The Velvet Claw

A Natural History of the Carnivores

David Macdonald

1992  BBC Books, London


The following has been abstracted for this website


Carnivores began over 65 million years ago as squirrel-sized creatures that fed on insects in the shadows of the dominant dinosaurs.  These creatures have led to all of today’s modern carnivores, ranging from flesh-eating leopards to bamboo-eating giant pandas.  What distinguishes carnivores from other mammals is that almost all of them have a set of scissor-like back teeth, called the carnassials, with which to shear through flesh.  Some living carnivores, such as the giant panda and the aardwolf, have lost these scissor teeth, but their ancestors had them. 

Meat-eating has a major advantage over a vegetarian diet in that converting prey flesh and bone into predator flesh and bone is simpler than the alchemy of turning plant material into flesh and bone.  Meat is also much easier to digest than vegetable. Thus, true carnivores, like lions and bobcats have guts only four times their body length, while more omnivorous carnivores, like foxes and wolves, have guts five times their body length.  However, this is also a high-risk lifestyle as well as high reward. 

Modern carnivores are split among eight families: the civets and genets (Viverridae), the cats (Felidae), the dogs (Canidae), the hyenas (Hyaenidae), the bears (Ursidae), the raccoons and coatis (Procyonidae), the weasels, otters, badgers and skunks (Mustelidae), and the mongooses (Herpestidae).  

Pangaea began to split up about 136 million years ago, with the separation of the northern Laurasia from the southern Gondwana.  About 130 my ago, Africa broke free of South America.  About 60 million years ago, Laurasia split, along the North Atlantic, separating Europe from North America, leaving Greenland in the middle.  Eventually Laurasia and North America had drifted far apart and were separated by a wide ocean, only to meet millions of years later on the other side of the world at the Bering Strait. 

For as long as the northern and southern domains remained apart, the story of predatory evolution had two separate strands.   As each of these continents separated, marsupials evolved into their own specialized forms.

Marsupials arose in North America and spread out over Europe and Asia before the breakup of Pangea.   By 25 million years ago, they had been eliminated in North America by the placental carnivores. 

In the southern continent of Gondwana, there were three groups of mammals.  One group was the xenarthras which included the sloths, anteaters and armadillos.  The second group was the didolodonts, which relied solely on herbivory.  This left the door of carnivory open to a third group, the marsupials. Between 100 and 75 million years ago, marsupials had populated Gondwana, which would become South America, Australia, and Antarctica.  Even today, fossils of these early marsupials can be found in Antarctica, who existed before the continent froze over about 40 mya.

The marsupial carnivores dominated South America for 30 million years during the Eocene and Oligocene, but about four million years ago, a new “thunder-bird” evolved that challenged the marsupials for the top of the food chain.  This flightless bird stood ten feet tall with a curved beak that was longer than a horse’s head.

In the northern domain, in Asia and North America, killer crocodiles threatened to dominate the landscape, leaving the waters to roam the lands about 50 million years ago.  However, these giant crocs were challenged by the placental mammals and withdrew back into the water.  Instead of letting their young develop in a pouch, these placental mammals grew their young inside them, in a womb.  The first big placental mammals to contend for the carnivorous niche in the north were, surprisingly, from vegetarian stock.  They came from a catch-all group of rooters and browsers called condylarths, which eventually gave rise to all modern large herbivores, from deer to elephants.  Incidently, about 53 million years ago, an early condylarth, a thinly furred animal slipped into a tropical lagoon, bobbed beneath the surface and them kicked violently as it swerved in agile pursuit of a fish.  By 35 million years ago, this line of "wolf-sheep" had become whales.

These condylarths never developed the meat-shearing scissor teeth, thus, by 35 million years ago, they were eventually replaced by two lineages of mammals that independently developed the carnassial scissor teeth: the creodonts and the carnivores. 

The predecessors of these two lineages began about 65 million years ago as small shrew-like creatures called Cimolestes.   It’s key legacy was the apparatus it used to chop up the prey.  By about 58 mya, the blueprint for carnassial scissor-teeth had been bequeathed to the creodonts and carnivores. 

The creodonts dominated throughout the world from about 55 to 35 million years ago.    The carnivores rose to become top predators on the northern continents 20 to 30 million years ago.  The eventual dominance of the carnivores over the creodonts is most likely related to the carnassial teeth.  The creodonts had no teeth left behind their carnassial scissors for grinding fibrous material, and this may have committed them too wholeheartedly to butchery.  The carnivores, on the other hand, had several teeth to the rear of their scissor teeth.  When the opportunity arose, these back teeth could be turned to other uses, such as grinding fruit and vegetables.  Consequently, the carnivores could become more abundant and diverse than creodonts (thus, ironically, it was their ability to diversify from carnivory that secured the carnivores their success).

A change in climate that affected the specialist creodonts more than the carnivores occurred just over 30 million years ago.  Leaves from warm, rainy climates tend to have smooth margins and pointed "drip-tips"; those from colder places are often smaller with serrated margins.  The proportions of these types in fossil forests changed radically at the end of the Eocene.  The shift (from warm and wet to cold with annual temperature extremes) resulted from a change in the inclination of the Earth's axis and thus the amount of sunshine falling at given latitudes.  Perhaps the initial loss of plant diversity also reduced the prey for creodonts, whereas the greater seasonality produced regular gluts of fruit and insects which could be exploited by the versatile carnivores.  Cycles in climate continued at about 9.5 million year intervals, with temperatures peaking at 13 to 16 degrees above the minima about 25, 15 and 5 million years ago (with the first and last of these corresponding with major changes in the history of the carnivores). 

About 55 mya, early arboreal carnivores gave rise to two main branches: the cat-branch and the dog-branch. The dog-branch, or Caniodea, evolved in the New World, while the cat-branch, the Feloidea, evolved in the Old World.  Each since has evolved into four of the eight modern families that make up the carnivores.  The cat-branch sprouted the four feliform families: civets (Viverridae), cats (Felidae), hyaenas (Hyaenidae), and mongooses (Herpestidae).  The dog-branch gave rise to the four caniform families: the dogs (Canidae), bears (Ursidae), raccoons (Procyonidae) and weasels (Mustelidae). 

Both the dog and cat branches evolved independently until about 30 mya, in the early Oligocene, when the Bering Strait formed a land bridge for these animals to migrate between America and Eurasia.    Although the true dogs remained in North America until about six million years ago, other dog-branch families became more common in Eurasia than in America.  Most cat-branch carnivores remained in the Old World, but the true cats subsequently crossed to America.  The two branches of the carnivores that settled in North America soon established their adversarial relationship which still characterizes cats and dogs today. 

In the northern hemisphere (North America and Eurasia), all the modern families of carnivores had arisen by the late Miocene (5 to 7 mya).  In the southern hemisphere (South America), the marsupials were still the dominant predators.  However, three million years ago, volcanoes provided a land bridge between North and South America, allowing the great showdown between the placentals of the north versus the marsupials of the south.  In the ensuing clash, the sabre-toothed cats obliterated the marsupial “Pouch-knives”.  As a result, half of all mammal genera in South America today are descendants of the early yankee invaders.  In stark contrast, only armadillos, opossums and porcupines from the original southern immigrants remain in North America, and even they have not diversified much. Only one of the  “thunder-birds” was able to make in-roads into North America, as far as Florida, but only for a short while.  (In a more recent example of a similar challenge, Man introduced the dingo to Australia some 8,000 years ago, producing a similar effect on the local marsupials, although the Tasmanian Devil still survives in Tasmania, an island never reached by the invading dogs.)

Scientists have tried to explain why pouched killers put up such a poor show against the carnivores.  As with the demise of the creodonts, the most probable proposal is related to teeth.  All the molars in the mouth of a marsupial killer started as scissors, with none free to develop other functions, while the carnivores have milk carnassial teeth, which are jettisoned when the permanent  teeth erupt.  The permanent teeth can then evolve for other purposes, such as grinding teeth.   So, like the creodonts, the pouched killer probably had to specialize in carnivory, limiting its dietary flexibility which, under changing circumstances, would render it less competitive than the flexible carnivores.



The Cat family evolved around 22 million years ago.  Of all the carnivores, only the cat family is exclusively carnivorous.  The oldest surviving member is the ocelot, which split off 12 million years ago.  The history of the small to medium cats is basically the same as it is today.  Females occupy a territory of 500 acres.  Males have territories 2 to 5 times greater, spanning the territories of three or more females.  These large territories are an adaptation of their diet of small rodents.  This prey is too small to share and doesn’t require cooperation among cats to capture.  Cats sharing hunting grounds would be disadvantaged because a cat reaching an area previously hunted will find the prey alarmed and hiding.  Additionally, prey populations take a while to breed again if several are killed, so that the cats will face lean times until populations rebound. 

The females are the sole caretakers of the young.  The males have opted out of child care and instead, focus on maximizing the number of females inseminated.  This translate in large territories and larger size to compete with other males.  Among small cats species, males may be 20% heavier than females and 50% heavier in larger species.  This difference in size among sexes is called sexual dimorphism.

Cats have paws that are evolved for killing and climbing, not for digging or running, thus, they have short attack range.  The canines function as long, sharp daggers, stabbing into their prey’s neck.  As evolution would have it, the space between the canines is directly related to each cat’s prey size.  The canines are sized to fit between the vertebrae and wedge in the gaps, prying them apart and severing the spinal cord.  The canine daggers can feel their way to the killing spot, being well supplied with nerves.  The larger the cat (and the distance between the canines) the larger the prey (and the distance between the vertebrae).  In Israel, caracal, jungle cat and wildcat all occur together and in each species males are bigger than females.  There is overlap between the species and sexes in the sizes of their skulls and carnassial scissors.  However, the diameters of their upper canines differ in a clear-cut sequence, allowing them to be ordered in neat, equal-sized steps from the smallest, female wildcat, to the largest, male caracal.  A similar pattern applies where these three species meet in India's Sind Desert, but there each canine is smaller.  The dagger teeth are apparently pushed down the size scale by the addition of a fourth, larger competing species, the fishing cat.  In South America, this gape differential exists between the jaguar, puma, ocelot and jaguarundi.  The jaguarundi and margauy are an exception to prove the rule: their gapes are similar but they use different habitats, the margauy being much more arboreal than the Jaguarundi.  Where similar-sized cats do compete, the relationships between them are strained: lion and tigers will kill leopards given the chance, and lion will seek out and kill cheetah cubs.   

To effectively and efficiently take down large herbivores, with large neck muscles and massive vertebrae, which have been around since 50 million years ago, mammals have independently evolved sabre-teeth by at least four mammal groups, on three continents.  These teeth are believed to have been used to slash the prey, not to be used as stilettos.  This sabre-tooth design dominated among large cats 5 to 6 million years ago, at the end of the Miocene.  Smilodon was one that arose in North America about three million years ago.  Then, the climate changed to a cooler environment, replacing forests with scrub and savannas.  The opening of the plains caused an explosion of rodents and, 4 million years ago, ungulates, like antelope and gazelle, that could outrun the ponderous sabre-tooths.  This led to the evolution of the pantherines, a new lineage of swifter, more agile cats.  With the soft hides of the new prey, the large sabre-teeth became a liability, and quickly were reduced in size.  The oldest surviving members of this line include the puma. 

The cheetah, the closest relative to the puma, broke the cat mold, to be more dog-like as a sprinter able to run after prey in the open.  For example, in the Serengeti Plains of Tanzania, almost 20% of males live in groups of three; 40% live in pairs, and the rest as individuals.  The earliest cheetah-like fossils date back 2.5 million years ago.  With a sprinter's lightweight build, the cheetah could only tackle small antelope.  So, when it evolved, the sabre-tooths were still able to monopolize the role of heavyweight  predators specializing in mega-herbivores.  Their nearest rivals were middle-weight and light-weight pantherines, such as the clouded leopard and serval.   However, large, agile prey, like wildebeest, oryx and zebra, were too fast for the sabre-tooths and too strong for these pantherines.  Light-heavyweights, combining strength and agility, were needed; and over 2 million years ago, an unknown ancestor grasped this opportunity.  One of its descendants became the lynx.  Another gave rise to the five big cats: the jaguar, the leopard, the snow leopard and the sister species, the lion and tiger.

For a while the sabre-toothed sumos and the light-heavyweight pantherines lived side by side on their respective prey.  Then disaster struck the mega-herbivores.  The weather became both colder and less predictable, which seems to have disadvantaged them.  Their absence from cave art suggests that most sabre-tooths were extinct 35,000 years ago and before the final collapse of their prey.  Perhaps the evolution of humans was also involved.  At first, early humans probably scavenged from the kills of large cats but then they started hunting with weapons.  The Australopithecines did not fair particularly well, with many punctured skulls showing the impact of big cats on early humans.  However, the Homo genus seemed to be able to handle the predatory cats.  In time, along with lions, leopards and spotted hyenas, human hunters migrated from Africa to Eurasia and around the world.  Although scientists remain divided on the evidence, the extinction of mega-herbivores and sabre-tooths followed suspiciously close in our wake.  Clearly, in addition to the human impact, the specialization of the sabre-tooth on the now disappearing huge, thick-skinned herbivores, led to their demise.

The first domesticated cats were from the African wildcats (Felis silvestris lybica), occurring in Egypt about 4,000 years ago.  These were introduced and hybridized with the stockier European wildcat (Felis silvestris silvestris) about 2,000 years ago.  Presently, taxonomists put the cat in your lap in its own species, Felis catusThe Egyptians mummified another species, the jungle cat (Felis chaus), documented as far back as 2,000 B.C. 




When the early carnivores came out of the trees (about 60 mya), two lifestyles soon emerged.  Some were intent on becoming hunters by stealth and ambush, maintaining a dependence on carnivory.  These became the felines of the cat family.  Others opted for a mobile, generalist and opportunistic lifestyle.  This group gave rise to the canids, including wolves, jackals, foxes and others. 

The first “dawn dogs” evolved around 35 million years ago in North America, still hunting in the canopies of trees (as all early carnivores did).   The oldest surviving dog is the grey fox, evolving some 6 -9 million years ago.  Territorial pairs move nimbly through deciduous woodland and fields with dainty steps, searching for insects, fruits, carrion and small mammals up to the size of rabbits.   Being able to rotate it’s forelegs, it is a relatively good tree climber.  At the same time as one of the openings of the Bering Strait (5-7 million years ago), dogs divided into the fox-like vulpine and the wolf-like lupine lineages.  The arctic fox is a recent Ice Age introduction, as is the channel island fox, which broke away from the mainland gray foxes about 16,500 years ago.  The arctic fox has 70% fine underfur, compared to 20% of the red fox.  In addition to the cold, the arctic fox has to deal with the boom/bust populations of it’s prey, the lemmings and voles.  As a result, this fox has developed the most prolific breeding rate of any canid.  The arctic fox also has 12-16 teats, more than any other member of the dog family (the African wild dog has 12-14).  The continental range of the arctic fox is white, with one in a hundred being blue.  However, along the coasts and on the islands (where snow doesn’t settle), the blue foxes outnumber the white foxes. 

Between five and seven mya, ancestral dogs crossed the Bering Strait to Eurasia, coincidental to the demise of most of the rivals (including the last of the creodonts).

The general rule is that larger dog species dominate, or even kill, smaller ones.  Gray wolves chase and kill coyotes, coyotes kill kit foxes, golden jackals kill red foxes, and red foxes kill arctic and gray foxes.  The arctic fox's range forms a circumpolar ribbon to the north of the red's, the two species overlapping in the Eurasian and Canadian tundra.  Both species are impressively adapted to the cold, and are remarkably similar in everything they do.  As a result, when they meet, red foxes seem to treat arctic foxes as smaller, and thus inferior, copies of themselves and overpower them.  However the heavier red foxes need to eat far more and, towards the north, food becomes too thin on the ground to sustain them.  The larger body that allows the red fox to bully the arctic fox further south gives it an appetite that cannot be satisfied to the north.  So, the red fox's brute strength sets the southern limit to the arctic fox's range, while its hefty appetite sets its own northern limit.  Red foxes and gray foxes can occupy the same habitat (sympatry) based on the difference in the space between the canine teeth, thus, different prey.

The early wolf-like lupine dogs spreading throughout Eurasia were probably generalists like their vulpine cousins.  These dogs were larger than foxes; more like modern coyotes, thus their prey were larger than the small rodents of foxes.  Originally, they were probably scavengers who grouped together to ward off other scavengers. Larger groups formed where carrion consisted of large deer, whereas, coyotes remained in pairs where they fed primarily on mice. Being social animals, coyotes formed alliances, not only with other coyotes, but, remarkably enough, with badgers to hunt ground squirrels.  The two would work together to flush out small prey, which may be caught by either one.  Even this alliance was tenuous, in that badgers occasionally kill and eat coyote pups and coyotes will gang up and kill a badger.

Such cooperation within a species is also seen among red and arctic foxes, in which females will care for the siblings of several families.  Among some dogs, males will tend to their pups; a situation unheard of in the cat family.  This may be due to the dog having a more omnivorous diet, such that food is less of a restricting element.  In fact, such feeding side by side may foster a closer link between father and pups and encourage cohabitation.

Around five to six mya, a cooling of the world’s climate caused the replacement of forest by scrub and savanna.  New fleet-footed antelope, gazelle and zebra were found on the new plains.  Unlike the tough-skinned mega-herbivore predecessors, these fast and powerful ungulates had soft bellies that could be torn by long-muzzled jaws.  This favored the evolution of fast, long-legged predators; not only in dogs, but in the new pantherine cats.

It appears that the first dogs to follow these new ungulates onto the plains preyed, not on the ungulates themselves, but on the insects found on the ungulate dung.  Although these dogs had speed and stamina, they were not large enough to tackle these animals (unlike the larger, muscular pantherine cats).  Thus, the solution arrived at was cooperation among the dogs.  Hunting as a pack was the answer. 

Gray wolves area the most widespread wild mammal in the world, spreading across Europe and Asia and crossing to America 700,000 years ago.  Like all dogs, pack size is dependant on prey size.  Where the 150 pound white-tail deer is the main winter prey of grey wolves, an average pack contains about seven wolves, but where 770 pound moose are the staple diet, packs average over nine members.  Solitary wolves tend to kill smaller prey, such as young caribou or smaller deer.  Among the pack, specialization in roles are found.  Fast dogs curve in long encircling arcs, strong dogs fling themselves at flailing hooves, skilled dogs dash for the nose hold.  It appears that most packs are larger than actually needed for the intended prey, but not all pack members take place in any given attack. 

Among the jackals, coyotes and gray wolves, howling in chorus is at the heart of family life (although foxes keep in contact with barks and wails, they do not call in loud unison as the previously mentioned dogs do).

One trait shared by all in the dog family is in the care of the young.  The general rule among wolves, jackals and foxes is that only one female breeds in the pack.  This behavior is so prevalent that it probably stretches back well before the split between wolf-like and fox-like dogs.  The breeding matriarch is normally the oldest female of the group, and the mother of her helpers.  The dominant (alpha) male is the only one to breed with the dominant (alpha) female.  These two alpha leaders block and intimidate all lower ranking members, thwarting all mating attempts.  Studies of a captive gray wolf pack, have shown that the females at least can be very successful in monopolizing breeding.  The dominant or alpha female produced pups whereas the subordinates, even when four or five years old, did not.  Analyses of the sex hormones revealed that the majority of subordinate females could have conceived.  They were cycling normally and ovulated, just like the dominant female.  Furthermore, they flirted with males as much as the dominant female would allow.  The main barrier to their reproduction was psychological: their bodies were ready for reproductive action but they were constrained to behave like neuters.  Indeed, when the alpha-female of one pack died during the breeding season, the most dominant of her daughters came into heat the same day.  Rarely, two female foxes or wolves will breed and nurse their cubs communally.  However, one such female has been observed killing the other’s cubs, relegating the other mother to a wet-nurse status to the killer’s cubs. 

During the advances of the Ice Cap some 250,000 years ago, the habitat of the swift fox changed from prairie to tundra, resulting in the evolution of the arctic fox.  The swift foxes, retreating southward ended in enclaves which isolated populations into separate species including kit foxes. 

The wolves of North America have successfully driven the smaller coyotes to limited, more marginal lands.  However, the advent of Man has removed most of the wolves, allowing the coyotes to make a remarkable recovery.  In large part, this is due to the adaptability of the coyote to survive in pairs, on a diet of squirrels and mice.

Wolves and coyotes interbreed (as do domestic dogs with wolves and coyotes).  The larger wolves easily intimidate puny male coyotes and find solace in the company of the numerous female coyotes invading their range.  The "coywolves" have formed a 300 mile hybrid zone around the Great Lakes.  The coywolves might integrate into either wolf or coyote population, but in fact only wolves with coyote blood are found, not coyotes with wolf blood.  The coyotization of America's wolves follows the spread of agriculture, and thus of coyotes, beginning in Minnesota and spreading to Quebec and Ontario in the East.

Coyotes in the southern states have crossbred with the rare red wolf for decades.  By 1970, seemingly pure red wolves were confined to southwestern Louisiana and the south-eastern corner of Texas, and by 1980 they were extinct in the wild.  In 1977, a breeding program came up with only 14 judged genetically pure red wolves (the rest, totaling 79, were deemed to be either gray wolves, coyotes or coywolves).  By 1988, 80 red wolves had been raised and reintroduced to eight locations including the Smokies.  While hailed as a conservation triumph at the time, this rescue has since become the center of a philosophical debate.  Scientists analyzed the genetic make-up of red wolves that had lived between 1905 - 1930 using skins that had lain fusty in museum vaults.  They also analyzed all the candidates for the captive breeding program, from stored blood samples, and the descendants of the 14 judged to be red wolves.  Their studies showed that none of the candidates rejected as gray wolves was in fact a gray wolf, being mainly coyotes or coywolves.  Furthermore, the red wolves---those that had lived from 1902 - 30 and the modern captive-bred ones---had no genetic characters that could not be found in either gray wolves or coyotes.  In fact, they were genetically indistinguishable from Louisiana coyotes.  So, either the red wolf had already cross-bred itself out of existence by the late nineteenth century, or it never existed as a true species but was always a hybrid between the gray wolf and coyote.



The bears, raccoons, pandas and mustelids (weasels) descended from the tree tops of North America about 40 mya.  The various families that evolved can be distinguished in part by the number of molar teeth they have on each side.  Dogs have three upper and three lower molars, bears have two upper and three lower, raccoons have two of each, and the weasels have one upper and two lower molars. 

In North America, bear dogs, or half-dogs, dominated the true dogs, and were the top carnivore between 25 and 15 mya.  But, with time, the smallers dogs evolved enough speed, and the true bears enough size, to out compete the omnivorous bear-dogs, which became extinct by 3 mya.  The bear dogs may had lasted longer if they had become more vegetarian, but the bears and raccoons beat them to that niche. 

The bears broke off from the dog branch about 35 mya, opting for a large size, giving them a life in the slow lane sustained mainly by fruits and nuts.  The raccoons also adopted a mixed diet but, being much smaller, evolved a frenetic, opportunistic lifestyle. 

Raccoon’s affinity for water may have helped it reach various Carribbean islands by rafting and forming five different species; the Barbados, Bahama, Guadalopes, Cozumel and Tres Marias raccoons.  Some 5 mya, when North and South America were still separated by water, some raccoons rafted from North to South America and became the first true carnivores to colonize South America.  They thrived, diversified, and grew large, becoming the ‘bears’ of South America.  However, this dominance only lasted until the land bridge between the two continents formed 3 mya, when the true bears, along with other carnivores, swept over the landbridge and annihilated the bear-like raccoons. 

Bear also evolved around an omnivorous diet of fruit and vegetables.  However, as opposed to the smaller size of the raccoon family, which has led them to radiate the most species in the tropics, members of the bear family chose to grow large, thus, occurring predominantly in cold or temperate climates.  Over time, bears became larger and the length and crushing surface of their molars increased while the blades of their scissor teeth diminished.  With their large size, it benefited them to walk on the soles of their feet (plantigrade) instead of their toes (digitigrade) like dogs.  It is believed that bears gave rise to sea lions and walruses about 20 mya.

Most bears evolved into more omnivorous eaters.  However, around 12 mya, the earth was becoming drier, with humid forests being replaced by temperate forests and scrubland.  This resulted in the new form of great running bears, or tremarctine bears, that could take advantage of the new open spaces.  Their success peaked during the Pleistocene, from 1.8 to 0.2 mya.  This included the 1300 lb. short-faced bear and the 900 lb. Florida cave bear in North America.  It was these running bears that traveled south and eliminated the bear-like racccoons when the land bridge was established.  Both these running bears themselves became extinct about 10,000 years ago.  It is thought they may have suffered from competition with the big cats, but the extinction of their large herbivore prey probably was the main reason (which also spelt the extinction of the dire wolf and North American lion).  Only one running bear survives today; that being the spectacled bear of South America; a much modified, accomplished opportunist. 

While the running bears were evolving in the New World, bears in the Eurasian part of the world were becoming more ponderous and omnivorous.  About 5 mya, the little bear appeared, which was the predecessor of six of the eight bears alive today: the sun, sloth, American and Asian blacks, brown and polar bears.  By 2.5 mya, the little bears became larger, possibly a result of the cooling climate of the advancing Ice Age.  From these larger forms, came the Etruscan bear, about the size of today’s brown bear, by the time of its extinction around 1.5 mya.  Before its demise, it lead to three lineages; the 900 lb European cave bears (living from about 500,000 to 10,000 years ago), the brown and black bears.  Originating in Asia, these bears came across the Bering Strait at least 1.5 mya.  Throughout the time of bears, there have never been a lot of species at any given time (only eight species exist worldwide today), due to their omnivorous diet, which give them huge geographical distributions. 

Today’s brown bears are the largest, due to their abundant food supplies.  Two subspecies exist in North America; the Grizzly and the Kodiak.  Grizzlies inhabit the inland forests, weighing between 350 to 700, with the largest weighing up to 990.  Kodiak bear are restricted to Alaska’s Kodiak Islands, cut off from the mainland some 10,000 years ago.  Males weigh from 1500 to 1800 pounds, with the largest reaching a weight of 2200 pounds.  Other species of brown bear include the Siberian brown bear, weighing in up to 1795 pounds (with an unsubstantiated record of 2495 lbs from Kamchatka in eastern Russia).

The large size of brown bear demands large appetites, which is reflected in large home ranges.  Similarly, their size is reflected in slow growth and reproduction rates.  Brown bear don’t reach sexual maturity until they reach eight years and only then produce one or two cubs every four to five years. 

Male black bear average 600 pounds (females; 300 lbs) in the eastern, acorn and beechmast-rich, North American forests.  Females breed between three and five years of age, producing two to four cubs every two years.  Those in the poorer habitats of western US, first breed between four to eight years, having 1.7 cubs every two to four years.  Females have a home range of 4 square miles, while the much larger males require a home range of 40 square miles.  These large ranges are too large for the males to defend, thus their ranges overlap with other males. 

The young leave their mothers after the second spring, when the mother is ready to mate again.  They will live together in a portion of the mother’s range that she will abandon for them to occupy.  After a few years in this “leased area”, the sons will leave the area altogether, but the female young may remain to occupy the margins of her mother’s range, much like the petals of a rose flower.  Young males will leave by four years of age, since the previous generation of males will kill any young they may find.  It is not clear if the sons are spared.  Perhaps, old males assume any lingering males must be from other male families.

In all species of bear, the female raises the young, while the male concentrates his efforts on fathering as many young as possible (polygyny).  This means direct competition with numerous other males, which translates into evolution of very large males compared to females (sexual dimorphism).  Males can weigh as much as 80 % more than females. 

The polar bear is one of the most recent species of carnivore, having arisen only 250,000 years ago from a northern population of brown bear.  These bear became carnivoran specialists, concentrating on seal.  In fact, the brown bear and polar bear are still close enough that they can still interbreed with fertile female offspring (males may not be fertile).  Their range has varied with the Ice Age comings and goings.  Some 70,000 years ago, polar bear roamed through the region south of today’s London, England. 

The largest polar bear just outweighs the largest Kodiak bear at 2204 pounds, although, male polar bears generally weigh between 770 - 1430 pounds.  Polar bears have adapted well to the arctic environment.  The white hair is hollow, acting like a fiber-optic filament, conducting warm UV light to the heart-absorbing black skin.  A four inch layer of blubber adds to their insulation.  In fact, they are in more danger of overheating than chilling.  They use up so much energy with their large bodies when they run that they cannot afford to take flightless snow geese in summer, since a chase of 12 seconds would consume all the energy the bear would gain from eating the goose.  More surprising, large males overheat so easily, an eskimo on foot can run them down in a few hours.  This overheating risk is why large males have large roman noses, which radiates heat like a beacon.  The grinding teeth of the omnivorous brown bear ancestors have become smaller and more scissor-like, while the canines have become larger.  Additionally, polar bear have shorter, more solid claws (less likely to break on ice), larger feet (which act as paddles in water - males are known to swim up to 25 miles between ice flows - and spread like snow shoes on thin ice), pimpled footpads (for better grip on ice), and little depressions in the sole of the foot (to add traction). 

In May/June, the northern population male polar bear and non-pregnant females travel north with the receding ice.  The remaining bear fill up on ringed seal in the spring and then live off the stored fat until the return of the seals in the fall.  However, the pregnant females, after doubling or quadrupling her early spring weight, tunnel into snow banks for an eight-month confinement (summer is the stressful period here, due to lack of food, as opposed to winter in the more temperate regions).  They give birth between late November and early January.  Their milk is uncommonly rich, containing almost 50% fat (compared to a cow’s 4% fat content).

Bear do not truly hibernate.  In hibernators, such as groundhogs, ground squirrels, jumping mice and some bats, body temperature drops to near freezing and the metabolic rate plummets.  Carnivores’ body temperature drops from 100 degrees to about 90 degrees, while their metabolic rate idles at about 55% of normal.  These “torpid” bear can be awakened extremely quickly, as one biologist discovered with his head pressed against the chest while listening to a dormant bear sleeping.  Black bear in harsh winter conditions will den for five to seven months, while those of Virginia will den for only three or fours months.  Bear of the deep south will not den at all.  In the north, bear will consume about 20,000 calories a day in the fall, five times its normal intake, building up a five inch thick layer of brown fat.  Only when the daily intake of calories falls below the level of energy expended foraging will the bear go into its den. 

Like the polar bear, the black bear and the brown bear give birth during denning, with the mother only rousing momentarily during the task.  Northern bear must be born early enough in the spring to take advantage of the lush vegetation and put on enough weight (and fat) to survive the coming winter.  With a three-month gestation period, this would mean mating in October, when northern bear are already dormant and southern bear are pigging out on hard mast, building their brown fat layer for the upcoming winter.  In order to avoid this conflict of interests, the brown and black bear mate in May or June, and the fertilized ova begin development.  However, the embryonic growth soon is arrested; held in suspended animation for five months, until the appropriate time for an early spring birth, normally well into the mother’s winter lethargy.  This is called delayed implantation.  This is also practiced among most of the weasel family.  Incidentally, if the mother fails to put on adequate fat in the fall, the embryos will not implant.

The northern bears not only face a prolonged period of lethargy, but, since their body temperatures are much higher than true hibernators, they face a real problem of not maintaining adequate body heat.  This favors a large body size, which loses heat more slowly. (This relationship between mammal size and northern habitat is so closely related, it even has been stated as Bregmann’s Law.)   This is not such an issue with the procyonids (raccoon family), since most of these family members are found in the tropics.  However, our North American raccoon is known to conserve heat by communal denning, with as many as 23 raccoons being found in one nest (usually such dens include only one adult male). 




This is the weasel family, which, in North America, includes not only the weasel, but the fisher, badgers, wolverines, skunks, polecats, martens and otters.  The familial name refers to the fragrances emanating from the anal glands.  This is the most diverse of the carnivore families, with the largest; the river otter at 66 pounds, being 1,000 times larger than the smallest; the least weasel.  The key to the mustelids’ diversity lies in its long thin body form which have enabled them to adapt to taking prey in trees, underwater, on the ground, through crevices and down burrows.

Mustelids are the most recent offshoot of the dog branch.  They share an ancestor with the procyonids, or raccoon, family.  Along with most of the related procyonids and bears, the mustelids developed in Eurasia, differing from the dogs that developed in North America.   The same cooling world climate that affected other carnivoran families about 30 mya, accelerated the opening of forests, enabling the spread of herbivorous mammals.  These mammals were small and became burrowers in order to escape from the larger carnivores.  The evolution of mustelids followed two directions.  The first were large bear-sized diggers, which has given rise to our wolverines and badgers.  These large mustelids abandoned the carnassial scissor-like teeth in favor of broad back-teeth suited to grinding vegetable foods.  They, thus are the most omnivorous of today’s mustelid family. 

The second direction of mustelid evolution took the form of adapting a tubular shape to follow the prey into their burrows.  One subfamily, the Lutrinae, or otters, become aquatic hunters.  With a heat loss 25 times faster in water than to air, otters (both river and sea) were obliged to become relatively large.  Sea otters also have developed the densest hair of any mammal to aid in maintaining body temperature.  Sea otters also spend hours daily blowing air into their fur, further maintaining their body temperature.  Finally, their high metabolism, required to generate necessary heat, demands a voracious appetite, consuming up to a third of their body weight daily. 

Another family, the Mephitidae, or skunks, evolved from ancestral stock 22 mya, became much smaller.  Although developing in Eurasia, by 5 to10 mya, skunks migrated to North America, where they are now found, while those staying in Eurasia are now extinct.  Their formidable front claws enable them to be predominantly carnivorous in diet, digging up small mammals and insects.  Skunks are nocturnal, and become dormant during periods of extreme cold or snow.  Females raise their young without assistance from the father.

Still, another family, the Mustelidae, have adapted the long tubular body shape to capitalize on a number of different habitats; from the arboreal marten, to the aquatic mink, the prairie polecats and ferrets, and the tundra weasels.  Their long and narrow body allows them to follow their prey into burrows and small tree cavities, nooks and crannies.  Many have specialized on hunting in the burrows of rodents and taking over the dens of their eaten prey. 

The pine marten can outrun a squirrel in the treetops, but spends most of its hunting time on the ground.  The largest of the martens is the fisher, with males that can weigh up to 13 pounds.  Fisher divide their time equally between the ground and in the trees, where they are more agile than cats, but less so than pine martens.  Fisher are known hunters of porcupine, whose low-slung body enables them to attack the porcupine on its own level, instead of looking down at the quills, like a cat would.  Fisher specialize on porcupine to the extent that they regulate the porcupine population.  In 1962, fisher were reintroduced to Michigan and Wisconsin, with a resultant 76% decrease in the porcupine population over the next 13 years.  Fisher have been reintroduced throughout many states in the east. 

North America’s only polecat representative is the black-footed ferret.  This mustelid specialized on prairie dogs.  It lived, hunted and bred among the prairie dog colonies, and as the prairie dog colonies were eliminated by ranchers, so went the black-footed ferrets.  From perhaps 500 million prairie dogs a century ago with an unknown, but flourishing ferret population, the ferret is now extinct in the wild, with only 10 individuals in 1986.  By 1991, captive breeding has increased that number to about 180.  (Several are being raised at the Smithsonians CRC station outside Front Royal, VA.)

The changing cooler climate 5 to 7 mya saw a replacement of forests for grassland, impacting all the existing cat and dog families as mentioned earlier.  These tundra grasslands prompted an explosion of small burrowing rodents, which resulted in an explosion of the tubular mustelids.  About 2 mya, the long-tailed weasel evolved in North America, while the stoat (short-tailed weasel) was emerging to fill the same niche in the Old World.  About 500,000 years ago (1/2 mya), during a polar ice advance, opening the Bering Strait land bridge, the stoat came across to North America, yet the long-tailed weasel did not make it to Eurasia.  During the subsequent warming (and closing of the land bridge), smaller weasels evolved in Eurasia and also came to North America during the next ice age about 200,000 years ago.  As a result, there are three weasels in North America now; the long-tailed, the short-tailed (or stoat, as they’re called in Europe), and the least weasel.  While the long-tailed is the largest, followed by the stoat and then the least, the relative sizes varies throughout the world, depending on what other predators coexist with them.  In northern North America where the long-tail is absent, the stoats are large.  In Spain, where stoats are absent, the least weasel is stoat-sized.  And where least weasels are absent, the stoats are smaller.  This is basically a reflection of shuffling of prey species among the predators and the resultant change in the diameter of the killing canine teeth (and subsequent head, then, body size).  This is also beautifully seen in the small cats throughout the world (see cats discussion).  Minks also fit into this sequence.  (It is also noted that latitude has some impact on mustelid size, with stoats generally bigger towards the northern range, whereas, the least weasel gets smaller towards the northern range.  As a result of all this, the least weasel in some areas may be larger than the stoat in other areas. 

Weasels can catch prey ten times its own weight (comparable to a lion taking down an elephant).  They can also lift ten times its weight.  They are known to “entrance” their prey by throwing themselves into convulsions in front of the prey, mesmerizing them until the weasel can get into striking distance.  Often, the prey is literally scared to death.  (These convulsions are known to happen with no prey in sight.  It is not known if the weasel is attempting to lure prey to the scene, or if the weasel is suffering the pain from skrjabingylosis, a parasitic worm that burrows in the head; a not too uncommon affliction of weasels.)

Northern weasels molt into white winter coats, while those to the south remain dark brown all year.  Along the border of this change, it has been found that stoats’ pelts change color in response to temperatures.  For example, the stoat’s flanks and rump will change to white at 36 degrees, while the head and back don’t change until the temperature reaches 30 degrees.  In the field, this means the stoat changes to the white form (then known as ermine) when snow covers the ground for at least 40 days.  At that threshold, “piebald” stoats are common.  Outside this border region, changes in pelage color is much less temperature dependent.  (A northern stoat was released into a southern habitat, and when winter came around, the northern stoat turned white, while all the local southerners remained brown.)  Both the long-tailed and stoat (short-tailed) turn all white, with a black tip on the tail.  This apparently deceives predatory hawks and owls to attack the black tip, often allowing the weasel to escape.  However, the smallest least weasel does not have a black-tipped tail.  Apparently, the least weasel is so small, a black-tipped tail will not prevent an attack from above from hitting the body.

Being small (thus, needing to replace lost heat through the large surface area), weasels have a very high metabolism.  This translates into a fast maturation; early to breed, and a short life span.  Least weasels have two litters per year, and the females from the first litter are likely to bear young later that same summer.  Their life span is about one year.  Stoats are a bit larger, and live up to 2 ½ years of age.  Both the stoat and the long-tailed weasel cannot grow fast enough to breed in the first summer, nor can they produce two litters in one season, so they maximize the growth by giving birth early in the spring.  This is accomplished by delayed implantation of the fertilized egg.  (It is believed even the least weasel’s ancestors practiced delayed implantation, but abandoned the habit in order to accelerate their birth rate.)  Another method employed by stoats to aid the population, is for the female to mate as an unweaned infant in the nest.  The infants can mate with the mother’s current mate, who probably is not the father, due to the delayed implantation and mating from the previous year. 

Fisher have a delayed implantation of 300 days.  American mink have four reproductive periods during spring, separated by six to twelve days between periods.  If a female is impregnated during the first period, it will delay implantation for two months, while a female impregnated in the last period will only experience a two week delayed implantation.  The result is that all females will give birth in May. 

Sexual dimorphism exists among the weasels (males are quite larger than females).  One reason is so the female doesn’t have to spend as much time hunting to meet her needs and can spend more time feeding and providing warmth for the large litters.  Additionally, the pregnant weasel is almost as wide as a male, thus limiting the size of the burrow the female can accommodate.  Thus, the female must be smaller the rest of the year to accommodate the limitations of the pregnancy girth. 

Regarding the males, larger males get the attention of the females and can ward off competitors for the females.