PELAGE
Classification of Hair
On the basis of growth, hair may be classified as either definitive or angora. Hair with definitive growth reaches a certain length typical for a species
and body
location and growth ceases. These hairs
are shed and replaced periodically, such as human eyelashes.
reaches a considerable length before being shed. Some angora hairs, such as a horse’s mane, are never shed but continue to grow through the life of the animal.
Growth patterns and hair functions are combined to classify hair into the various types recognized on mammals.
Vibrissae
Vibrissae are long stiff hairs with well innervated bases. They primarily serve as tactile receptors. The best known vibrissae are the “whiskers” on a
mammal’s face, but vibrissae may also be located on the legs and elsewhere on the body.
Guard Hairs
Guard hairs or overhairs are the most conspicuous hairs on most mammals. They serve primarily for protection. The three major types of guard hair
recognized are given below. However, intermediate types do occur. Spines (commonly also called “quills”) are greatly enlarged, stiff guard hairs with definitive
growth. They serve primarily as defense from
predators. The
barbs on the tips of the spines. Once embedded in a predator’s skin, such a spine cannot be easily removed, and the victim’s movements will actually cause the
spine to embed even more deeply. If it works its way in deeply enough it can cause major infection, or even penetrate a vital organ. Spines on other groups of
mammals
like the
barbs. Bristles are long, firm hairs with angora growth (e.g., horse and lion manes). Awns are hairs with definitive growth that have a firm, expanded distal
portion and a smaller, weaker base. These are the most noticeable hairs on most mammals.
Underhairs
Underhairs (or underfur) function primarily for insulation. Three major types are generally recognized but, again, intermediate types do occur. Wool is
angora underhair. It is usually long, soft and curly. Fur is fine, relatively short, hair with definitive growth that grows densely over the body.
Velli, down or fuzz, are very fine, short hairs that are velvety in appearance. Embryonic hair, lanugo, is a type of velli.
Examine a variety of mammals. What types of hair are found on each? What is the function of each type
of hair?
Hair Replacement
Some angora hairs grow continuously for the life of the mammal and continuously are worn away at the tips. Most hair, however, is shed and replaced
periodically in a process called molting.
Molts may occur continuously with some hairs being replaced at all times, e.g., human eyelashes. But most mammals, particularly those living in temperate
or arctic climates, have an annual molt during which all hairs are replaced in a short period of time. Such molts usually begin in a specific region of the body and
progress in orderly fashion until all hairs have been replaced. The molt pattern varies with the species and occasionally with the age of an individual. Some
mammals have seasonal molts with more than one molt per year. This is most conspicuous in species that cahnge from a brown summer pelage to a white winter
pelage. In northern populations of the long-tailed weasel, Mustela frenata, for example, the spring molt begins along the dorsal surface and brown hairs replace
the white winter hairs over the dorsal parts of the animal, progressing downward until his entire pelage, except for his belly, is brown. In the fall, the molt pattern is
reversed with replacement of brown hairs by white ones progressing dorsally from the perpetually white belly. In southern parts of the range of this species, the
molts occur but do not result in a color change, and the weasel has a brown dorsal pelage with a white belly throughout the year. The change from winter to
summer pelage is influenced by hormones, affected by photoperiod and temperature.
A distinctly juvenile pelage is recognizable in many mammals. This is usually grayer and duller than the adult pelage. The juvenile pelage can also be
variously striped or spotted while the adult is more or less uniform. In some groups a distinctive subadult pelage may also occur between the juvenile and
adult pelages.
Color
The color of an individual hair is affected by numerous factors. The kind, amount, and distribution of pigment granules in a hair can all vary to produce
different effects. In addition, hair surface texture, the thickness of the hair, and the amount of air space in the medulla can all alter the way in which light is reflected
by the hair, and, therefore, change the appearance of the pigments present.
The overall coloration of a mammal is determined by the coloring of individual hairs and the relationships among these hairs in the pelage. An animal may,
therefore, be red and brown speckled because each hair has both red and brown pigment bands or because the pelage has a mixture of both red hairs and brown
hairs.
There are two types of pigment in mammalian hair. Eumelanin in various concentrations produces blacks and browns. Pheomelanin in various
concentrations produces reds and yellows. White is the complete lack of pigment. Each hair usually has a series of color bands. The agouti hair has a black tip
followed by successive bands of pheomelanin and eumelanin. This is the most common coloration pattern among mammalian hair and is thought to be primitive.
Examine individual hairs from a
cottontail rabbit (Sylvilagus)
or an opossum (Didelphis) and note the
sequence of color
bands – these are typical agouti hairs.
Examine the color banding on hairs of
other mammals that are generally considered brown. What kinds of
variation do you
find? How do they compare in number,
width, sequence and hue of the bands? Do
all the
hairs from a
given animal exhibit the same banding?
Mammalian hair and skin coloration serves several functions. Among these are concealment, communication and protection from UV radiation. Many
predators and many prey have concealing or cryptic coloration that allows them to blend with their habitat to avoid detection. The primitive agouti pattern
provides coloration that is usually very similar to the color of the earth and dead vegetation.
Mammals such as the tiger and zebra are strikingly marked with sharply defined light and dark colors. This would seem to stand out, but in their normal
habitats these examples of disruptive coloration obscure the body contours and cause the animal to blend into the patterns of light and shadow caused by sunlight
on tall vegetation. Facial stripes, present in many mammals, are also disruptive coloration, usually intended to conceal the eye. Most mammals have a ventral
surface that is more lightly colored than their back – this is called countershading. Why is that important?
Coloration is also important for intra- and interspecific communication. The color pattern typical of a species serves to identify each individual to others
of his species. In some species, display of distinctive color regions, such as the red genital area of baboons, is an important part of courtship and mating.
Conspicuously colored “flags” such as the white underside of the tail in many rabbits and deer, may be exposed to alert others of a group or herd to dangerous
conditions. These flags may also offer protection since predator’s eyes will be drawn to the white tail and he may lose sight of the prey when it quickly stops
and lowers the flag. Warning or aposematic coloration is present in some species that have other special means of defense. The striking black and white
striping of a skunk leave the would-be predator with a lasting impression of this most disagreeable prey.
Prolonged exposure to UV can produce many health problems for mammals of all kinds. Melanins filter this harmful UV radiation. Since UV is highest
near the equator and decreases toward the poles, in many cases mammals near the equator tend to be more darkly pigmented than those at the poles. This holds
true if other factors like concealment and communication don’t produce alternate patterns.
Terrestrial vertebrates inhabiting arid regions are usually paler in color than closely related forms inhabiting more humid regions. This phenomenon is known
as Gloger’s Rule. The factor that most likely produces this variation in color pattern is the background color of the habitat. As soil becomes more arid, vegetation
becomes sparse, leaving relatively little decaying humus in the soil. So the soil tends to be light in color. In humid environments, the large amount of humus in the
soil makes it relatively dark (go wander thru VanLandingham Glen and look at the soil color). Thus, the mammals are subjected to natural selection by predators
to blend
in with their background. In desert
areas where there are large expanses of black volcanic rock (as in northern
mammals are usually black like the rock rather than light as Gloger’s Rule would predict.
Albinism, the complete lack of pigments in the skin or hair, results from mutation and is found in many species of mammals. Since true albinos are not well
adapted for camouflage, communication or UV protection, they generally do not survive to establish themselves in the wild population. Partial albinos may survive
better, but most species in which the pelage is normally white (e.g., polar bears) are not true albinos. They have pigments in the eyes and skin, but pigments do not
get deposited in the hair.
Melanism, another mutation, causes an increase in the amount of eumelanin in the skin and hair. These are typically very darkly colored individuals, and
can much more commonly survive within their wild population than can albinos. So melanism is very common among mammals – e.g., melanistic individuals among
red foxes, fox squirrels and grey squirrels are very common in this area.
A few kinds of mammals have coloration that is not dependent upon pigmentation. Sloths, for example, have coarse overhair with numerous external
grooves. Algae grow in these grooves and give the animals a greenish color that allows them to better blend in with their forest environment. The color of many
mammals is also frequently altered by layers of dust (e.g., elephants).
Examine the specimens at various
stations to note examples of Gloger’s Rule, countershading,
disruptive coloration,
etc.
Integumentary Glands
There are two basic types of glands in the skin of mammals, sebaceous glands and sweat glands. All other glands are considered to be modifications
of one of these two types.
Sweat glands are found only in mammals, although several kinds of mammals – echidnas, moles, sirenians, cetaceans, etc. – have none. There are two
basic types of sweat glands. Sudoriferous (=apocrine sweat) glands are highly coiled and usually located in the vicinity of hair follicles. These produce the
odorous component of perspiration, and are usually located in very specific areas of the body. In humans, for instance, they occur in the armpits, naval, ano-genital
areas, nipples and ears. Eccrine sweat glands are also highly coiled but open directly onto the skin surface, independent of hair follicles. Eccrine sweat glands
are responsible for most of the fluid portion of sweat and also excrete some metabolic wastes and salts. Evaporation of sweat from the surface of the skin is a
cooling mechanism for the body and perspiration also improves tactile sensitivity and grip when secreted onto the palms and soles of the feet. The wax-producing
glands of the external ear are modified sweat glands. Their wax helps protect the tympanic membrane from drying out and losing flexibility.
Sebaceous glands which are usually associated with hair follicles, serve primarily to keep the hair from becoming too dry and brittle. In many mammals
they are also important in waterproofing the pelage. Sebaceous secretions in the hair of otters and fur seals, for instance, keep cold water from contacting the skin
and thereby retard heat loss. Some sebaceous glands, as in the upper lip, nose and upper cheek areas of humans, open directly onto the skin surface rather than
into hair follicles.
Mammary glands were long considered to be derived from sweat glands, but new evidence suggests that they are new structures which share some
features in common with sebaceous glands. We may never know their true homologs.
Scent glands are complex odor-producing glands found in many mammals. They are often composites of both sebaceous and sudoriferous glands. Much
remains to be learned about their functions and secretions, but generally the functions can be divided into three general categories: defense, recognition of territory,
and social interactions.
In defense, skunks (Carnivora; Mustelidae) discharge a mercaptan-based musk from anal glands. Wolverines (Carnivora; Mustelidae) and peccaries (Artiodactyla;Tayassuidae) are examples of other mammals that emit a musk when they are in danger.
Many mammals establish a scent trail in their territory. This not only helps them find their way around as they forage (like dropping a trail of bread
crumbs), it also allows them to lay personal claim to the territory, warning others to stay away. This process of labeling an area with scent is called scent marking.
The interdigital glands of deer (Artiodactyla; Cervidae) and the musk glands of badgers (Carnivora; Mustelidae) apparently are used for scent marking. In rodents
and some primates, urine and anal gland secretions are important for marking territories.
As we said, scent marking can also be used in social interactions within a species. A pheromone is an odor that has a behavioral or physiological effect
on other individuals of the same species. For example, pheromones in the urine of many rodents are known to influence the onset of estrus and other reproductive
events. Musk produced by musk glands or anal glands in many mammals is also a type of pheromone. For instance, male elephants go into a kind of male “heat”
(called musth) and their musk glands produce prolific amounts of musk that simulate the females who are nearing estrus. Pheromones in the female elephant’s urine
signal that she is ready to mate and stimulate increased fighting among the males. An alarm pheromone is released when an animal is in danger. The secretions
of the metatarsal glands of deer are thought to be alarm pheromones that warn the entire herd when danger is near.
An individual encountering a scent mark may have several possible responses. A mark made by the individual or a member of his own population signals
that he is “at home”, and may reduce anxiety. A foreign mark, one made by a member of another population, typically results in increased aggression (if it’s found
on your own territory) or an increased readiness to flee (if you have ventured onto another’s territory). The position of an individual within a social group may be
influenced by the amount of scent produced. In some rabbits (Lagomorpha; Leporidae), individuals with well-developed scent glands enjoy high social status and
engage in more territorial marking than individuals with smaller scent glands.
The secretions of scent glands are sometimes used by humans in commercial enterprises. Some scent glands of mammals are removed by trappers and
used to prepare scent baits or attractants in hunting and trapping. The musk produce by the anal glands of certain species of civets (Carnivora, Viverridae) is
utilized as a base in the manufacture of fine perfumes.