Unique Mammals of North America

 
Photograph © Glen and Rebecca Grambo/First Light

Prairie dogs are among the most social of North American squirrels, and their behavior is the most elaborate of all. One prairie dog town in Texas contained an estimated 400 million inhabitants and covered an area that extended 25,000 square miles.

Named for their barklike warning call, black-tailed prairie dogs are at the extreme end of the scale of ground squirrel evolutionary tendencies. They are stouter than their cousins, they remain active except during the very coldest winter weather, and instead of using cheek pouches to carry and cache seeds, they eat large quantities of leafy vegetation and have evolved bigger incisors and molars for the job.

The densely packed “towns” in which they live alter the landscape of western grasslands. Before poisoning campaigns exterminated wholesale numbers of prairie dogs, their towns used to reach staggering sizes. Such large colonies may have had a mutually beneficial relationship with the vast bison and pronghorn antelope herds that ranged the same areas. The herds are thought to have grazed plants that the prairie dogs shunned, and the buffalo wallows encouraged the grassy and herbaceous vegetation favored by the prairie dogs.

Prairie dogs’ feeding habits and mound building leave their mark on the landscape. Their selective grazing favors the growth of certain species of woody shrubs, and sometimes a town can be spotted from miles away by the silvery blue shine of the sage growing in the colony. Grass and broad-leafed herbs are so heavily cropped that towns are often depleted of favored foods. When this happens, prairie dogs will dig roots and eat insects.

The entrance to the burrow of a prairie dog is unmistakable. It is in the crater of a volcano-shaped cone built of soil removed from the tunnel system. The cone serves as both a lookout perch and a dike to prevent surface water from pouring down through the entrance. The burrow is a remarkable excavation that begins with a straight plunge of 9 to 15 feet, a depth requiring that towns be located in areas of deep and malleable soils. The burrow then turns and runs horizontally for 50 feet or more. Short side tunnels lead to nesting chambers or tunnels used for defecation, and there may be multiple exits. Burrow systems are usually 25 to 50 yards apart.

Towns are organized into extended female kin groups called coteries. A coterie typically consists of a single adult male, three to four adult females (often sisters or cousins) and larger numbers of juveniles of both sexes. A male maintains more or less exclusive sexual access to the breeding females in one coterie, but his territory may expand to include a second. In this case, the females of one coterie remain loyal to their own and are hostile to females of the other. A male’s behavior is thus subject to the outcome of female-female interactions.

Cooperation, sociability and egalitarianism are the most striking features of a coterie. The members usually sleep communally. They greet each other with a hug, placing their forepaws around each other. Their social gestures also include nose or teeth touching and tail flicking. And they all give alarm calls, defend their territory and build the burrow system. Neither males nor females in a single coterie dominate relationships with one another through violence or threats. However, as in any group, there is a mix of cooperation and competitiveness. Some subtle forms of competition may exist among females, because not all females within a group breed and because the average reproductive success per female declines as the size of the group increases. If competition does exist, more obvious co-operative behavior generally overshadows it. For example, nonbreeding females assume more than their share of colony defense, and mothers may also feed infants that are not direct descendants. On the other hand, pregnant and lactating females defend their nests against other members of the group, and females may kill the offspring of other females within the group.

Both males and females breed at 2 years of age. Breeding occurs in late winter. The litter size is small as ground squirrels go—an average of three pups weaned per female. The pups first emerge above ground in May and June. Female pups will remain in the group for life, and males will disperse the next summer at the age of 12 to 14 months. Why males disperse is not clear. It may be because the residents evict them, because females refuse to breed with closely related males or because the males are genetically encoded to disperse to avoid inbreeding.

After dispersing, a male must set up his own burrow system and attract females. Only the largest coteries have more than one breeding male. They will tolerate and may even groom each other and share burrows, but one male fathers the bulk of the offspring.

Colonies expand as mature males move to the periphery. Food depletion is lower there, but pioneering males and the females that they attract face a greater risk from predators such as coyotes and hawks than the centrally located residents do. Like all ground squirrels, prairie dogs give alarm calls, providing advance warning to interior residents. The warning call is one of at least nine vocalizations that the black-tailed prairie dog uses. Among its other greetings, threats and social signals is the territorial “bark,” from which it derives its common name.

Many carnivorous mammals and raptors feed on prairie dogs. Badgers are sometimes able to excavate them, and coyotes working in pairs can sometimes catch them. The endangered black-footed ferret used to feed mainly on prairie dogs, but the poisoning campaigns that eradicated most prairie dog towns destroyed most of the ferrets as well. Rattlesnakes and burrowing owls also eat prairie dogs and use their burrows. The owls nest in them, and the snakes den and overwinter in them.

The black-tailed prairie dog does not hibernate, although it sometimes becomes inactive during very cold spells. This is attributed to its ability to meet all its water needs from grazing plants. By contrast, the white-tailed prairie dog, which has a relatively high water requirement, is an obligate hibernator. In winter, it minimizes its water needs by turning its metabolism down and sleeping the season away.

Extended families cooperate to outcompete other prairie dogs for grazing and burrowing territory, the key to success for squirrels such as the black-tailed prairie dog.

Mammal: Cynomys ludovicianus, black-tailed prairie dog

Meaning of Name: Cynomys (dog mouse);
ludovicianus indicates that it was found in Louisiana; known as prairie dog because
of its sharp, barking alarm call

Description: pinkish tan dorsally; whitish
or pale buff underparts and lower part of face; short ears may be hidden in fur; larger and stouter and has a shorter tail than squirrels

Total Length: 14.2 to 16.3 inches

Tail: 3 to 4 inches

Weight: male, 3 pounds; female, 2.5 to 2.8 pounds

Gestation: 30 to 35 days

Offspring: 2 to 8 (usually 5)

Age of Maturity: 2 years; some mature as yearlings but have smaller litters

Longevity: 3 years in the wild; 7 to 8.5 years in captivity

Diet: primarily forbs and grasses; some animal matter; opportunistic but does select for certain plants according to nutritional requirements

Habitat: dry, open upland prairies and shortgrass plains; likes river flats and coulee bottomlands

Image © Robbie Cooke

Predators: rattlesnake, burrowing owl, eagles, hawks, badger, coyote and black-footed ferret

Dental Formula: 1/1, 0/0, 2/1, 3/3 = 22 teeth

Incest: How to Avoid It

Photograph © Jeff Foott

Animals that live in dense colonies run a high risk of incest: mating with a close relative. In most animals, incest results in a phenomenon that geneticists call inbreeding repression.

Every mammal has two sets of genes, one set from each parent. Some of these genes may be defective, but if the copy from either parent is functional, it can mask the effect of the defective gene. For example, an animal might inherit from its father a defective skin-pigment gene, but if the copy of the gene it receives from its mother is functional, the animal will grow up to be normally pigmented.

Defective genes that are not expressed when a functional copy of the gene exists in the same body are called deleterious recessives. They are hidden, transmitted from generation to generation, and are exposed and removed by natural selection only when an individual receives two identical copies, one from each parent. An albino organism might be the result of such a pairing.

Mammals carry large numbers of deleterious recessives, and their effects on an individual’s fitness vary. A gene that reduces fitness and reproductive success by 50 percent is half a lethal equivalent. Humans carry an average of two lethal equivalents, and if expressed, they could cause death. Other species have been shown to have similar numbers of deleterious genes hidden in their gene pool, and virtually every study of every mammal subject to inbreeding has shown severe costs when these genes are expressed.

Inbreeding increases the chance that both copies of a gene are identical. If an animal carrying a defective gene breeds with a brother or sister or one of its parents, the chances are 50-50 that the mate carries the same defective gene. Simple laws of probability predict that the resulting offspring are highly likely to express a defective trait. For example, a deleterious recessive gene in humans can produce a genetic disease called phenylketonuria, which suppresses production of a metabolic enzyme and can cause severe mental retardation and grave illness. The gene occurs in 1 person in 100, but it is carried and transmitted from generation to generation mostly in the recessive state. Individuals who carry the defective gene are unaware of its presence, because they usually also carry a working copy of the gene that produces the necessary enzyme. Under the laws of probability, the chance that an individual will inherit two copies of the gene from parents who are not related is 1-in-100 times 1-in-100, or 1 in 10,000. Inbreeding between brothers and sisters increases 500-fold the chance that their offspring will receive a defective copy from each parent and express the disease.

How can prairie dogs living in such densely populated towns avoid inbreeding? One study found several answers. Newborn males disperse from their coterie before they reach breeding age, and their sisters and female cousins remain behind. This eliminates incest between siblings of the same age. A father also usually leaves the coterie before his daughters begin mating. If a father remains, the daughters tend not to come into estrus. (Nine percent of the offspring in this study were sired by a male from outside the coterie, so females do have some chance of recruiting an unrelated mate if necessary.) And finally, if a female does come into estrus when one of her close male relatives is around, she will avoid mating with him. This avoidance is based on the ability to distinguish kin from nonkin by their scent—which may explain why prairie dogs spend so much time hugging and grooming each other.

Two cases of inbreeding did occur in this study. No birth occurred in one instance. In the second, the litter died shortly after weaning. The avoidance of incest, then, appears worthwhile.

Bobcat

Lynx rufus

Photograph © Thomas Kitchin / First Light

The bobcat is the slightly smaller southern counterpart of the lynx. The long legs, very wide feet and long ear tufts that make the lynx such an admirable winter hunter are lacking in the bobcat, as it adapted to a less snowy habitat. Its fur has a distinctively mottled pattern that provides camouflage
in the brushy scrubland where it is commonly found.

Lynx and bobcats do not share the same territory. Where bobcats have been introduced into the range of the lynx—as on Cape Breton Island—they have tended to segregate, bobcats gravitating to low coastal areas, where winter snow is light, and lynx moving up into the snowy highlands.

What snowshoe hare is to lynx, cottontail rabbit is to bobcat—that is, the most important element of its diet and the basis of its prosperity. However, the bobcat is much less specialized than the lynx, in terms of both food and habitat. Bobcats are found everywhere from the dry deserts of the southwestern United States to well into the cold boreal forests, wet bogs and mountain meadows of the north, although they do not thrive in deep snow. Their only requirement is woody cover. They prefer rough terrain with rocky caves and ledges for their dens, but where these are lacking, they are flexible enough to set up house-keeping in protective clumps of brush.

Bobcats eat mostly rabbits and hares, but they supplement their diet with large numbers of birds and small mammals, and occasionally, they prey on young white-tailed deer and pronghorn antelope. Rabbits are commonly ambushed by bobcats as they pass along well-traveled game runs, while deer are usually attacked when they have bedded down. There are records, however, of the bobcat leaping down on its prey from tree limbs. It is the flexibility of the bobcat, in terms of both food and habitat, that has allowed it to increase its range at a time when most other cat populations are declining.

The social behavior of the bobcat is similar to that of the lynx, especially when it comes to territoriality. There is one record of a female bobcat killing and eating a juvenile, apparently a trespasser on her home range.

The breeding behavior of bobcats tends to follow the same pattern as that of other solitary cats. Males are polygynous, offering no parental care, and they drive other males away from the estrous female. Sometimes several males will contest for one female, and many bobcats wear battle scars from the breeding season.

A bobcat kitten, is curious about almost everything. 

Photograph © Victoria Hurst/First Light

Mammal:Lynx rufus, bobcat

Meaning of Name:Lynx (lamp, to see)
refers to the animal’s bright eyes and keen sight; rufus (reddish) refers to the general
coloration of the body

Description: shorter legs and smaller feet than lynx; coat is tawny brown above with numerous black spots, especially along the midline from the head to the base of the tail; tip of the tail is blackish above and white below, with subterminal black bars; underparts are white with black spots; tawny limbs have black orizontal bars; a prominent streaked ruff on each cheek extends down the side to below the lower jaw; short blackish ear tufts

Total Length: male, 2.5 to 4.1 feet;
female, 2.5 to 3.1 feet

Tail: male, 5.2 to 6.7 inches;
female, 5 to 6.5 inches

Weight: male, 14.1 to 40.3 pounds;
female, 9 to 33.7 pounds

Gestation: 50 to 70 days

Offspring: 2 to 5 (usually 3); in southern
part of range, 2 litters per year

Age of Maturity: female, 1 year,
but does not mate until second year

Longevity: 12 to 14 years in the wild;
maximum 32 years, 4 months in captivity

Diet: primarily a small-game predator (rabbits, rodents and birds); larger prey such as deer are occasionally taken; also reptiles, insects and snails; will eat untainted carrion

Habitat: hardwood forests, mountainous areas, semideserts, brushland, rocky hillsides and swamps; occupies agricultural lands and the outskirts of cities; only requirement is woody cover

Image © Robbie Cooke

Predators: puma, coyote, wolf and man

Dental Formula: 3/3, 1/1, 2/2, 1/1 = 28 teeth

Red Fox

Vulpes vulpes

Sharp vision, acute hearing and a keen sense of smell are essential senses for this hunter of small mammals. Extended whiskers orient the red fox and help it deliver a deadlybite.

Photograph © Wayne Lynch 

The red fox enjoys the largest geographic range of any living carnivore. It is widely distributed throughout the northern parts of both the Old and New Worlds wherever the habitat is relatively undisturbed.

The red fox has only recently become common throughout North America. It is now found everywhere from the high Arctic to the deep south, except in dry plains and deserts. The fox may not be native to the continent, however. Some scientists believe that North American red foxes are descended from the European species introduced into New England in 1750, but fossil skeletons have been found that predate that import, so other scientists argue for a relatively rare breed of native red fox that may have interbred with the European import. The resulting hybrid may have then proliferated; alternatively, the formerly rare native fox may have spread in the wake of increased human activity.

The diet of the red fox is limited only by what it can find or catch. Scientists prodded by hunters suspicious of fox depredations of game birds and small animals have learned much about the fox’s diet, discovering, for instance, that most foxes are omnivorous: When wild grapes, blueberries, cherries and other small fruits are in season, they may eat only fruit; at other times, they feed primarily on small mammals up to the size of cottontails and groundhogs. Their diet is supplemented with insects, frogs, snakes, birds and bird eggs. Red foxes also scavenge the carcasses of large animals that they cannot kill themselves.

The red fox’s habitat ranges from tundra and boreal forest all the way to prairie. It is rarely found deep in mature forests, however, preferring to roam the open country and live at forest edges.

Red foxes breed as monogamous pairs once a year, appropriating the burrows of other small mammals such as groundhogs for dens where their pups can be born. In spring, the females give birth to a litter of about five. Much variation in social and breeding behavior exists, but in some areas, the male stays with the family and assists in rearing the young. When many den sites are clumped together, though, males may mate with more than one female and neglect to provide parental care.

As the pups grow older, the parents bring them half-dead animals to play with to begin their education as hunters. Soon, the young accompany their parents on foraging trips during which they acquire the skills they will need for survival. By the end of summer, the new generation disperses, and by spring, they are themselves ready to breed.

Except during the reproductive season, foxes are solitary and hunt alone. They stay in one home range, marking the boundaries with scent and urine. Although they are not overtly territorial, males may exclude other males during the breeding season.

In habitats shared with coyotes, there may be antagonism between the two species. An adult fox will approach and bark at coyotes that come too close to its breeding den; coyotes respond by chasing the agile fox, but they are seldom able to catch it. Pressure from coyotes, however, does cause foxes to favor denning near human habitation, since foxes tolerate human disturbance much better than coyotes do.

Red foxes are the main carriers of rabies in eastern North America, and their population periodically suffers epidemic outbreaks of the disease. Rabies is often transmitted to livestock or attacking dogs and thus to humans. Recent attempts to inoculate wild foxes against the disease have involved dropping medicated meatballs from airplanes. Experiments have shown that most foxes in a given area will come across one of these meatballs and eat it. This has made it possible, for the first time, to control the scale of rabies epidemics.

Mammal:Vulpes vulpes, red fox

Meaning of Name:Vulpes (fox)

Description: long, silky fur; three distinct color phases:

1. red fox: face, top of head and nape are yellow to rusty red; back is yellowish red or fulvous; lips, chest, abdomen, inside of ears and tip of tail are creamy white; back of ears and anterior portions of legs are black;

2. cross fox: grayish brown, long black guard hairs form a cross from shoulder to shoulder;

3. silver fox: totally black except for white tip on tail and a variable amount of frosting; also a black phase, which is entirely black; all other North American canids lack white tail tip

Total Length: 3 to 3.6 feet

Tail: 13.8 to 16.5 inches

Weight: 7.9 to 15 pounds
(up to 30.9 pounds)

Gestation: 51 to 53 days

Offspring: 1 to 10 (usually 5 to 7);
1 litter per year

Age of Maturity: 10 months

Longevity: few live more than 3 to 4 years; has potential to live 12 years

Diet: omnivore, but more of a carnivore,
as it prefers animal matter; small rodents,
frogs, insects, birds, snakes and plant material such as acorns, grasses, corn, fruits

Habitat: open regions such as farming areas, prairie, alpine and Arctic tundra, meadows, bushy fencelines, woody stream borders and forest clearings, low shrub cover and along beaches bordering larger lakes

Image © Robbie Cooke

Predators: man is chief predator; also wolf, coyote, dog, lynx, bobcat; occasionally bear and wolverine, as well as hawks and owls

Dental Formula:
3/3, 1/1, 4/4, 2/3 = 42 teeth

Moose

Alces alces

A thick network of blood vessels is needed to grow the massive antlers seen on the adult bull moose. The animal must also build huge neck muscles to support their weight.
Photograph © Thomas Kitchin/First Light

Reaching a weight of 1,800 pounds, the moose is the largest cervid and, next to the bison, the largest land animal in North America.

The moose is a boreal-forest specialist. Not common in mature coniferous forests, which are dark and support little shrubbery in the undergrowth, the moose favors mixed forest with open areas and the edges of watercourses. Its common name is an Algonkian word meaning twig eater or bark stripper, and it is indeed a browser, clipping a wide variety of trees and shrubbery. In winter, however, when snow is heavy, the moose will forage in thick mature coniferous forest in which the understory is relatively clear of snow and offers protection from the wind.

The moose’s long legs seem to be an adaptation for snowy winters. Moose can high-step their way through deep snow, and their broad hooves provide good support in wet, boggy areas. The boreal forest is crisscrossed with lakes and rivers, and moose are excellent swimmers. They will take to the water to escape predators such as wolves, and they feed heavily on water plants in summer. Moose have even been seen diving and swimming underwater in search of water plants.

Like other creatures of the boreal forest, the moose is solitary, more of a loner than any other North American cervid. Although a few moose may yard together in winter, they travel alone most of the year. In spring, females give birth to a single calf, which they defend with great vigor. The calf is weaned by autumn and stays with the mother through the winter and spring until she gives birth again. Females may tolerate the presence of the calf for as long as a year, but otherwise, they solicit company only during the autumn rut. Un-like other cervids, the female takes the most vocal role in the rut. She bawls out a long, loud moan that summons any bulls in the area. The call is effective and has long been imitated by hunters.

Since moose are generally widely dispersed, males do not attract and defend a harem of females as do open-habitat cervids such as elk. Instead, the bull patrols the area, advertising his presence with urine and other scent marks, thrashing the vegetation with his antlers and creating wallows. If he encounters a male, he attempts to chase him out of the area. If he encounters a female, he mates with her if she is receptive, and if she is not, he stays with her and waits up to several weeks for her to come into estrus. The dispersion of the females limits a male’s opportunity for polygyny. Nevertheless, successful mating seems to depend heavily on the male’s ability to defend a mating territory. In some areas, small groups of males and females may congregate for mating, but this has not been well documented.

Moose antlers are among the most massive of all animal antlers; some may stretch 6 feet and weigh more than 70 pounds. They represent a tremendous investment of nutrients, even though they are cast off every season. To support and wield their antlers, bulls develop huge neck muscles. Their weight may increase by almost a third before the rut and drop to 10 percent lower than normal afterward. During the rut, males can be extremely belligerent and will charge humans and even cars.

The expanding range of white-tailed deer has brought them into a dangerously increased contact with moose. White-tails are host to the parasitic meningeal worm, Parelaphostrongylus tenuis. Although of little consequence to deer, it attacks the moose’s nervous system and will cause paralysis and eventual death. Contact between white-tails and moose can fuel infections and lead to a serious moose-population decline.

A Taste for Salt

It’s a good bet that moose would like the taste of junk food, especially potato chips, as most herbivores in the north have a difficult time getting enough salt. The mammalian body uses the sodium molecules of salt in all sorts of jobs: transmitting electrical impulses along nerves, maintaining pressure within cells and effecting the movement of compounds through membranes. Meat eaters get plenty of salt in their diet, but northern herbivores usually have to feed on terminal twigs and branches of conifers and deciduous wood plants all winter long. These are low in sodium, so by the time spring arrives, the animals’ salt reserves are greatly reduced.

This accounts for some strange behavior in browsers. It explains why porcupines love to eat canoe seats, outhouses and virtually anything that has come in contact with sweat or urine. It is why sheep and deer will travel many miles to salt licks, even though they are exposed to predators along the way. And it explains why moose go swimming and diving in the summer. Moose have been seen diving to a depth of 15 feet in search of underwater plants that contain salt.

Aquatic plants concentrate salt and other minerals. Some species, such as milfoil and the Potamogeton pond weeds, have sodium concentrations 10 to 400 times higher than in the woody plants moose browse on in winter. Moose have definite preferences for these over other kinds of water plants, and they have favorite lakes that they visit to the exclusion of others. The ideal lake for a salt-hungry moose is a shallow one with a high mineral content and a good flow of water in and out of it. The degree to which it has these features and the density of the water plants in it determine how much salt a moose can acquire over the summer. There is some suggestion that moose may build up a sufficient reserve of sodium to get them through the winter. They seem to store sodium in their large rumen, whose fluids make up 15 percent of their total body weight. Over the winter, they deplete this reserve.

It has also been suggested that the need for sodium and the intense use of aquatic plants may cause population cycles in moose. In years of moose abundance, they eat, trample and otherwise destroy aquatic-plant populations. They also strip the bark from the trees and eventually kill them. As their sodium sources decline, so does the moose population, which will rebound once the plant populations recover.

Not all the moose’s foraging ecology revolves around sodium. They must meet other nutrient demands for protein, vitamins and carbohydrates. When aquatic plants are available, moose pursue a foraging strategy that maximizes energy intake rather than minimizing feeding time. An energy-maximizing strategy might be expected in large adult moose that are faced with building up fat reserves.

Compared with smaller mammals, moose have few predators to fear aside from wolves, and a healthy adult can usually fend off wolves in the summer. Mothers with calves, however, must compromise. They often swim to islands where plants are less rich in energy and sodium but where there are no wolves, apparently taking the risk of predation into account in their foraging tactics.

Mammal:Alces alces, moose; the largest
living member of the deer family

Meaning of Name:Alces (elk); moose is
derived from the Algonkian word moos (“eater of twigs” or “he strips off bark”)

Description: prominent drooping snout and a dewlap, or “bell,” hanging from throat; males have large, flat, palmate (shovel-shaped) antlers with small prongs projecting from the borders; brittle, stiff pelage; reddish brown to black, with gray legs; lower belly and underside of legs are whitish; males have brownish foreheads, females more gray; first year, animals are reddish

Total Length: male, 7.5 to 9.2 feet;
female, 6.5 to 8.5 feet

Tail: 2 to 4.7 inches

Weight: male, 849 to 1,800 pounds;
 female, 727 to 873 pounds

Gestation: 226 to 264 days

Offspring: 1 (sometimes 2, rarely 3)

Age of Maturity: male, 2 to 3 years
but rarely has the opportunity to breed
until 5 or 6 years old; female, 2 to 3 years

Longevity: up to 27 years

Diet: primarily a browser; in winter, twigs and shrubs, bark of saplings; in summer, leaves from upland plants, large quantities of water plants, forbs, grasses and foliage

Habitat: wooded areas and early successional stages of evergreen forests; swamps, lakeshores adjacent to forests, muskegs and streams of great boreal forests; also tundra; in winter, found in mosaics of mature and young coniferous, deciduous and mixed stands

Image © Robbie Cooke

Predators: wolf is major predator; grizzly bear, black bear, wolverine and mountain lion prey on calves

Dental Formula: 0/3, 0/1, 3/3, 3/3 = 32 teeth 

Reprinted from Mammals of North America and Published by Firefly Books Ltd. / Copyright © 1999 Adrian Forsyth.