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2 Form and Function
- Johns Hopkins University Press
- Chapter
- Additional Information
What are the largest and smallest squirrels? 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 “groundhog ” 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. How fast does a squirrel’s heart beat? Mammalian heart rate is inversely related to body size, thus smaller squirrels have faster heart rates than larger ones. For example, the 13-lined Chapter 2 Form and Function 26 Squirrels: The Animal Answer Guide 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. Can squirrels see color? 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 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. [3.239.59.193] Project MUSE (2024-03-19 08:07 GMT) 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...