One of the features shared by nearly every mammal species on Earth — from antelopes to zebras, and even humans — is that their bodies are covered in structures known individually as "hairs" and collectively as "fur."
Fur can be dense or sparse; soft or coarse; colorful or drab; monochromatic or patterned. However, regardless of what it looks or feels like, fur is an evolutionary characteristic that defines the mammalian lineage.
But what makes a lion's mane different from a polar bear's coat, a boar's bristles or a ram's fleece — or even the hair on our own heads? [The World's 5 Smallest Mammals]
According to Kamal Khidas, curator of the vertebrate collection at the Canadian Museum of Nature, there are three types of hair in mammals that make up their fur: vibrissae, which are sensitive tactile receptors, such as whiskers, used for sensing the environment; guard hairs, the most conspicuous hairs, which serve as protection; and underhairs, whose primary purpose is insulation.
The length, thickness and density of these hair types contribute to the incredible diversity we see in mammals' furry pelts.
"Hair is the basic unit," Khidas told Live Science. Hair is made of keratinized filament — the same substance that makes up our fingernails — and can vary in length from just a fraction of an inch to about 3.3 feet (1 meter).
What is commonly called "fur" is typically recognized as "the relatively short hair with definitive growth that grows densely over the body," Khidas said. The type of fur known as wool is a kind of underhair — soft, thin, curly, flexible hair that never stops growing.
Human hair is less differentiated than the hairs on other mammals, having characteristics of both guard hairs and undercoat hairs, according to a manual on hair microscopy published in 2004 by the Federal Bureau of Investigation (FBI).
But to begin to understand how fur diverged into the variety grown by animals alive today, we first need to take a step back in time, to about 310 million to 330 million years ago, to an era when something akin to fur is thought to have first appeared.
A scaly start
The first type of "hair" to emerge in mammalian ancestors was perhaps a modification of scales, "or some sort of hard, nonhair epidermal structures," Khidas told Live Science in an email.
"What seemed to have happened was that some sort of dormant genes that already existed in mammal ancestors later played a role in hair formation," Khidas said.
A need for insulation likely drove fur's evolution in early mammals, as it developed alongside another trait that differentiated them from reptiles: a consistently high body temperature that had to be maintained, using a process known as thermoregulation. [In Photos: Mammals Through Time]
Rob Voss, a curator in the mammalogy department at the American Museum of Natural History in New York City, told Live Science that fur's most important role for mammals is to help with thermoregulation, preserving their internal temperature regardless of external conditions.
In especially cold environments, terrestrial mammals such as the musk oxes, arctic foxes and polar bears rely on their thick coats to stay alive in frigid temperatures; dense fur traps a layer of air close to their skin, which helps to keep them warm. Semiaquatic mammals, such as fur seals and otters, also have a thick covering of fur, with sea otters sporting up to 1 million hairs per square inch of skin — more than any other mammal.
Slick-skinned marine mammals such as whales, dolphins and elephant seals lost their furry coverings long ago but replaced the fur's insulation with a thick layer of blubber that shields them from the cold, Voss explained.
But in warmer climates, larger mammal species tend to have sparser coverings of hair, as big animals are generally able to maintain their core body temperatures without much insulation, Voss said. Smaller animals with higher metabolic rates tend to have body temperatures that fluctuate more dramatically, and are therefore more reliant on furry insulation to protect them from dips in external temperatures, he added.
More than just warmth
However, a mammal's fur can serve many purposes in addition to insulation. In some species, Voss told Live Science, guard hairs evolved into highly specialized protective structures — like the porcupine's and hedgehog's quills, or the pangolin's armor, where hairs fuse together to form tough plates.
Fur can also be a source of camouflage. For example, Voss said, small mammals' coats generally match the color of the soil in their environment so they'll blend in with the dirt. Fur coloration can be used for sexual selection, or to serve as a warning to predators that an animal carries toxic chemical weapons — as is the case with the skunk.
"Rodents that have odors or toxic chemicals in [their] skin tend to be marked in black and white," Voss said. "Most of them are nocturnal, so colors like black and white stripes stand out."
And a recent study of zebras' distinctive striping suggested that their patterns might have evolved to deter biting tsetse flies.
Considering that mammals are so reliant on their fur, it's no wonder that they also work hard to keep it in good condition. Grooming isn't a high-maintenance luxury — it can be a matter of life and death, Voss noted.
"Most mammals invest an enormous amount of time in maintaining their fur, to preserve quality, function and insulation, and to weed out ectoparasites," Voss said.
The dull, dirty or matted fur also sends a warning signal to prospective mammal mates, he added. "Hair is a good indicator of health in most mammals," he said. "Strong, healthy mammals have glossy coats, while sick mammals have shabby-looking coats."
And what about humans? Our own hair — even though we don't call it "fur" — is an intrinsic part of our mammalian heritage, though perhaps we have less of it overall than some of our fuzzy friends.
And while one aspect of our cranial hair is, in fact, rare among mammals — it grows continuously and isn't shed seasonally as most mammal fur is — when it comes to sexual selection, a glossy, healthy head of hair may be just as important to us as it is to our mammalian relatives.
"Most of the things we find beautiful are markers of youth and health," Voss said. "This could be one of the cues that humans use unconsciously to assess youth."