FURTHER READING
Elbroch, M. 2003. Mammal Tracks & Sign: A Guide to North American Species. Stackpole Books, Mechanicsburg, PA.
Glass, Brian P., and Monte L. Thies, 1997. A Key to the Skulls of North American Mammals, 3rd edn. Oklahoma State University.
Nowak, Ronald M., 1999. Walker’s Mammals of the World, 6th edn, vols 1 & 2. Johns Hopkins University Press.
Wilson, Don, and Sue Ruff (eds), 1999. The Smithsonian Book of North American Mammals. Smithsonian Institution Press, Washington.
Wilson, Don E., and DeeAnn M. Reeder (eds), 2005, Mammal Species of the World. Johns Hopkins University Press.
Regional Tracking guides, by J. Halfpenny, published by Pequot Press.
The Journal of Mammalogy, published 6 times a year. Hard-core science, not for the faint of heart.
RECOMMENDED INTERNET RESOURCES
The Animal Diversity Web: http://animaldiversity.ummz.umich.edu
Mammal Species of the World Web Site: http://www.bucknell.edu/msw3/
The American Society of Mammalogists: http://www.mammalogy.org
North American Mammals, including interactive maps: http://www.mnh.si.edu/mna/
USING THIS BOOK TO IDENTIFY A MAMMAL
1. WHAT GENERAL TYPE OF MAMMAL IS IT?
Whale? Bat? Mouse? Consult the mammal chart on the next page for hints on general mammal classification, and appropriate page numbers.
2. LOOK AT THE MAMMAL PICTURES.
Examine the artwork that covers your mammal type. Look for pictures similar to your mystery mammal.
3. CONSIDER RANGE MAPS.
Species that don’t live where the mystery animal originated are unlikely, but not impossible, candidates.
4. COMPARE THE CANDIDATES.
Look carefully at the illustrations of species that resemble your mystery mammal and are known to live in your area. These should be on the same, or neighboring pages, allowing easy comparisons.
5. READ THE DETAILS.
The text for each species provides additional details that may help in species identification. If no illustrations match your mystery mammal, look here for details about known variation in size and color. If more than one species seems to match your mystery mammal, look here for details about small characters or measurements that distinguish similar species. Sometimes the distinguishing character is quite technical, and may require examining features of the bones or teeth (e.g. some shrews, some western chipmunks), which may not be practical for all situations. Don’t forget to consider habitat type, which is always described in the final sentence of a paragraph, and can sometimes be quite specific and useful in identification.
6. MAKE IDENTIFICATION.
In most cases this book will help you precisely identify your mystery mammal to the species level. However, the species of some groups are very difficult to identify, requiring skeletal material, or even genetic tests (e.g. some gophers and ground squirrels). If this is the case, you may have to settle with a genus level identification (e.g. Thomomys sp.) or a species group identification (e.g. the Richardson’s Ground Squirrel Species Group).
HOW ARE MAMMALS RELATED?
The family tree of mammals below is arranged to show the basic phylogenetic, or evolutionary, relationships of all of the major orders of mammals. Our understanding of these relationships is changing rapidly, as mammalogists bring to bear a series of new techniques. Significant advances in molecular genetics in recent years have greatly increased our ability to study the evolution of all mammals. In the schematic diagram below, the length of the lines connecting the groups is a relative indication of how closely related each group is to another. This diagram is a composite, based on several recent studies. The actual degree of relatedness is poorly known for most groups. However, ongoing studies not only document changes, but add to our confidence in our ability to recognize phylogenetic relationships.
The study of phylogeny forms part of the field of systematic biology, and scientists doing this type of research are known as systematists, or taxonomists. Understanding systematic relationships at the species level, and at higher taxonomic levels such as the orders shown below, is fundamental to all other studies of the biology of any organism.
MAMMAL MEASUREMENTS AND ANATOMY
Because we rely on differences in morphology to identify mammals, some knowledge of their anatomy is necessary for distinguishing similar species. Traditionally, mammalogists have used measurements of different parts of the body to make these distinctions. Externally, the measurements most often used are Total Length, Tail Length, Hind Foot Length, and Ear Length. For bats, forearm length is also sometimes a useful indicator. We have also included at least Total Length, Tail Length, and Weights for each species; where useful for identification, we have included other measurements in the text. Obviously, these very specific measurements were done on mammals in the hand, and in most cases, on museum specimens. Translating them into useful gauges of the size of an animal you see scampering away from you will be much more difficult. While most measurements and descriptions we give are nontechnical, bats are so specialized that some unfamiliar terminology is needed. The figures on the right illustrate these terms.
NOTE ON MEASUREMENTS
The main section of the book describing each species uses metric measurements. The following conversions link metric with Imperial measurements:
PLATE 1
OPOSSUM AND ARMADILLO
OPOSSUMS - This is the only one of the 94 species of New World marsupials that extends its range northward into the United States. The Virginia Opossum is still spreading north in association with human settlements. Frostbite regularly nips off their ear and tail tips on cold winter nights.
VIRGINIA OPOSSUM Didelphis virginiana 350-940mm, 215-470mm, 800-6500g; 300-3700g
Unique with white head and long, scaly, prehensile tail. A medium-sized, rather ponderous-looking mammal with a long pink-tipped snout, white toes, and leathery, white-tipped ears. Body fur is gray with long white and gray guard hairs giving an overall scruffy appearance. Has bright yellow-green eyeshine at night. Known for “playing possum,” a catatonic state assumed in the face of danger. Although it is omnivorous, its slow reflexes make it a better scavenger than active hunter. Diet typically includes a mixture of locally abundant fruits, grains, grubs, and carrion. Nocturnal and primarily terrestrial, opossums are also adept climbers and can exploit fruiting trees. For a mammal of its size, opossums have remarkable reproductive potential and very high turnover in their population. Females typically have one litter of 7-9 young, although in the south some females may have a second litter. Babies are born after only a 2-week pregnancy, but grow for another 8 weeks in their mother’s pouch. Survival is low and the oldest known wild opossum was 36 months old when last captured. These slow mammals are frequent roadkills, and are often preyed on by predators such as coyotes. Surviving the winter in northern areas requires the use of human houses, barns or sheds as dens. In the south they use a wide variety of woodland habitats. In the northern portions of its range they primarily occur in lower elevation areas with less forest cover and more human development.
ARMADILLOS - Only one of the 21 New World armadillos inhabits the United States. This insectivorous group specializes on ants and termites, and all species have very reduced, peglike teeth.
NINE-BANDED ARMADILLO Dasypus novemcinctus 615-800mm, 245-370mm, 5.5-7.7kg; 3.6-6.0kg
Bony skin plates are unique. Long head, prominent ears, short legs, and short, tapered, scaly plated tail are distinctive. Body has nine moveable bands encircling the mid-section. It digs burrows with its nose and forefeet, and lines them with vegetation as a sleeping den. Primarily nocturnal and crepuscular, it is sometimes active during the day in the winter. One of the few mammals with no eyeshine. Surprisingly strong swimmers, can float with their head above water or exhale and sink to walk along stream bottoms. Breed in mid-summer, but implantation of the embryos is delayed, and young are not born until March or April. All l
itters consist of a set of identical quadruplets that come from a single egg. Although clumsy looking when rooting through leaf litter for insects, it can escape quickly by bounding straight up and then running away quickly. Searches for food with its nose, and digs small conical holes when extracting insects from underground. Armadillos are expanding their range east and northward; they were known only from southern Texas in the 1800s. Their colonization of Florida resulted from a combination of introduced animals in the Miami area in the 1920s and 30s and the natural eastward expansion of animals from Alabama in the early 1970s. Armadillos are adaptable to a wide variety of habitats including woodlands, fields, and brushy areas.
PLATE 2
EASTERN SOREX
SOREX SHREWS - A group of long-tailed, insect-eating shrews. Because of their small size, most are rarely captured in box traps. Pitfall traps have revealed surprising diversity and abundance in many areas. Many of these species are difficult to identify, even for experts. Geographic range, color, and tail distinguish most species, but dental characters are needed for some.
CINEREUS SHREW Sorex cinereus 75-125mm, 30-50mm, 2-5g
Tip of tail black. Medium-sized shrew lacking distinctive markings. Back brownish, fur of underparts is gray at base, paler at the tips. Darker color in winter. Tail is long (roughly 40% of total length), brown above and slightly paler below; dark tail tip not always obvious. (For dental characters see page 26.) Nocturnal and rarely seen. Common in northern forests, shrublands, grassy areas, and herbaceous habitats.
SOUTHEASTERN SHREW Sorex longirostris 75-100mm, 25-40mm, 2-6g
Only long-tailed shrew in most of range. Small brown body with long and narrow rostrum. Broadly distributed in southeastern United States, but rarely seen. Favors moister areas with dense ground cover, but can occur in pine woods and scrub habitats as well.
AMERICAN PYGMY SHREW Sorex hoyi 60-105mm, 20-40mm, 1-7g
One of our smallest mammals, with small, bright eyes and obscure ears. Long snout has conspicuous whiskers. Color varies from coppery brown (summer) to grayish (winter) above, underparts are paler, grayish brown or drab tinged with copper or tan. Tail is dark brown above, paler below, and muzzle is paler than the crown and back. Tail is less than 40% of total length. (For dental characters see page 26.) Often stands on hind limbs in kangaroo fashion. Runs quickly with the extended tail slightly curved. Most abundant in boreal habitats.
LONG-TAILED SHREW Sorex dispar 105-135mm, 45-65mm, 3-8g
Slender, dark-gray shrew with a long tail. Belly only slightly paler than back, tail not distinctly bicolored. Smaller than American Water Shrew; larger than Gaspé Shrew; tail longer than Smoky Shrew. Typically smaller in northern parts of range. Rarely encountered due to activity in passages and runways below rocky surface. Procumbent incisors may aid in probing rocky crevices for insects. Favors rocky areas in cool, moist, boreal forests.
SMOKY SHREW Sorex fumeus 110-125mm, 35-50mm, 6-11g
Short, heavy-bodied shrew with a dark back, light belly, and long tail. Back is gray in winter and brown in summer. Tail shorter than American Water Shrew and Long-tailed Shrew. Nocturnal and active year-round. Nests of shredded vegetation are often in rotten logs or under rocks. Common in wet forests, where it is active on forest floor and under litter.
GASPÉ SHREW Sorex gaspensis 95-125mm, 45-55mm, 2-4g
Very limited range in Quebec and Nova Scotia. Like Long-tailed Shrew but smaller, with paler pelage. Almost never seen, except in pitfall traps. Uses rocky boreal habitats preferring cool, rocky stream edges.
PLATE 3
NORTHERN SHREWS
ARCTIC SHREW Sorex arcticus 100-125mm, 35-45mm, 5-13g
Dark tricolored pelage. Back is very dark brown to black, sides are paler brown, and belly is grayish brown. Winter pelage is more uniformly dark brown, and young of the year may be bicolored (brownish above with pale brown underparts). The tail is indistinctly bicolored. (For dental characters see page 25.) Often uses vision to hunt small insects. A common species, occurs primarily in marshes and grassy clearings within northern boreal coniferous forests.
MARITIME SHREW Sorex maritimensis 105-117mm, 40-45mm, 7-8g
A tricolored shrew from Nova Scotia and New Brunswick. Compared with the slightly larger Arctic Shrew, the Maritime Shrew is more brown than black on its back, has much lighter shades of brownish gray on the sides, and is lighter gray on the belly. (For dental characters see page 25.) Originally described as a subspecies of the Arctic Shrew, it is now considered a full species. A rare species found primarily in wetlands such as marshes and wet meadows.
ALASKA TINY SHREW Sorex yukonicus 68-75mm, 23-27mm, 1-2g
One of our smallest mammals. Tricolored, with dark brown back, gray-brown sides, and gray belly. Smaller than other tricolored shrews. Alaskan subspecies of American Pygmy Shrew is slightly larger, without tricolored pelage. (For dental characters see page 25.) Close relative to the similar Miniscule Shrew (S. minutissimus) from Siberia. Most common in vegetation close to streams and rivers.
TUNDRA SHREW Sorex tundrensis 85-120mm, 20-35mm, 5-10g
Contrasting, tricolored pelage, lighter colored than most other shrews. Light brown back contrasts with pale brown or grayish sides and pale belly. Larger than Barren Ground Shrew, lighter colored than Arctic Shrew with a relatively shorter tail than both. Winter pelage is longer and more uniform in color. (For dental characters see page 25.) Occurs in dense vegetation including grasses, shrubs, and dwarf trees in tundra zone north of boreal forests.
PRIBILOF ISLAND SHREW Sorex pribilofensis 90-95mm, 30-35mm, 3-4g
Only shrew on St. Paul Island, Alaska. Superficially similar to the closely related Barren Ground Shrew. These small, bicolored shrews have been isolated in the Pribilof Islands for about 16,000 years. It may have occurred at one time on Unalaska Island also, but no recent specimens are known. Nothing is known about the life history of this species.
BARREN GROUND SHREW Sorex ugyunak 75-105mm, 20-30mm, 3-5g
Small tricolored shrew. Distinct line separates dark fur on back from pale fur on sides. Once considered a subspecies of S. cinereus, but the coloration is more like S. tundrensis, which is much larger. The brown back of the Barren Ground Shrew looks like a well-defined median dorsal stripe. Tail is pale brown above and whitish below, with a pale buff to brown terminal tip. (For dental characters see page 25.) Immatures are more uniformly colored than adults. Lives in short grass meadows and woody thickets of northern tundra.
PLATE 4
AQUATIC SHREWS AND SKULLS
AMERICAN WATER SHREW Sorex palustris 130-170mm, 57-89mm, 8-18g
Large shrew with black or grayish back. The long (>18mm) hind feet have fringes of stiff hairs. The subspecies from Glacier Bay, Alaska (S. p. alaskanus) is sometimes considered a full species based on their smaller size and more ridged skull; recent efforts to study this subspecies have not turned up any water shrew populations. Capable of skipping across surface of water by taking advantage of the bubbles trapped in the hairs on its feet, or diving to stream bottoms in search of food. Can be common along fast flowing streams, less common in slower moving rivers and lakes. Found throughout boreal forests in the north of its range, restricted to montane areas in the south.
MARSH SHREW Sorex bendirii 128-174mm, 58-80mm, 7-21g
Large, velvety, brownish-grayish dark shrew with relatively uniform belly and back. Similar to American Water Shrew, but without stiff hairs on hind toes. Olympic Peninsula subspecies (S. b. albiventer) has white belly. Forages both on land and in water, but eats prey on land. Occupies coastal forests, typically associated with skunk cabbage marshes, riparian alder, small streams, and beach debris. May temporarily move into new habitats during rainy times (red and orange areas on map).
ROHWER’S SHREW Sorex rohweri 90-113mm, 32-50mm, 2-5g
Newly described shrew from northwestern North America (not pictured), externally similar in appearance to the Cinereus Shrew. In front view, incisors much more angled, like an upside-down V,
than in the Cinereus Shrew (orange area on map of Marsh Shrew).
ARCTIC SHREW Sorex arcticus (See page 22 for more details)
Skull has five unicuspids. The first four gradually decrease in size from front to back, while the fifth is significantly smaller. Most similar to the Maritime Shrew.
MARITIME SARITIME SHREW Sorex maritimensis (See page 22 for more details)
The skull is nearly identical to the Arctic Shrew, except in being smaller and flatter, with a less tapering rostrum and a more noticeably arched maxillary tooth row.
TUNDRA SHREW Sorex tundrensis (See page 22 for more details)
Skull has five visible unicuspids, with the first two being larger than the last three. The third, fourth, and fifth are similar in size, but gradually become smaller from front to back. The fifth unicuspid is relatively larger than in the Arctic Shrew.
Mammals of North America Page 2