The largest squirrels are the marmots (Marmota), which accordingly can be called “the giant ground squirrels.” In North America, the best known are the yellow-bellied marmot of the Rocky Mountains and the woodchuck of the eastern United States and Canada, which is the proverbial “ground-hog” of February 2. The largest of all is the gray marmot found in the mountains of Khazakstan. All marmots put on weight before they enter hibernation—some of them double their weight—so the animals are heaviest at the end of the summer. At this time, the largest gray marmots may weigh more than 8 kg (18 lbs), truly a giant squirrel! Among tree squirrels, the giant tree squirrels of Southeast Asia (Ratufa) are not nearly as big as the marmots, but they are still quite large, ranging from approximately 2 to 3 kg (4 to 6 lbs). With their beautiful long tails and striking coloring, these squirrels are impressive as they bound above you through the trees.
In contrast, the smallest squirrels are the pygmy tree squirrels, such as the ones in western Africa (Myosciurus pumilio) and the ones in Southeast Asia (Exilisciurus spp.), which are smaller than some mice. The smallest adults of both genera average approximately 14 or 15 grams (approximately half an ounce). They are so tiny you could mail two of them first class from New York to Los Angeles for a 39¢ stamp.
Mammalian heart rate is inversely related to body size, thus smaller squirrels have faster heart rates than larger ones. For example, the 13-lined ground squirrel, weighing approximately 140 grams (5 oz), has a heart rate of about 280 beats per minute. The heart of a 15-gram (half an ounce) pygmy squirrel probably beats about 500 times each minute, whereas the heart of an 8-kg (17.6 lbs) gray marmot (nonhibernating) is expected to beat approximately 145 times a minute, although neither has been measured. (For comparison, a human heart beats about 75 times each minute.) The heart rate of a hibernating squirrel can be amazingly slow, dropping to only 3 to 15 beats per minute. An individual squirrel’s heart rate will vary, and a chipmunk fleeing a hawk, unsurprisingly, will have a much higher heart rate than a one sunning itself on a log.
The skull of the largest and smallest tree squirrels in Africa. (Top) The African pygmy squirrel (Myosciurus pumilio), weighing approximately 15 grams. (Bottom) The forest giant squirrel (Protoxerus stangeri), weighing approximately 600 grams.
Yes. Squirrels have dichromatic color vision. They can distinguish color much like a human who has red-green color blindness—which means they can differentiate red or green from other colors, but cannot distinguish red and green from each other.
Several other aspects of squirrel vision are worth noting. Many squirrels have yellow-tinted eye lenses. Ground squirrels have dark-yellow lenses and tree squirrels have paler-yellow lenses. These yellow lenses, much like sunglasses, reduce glare from bright light and increase the contrast between colors, giving the squirrel sharper vision. Flying squirrels, however, have clear lenses. Because they are nocturnal and seldom encounter bright light, they have no need for a tinted lens.
Squirrels also have exceptional focusing ability. Human eyes have a fovea centralis or a small area of the retina where cones are most densely packed and vision is most acute. This is the part of the retina you use when you read. Squirrels, on the other hand, have sharp vision across the entire retina, which allows a motionless squirrel to see clearly what is next to it and above it at the same time without moving its head. Thus, a squirrel could read the small print of a newspaper with its peripheral vision.
As in most mammals, the squirrel retina contains both rods and cones. Rods are specially adapted cells that enable vision in low light, and cones are specially designed cells for daylight vision, color vision, and the discrimination of detail. That the retina of diurnal squirrels contains both rods and cones makes sense, because although they are primarily active during the day, they also need to see at dusk and dawn and in shaded areas. Ground squirrels, such as the prairie dog, are superbly adapted to bright light and have many more cones than rods. In fact it was once thought that they had no rods at all. The retinas of the nocturnal flying squirrels, on the other hand, have mostly rods and few cones, which gives them excellent night vision.
Good vision is very important to squirrels. It helps tree squirrels safely navigate through a complex three-dimensional environment and helps social ground squirrels identify and interact with each other. Most importantly, good vision is crucial for spotting and avoiding potential predators.
No, cheek pouches of various sizes are found only in ground squirrels— specifically in the chipmunks, antelope ground squirrels (Ammospermophilus spp.), rock squirrels, and other ground squirrels (Spermophilus spp.), marmots, and prairie dogs. No tree squirrels or flying squirrels have cheek pouches and neither, surprisingly, do any of the African ground squirrels.
There is a muscle in the cheek pouch that helps with emptying it, and this muscle attaches to the squirrel’s skull just behind the upper incisors. This muscle leaves a prominent scar on the bone where it attaches. When paleontologists find a fossil squirrel skull, they can infer that the squirrel had a cheek pouch if this scar is present. Accordingly, they have been able to document that cheek pouches evolved very early in the history of ground squirrels. From this evidence, we assume that early ground squirrels collected small seeds, carried them in their cheek pouches, and probably hoarded food in their burrow, the way chipmunks do today.
A foraging eastern chipmunk (Tamias striatus) fills its cheek pouches with sunflower seeds, which it will bring back to its burrow for storage. Photo © Phil Myers
Marmots, which evolved more recently and do not store food, have smaller cheek pouches than other squirrels in their lineage. We presume that the habit of feeding on the softer, vegetative parts of plants, the way marmots do, evolved later and led to the reduced usefulness of a cheek pouch and its reduction in size.
Yes, there are many records of tree squirrels swimming. During their migrations, eastern gray squirrels may swim across relatively large rivers, as commonly reported in past centuries. Bill Hamilton, at Cornell University, reported an abundance of squirrels in Connecticut and New York in the fall of 1933, with more than a thousand squirrels estimated to have attempted to swim across the Connecticut River near Hartford. During September 1968, population densities of eastern gray squirrels were very high throughout the eastern United States, and Vagn Flyger, at the University of Maryland, reported that squirrels were seen crossing rivers and reservoirs in a number of areas, including reservoirs in Tennessee and the Connecticut River near Hartford. In the autumn of 1990, there were dense populations of eastern gray squirrels in the vicinity of Washington, D.C., and they were observed swimming across the Potomac River. Similar observations have been reported for the Eurasian red squirrel. Robert Hatt cited several accounts of swimming in the North American red squirrel. Most recently, in 2005, Jonathan Pauli of University of Wisconsin-Stevens Point was kayaking in Lake Superior and noticed a North American red squirrel swimming. He followed the squirrel as it swam 30 minutes, about 1.5 km (approximately 1 mile) from the mainland to a nearby island through choppy water. This is the first report of a squirrel swimming in the Great Lakes, as well as the longest reported swim of a North American red squirrel.
A woodchuck (Marmota monax) forages along the water of the Tred Avon River off of the Chesapeake Bay. Although reports of ground squirrels swimming are rare, it is probable that those living near water do wade into the water to get vegetation. Photo © Peter Wainwright Thorington
Reports of ground squirrels swimming are more scanty. In 1883, renowned American zoologist C. Hart Merriam observed a juvenile woodchuck swimming across a lake in the Adirondacks but considered this an unusual observation. Golden-mantled ground squirrels have been seen swimming across small streams, and under experimental conditions, Richardson’s ground squirrels swim capably. All ground squirrels probably can swim but few choose to do so. In contrast, flying squirrels are not good swimmers, and there are some reports of them drowning when they landed in water.
This is one of those simple questions that is very difficult to answer. Nature writer John Burroughs described a squirrel that was released at the edge of a canyon and leaped off into space. He peered over the edge to see that the squirrel had safely caught itself on a small ledge, far below. That squirrel probably accomplished a world record leap, but it is not one that we would count because it was really more of a fall. Therefore, we must define the question better, perhaps by adding “without losing height”— meaning the squirrel must land on an object at the same height as its take-off point. Reports in the literature about distances jumped are very rarely that specific, so our answer is that we really do not know how for squirrels can jump.
An Indian giant squirrel (Ratufa indica) makes a leap in Bandipur National Park, Bandipur, Karnataka, India. Photo © Sudhir Shivaram
Thorington approached this problem experimentally with the eastern gray squirrel by placing a feeding platform full of seed beside a stump. Gradually, the platform was moved farther and farther away from the stump. Because it was a hanging platform and too high for them to reach by leaping from the ground, the squirrels had to climb the stump and then leap to the feeding platform. As the distance between stump and platform increased, the jump became an elimination contest. First the younger squirrels and then others refused to jump and instead would forage under the platform. Of those squirrels that jumped, he never saw one miss–they seemed to judge their limitations and refuse to try. Eventually, only a single squirrel was making the leap, at a distance of more than 2.5 m (8.2 feet), approximately 10 times the length of its head and body!
His intention was to conduct a comparative study of leaping in different types of squirrels. When he set up a similar experiment at a campground in California to test the jumping abilities of chipmunks, he encountered a problem. A deer would always find the feeding tray before the squirrels did and lick it clean. So his experimental work is incomplete. He became involved in another project and we still do not know how far different species of squirrels can jump.
Ernest P. Walker, former director of the National Zoo in Washington, D.C., is entertained by one of his pet flying squirrels. Photo © Smithsonian Institution
The term flying squirrel can be misleading. Flying squirrels do not fly the way bats and many birds do. Instead, they glide. With the aid of a special membrane, they can glide from a higher perch to a lower one. The physics behind the gliding of the flying squirrels is fascinating. By starting at a high point and moving toward a low point, a gliding squirrel converts the energy created by the vertical drop into forward movement and can glide three to four times as far as it drops. In other words, a flying squirrel can glide half the length of a football field from a 15 m (50 feet) tall tree. Under some conditions, in particular, if there is an updraft, they can go farther. Gliding, however, is not the only means of locomotion that the flying squirrels employ. Flying squirrels can race through trees and bound across ground much like nonflying tree squirrels.
Flying squirrels are not the only mammals that glide. At least six groups of mammals, from marsupial possums to colugos, have evolved gliding flight, but these other gliding species do not enjoy the wide geographic distribution of the flying squirrels.
The largest flying squirrel, and in fact the largest mammalian glider, is the woolly flying squirrel (Eupetaurus cinereus) of northern Pakistan, Afghanistan, and northwestern India. This is a poorly known species, measuring from 900 to 1,200 mm (35 to 39 inches) from head to tail; adult animals of up to 2.5 kg (5.5 lbs) have been weighed. Some of the so-called giant flying squirrels (Petaurista spp.) are almost as large.
The skulls of some of the largest and smallest flying squirrels. (Top) The Selangor pygmy flying squirrel (Petaurillus kinlochii), weighing approximately 19 grams. (Bottom) A giant flying squirrel (Petaurista spp.), weighing approximately 2,000 grams.
The smallest flying squirrel is undoubtedly the pygmy flying squirrel, Petaurillus emiliae, of Borneo, which probably rivals the pygmy tree squirrels in diminutiveness, but it is known only from two specimens, which were not weighed. Thorington examined these specimens in the British Museum and found them considerably smaller than the closely related species, Petaurillus kinlochii, which weighs 19 to 22 grams (7 to 8 oz).
We excluded a more detailed discussion of flying squirrel anatomy from the “What are squirrels?” question, because flying squirrels are special, and their anatomy includes unique features that can best be explained in the context of the question “How do you make a flying squirrel?”
WINGS. The obvious first answer to our question is that a flying squirrel must have wings, but flying squirrels do not have wings like birds. Instead, they glide with a parachute-like membrane, called a patagium, which extends from the forelimb to the hind limb. Large flying squirrels (more than 1 kg or 2 lbs) have an additional membrane, called a uropatagium, between the hind limb and the tail. The patagium and uropatagium are made up of two layers of skin with fur on the outside and a thin layer of muscles and nerves in between. When the squirrel is not gliding, it uses these muscles to gather the membrane close to the body, out of the way, so that it can run and climb easily. There are also thin cordlike muscles bordering the patagium. One of these extends from the wrist to the ankle, and in some flying squirrels there is a small bump on one of the leg bones, the tibia, where it inserts. This is of potential interest to paleontologists for identifying fossil bones. Of interest to anatomists is how one of the thigh muscles lines the edge of the uropatagium in large flying squirrels. By comparing this muscle in different flying squirrels, we can see how it has evolved from a muscle of the thigh to a completely separate muscle, extending from the ankle to the tail in the largest flying squirrels.
LONG LIMBS. Flying squirrels have the longest limbs relative to body size of all the squirrels and this appears to be a necessary part of the flying squirrel adaptation. If you stretch a patagium between the limbs of a modern tree squirrel, you get a rectangular shape. If you elongate the squirrel’s arm and leg bones the patagium becomes square shaped. This change in shape is important, because it affects the gliding ability of the squirrel. A rectangular-shaped patagium does not provide as much surface area, resulting in less lift and more drag, whereas a square-shaped patagium enables the squirrel to glide further horizontally relative to the distance it drops vertically. This shape also allows the squirrel to land at a relatively slow speed and with a high angle of attack (with its head up and feet forward), instead of a low angle of attack (head first).
STEERING AND STABILIZATION. Being able to glide longer distances is only one part of the adaptation. A flying squirrel must also be able to steer and to land safely. It appears that small flying squirrels use their tails to steer but that large flying squirrels probably steer by adjusting their arm positions. Although the aerodynamics of steering and landing are not well known for flying squirrels, stability clearly plays an important role.
Greater wing tip stability in flying squirrels is accomplished by reducing movement between the forearm bones, the radius and the ulna, and the three wrist bones, the scapholunate, triquetral, and the pisiform. This stabilization should come with a cost, however, because it should compromise manual dexterity. To understand this, sit in front of a table and place your hand palm down on the table. Now, turn your hand palm up. This movement is called supination. Now, turn the palm down again. This movement is called pronation. If a surgeon were to pin your radius and ulna together near the wrist, you would not be able to pronate or supinate your hand, which would greatly reduce your manual dexterity.
(Left) A museum specimen of the Selangor pygmy flying squirrel lies on top of a giant flying squirrel, illustrating the range in body size among flying squirrels. (Right) An x-ray of the complete arm of a southern flying squirrel (Glaucomys volans). Note the styliform cartilage, where the gliding membrane attaches. Also see the minuscule thumb and how the radius and ulna are bound together near the wrist.
If we look at the anatomy of the southern flying squirrel (Glaucomys volans) we see that (1) the radius and ulna are tightly bound together at the wrist and (2) the pronator and supinator muscles of the forearm are large. Finally, if we observe living flying squirrels it is apparent that they have lots of manual dexterity and can pronate and supinate their hands readily. We have determined that pronation and supination are occurring at the elbow instead of the wrist in these flying squirrels, with the radius and ulna rotating together as a whole. This is an extraordinary way to increase wrist stability without compromising manual dexterity.
The tails of flying squirrels also are important for steering and stabilization. Small flying squirrels (smaller than1 kg or 2 lbs) have broad tails with long hairs on each side (distichous), much like the tails of many tree squirrels. Larger flying squirrels (larger than 1 kg or 2 lbs) have long round tails, with all hairs of equal length. In large flying squirrels, the base of the tail, for a short distance, is connected by the uropatagium to the hind legs. We think that the broad tail of a small flying squirrel is used much like the rudder of a boat, whereas the uropatagium in large flying squirrels is probably used more like the flaps on the back edge of an airplane wing.
TURBULENCE. A gliding squirrel needs to be able to deal with air turbulence. If you have ever watched a kite or a glider come crashing to the ground, you have probably observed the devastating effects of air turbulence. An airfoil, however, like a kite or gliding squirrel, can be self-correcting in the face of air turbulence, if its wing tips point slightly upward. This is because of the dihedral effect—when a dihedral-shaped airfoil is tipped too far to the right, the right side provides more lift and tips it back to the left; if it is tipped too far to the left, the left side provides more lift and tips it back again to the right.
The flying squirrel flies with the tips of the patagium turned up, much like the wingtips of some airplanes. This affords the squirrel greater stability and steering ability. Photo © Karolyn Darrow
Flying squirrels glide with the tips of their patagium turned up and so benefit from the dihedral effect. How they go about elevating the tip of their patagium is pretty ingenious. The tip of the patagium is supported by a flexible rod of cartilage, which is attached at the wrist to the pisiform bone. This cartilage holds the tip of the patagium up when the wrist is cocked toward the thumb side (radially abducted) and then flexed backward (dorsiflexed). Anatomists had been puzzled by the fact that flying squirrels have a very small thumb, like tree squirrels, but a prominent muscle for abducting the thumb. Examining the hand anatomy carefully, we found a ligament that extends across the palm from the thumb to the pisiform bone, so that when the abductor muscle pulls on the thumb it also pulls on the base of the tip of the patagium, causing it to be extended and raised. This seems like a bizarre arrangement, whereby a muscle of the thumb controls the tip of the patagium on the other side of the hand. We can see how it evolved from the anatomy of a tree squirrel hand in which no other muscle is positioned to extend and raise the tip of the patagium.
It seems that most flying squirrels can glide a distance at least three times as far as the distance they drop in height. Thus, the longest glides are initiated at the greatest heights—the tallest trees, the tops of hills, cliff tops, etc. Under normal circumstances in the wild, accurate measurements of the distances covered are difficult to obtain—the observer is never close to both the origin and termination of the glide. We do have some facts about the distances of observed glides.
Upper tooth rows of four species of flying squirrels showing the diversity of tooth patterns. (A)Eoglaucomys fimbriatus. (B) Petaurista petaurista. (C) Trogopterus xanthipes. (D) Aeromys tephromelas.
Good data are available on the gliding of the small North American flying squirrels, both the northern species, Glaucomys sabrinus, and the southern species, Glaucomys volans. John Scheibe and his students at Southeast Missouri State University have studied the southern flying squirrel in Missouri; they report the lengths of several glides, with a maximum of 45 meters (148 feet). Karl Vernes and his students at Mount Allison University studied northern flying squirrels in New Brunswick and recorded the distances the squirrels glided in the wild, when released from their live traps. The males averaged 19 meters (62 feet) and the females, 14 meters (46 feet), but the maximum distance was also 45 meters (148 feet).
The large flying squirrels in southern Asia can glide phenomenal distances. The Japanese flying squirrel (Petaurista leucogenys) has been recorded to glide 115 meters (377 feet). The squirrels do not normally seem to choose to glide long distances. When possible, they prefer to make a series of shorter glides rather than one long glide. The average glides recorded by Japanese biologists Motokazu Ando and Satoshi Shiraishi at different study sites ranged between 17 meters (56 feet) and 33 meters (108 feet).
Peter Zahler, while at the University of Massachusetts, studied the woolly flying squirrel (Eupetaurus cinereus) in northern Pakistan, and on one occasion he estimated a glide to be 150 meters (492 feet), with a 50-meter (164 feet) drop. Again, the average glides for this species were much shorter.
We have the uncomfortable feeling that we are underestimating the capabilities of the large flying squirrels, because there are too many stories about glides too long to be measured. Illar Muul, formerly of the U.S. Army Medical Research Unit in Malaysia, has described flying squirrels (Petaurista petaurista and others) living in caves in limestone cliffs above a village in Malaysia and gliding from these caves in the evenings down to their feeding trees near the village—distances considerably longer than those listed above. Similarly, Smithsonian researcher Brian Stafford saw some very long glides of the Japanese flying squirrel that he was unable to measure.
The surest way to determine if a fossil squirrel was a tree squirrel, a flying squirrel, or a ground squirrel is to study the limb bones. Unfortunately, most fossil squirrels are represented only by teeth and jaws, without associated limb bones. When limb bones are present, flying squirrels can be recognized by their long and slender bones, tree squirrels by their shorter bones with more prominent muscle attachments, and ground squirrels by their more robust bones with extremely prominent muscle scars.
In flying squirrels the radius and ulna are tightly bound together at the wrist, and in some flying squirrels there is a small bump on one of the leg bones, the tibia, where the muscles lining the patagium insert. These characters may be diagnostic for paleontologists.
Some small bones can also be very distinctive, such as the wrist bones of flying squirrels or the short robust finger bones of ground squirrels. Anatomical features associated with the stabilization of a flying squirrel’s patagium also should be evident in the fossil record, if the relevant bones are present, and may enable paleontologists to identify early flying squirrels.
Teeth and jaws, although not as diagnostic as limb bones, can also be indicative of the way of life of a fossil squirrel. Paleontologists frequently use teeth to help create a more complete picture of how fossil squirrels were related to one another.
