concealed such that it is hard for predators to locate. If a nest is disturbed, the mother squirrel may relocate the young, carrying them one-by-one to another nest.
SOCIALITY. Most species of ground squirrels and prairie dogs and all marmots (except the woodchuck) live in social groups of varying size. Researchers have shown that social living among squirrels offers a range of protection against predators. John Hoogland, a prominent prairie dog researcher, found that prairie dogs living in large groups detected predators more quickly than those living in small groups. He also found that prairie dogs in large groups spent less time scanning for predators than those in small groups. David Barash, in his study of marmots, found the same re- sults, with more social-living marmots, such as Olympic marmots, spending less time alert than solitary living marmots, such as the woodchuck.
Black-tailed prairie dogs, like these in captivity, are considered one of the most social of the ground squirrels. One of the main benefits of living in a social group is increased protection from predators. Photo © Shirley Curtis, www.scarysquirrel.org
Another benefit of social living, commonly called “protection by dilution,” is that as group size increases, the probability of any one individual being preyed upon decreases. In some group-living squirrels, individuals work collectively to confront, harass, and discourage certain predators, most commonly those that prey on baby squirrels. This mobbing behavior would not be possible without the close association of other squirrels. Finally, the more squirrels living in a given area, the more burrows there are to use as hiding spots when danger threatens.
ALARM CALLS. Most squirrels vocalize when they are startled, threatened, or detect a potential predator nearby. These alarm calls, as they are known, vary in intensity and, depending on the species, can include chucks, whistles, chips, “kee-yaws,” trills, and even ultrasonic calls.
Types of alarm calls. The eastern gray squirrel gives a familiar “chuck” and “chuck, chuck, qwaaa . . .” alarm call, most commonly in response to cats, dogs, or humans. They usually give this call from the safety of a tree branch, looking down at the intruder and flicking their tail up and down. Other squirrels in the area respond to these calls by becoming alert, scanning around, and retreating to the safety of a tree. Calls of the European red squirrel (Sciurus vulgaris) are comparable to the eastern gray squirrel, and the fox squirrel gives a combination of similar chucks and chatters.
Eastern chipmunks have three distinct alarm calls: a chip, a chuck, and a trill. The chip and chuck calls are given in response to any detection of a predator, either aerial or terrestrial. These calls usually are given in bouts, which can sometimes last up to 30 minutes. The trill, however, is a short call given when a chipmunk is being pursued by a predator, or escaping into its burrow. While a researcher at the Smithsonian, Lang Elliott commented that he could tell the direction a hawk was flying through the woods by following the chipping of the chipmunks.
North American red squirrels, like eastern chipmunks, have three distinct alarm calls. Unlike chipmunks, though, red squirrels discriminate between aerial and terrestrial predators with their calls. The red squirrel gives a “seet” and “seet-bark” call in response to aerial predators but a sharp bark call in response to terrestrial predators.
Among social squirrels, such as marmots and prairie dogs, some use separate calls for different predators and others do not. California ground squirrels use a multiple note chatter call for terrestrial predators and a high-pitched whistle call for aerial predators. Hoary and Alpine marmots and Uinta, Arctic, Richardson’s, and long-tailed ground squirrels also have separate calls for terrestrial and aerial predators. Yellow-bellied marmots have two different alarm calls—the trill and the whistle. They do not use them to discriminate between aerial and terrestrial predators, but instead to communicate the level of risk. For example, Daniel Blumstein and his colleagues found that yellow-bellied marmots whistle longer, louder, and faster when the risk is higher. Similar use of variation in call rate and volume to communicate relative risk has also been found in Olympic and Vancouver marmots.
Three species of Southeast Asian tree squirrels, Callosciurus, were studied by Noriko Tamura and colleagues from the Tokyo Metropolitan University. All three species were found to have separate calls for aerial and terrestrial predators. When faced with potential terrestrial predators, these squirrels run up a tree, look down at the predator, flick their tails, stamp their feet, and make continuous staccato barks. When an aerial predator is detected, however, the squirrels remain motionless and emit a single bark or chuckle. If the bird flies very close the squirrel sometimes emits a rattle. Other squirrels hearing these calls freeze in place. These three species of squirrels also have a unique call they direct specifically at snakes. This squeak call elicits other squirrels (even of other species) to come into the area, where they all begin squeaking and mobbing the snake.
Smithsonian researcher Louise Emmons studied the vocalizations of nine species of African tree squirrels in the rain forests of Gabon and found, in general, that these squirrels used two main types of alarm calls: high intensity and low intensity. Low-intensity alarm calls, for the most part, were accompanied by tail and foot movements, in situations not presenting immediate danger. High-intensity alarm calls, however, were used in immediate danger situations and were emitted by a motionless or hidden squirrel.
Purpose of alarm calls. The real question surrounding alarm calls is why squirrels call at all. It seems that by calling, a squirrel makes itself more conspicuous to predators. In fact, during one study Paul Sherman, at Cornell University, recorded that calling Belding’s ground squirrels were chased more often by predators, and of the six predations he saw, three of those happened to squirrels who just called.
In social squirrels, alarm calling seems to function primarily as a means of nepotism. Many studies have shown that squirrels with kin nearby call more often than those without kin nearby. In some species such as round-tailed and Belding’s ground squirrels, only females with babies or grown young nearby call, whereas in other species, such as black-tailed prairie dogs, males with nearby kin (adult or babies) also call. Some species call when even more distant kin is close by (e.g., cousins). Squirrels commonly give alarm calls in bouts, and it is suspected that these recurring calls serve to maintain vigilance among other group members. Studies have found that some squirrels are able to tell when another squirrel is “crying wolf.” Yellow-bellied and steppe marmots and California and Richardson’s ground squirrels, have been shown to discriminate between reliable and non-reliable individual alarm callers. This means a squirrel can identify and react differently to the individual that gives an alarm call to every movement in the grass, and the individual that gives an alarm call only to authentic predators.
Similar to ground squirrels, mother tree squirrels call more frequently when they have young nearby. Among adult tree squirrels, however, it is hard to determine whether alarm calling is playing a nepotistic role. Adult tree squirrels do not live together in compact, easily observed groups similar to the social ground squirrels; therefore, determining the exact relationship between tree squirrels in a given area is more difficult. The main function of the continuous chucking and barking alarm calls given by tree squirrels is probably not nepotism but predator deterrence. Squirrels commonly give these calls while observing the predator from the safety of a tree branch, flicking their tail. This style of calling informs the predator that it has been seen and can no longer sneak up on the squirrel. The continuous nature of these calls, sometimes lasting up to an hour, simply reinforces this idea. “Vigilant squirrel here!” “You can’t sneak up!” “Vigilant squirrel here!” Other squirrels in the area hear these calls and take appropriate action, but it does not appear that the main purpose of the squirrel’s calling is to warn its neighbors.
SNAKE DEFENSE. Instead of fleeing from snakes like they do other predators, many squirrel species approach snakes and harass them. The nature of this harassment is similar among all species of squirrels who do it. They approach the snake on all fours—with their bodies stretched long, tail hair erected—and make alarm calls. At the same time they flip their tail side to side rapidly. Depending on the risk posed by the snake, squirrels may continue to approach the snake, kick dirt or sand at it, and even attack the snake. The squirrel may harass the snake only a short time, or it may continue vigorously until the snake leaves the area. Commonly, other squirrels in the area hear the alarm calls, congregate around the snake, and participate in the mobbing.
South African ground squirrels (Xerus inauris), from southern Africa, are just one of the many species of ground squirrels around the world that mob snakes. (Top) Squirrels might mob a snake individually, approaching the snake with a pilo-erected tail. (Bottom) More commonly, squirrels will mob snakes in groups (snake in grass to left of picture, not visible). Photo © Jane Waterman
To the casual observer snake harassment appears to be a risky, perhaps even foolish, behavior on the part of the squirrels. Research has shown that snake harassment actually serves multiple purposes. First, snakes rely on camouflage and stealth to capture prey. When a squirrel detects a snake, it alerts other squirrels in the area by vocalizing. With other squirrels on alert, the snake has little chance of successfully ambushing them. By congregating around the snake and continuing to harass it squirrels are able to drive the snake out of the area.
Second, by approaching the snake, a squirrel is able to see it better and gather important information about it, such as its size and type. California ground squirrels can visually discriminate venomous rattlesnakes from nonvenomous gopher snakes and change their behavior accordingly. This type of information is important, as a large snake poses a greater risk than a small one. And a venomous snake, like a rattlesnake, needs to be approached more cautiously than a nonvenomous one.
Third, by looming near a snake and kicking dirt or sand at it, a squirrel can elicit information from the snake. When threatened by a squirrel, rattlesnakes shake their rattle, and the sound of the rattle can tell the squirrel a lot about how dangerous the snake is. A warm snake rattles faster than a cold snake, and a large snake rattles at a lower frequency than a small snake. Studies by Donald Owings and colleagues at the University of California, Davis, have shown that California ground squirrels alter their harassment behavior based on the nature of the rattling sounds, approaching warm, fast-rattling snakes more cautiously than cold, slow-rattling snakes.
Finally, in most species of squirrels that mob, the juveniles and babies, not the adults, are most at risk from snakes. This is because adult squirrels are generally too large for the snakes and the adults of many ground squirrel species have some level of venom resistance to the snakes they encounter. So, in many cases the adult squirrels doing the harassment are at less risk than it first appears.
There is a long evolutionary history linking certain squirrel and snake species. For example, present-day California ground squirrels, Pacific gopher snakes, and Northern Pacific rattlesnakes have coexisted for thousands of years. According to fossil evidence, the ancestor of the California ground squirrel, a species of Spermophilus (Otospermophilus), coexisted with the ancestors of both gopher and rattlesnakes as far back as the middle Miocene, some 10 million years ago. Rock squirrels (Spermophilus variegatus) and Mexican ground squirrels (Spermophilus mexicanus) have similarly long evolutionary history with various species of rattlesnakes. One result of this long history is that these ground squirrels have evolved resistance to the toxic venom of the species of rattlesnakes they encounter most frequently. This venom resistance allows the adults to engage snakes offensively with little or no risk, to protect their young. Venom resistance is not consistent within a species but varies between populations depending on the prevalence of snakes in their area.
Naomie Poran, then at the University of California, Davis, and colleagues researched the variation in venom resistance among different populations of California ground squirrels. They found that California ground squirrels from a high-density rattlesnake area were able to neutralize the highest amount of venom, with few side effects, and heal quickly. Whereas the same species of squirrel from a snake-free area in Oregon could not neutralize a similar dose of venom, suffered severe tissue damage, healed slowly, and in some cases died. In another study, Richard Coss and colleagues found that a population of California ground squirrels that nized a snake-free valley 9,000 years ago has only half the venom resistance of their nearest neighbors in a high-snake area. Another population of the same species living in the San Joaquin River Delta, a snake-free region for more than 60,000 years, has almost no venom resistance.
Another result of the long evolutionary history of the co-occurrence of snakes and squirrels is that squirrel pups from such areas innately recognize snakes. Even squirrels raised in a lab can recognize snakes and behave appropriately.
