Squirrels are among the most widely known and recognized mammals, inhabiting all continents except Antarctica and Australia. In many parts of the world they occupy human habitats, gladly sharing our lunches in a city park, helping to empty our bird feeders, or feeding on our crops. Squirrels belong to a family of rodents, the Sciuridae, whose common ancestor lived some 30 to 40 million years ago and gave rise to the 278 species we currently assign to the family. All squirrels share a number of anatomical features—teeth, jaw musculature, skull, and other bones—that scientists use to identify recent and fossil squirrels. There are also important differences in anatomy between the main groups of squirrels. Below we discuss some basic components of squirrel anatomy, including similarities and differences between tree squirrels, flying squirrels, and ground squirrels.
TEETH AND SKULL. Squirrels, like all rodents, have four chisel-like incisors at the front of the mouth—two above and two below—which they use for gnawing. These incisors grow continuously and have roots that extend well back into the maxilla and mandible, unlike human teeth, which have short roots and grow only so long. Constant gnawing keeps rodent incisors short and sharp, because the enameled layer on the outside of the tooth wears much more slowly than the inner layers of dentine. Squirrels use their incisors to gnaw almost anything, but under natural conditions they use them most commonly to cut vegetation and to gnaw hard objects like nuts. If for some reason a squirrel cannot gnaw, its incisors will continue to grow until the animal dies because it can no longer eat.
Eastern gray squirrels (Sciurus carolinensis) are perhaps the most widely recognized squirrel species in the world. Photo © Donald Reeve,www.scarysquirrel.org
Cheek teeth are separated from the incisors by a space called the diastema. A squirrel’s cheek teeth consist of one or two premolars and three molars in the upper jaw, and one premolar and three molars in the lower jaw. These have short roots and determinate growth, meaning they grow for a short while and then stop. Squirrels use their molars and premolars for grinding up food before they swallow it, just as you do. The molars and premolars have cusps (called cones on the upper teeth and conids on the lower teeth) connected by ridges (called lophs on the upper teeth and lophids on the lower teeth). The differences and similarities in patterns of these cusps and ridges on teeth are used extensively by paleontologists to identify fossils and to recognize which fossil mammals are closely related to which others.
A squirrel must be able to move its jaw forward to bring its incisors together to gnaw, but it also must be able to move its jaw backward to bring the molars together to grind. To move the jaw forward to gnaw, some of the jaw muscles must attach farther forward on the skull than they do on the lower jaw. Different kinds of rodents have evolved jaw muscles that attach farther forward on the skull in several different ways. In all but the most ancient fossil squirrels, these muscles attach below and in front of the orbit of the eye, and also (in most squirrels) on the side of the nose. This type of jaw muscle attachment is called sciuromorphy. It is also found in a handful of other rodents, like beavers, pocket gophers, and kangaroo rats.
HANDS AND FEET. The hands and feet of squirrels are relatively unspecialized, with five digits on each, although the thumb ranges from small to diminutive. Like you, they have three phalanges, or bones, on each finger and toe, except for the thumb and the first toe, each of which has two. The relative proportions of the phalanges of squirrels, particularly of ground squirrels, are different from yours, and they have claws instead of nails on the terminal phalanges of all digits except for the thumb. There is a small nail on the thumb, but it may not be large enough to be seen. The hands of burrowing squirrels tend to be wider than those of tree squirrels, with the middle digit the longest, presumably because a wider hand is more useful for digging. In tree squirrels, the fourth digit is the longest and serves as an effective grappling iron when the squirrel climbs a vertical trunk. The hands and wrists of flying squirrels also have an extra addition, the styliform cartilage, which extends laterally from the heel of the hand and assists in the support of the gliding membrane.
X-ray of the skull of the southern Asian tufted ground squirrel (Rheithrosciurus macrotis), with its ever-growing incisors.
One of the most striking features of tree and flying squirrels is their ability to turn their hind feet around when they are coming down a tree headfirst. For you, the equivalent move would be to stand on your tiptoes, rotate your ankles so that the soles of your feet face each other, then keep rotating them until the soles of your feet point forward–without moving the rest of your leg! Most squirrels accomplish this with the extraordinary mobility of the joint between the most proximal ankle bone, the astragalus or talus,and the heel bone, the calcaneus. The ankle anatomy of some squirrels (forexample, the Holarctic tree squirrels: those found in the northern parts ofNorth America and Eurasia) shows certain specializations for this move,whereas in other squirrels (the Southeast Asian tree squirrels) the same specializationsare not present. However, these squirrels still are able to turn their feet around. We just aren’t yet sure how they do it.
Another notable difference in squirrel anatomy is limb length. Ground squirrels usually have shorter limbs than tree squirrels for their body size. Tree squirrels need long, muscular legs for leaping from branch to branch and long arms to reach around a tree trunk. By contrast, a burrow-dwelling ground squirrel has short, stout limbs well suited for digging. Flying squirrels have the longest limbs relative to body size of all the squirrels. This appears to be a necessary part of the flying squirrel adaptation. (For moreinformation on flying squirrel anatomy see “How do you make a flying squirrel?” in chapter 2.)
X-ray of the forearm of a ground squirrel and a tree squirrel of comparable size. On the left is a Richardson’s ground squirrel (Spermophilusrichardsonii); on the right is the North American red squirrel(Tamiasciurus hudsonicus). Notice how digit no. 4 is much longer on the tree squirrel than on the ground squirrel.
Some Holarctic ground squirrels have another specialization, not seen in tree squirrels: an extra layer of the deltoid muscle of the shoulder that extends to the forearm. This addition assists in flexing the forearm and is presumablyimportant in digging. It is not found in chipmunks (Tamias) or antelope ground squirrels (Ammospermophilus), so it must have evolved among the other ground squirrels (Spermophilus, Marmota, and Cynomys) after the chipmunks and antelope ground squirrels diverged from the lineage. It also is not found in any of the African or southern Asian ground squirrels.
TAILS. Many ground squirrels have relatively short tails, which are probably useful in a burrow, particularly for feeling their way when backing up. Tree squirrels, on the other hand, have longer tails, which are useful for balance when leaping and running across narrow branches. All squirrel tails are most flexible at the base of the tail. This is because they have many short vertebrae, thus more intervertebral joints, at the base of the tail, and longer vertebrae toward the middle and end of the tail. The characteristic way a tree squirrel holds its tail, curved up against the back, demonstrates this attribute quite nicely. The hairs on the tails of most tree squirrels are haired distichously, meaning the hairs along the sides are much longer than the hairs along the top or bottom. Among the giant tree squirrels, the hairs on the tail are instead approximately the same length in every direction. We think this reflects the effective wind resistance of a broad tail, thus assisting with balance in a small squirrel (perhaps up to 1 kg, or 2.2 lbs), contrasting with the need for a longer, more massive tail in a large squirrel (more than 1.5 kg, or 3.3 lbs). The longer the hairs, the more the hairs bend and the less wind resistant they are. A large squirrel, like a giant tree squirrel, cannot grow long enough, thick enough hair to be wind resistant and thus effective for assisting balance.
An eastern gray squirrel outside Washington, D.C., pauses while descending a tree headfirst–a feat made possible by its specialized ankle anatomy. Photo © Caroline Thorington
Another important function of tails is in heat conservation or dissipation. Long cylindrical objects, like rodent tails, are difficult to insulate and they lose heat rapidly because they have so much surface area per unit of length. Although this is convenient if the animal is overheated, it is inconvenient if it is trying to keep warm. Squirrels, like other rodents, have a complex vascular system in their tails—a “countercurrent heat exchange system” within a vascular bundle on the ventral side of the base of the tail. In cold conditions, the warm blood entering the tail by the large ventral arteries warms the venous blood returning to the body in the adjoining ventral veins, and in so doing, the arterial blood cools. In other words, heat is returned to the body via the venous blood, not lost to the environment. In warm conditions or when the animal is overheated, much of the venous blood is returned to the body through large lateral veins. This blood is not warmed in the countercurrent system, and so the arterial blood to the tail is not cooled. The warm arterial blood warms the skin of the tail, which then loses heat to the environment. The result is that under cold conditions, the tail itself is not warmed up, and little body heat is lost. But under warm conditions, the tail is warmed and heat is lost from the whole surface to the air.
The clavobrachialis muscle, found in the Holarctic ground squirrels (tribe Marmotini) but not in African ground squirrels (tribe Xerini). Photo © Karolyn Darrow
The tail is also used in thermoregulation by serving as a blanket over the squirrel in cold weather and, among some squirrels, as a parasol to protect them from the heat of the sun in hot weather.
VIBRISSAE. Hairs are very sensitive to touch, as everyone knows, and squirrels have long, stiff hairs, called vibrissae, dedicated to the detection of tactile stimuli. There are a dozen to two dozen vibrissae on the side of the nose above the mouth and three to six vibrissae on the side of the cheek. There are usually four vibrissae above the eye, rooted at the front edge, several on the underside of the chin, and several farther back but above the throat. There are also vibrissae near the wrist, at the lower end of the forearm just above the lateral side of the wrist. All whiskers are vibrissae, but not all vibrissae are whiskers.
A world map showing the distribution of squirrels. The distinction between tree squirrels and ground squirrels is arbitrary in some cases.
We have recently estimated that there are 278 species, or unique types, of squirrels. This number is likely to increase as we continue to study squirrels more carefully, in particular, as we learn more about their DNA. The ecological diversity of squirrels is also large. In North America, there are tree squirrels, flying squirrels, ground squirrels, chipmunks, marmots, prairie dogs, and antelope ground squirrels (so named because they are speedy little fellows and show pale rumps when they bound away). In South America there are tree squirrels and pygmy tree squirrels, but no flying squirrels and no ground squirrels. The lack of diversity in South America is not surprising because squirrels have been on that continent for only a few million years. In Africa, where squirrels have been for 20 million years, there is a greater diversity of squirrels. There are tree squirrels, a pygmy tree squirrel, ground squirrels, and bush squirrels, but no flying squirrels. (There are scaly-tailed flying squirrels in Africa, but they are a completely different group of rodents from the true squirrels.) In Eurasia, there are tree squirrels, marmots, and chipmunks—all closely related to the North American ones—and a few species of flying squirrels. In Southeast Asia there are the beautiful tree squirrels, the giant tree squirrels, four species of pygmy squirrels, a large number of flying squirrels, and several more specialized squirrels. We frequently refer to Southeast Asia as the “squirrel headquarters of the world” because of the extraordinary diversity there.
A marmot, better known for its ground-dwelling habits, demonstrates its talent for tree climbing. Photo © National Park Service
Squirrels are found in an extraordinarily diverse range of habitats, from desert conditions to extremely wet rain forests. The greatest diversity of squirrel species is found in warmer climates, specifically the tropics, but species of ground squirrels and marmots occur in the Arctic, where they hibernate through the long, cold winter. Squirrels occur naturally on all continents except Antarctica and Australia. As far as we know, squirrels never have reached Madagascar, Greenland, or any of the truly oceanic islands, like Hawaii or the Galápagos Islands. However, squirrels are found on the Philippine Islands and Sulawesi.
We commonly divide the squirrels into three groups: the tree squirrels, the ground squirrels, and the flying squirrels. For the most part, this division is a good description of the animals’ habits. Tree squirrels and flying squirrels usually live and nest in the trees, sometimes in tree hollows and at other times in leaf nests outside of hollows. Many of them, however, forage for food on the ground, like the eastern gray squirrel (Sciurus carolinensis) and the northern flying squirrel (Glaucomys sabrinus), and some of them, like the North American red squirrel (Tamiasciurus hudsonicus), may even nest underground. Ground squirrels usually nest in underground burrows and forage on the ground. Chipmunks (like the eastern chipmunk, Tamias striatus, and the Townsend’s chipmunk, Tamias townsendii, in the West) and even marmots (like the woodchuck, Marmota monax) will climb trees, and some chipmunks will nest in trees, for example, the shadow chipmunk (Tamias senex).
A rock squirrel (Spermophilus variegatus) at Grand Canyon National Park in Arizona. Photo © Tim Coffer
Outside North America, squirrels show even greater diversity in their habits. Some members of the Southeast Asian tree squirrel group are strictly arboreal, while others are strictly terrestrial. The woolly flying squirrel (Eupetaurus cinereus) and the complex-toothed flying squirrel (Trogopterus xanthipes) spend the daytime sleeping in caves, not trees, and the Asian long-clawed ground squirrel (Spermophilopsis leptodactylus) sometimes burrows in sand dunes. The Himalayan marmot (Marmota himalaya) inhabits parts of the Himalayan Mountains of Nepal, Tibet, and India at high elevations from 4,000 meters to 5,500 meters (13,123 to 18,035 feet) and is considered one of the highest-living mammals in the world.
There is a well-known “Harvard Law of Animal Behavior” which states, “Under the best controlled experimental conditions, the animal will do whatever it darn well pleases.” Squirrels in the wild seem well acquainted with this law! They opportunistically exploit their environment, in the trees, on the ground, and under the ground, irrespective of how we categorize them.
Lush vegetation provides food and shelter for eastern gray squirrels in the tropical habitats of southern Florida. Photo © Donald Reeve www.scarysquirrel.org
With the possible exception of the pygmy squirrel (Sciurillus), squirrels did not get to South America until relatively recently, in geological terms, after the Panama land bridge formed between North and South America. At that time, there most likely were no flying squirrels or ground squirrels in southern Central America, just like today. Consequently, since only tree squirrels were present in the southernmost part of Central America, it could only be tree squirrels that would cross the Panama land bridge into South America, which probably occurred within the last 3.5 million years. At present, the southern flying squirrel (Glaucomys volans) has a range only as far south as Honduras, and the southernmost ground squirrels (Spermophilus) reach no farther south than Mexico.
Classification is a way to organize large amounts of information. To classify life on earth, scientists use a hierarchical classification system that starts broadly and becomes more specific. Organisms, on the basis of similarities or differences, are placed first into kingdoms, which are subdivided into phyla or divisions. These in turn are divided into classes, then into orders, then families, and finally into genera and species. Every known organism is identified by two Latin terms: the genus name and the species name. These two names together are the official scientific name of the species. For example, the scientific name for the eastern gray squirrel is Sciurus carolinensis. (Notice that the name of the genus is capitalized, but the name of the species is not.) This system, called binomial nomenclature, was developed by biologist Carolus Linnaeus in 1758. Other systems of classification and nomenclature are being debated, but they will not be treated in this book, with the exception of cladistics, which emphasizes groupings (clades) of organisms sharing a common ancestor and includes that ancestor and all its descendents in the clade.
Grasslands are the habitat of choice for Richardson’s ground squirrels (Spermophilus richarsonii) in southwestMontana. Photo © Caroline Thorington
Squirrels, like all organisms, are classified by the way they are related to one another. All squirrels are more closely related to one another than they are to any other mammals, and so we put them together into a family, the clade of all squirrels. Similarly, families can be divided into subfamilies that contain squirrels that are more closely related to each other than they are to any other squirrels. The same is true for every tribe, genus, and so on. The trick is to determine how different species and groups of species are related. We determine these relationships in two main ways: by looking at similarities and differences in morphology and in DNA. With correct interpretation of which features are ancestral and which features are derived, we can determine the genealogical relationships, or the phylogeny, among squirrels. These relationships are commonly represented as cladograms, which are stick-figure trees showing how the species and clades of species are related.
DNA is a particularly useful tool for determining phylogenies, because there are so many parts of the molecule (specifically, base pairs) subject to change. The DNA of an organism is replicated every generation, so every squirrel has DNA almost exactly like that of its father and mother. Small mistakes in the replication do occur, and these mistakes are passed on to the subsequent generation when the squirrel breeds. We like to use the analogy of medieval scribes, who, in the time before printing presses, copied manuscripts by hand. If a scribe made a mistake in copying, the subsequent copies of a manuscript made from his original would incorporate the same mistake. Subsequent scribes might make additional mistakes not found in the one from which they copied but incorporated into later copies. Thus, historians can trace the origins of differing manuscripts from the mistakes that have or have not been incorporated into them. Similarly, we can determine the evolution of squirrels from the errors of DNA replication that have or have not been passed on to subsequent generations.
The process is actually more complicated than we have portrayed it. DNA degrades over time, so all the DNA used in our molecular research is from modern squirrels. This means that the squirrel family tree must be constructed by working backward from the tips of the branches. If an animal or a group of animals, like the giant tree squirrels, does not have any close relatives, then it will be represented as the tip of a long branch. Long branches can be problematic, because they can conceal repeated replication errors. To return to our analogy of medieval scribes, it is as if a series of scribes wrote “the” instead of “two” and “two” instead of “the.” After a series of such replication errors, these words would be almost useless for interpreting the lineage of the resulting manuscript. Computer programs are used to assess the probabilities that such problems occur and the likelihood that a particular tree is the best estimate of the true tree.
Some genes evolve more quickly than others and are better for estimating recent divergences between closely related species. Mitochondrial genes are commonly used for this purpose. These genes are found in the mitochondria of the cell and are inherited solely from the mother. Other more slowly evolving genes, usually nuclear genes, are better for resolving more ancient divergences. The nuclear genes are found on the chromosomes within the nucleus and are inherited from both parents. We will not go into further description of the techniques used for analyzing DNA.The analytic techniques themselves are evolving rapidly, and there is much experimentation and ongoing discussion about which methods are best.
Current squirrel phylogeny based on research by Scott Steppan et al., 2004.
Two recent studies of the DNA of squirrels, one by John Mercer and Louise Roth at Duke University and a second by Scott Steppan and his colleagues at Florida State University, have greatly increased our knowledge of squirrel phylogeny. Their conclusions are in good general agreement with one another and with previous phylogenies based solely on anatomical features, but there are some differences.
Squirrels belong to the order Rodentia, and within that belong to the family Sciuridae. Based on the current morphological and molecular studies, the family Sciuridae is presently divided into five subfamilies: Sciurillinae, Ratufinae, Sciurinae, Callosciurinae, and Xerinae.
Squirrels evolved as a distinct lineage of rodents approximately 36 million years ago, at the end of the Eocene or beginning of the Oligocene geological epic. In both of the DNA studies, the most ancient squirrels are the pygmy tree squirrels of South America (Sciurillinae) and the giant tree squirrels of southern Asia (Ratufinae). Sciurillinae contains only a single recognized species, but we believe that further study will prove this single species actually is a collection of several closely related species. Ratufinae contains the four species of giant tree squirrels of Southeast Asia. Among these, the Indian giant squirrel is one of the most beautiful squirrels in the world, with dark maroon and black hair on the back and tail, cream-colored legs, feet, and tail tip, and maroon ear tufts. In the Mercer and Roth molecular study, the Sciurillinae and Ratufinae seem to have branched off independently, the pygmy squirrels diverging first. Scott Steppan and his colleagues contested this with their data, which suggest that the two sub-families diverged from a common branch of the squirrel tree. It is a bizarre idea that the South American pygmy squirrel and the Asian giant tree squirrel are each other’s closest relatives, but it is possible, considering that they have probably evolved independently for approximately 30 million years. There is, however, little question that these two groups are ancient lineages, because they both have anatomical peculiarities, notably in skull morphology, that have long been recognized.
The other squirrel subfamilies diverged later but still in the late Oligocene or early Miocene. The subfamily Sciurinae includes all flying squirrels (tribe Pteromyini) and all tree squirrels (tribe Sciurini) of North America, Europe, northern Asia, and South America (except the pygmy squirrel)—a total of 81 species. It has been debated for a long time whether gliding evolved once or twice among squirrels. The molecular data in both investigations clearly support the thesis that gliding evolved only once in the family Sciuridae. This conclusion agrees with the anatomical data, but the close relationship between tree squirrels and flying squirrels is not in accord with some interpretations of the fossil record of flying squirrels. We think that this calls for reinterpretation of the fossil record (which unfortunately is based almost entirely on teeth) because there are some striking anatomical similarities between the Pteromyini and the Sciurini.
A group of tree squirrels in southern Asia forms a fourth subfamily, the Callosciurinae, which includes 64 species. Callosciurinae means “beautiful squirrels,” and some species of these, such as the tricolored Prevost’s squirrel of Malaysia, rival the dramatic coloration of the Indian giant squirrel. This subfamily was first recognized and described by Reginald Pocock at the British Museum of Natural History in 1923, based on his study of the penis bones of squirrels. His conclusions are completely concordant with the results of the two DNA studies. The fifth subfamily, Xerinae, is the largest group and includes 128 species. Almost all the ground squirrels belong to it: the North American and Eurasian ground squirrels, marmots, chipmunks, and so on (tribe Marmotini), and the African ground squirrels (tribe Xerini). The subfamily Xerinae also includes the 30 species of African tree and bush squirrels (tribe Protoxerini). There has been some speculation that two different squirrels invaded Africa, with one the progenitor of the African ground squirrels and the other the progenitor of the African tree and bush squirrels. The molecular data support this hypothesis.
Table 1.1. Classification of the Sciuridae
order RODENTIA
family Sciuridae
subfamily Sciurillinae (1 species)
South American pygmy squirrels
subfamily Ratufinae (4 species)
giant tree squirrels of southern Asia
subfamily Sciurinae (81 species)
tribe Sciurini
North and South American, northern Asian, and European tree squirrels
tribe Pteromyini
all flying squirrels
subfamily Callosciurinae (64 species)
southern Asian tree squirrels
subfamily Xerinae (128 species)
tribe Marmotini
North American and Eurasian ground squirrels
tribe Xerini
African ground squirrels
tribeProtoxerini
African bush and tree squirrels
The giant tree squirrels of southern Asia (subfamily Ratufinae) are twice to three times the size of the largest tree squirrels elsewhere. They frequently average 1,800 grams (4 lbs) and individuals may exceed 2 kg (4.4 lbs). To compare, the largest tree squirrels in North America, the fox squirrels, weigh approximately 1 kg (2.2 lbs), the largest squirrels of South America are somewhat smaller, and the largest tree squirrels of Africa are only 600–700 grams (1.3–1.7 lbs). The Ratufinae have extraordinarily long tails, which are used to counterbalance the mass of the body. They are impressive animals, and visitors to the Smithsonian, who are not familiar with the giant tree squirrels, are always awed when we show them specimens that are more than 76 cm (30 inches) long, from tip of nose to tip of tail. In the wild, the squirrels frequently sit crosswise on a branch with their body hanging down on one side and their tail on the other side, instead of sitting up on their haunches like other tree squirrels. The giant squirrels also have larger thumbs and manipulate their food with them more than do other tree squirrels.
The South American pygmy squirrels (Sciurillinae) are small (33–45 grams; 1.2–1.6 oz), active, normal-looking squirrels, at least on the outside. They are the smallest South American squirrels, about the same size as the least chipmunk (Tamias minimus) of North America. Smithsonian researcher Louise Emmons describes them feeding from close to the ground to high in the trees in the rain forest. They are bark gleaners, which forage on tree trunks, prying or gouging small pieces of loose bark, and scraping food off the undersurface. Anatomically, they are very distinct. When he was a researcher at the American Museum of Natural History, the late Joseph Moore considered that these squirrels were some of the most unusual squirrels and listed 12 distinctive features of skull morphology, noting that the South American pygmy squirrels differ from every other group in at least six of these features.
The Indian giant squirrel (Ratufa indica) shows the feeding posture characteristic of the giant tree squirrels. Photo © Sudhir Shivaram
The common tree squirrels of southern Asia are the squirrels in the subfamily Callosciurinae, which were formerly confused with the American and the Holarctic tree squirrels (tribe Sciurini of the subfamily Sciurinae). In 1923, Pocock reported on his study of the baculum (the penis bone) of squirrels. He demonstrated that this bone is very useful for sorting out the taxonomy of squirrels. He found that all Callosciurinae have an extra blade on the baculum seen in no other squirrels. This feature evolved in the lineage of this group, shortly after it diverged from the lineage of the Indian striped squirrels (Funambulus), just as suggested by Pocock. Recent molecular studies of squirrel DNA have completely supported Pocock’s findings, and the subfamily is now considered to include Funambulus and all the other tree squirrels of southern Asia, except of course, the Ratufinae. You do not need to dissect one of the squirrels to distinguish it from an American or Holarctic tree squirrel. In a zoo or elsewhere, just look at the size of the ear. The Callosciurinae usually have much smaller external ears than the North American or Eurasian tree squirrels, and only one, the diminutive pygmy squirrel, Exilisciurus whiteheadi, has tufted ears. Many species of the Callosciurinae are very distinctive, like the dramatic tricolored Prevost’s squirrel (Callosciurus prevostii), the ant-eating, long-nosed shrew-faced squirrel (Rhinosciurus laticaudatus), and the pygmy black-eared tree squirrel (Nannosciurus melanotis) with its white mustache mark. Others are less distinctive, like the Indian striped squirrels (Funambulus) and the striped squirrels of Southeast Asia (Tamiops), which look like North American or Asian chipmunks (Tamias).
The tricolored Southeast Asian Prevost’s squirrel (Callosciurus prevostii). Photo © Jesse Cohen, National Zoo
The Sciurinae is the subfamily that includes the flying squirrels (tribe Pteromyini), the tree squirrels of the Americas (excluding the South American pygmy squirrel), the tree squirrels of Europe and northern Asia, and one squirrel (Rheithrosciurus) of Borneo (tribe Sciurini). All Pteryomyini are principally nocturnal. They range in size from the 15-gram (approximately half an ounce) pygmy flying squirrel (Petaurillus emiliae) on Borneo to the 2.5-kg (5.5 lb) woolly flying squirrel (Eupetaurus cinereus) of the mountains of north Pakistan and Tibet. They are widely distributed in North America, across northern Eurasia, from Finland to Japan, and are most speciose in southern Asia, in particular, in Borneo.
The Pteromyini are easily recognized by the skin extending from the wrist to the ankle, the patagium, which serves as a wing or parachute when they glide. Larger flying squirrels also have skin extending from the ankle to the tail, the uropatagium, that acts like a rear flap on a plane’s wing. Small flying squirrels don’t have much, if any, of this rear flap.
The northern palm squirrel (Funambulus pennantii) from South Asia has adapted well to living in a human-altered environment. Photo © Dr. S. N. Naik
There are distinctive marks on the wrist and ankle bones of flying squirrels associated with the attachment of the patagium muscles. In the wrist there is a special arrangement of three bones (the pisiform, triquetrum, and the scapholunate), which provides increased stability at the wing tip, at the cost of reduced mobility of the wrist. Just above the ankle, on the long bone (the tibia), many flying squirrels have a small extra bumpf for the attachment of a special wing muscle. On other flying squirrels, this muscle attaches on the foot, but there is no bump there. Smaller flying squirrels (less than 1 kg) usually have tails with long lateral hairs (a pattern called distichous), but bigger flying squirrels have relatively longer tails that are round (a pattern called terete)—with lateral hairs the same length as those above and below.
The tree squirrels (tribe Sciurini) are more difficult to characterize. Their geographic range includes South America, North America, Europe, northern Asia, and a single species in Borneo. They are typical tree squirrels in having usually long, distichously haired tails, but in contrast with most of the Callosciurinae, almost all species have larger ears, which are frequently tufted. The Sciurini also lack the extra blade on the baculum that all Callosciurinae possess. The bacula of the South American pygmy squirrel (Sciurillus) and squirrels in the genus Sciurus, however, are very similar, suggesting that the bacular morphology of the Sciurini is similar to the ancestral form. In 1959, Joseph Moore listed four somewhat distinctive features of the skull in the Sciurini, but cranial anatomy is not very diagnostic. There is an interesting and distinctive feature of the ankle joint that characterizes both the flying squirrels and the tree squirrels of the sub-family Sciurinae. There is normally a prominent sulcus or groove on the underside of the first ankle bone, the astragalus. It is quite prominent in the Ratufinae and the Callosciurinae, and only slightly less prominent among the African tree squirrels. However, the sulcus is nearly obliterated in the Sciurini tree squirrels and it is even less evident in the flying squirrels. We think it has to do with ankle rotation and the way these squirrels turn their feet around when descending a tree head first, because obliteration of the sulcus is associated with an increased joint surface between the astragalus and the calcaneus, where ankle rotation occurs. Chipmunks and some other North American ground squirrels that climb have the sulcus obliterated also, however, complicating the interpretation and diagnostic utility of this feature.
The basic parts of a flying squirrel
The subfamily Xerinae includes such a heterogeneous group of squirrels that it is the most difficult of all subfamilies to characterize. Included are the African ground squirrels (tribe Xerini), all the ground squirrels of North America and northern Eurasia—from the chipmunks to the marmots and prairie dogs (tribe Marmotini)—and all the African tree squirrels, including the sun squirrels, the rope squirrels, and the bush squirrels (tribe Protoxerini). Only molecular biology unites these three tribes into a single subfamily, but on the basis of two separate studies utilizing five different genes. Joseph Moore did note five cranial characters shared by squirrels of the tribes Xerini and Marmotini, but he considered these similarities to be due to convergent evolution not common ancestry.
The tribe Xerini includes four species of African ground squirrels and the long clawed ground squirrel (Spermophilopsis leptodactylus) from Turkmenistan, Uzbekistan, and neighboring countries. Spermophilopsis and Xerus rutilus are unstriped, whereas the other species have prominent white stripes along their sides. Spermophilopsis is the only one with a short tail. All four species share seven distinctive cranial features, four of which distin guish them from the Marmotini. One of these is a bony palette that extends well behind the cheek teeth, a feature that distinguishes them from most other squirrels.
The unstriped ground squirrel (Xerus rutilus) of East Africa. Photo © U.S. Fish and Wildlife Service
The tribe Marmotini is difficult to characterize, because it includes animals as different as chipmunks (Tamias) and marmots (Marmota). However, it has long been recognized as a natural group on the basis of several characteristics. All species, for example, have cheek pouches, which are not found in any other squirrels. Another anatomical feature is the layer of the deltoid muscle that extends from the clavicle to the forearm in the marmots, ground squirrels, and prairie dogs. This is unique among squirrels. The species of Marmotini come in a bewildering variety of colors and patterns. Chipmunks are striped, as are many ground squirrels. Some ground squirrels are spotted, or even striped and spotted (the 13-lined ground squirrel, Spermophilus tridecemlineatus). Marmots and prairie dogs are unstriped and relatively uniformly colored. Size differences are also extreme. The least chipmunk may average 40 grams (1.4 oz), but the largest marmots may weigh 8 kg (almost 18 lbs). Tail length is also variable: The tail is 45% of the total length of the Baja California rock squirrel (Spermophilus atricapillus), 20–25% of the total length of the woodchuck (Marmota monax), and less than 20% of the total length of the white-tailed prairie dog (Cynomys leucurus).
The tribe Protoxerini includes the African sun squirrels (genus Heliosciurus), rope squirrels (Funisciurus), giant squirrels (Protoxerus and Epixerus), pygmy squirrel (Myosciurus), and bush squirrels (Paraxerus). The squirrels of the Protoxerini are also diverse and are combined into one tribe by DNA data but not by similar appearance or morphology. They range in size from the 16-gram (approximately half an ounce) African pygmy squirrel (Myosciurus pumilus) to the 700-gram (1.5 lbs) giant forest squirrel (Protoxerus stangeri), some are striped (species of Funisciurus and Paraxerus), most are not, and neither the study of bacula by Pocock nor the study of skulls by Moore suggested a close relationship among them all. It is likely that the genetic studies correctly identify their common ancestry, and the morphological studies document the evolutionary divergence since their ancestor invaded Africa, probably 20 million years ago.
Yellow-bellied marmots (Marmota flaviventris) range from southern British Columbia and Alberta south to northern New Mexico. Photo ©National Park Service
Prairie dogs, like this black-tailed prairie dog, inhabit the great plains of southern Canada, mid-America, and northern Mexico. Photo © U.S. Fish and Wildlife Service
Golden-mantled ground squirrel(Spermophilus lateralis) with stripes and white eye ring, characteristic of some ground squirrels. Photo © NationalPark Service
Weighing only 16 grams (or slightlymore than half an ounce), the African pygmy squirrel (Myosciurus pumilio)is one of the smallest squirrels in the world. Photo © Alain Devez
A comparison showing the relative similarities between the rear foot of the 36-million-year-old Douglas-sciurus jeffersoni and the rear foot of a modern North American red squirrel (Tamiasciurus hudsonicus). Notice how the fourth digit of both squirrels is the longest, which is typical of tree squirrels.
Squirrels evolved from more primitive rodents approximately 36 million years ago, at the end of the Eocene or the beginning of the Oligocene epoch. At this time, the earth was much warmer than it is now, and consequently it was more heavily forested. This probably provided an arboreal niche, for which the ancestral squirrels became adapted.
There is a difference of opinion about which is the oldest fossil squirrel, resulting more from differences in definition than from disagreement about the fossils. Jefferson’s squirrel (Douglassciurus jeffersoni) dates from the late Eocene of Wyoming, 36 million years ago. It is an extraordinarily complete skeleton, found inadvertently during a 1975 field season by Jennifer Emry, the wife of Smithsonian paleontologist Bob Emry. Jefferson’s squirrel has a skeleton surprisingly like that of a modern tree squirrel.In fact, Thorington mistook it for a modern squirrel when he first saw it.However, the skull of Jefferson’s squirrel exhibits an ancestral anatomy of the jaw muscles, not the anatomical arrangement found in all later fossil squirrels and all modern squirrels. Therefore, some paleontologists exclude it from the squirrel family, although they may consider it to be ancestral or closely related to the ancestor of the family. Others, like us, consider it to be a squirrel despite the primitive jaw musculature. There is no question, however, that it is the oldest fossil.
The next oldest fossil, described by French paleontologist Monique Vianey-Liaud in 1974, is the 31-million-year-old Paleosciurus goti from the early Oligocene of southern France. It is also a surprisingly complete skeleton and is the oldest fossil to exhibit the typical jaw musculature of squirrels, hence the claim that it is the oldest fossil squirrel. Unexpectedly, on the basis of the relative lengths of the arm bones, some have claimed that it was a terrestrial squirrel. We are uncertain about this claim, however, because the bones look very much like the bones of a tree squirrel, based on the published illustrations. Also, the illustration of the radius shows that this forearm bone is slightly damaged at one end, causing it to be shorter than expected of a tree squirrel and more like a ground squirrel, as reported.
