THE MARYLAND NATURALIST (July/December 1997) - A long-term mammal study of Fort Belvoir, VA, was conducted from 1987 to 1995.  The following was found:


SPECIES                                           NUMBERS


Virginia Opossum                              Abundant

Northern Short-tailed Shrew             369

Least Shrew                                        1

Pygmy Shrew                                    82

Southeastern Shrew                         254

Starnosed Mole                                  3

Eastern Mole                                  ~10

Big Brown Bat                                 187

Silver-haired Bat                                  1

Red Bat                                             77

Eastern Pipistrelle                               16

Eastern Cottontail                                Abundant

Beaver                                                4 sites

Meadow Vole                                    18

Pine Vole                                           105

House Mouse                                    Abundant

Muskrat                                              Abundant

Marsh Rice Rat                                  20

White-footed Mouse                        275

Deer Mouse                                        5

Norway Rat                                        Abundant

Southern Flying Squirrel                       5

Woodchuck                                        Abundant

Gray Squirrel                                      Abundant

Eastern Chipmunk                               19

Meadow Jumping Mouse                    13

Gray Fox                                            Present*

Red Fox                                             Abundant

Bobcat                                                Present*

River Otter                                          Present*

Striped Skunk                                     Present*

Mink                                                    7

Raccoon                                             Abundant

White-tailed Deer                                Abundant

 *  "Present" indicated a very few observations were made.

NOT FOUND, but expected, was the Little Brown Bat, hoary bat, and the long-tailed weasel.

 Efforts to capture larger mammals were minimal, with tracks and visual sightings used to document their existence.


NATURAL HISTORY (5/94) - Three major groups of mammals have evolved back to a life in the sea.  These are the suborder Pinnipedia (seals, sea lions, and walruses) within the order Carnivora, and two entire orders; the Sirenia (dugongs and manatees) and Cetacea (whales and dolphins). 

NATIONAL GEOGRAPHIC (11/94) - From 30 million buffalo (Bison  bison), in 1870, only 1,000 existed in 1889.  4,000 now exist (1994) in Yellowstone National Park.  At the Flying D Ranch near Bozeman, MT, Ted Turner has about 5,700, with another 2,100 at a ranch in New Mexico. 

NATURAL HISTORY (4/94) - Mammal evolution started with ancestoral shrew-like creatures over 200 million years ago.  Thus, two thirds of the time line of mammals occurred in the Mesozoic period, the period of the dinosaurs.  While both marsupial and placental species existed, they were limited in adaptability, being primarily very small mammals and all noses and ears.

70 to 63 million years ago, placentals and marsupials arrived in South America; the placentals being the herbivores, and the marsupial, the carnivores.  With the lowering ocean level creating the Panama land bridge about 3 million years ago, the placental saber-toothed tigers migrated south from North America and wiped out the marsupial carnivores.  This land bridge was responsible for the northward migration of opossum, and porcupines and the southward migration of raccoons, weasel, dogs, bear and cats (including the saber-toothed cat).  Mastodons, tapirs, horses, llamas and deer were the hoofed emigrants southward.  Rabbits and rodents (including the very successful mice) also went south.  Today, fully half of the land mammal genera of South America came by way of this land bridge, while only three genera (the porcupine, opossum and armadillo) have found permanent homes in North America from the south.

 North America is the ancestral home of horses.  As part of the mixing of continental members 3 million years ago, horses traveled over the Bering Strait to the Old World and over the Panama isthmus to South America.  Dramatic climatic shifts and the arrival of man contributed to the extinction of Equus in the Americas.  Only one genus world-wide exists in the Old World from their maximum in the Miocene.  This genus, includes all the world's horses; zebras, asses and onagers - the ancestor of the domesticated horse. 

NATIONAL WILDLIFE (12/1 - 93) - Hair, along with mammary glands, are the hallmark of mammals.  It is felt that hair was first evolved in early mammals to help regulate temperatures.  The earliest mammals, which evolved from reptilian therapsids about 160 million years ago, grew these sensory cones between the scales of the evolving reptiles, which, when brushed on objects, gave a stimulus to the brain.  Certain remnant scales still exist on rats' tails, armadillo shells and the backs of pangolins.  Animal whiskers are still sensory receptacles.  Hair is mainly the protein keratin, a form of skin, which is the same material making up the epidermis, feathers, fingernails, horns, hooves and claws.  Humans have about 100,000 hairs on our head, while sea otters have 170,000 to 1,000,0000 hairs in their underfur; only one of the otters’ three types of hair.  Polar bear hairs have hollow cores. The cores scatter light, making the hairs seem white.  Actually, they are colorless.  Sunlight passes through the bear's long guard hairs to its black skin, which absorbs the radiation as heat.  The inner insulating hair prevents the heat from being radiated back to the air (similar to glass of a greenhouse).  Most mammals can cause their hair to become erect, thus increasing the air, (i.e., heat) retention.  Although humans have lost most of their body hair, goose bumps are the results of our own relic muscles that still exist which used to control these hairs.  White-tailed deer grow winter coats four times thicker than summer coats. 


ANS MAMMALS PROGRAM (Rob Simpson - 3/89) - Reptile teeth can be either replaceable (rattlesnake fangs) or one set (crocodiles/alligators).  Mammals have growth stage teeth.  Hair on mammals is usually dark on top and light on the belly, so sunlight on top makes it shiny (light), like the bottom.  Caribou hair is hollow for better insulation and aid in swimming.  Forest dwellers are usually dark haired, meadow mammals are light colored.  Porcupines and fisher have been reintroduced to Dolly Sods and Spruce Knob areas.  Bering hare (snowshoe rabbit) exists both in Dolly Sods/Spruce Knob as well as parts of NW Virginia (but not the Shenandoah National Park).  New World rats have hairy tails, Old World rats have nearly hairless tails.  Jumping mice hibernate, meadow mice don't.  Eastern woodrat lives in caves (lots in rocky areas in SNP-especially on Old Rag).  Man used to mine excrement (basically KNO4), using it 3 to 1 with wood ashes to make salt peter (gun powder - how the original Dupont fortune (Winterthur-Longwood fame) was made).  Pine voles are an emerging problem for orchard trees (voles girdle them). 


MAMMALOGY (1972) - Ungulates are the hoofed animals, although this group has no taxonomic status.  Ungulates include the odd-toed Order Perissodactyla (horses, rhinos, and tapirs) and the more advanced even-toed Artiodactyla (pigs, camels, deer, antelope, sheep, goats, and cattle).  Exceptional running abilities dominate the ungulates that evolved among the expanding grasslands of the increasingly cooler and drier era of the Miocene (25 – 5 mya).  In these cursorial mammals (runners), the limbs have lengthened while the clavicle has been greatly reduced, allowing a greater stride.  Many ungulates have become large to protect them from predators and to allow temperature regulation.  Being herbivores, vegetation is far less concentrated food than is meat, and is more difficult to digest.  In addition, plant material is frequently low in protein.  Thus, herbivores have developed specialized adaptations to aid in food consumption efficiency.  In ruminant artiodactyls, a four-chambered stomach allows undigested foods to be regurgitated, allowing the animal to chew its cud.  (Lagomorphs, which includes rabbits, increase food efficiency by eating their feces, called coprophagy.  God knows why dogs do it.)  This cud chewing makes for longer time for food to be processed (70 - 100 hours through the gut) than, say, for a horse (30 -45 hours).  The central difference, then, is that the digestive system of the horse is less efficient than that of the ruminant, but in compensation the horse eats greater quantities of food; the emphasis in ruminants is on highly efficient digestion and on more selective feeding, but not on high rates of food intake.  Given food in short supply, the ruminant will probably survive after the horse has starved to death. 

 Within the Suborder Ruminantia, antlers and horns are found in the most progressive families.  The antlered artiodactylas include the deer, elk, caribou and moose.  Antlers are usually present and occur in males only, with the exception of caribou, where both sexes have antlers.  Antlers differ from horns in being annually shed and regrown each year.  In the Giraffadae (giraffe and okapi), both male and female have fur covered bone protuberances.  In the Antilocapridae (pronghorn sheep), both male and female have a boney spur with an annually shed sheath.  Finally, in the Bovidae family (antelope, bison, sheep, goats, and cattle), usually both male and female have horns.   The horns are never shed and in some species grow throughout the life of the animal.  Bovid horns are never branched, but can be spectacularly curved or spiralled. The domestication of bovids about 8,000 years ago was one of the major factors responsible for the first civilization in the Tigris/Euphrates River valley in Mesopotamia (annual grain crops, including wheat and barley were another reason).  A few bovids reached the New World in the Pleistocene via the Bering Strait land bridge.  Because this avenue of dispersal was under the influence of severe boreal climatic conditions at this time, it functioned as a filter bridge, and only animals adapted to these conditions dispersed across the bridge.  Migrators to the New World included bighorn sheep, mountain goat, musk ox, and the bison.  Other Old World bovids that couldn't make the cold crossing included antelopes and the gazelles.   Bovids inhabit grasslands, making their greatest stand currently in the savannas and grasslands of Africa.


DELAYED FERTILIZATION---Occurs in many north temperate zone bats.  Sperm is stored overwinter.  Seems to occur with species having continuous or periodic winter dormancy.  Advantageous for species due to males in Fall being in excellent condition and with plenty of food, rather than in Spring, when the animals are in their poorest condition and when food (insects) may not yet be abundant.  Early ovulation in spring allows for maximum time for development before winter. 


DELAYED IMPLANTATION---Occurs with many carnivores; bear, marten, fisher, badger, otter, mink, long-tailed weasel.  Also several seal, walrus, Roe deer, armadillos and two species of bat.  Many different Orders, thus probably evolved separately. 


WINTER ADAPTATIONS---Insects utilize supercooling-body fluids remain as liquid below freezing (some Alaskan insects do this to 70 Below).  The Arctic ground squirrel is the only mammal that can do this.  Some reptiles freeze solid---wood frogs, box turtles and garter snakes by flooding the body with glucose.

Garter snakes are the last snake in and first out of winter hibernation (sometimes will bask on snowbanks).  They are also known to hibernate underwater, with their heart rate dropping from 60 to one beat per minute (getting air through their skin).  Painted turtles may take one breath every five months, respiration dropping from a summer 35 to one every 10 minutes.  Even hibernators wake up occasionally for a few hours to a day.  Some rodents, shrew and rabbit family members produce brown fat, full of mitochondria, the engines in cells that convert food into energy, and its only functions is to generate heat. 

AUDUBON (1-2/00) – Some small mammals, like voles and mice live solitary lifestyles in summer, then become social in winter, building communal nests six to eight inches across.  The advantage is shared warmth (a study found nest temperatures up to 50 degrees warmer than the air above the snow).  Relative humidity is also higher; reducing heat loss.  In fact, the red-backed vole does so well, it gives birth in winter.  The disadvantages of communal nests include being easier prey (sound and smell) to weasels, greater disease and parasite transmitters, and more competition for food. 

Mammalian adaptations to the stresses of winter generally take on three different strategies; migration, hibernation, or continue in a greater or lesser level of winter activity, by developing mechanisms to minimize the impacts of both the stresses of the cold and lack of food sources.

Migration and hibernation are costly strategies.  In the case of migration, a large amount of energy must be consumed to make the trip south and the returning spring trip back north.  And, of course, migration to warmer latitudes is a feasible alternative only to those species capable of long distance travel.  With the exception of marine mammals, our only migratory mammals are three species of bats that inhabit our insect-laden deciduous forests.  These bats; the red, hoary, and silver-haired bats, travel to southern regions of eastern U.S. and Mexico, and then, normally will hibernate.  However, some of these species will stay in their northern (summer) ranges and will hibernate there in protective locations.  To make the distinction between migrating and hibernating species more ambiguous, all species of bats have some individuals that will migrate, with individuals of many species migrating 100 miles (more or less) before entering hibernation.

Hibernation, in the truest sense, is only practiced by woodchucks, bats, jumping mice and western ground squirrels.  This strategy is costly in the need to store an adequate amount of energy, in the form of brown fat that will fuel the body's system over the duration of the winter season.  This strategy is only suitable in mid-latitudes, since more northern habitats preclude animals from safely sleeping through the winter without freezing or running out of stored energy. 

Woodchucks are the textbook hibernators, with the oldest (and fattest) entering the burrow first (late October). In Autumn, after gaining 30% of it's summer weight (mostly in the form of a half-inch layer of brown fat stored over much of the body, especially the back and shoulders), the ground hog will line it's hibernaculum with grass and leaves and then plug the entrances (to maintain constant temperatures and to keep out curious visitors) before curling into a ball. During hibernation, heart rate drops from 100 to four beats per minute, respiration rate drops to one breath per three or four minutes, and the body temperature drops from 98 to about 40 degrees.  Males usually exit a few days earlier than females in March (similar to the hibernating jumping mice).  They will awaken occasionally throughout the winter (about every two weeks) to defecate.  This may occur in special chambers below ground or, less frequently, on the surface.  They will emerge having lost approximately one half their fall weight.

All bats of the eastern US are capable of hibernation in winter, and they will also reduce metabolism and enter torpor during cool days, as an energy-saving mechanism.  Brown bats store energy as brown fat.  About 75% of the stored fat is used during the wakeup periods, with the other half-gram serving to maintain the bats life functions through the entire winter of hibernation.  Many studies of big brown bats in hibernation have been conducted.  While normal awake body temperatures are 99 degrees, apparently the body temperatures match the environment down to a temperature of 30 degrees while breathing can drop from 200 times a minute to once every four to eight minutes. As all true hibernators, bats will occasionally awaken during winter to prevent muscle atrophy and defecate, and may even change caves.

Jumping mice are profound hibernators.  Meadow jumping mice put on a substantial layer of fat (about 6 grams - increasing in total body weight from 15 to 21 grams) and wake every two weeks to stretch and defecate.  Woodland jumping mice put on 7-8 grams (from 20 to 28 grams total weight).  The woodland jumping mice will spend from October to April or May in deep torpor in underground moss/grass/leaf nests. They will put on one third their weight within two weeks in fall.  The biggest cause of death in jumping mice is the inability to put on enough brown fat in the fall to carry them through to spring.

Whether other animals, like bears and chipmunks, hibernate or not depends on your source and definition.   Animals (as well as plants and all living material) don't play by any definitive rules, and thus individual species (in fact, individuals within a species, as our bats have illustrated) have developed  characteristics that are more of a "smearing" of possible strategies and physical adaptations, covering an endless number of variations on a common theme. There is a continuum of sorts between the true hibernation of ground squirrels and woodchucks, in which all bodily functions are greatly slowed, and the deep sleep of raccoons, opossums, and gray squirrels.

Bears are said not to truly hibernate, because although their bodily processes are slowed, they are not suppressed to the extent found in the deep hibernators. Their metabolism drops by half, and their digestive system tightens into a knot, with the limited waste products reprocessed into the bloodstream in the form of proteins.  Black bears, however, if not true hibernators, are certainly close.  Various terms - dormancy, ecological hibernation, and carnivoran lethargy, for example - have been used to indicate the black bear's various modifications to hibernation.  They can exhibit continuous dormancy for up to about seven months, without eating, drinking, defecating, or urinating. (In the Shenandoah National Park, bears enter their dens around December 1, and emerge in mid-April.)  Their temperature does not fall to the extent it does in true hibernators, nor is respiration as greatly retarded.  Body temperature decreases from a summertime 99-101 to 88-95, and the heart rate drops from about 40 to about 8 to10 beats per minute.  Bears feed heavily and become extremely fat, thus well equipped for winter.  They may increase their weight by up to 100% (one bear went from 110 to 220 lbs).  Average loss of mass in one study was 260 grams per day; over the winter, that amounted to a drop of 23.1% from peak weight.  Most hibernators wake periodically, but bears do not wake during winter unless disturbed.  If they are disturbed, they will rouse readily.  The ability of bears to recycle urea and to desist urinating, defecating, eating, or drinking during the entire denning period is unique.  It has been said that, while bears may not be true hibernators, they are digestive hibernators. 

Chipmunks do not develop a layer of brown fat as does a true hibernator. Unlike true hibernators, who overwinter on the brown fat alone, chipmunks must wake occasionally to eat from their caches. They will go through periods of torpor of variable length and frequency during periods of stressful weather.  During these periods of torpor, their body temperature drops, and their heart rate and respiration slows for several days. Chipmunks' respiration rate drops from 60 to less than 20 per minute and a drop in temperature from 100 degrees to 45 degrees. Chipmunks will come out in mild weather any time.  They are also known to overwinter among rattlesnakes, benefiting from their warmth, while entering and leaving the rattlesnake hibernaculum as the snakes are dormant.

By far, most mammals "stick it out" in winter by various adaptive methods.  Development of brown fat is not uncommon.  Many mammals produce brown fat, full of mitochondria, the engines in cells that convert food into energy, whose only function is to generate heat.  Winter grouping allows for mutual warmth sharing, allowing a further northern range.  (Raccoon are known to den together in winter communal “piles” of up to 23 individuals in a single den.  Skunks will also den together, but only female and young.)  Thicker winter coats, or pelage, provides further cold protection (the winter coat of the mink is highly prized and is much thicker and heavier than the summer coat), as does the winter white fur of weasels and snowshoe hare.  White hairs, without the pigment melanin, has more air spaces within the hairs and thus has greater insulation (Snowshoe hares' white winter pelage has 27% better insulative qualities than the summer brown coat).  A change in diet is also utilized to provide the necessary energy supplies. (Red fox consume primarily insects and fruits in summer and fall, and then rodents in the winter. The mink's diet in spring and summer includes muskrat, crayfish, frogs, fish, snakes, small mammals, and waterfowl.  Winter mink diet focuses on muskrat, but small rodents make up a portion of their diet. Raccoons will feed on anything being omnivorous, but favor crayfish in the spring, and adding insects in the summer, but switch to primarily acorns and corn in the winter.)  Most carnivores assume a larger home range to assure an adequate food source (bobcat and mountain lion).  Some animals will cache food for winter food sources. (Red squirrels cache their food supplies in "middens" while gray and fox squirrels prefer to store their acorns individually.  Members of the weasel family also cache their prey.  One winter mink cache was found to contain over a dozen muskrats, two mallard, and an American coot).   Gray wolves and red foxes become more diurnal in response to weather conditions and the nature of their winter prey (red foxes feed almost exclusively on voles).  And practically all mammals lay low and minimize energy loss during extremes of winter cold and food scarcity. 

An interesting secondary adaptation to winter stress developed by many mammals are the phenomena of delayed fertilization and delayed implantation.  Delayed fertilization occurs among most of our native bats.  Delayed fertilization is seemingly a highly advantageous adaptation in mammals with long periods of dormancy.  In the case of bats, copulation, requiring considerable energy, occurs in the late summer and autumn, when males are in excellent condition and have abundant food, rather than in spring when the animals are in their poorest condition and when food (insects) may not yet be abundant.  Ovulation and zygote formation occur almost immediately upon emergence from dormancy, rather than being delayed until after males attain breeding condition and copulation occurs.  Perhaps the major advantage is that of hastening the time of parturition and allowing the longest possible time for development of young before the winter period of dormancy.  Delayed implantation occurs in bear, marten, fisher, badger, river otter, mink, long-tailed weasel, certain seal and roe deer.  The advantages are similar in nature to those found for delayed fertilization.  In this case, fertilization occurs normally with early cell cleavage, but then, the embryonic development is arrested at the blastocyst stage until a more suitable time of development and ultimate birth can occur.  In the case of the black bear, mating takes place in early summer, when the males are in prime condition and food supplies are not limited.  Without delayed implantation, mating would have to occur in the fall, when bear must concentrate its efforts on building up a fat reserve to allow it to pass through the winter dormancy.  The delayed implantation of five to six months allow for a late fall implantation and mid-winter birth, allowing the maximum length of summer growth period for the newborn cubs before the subsequent winter.  The longest period of blastocyst dormancy is attained by the fisher and river otter, which is ten to eleven months.  (White-tailed deer don't follow this trait since they are active all winter, commensurate with the availability of their food source.)