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
catus. The
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.