Mammals Introduction

WHAT ARE MAMMALS?

An answer to the above question only becomes clear when the characteristics of mammals are known and understood. Mammals are a diverse group that inhabits a great diversity of habitats: from temperatures well over 100 degrees to well below minus 30 degrees, from very dry deserts to life in a pond or stream. There are strict vegetarians to strict carnivores. Their characteristics are the result of a wide range of adaptations that allow them to survive in the wide variety of environmental conditions they live in.

It is also important to consider their history. From a few small animals existing in a limited number of ecological niches, small mammals have evolved and adapted to a broad diversity of niches. Mammals evolved during the warm, wet climate of the geological era called the Mesozoic, which existed from 200 to 70 million years ago. The late Mesozoic, known as the "Age of the Dinosaurs" was a time when reptiles were most abundant and numerous. A few mammals lived then, but they were small, weasel-like insect eaters. Their small size and quickness might have helped them to avoid larger and slower reptilian predators. The evolution of hair for insulation and endothermy,(warm bloodedness) gave them opportunities to occupy more niches, especially when climates during that era became colder. Because reptiles were ectothermic (cold blooded), they could only be active during warmer periods when temperatures allowed their physiological systems to function efficiently. We can see this temperature restriction today when we compare the number of mammal species in Idaho to the number of reptile species; there are many more mammals than reptiles. When the large, numerous reptiles began to diminish in numbers during the late Mesozoic, the mammals were well adapted to expand their range and diversity. They could reproduce at maximum rates, acquire food efficiently, and survive in the climates and ecosystems of that time. During the most recent geologic era, the Cenozoic, which has been called the "Age of Mammals", they diversified and their numbers increased rapidly. Today the trend may be reversed; human destruction of habitat has accelerated the extinction of mammal species and there is concern that we are losing mammal diversity .

The first characteristics we usually think of when we think of mammals is "hair" and "nursing their young". Hair grows from the epidermis of the skin, contains the protein keratin, which gives it resiliency, and hair provides insulation and protective coloration. Most mammals have thick coats that insulate them well. A river otter foraging along a river such as the Salmon River, in mid-winter when it is minus 350 , attests to the insulative value of hair. Mammal milk provides young mammals with a good start in life. They can acquire it easily and it is highly nutritious. In provides an abundance of nutrients and even some antibodies from the mother, which helps the young resist infections and diseases. Nursing insures that the mother is providing intensive care of the young.

Certain physiological adaptations are important also. The mammal heart is four-chambered and capable of rapidly circulating a high volume of blood. This rapid circulation accommodates a higher metabolic rater and the maintenance of a constant body temperature. Mammalian heart rates vary, but generally, are more rapid in smaller mammals. Below are some heart rates of selected mammals.

Mammal Heart Rate	(Beats per minute)
Masked shrew (Sorex cinereus)	588 -1320
Least chipmunk (Eutamias minimus)	660 - 702
Mink (Mustela vison)	272 - 414
Human (Homo sapiens)	55 - 75
Horse (Equus caballus)	34 - 55

As you can see, smaller mammals have a much higher heart rate than larger mammals. This is related to the "surface to volume ratio" difference of small versus large mammals. Smaller mammals have a proportionately larger surface area, relative to their volume, exposed to the environment than larger mammals. Thus, they lose proportionately more body heat to the environment. A loss of body heat is a loss of energy, and smaller mammals must compensate by being more active, and eating proportionately more than larger mammals. The sense organs of mammals are very well developed.

The sense of smell is acute, hearing is quite variable but generally well developed (and much better than that of humans), and eyesight is typically very good. It has been reported that when bears first emerge from hibernation, they search for carrion(dead animals) which often is plentiful in early spring from winter deaths. There is some evidence that a bear can smell the carcass of a dead animal such as an elk from many miles away, perhaps up to 10 miles. Small insectivores, such as shrews, have poor eyesight and rely on hearing. Some shrew species even use echolocation to help them navigate about their environment. Pronghorns, which live on prairies, have very specialized eyes. The rods and cones of their eyes are arranged on a horizontal plane in the back of the eye, which allows them to see movement and objects that are quite far away on toward the horizon. Bats, of course, rely very heavily on echolocation. Echolocation demands a very keen sense of hearing. The tactile sense, or touch, of many mammals is very good. Vibrissae, or long whiskers, are tactile organs and may be very important, especially for nocturnal mammals.

Mammal skeletons are variable, and especially adapted to the various modes of locomotion. Consider that mammals can fly (bats), glide (northern flying squirrel), climb (tree squirrels), swim (beavers and muskrats), run and gallop (hoofed mammals), dig and live underground (pocket gophers), etc. Their skeletal modifications include long, strong legs for running, different foot structures for climbing, digging, and running, and modifications of the front and hind limbs for flying and gliding. Teeth are also an important adaptation. The hardest part of the body, teeth persist in the environment long after the animal is dead, and years later they are often the only part of the animal we find. Because teeth reflect the diet of the animal very closely, then can usually indicate what the mammal ate.

[insert photo or drawing here - with explanation of the type of teeth].

Mammals have evolved various reproductive strategies that ensure high survival rates for long numbers of young. Internal fertilization and development in the uterus provides a safer environment than that for eggs laid externally by distant mammal ancestors such as amphibians and reptiles. The reproductive season is controlled by hormones that produce the estrous cycle. During estrus, when the mammal is "in heat", the uterus is prepared for implantation of a fertilized egg.

We can think of two basic reproductive strategies, quantitative and qualitative. Rodents are quantitative: female rodents often breed shortly after giving birth to a litter of young. Four weeks later they can be nursing another litter. Qualitative species, such as weasels and bats, only have one small litter per year. The survival rate of their young is much higher than that for rodents, chiefly because of more intensive and long-term parental care. Some larger mammals such as bears, have young only every 2 or 3 years, and the young stay with the mother for about 2 years. While their young production is low, the level of parental care is intensive, which greatly increases the survival of the young.

Order: Insectivora (Shrews & Moles)
Shrews, the smallest mammals, are also important because they represent the most primitive mammals. Their characteristics most closely resemble primitive mammals that fossil evidence indicates evolved during the era of dinosaurs. As the name implies, they feed primarily on insects and other small invertebrates. This order includes two families, Soricidae, the shrews and Talpidae, the moles. In Idaho, we have relatively few species.

Order: Rodentia (Rodents)
Most are small, secretive, nocturnal, abundant, and difficult to observe. Without a doubt, the majority of mammals in Idaho are rodents, and about one-third of all mammals, about 1,700 species, are rodents. Their abundance is due partly because they occupy a wide diversity of niches; from tree tops, to undergound burrows, to the water, to human shelter, such as cabins, barns and garages. Their primary distinguishing characteristic is large, ever-growing, chisel-like incisors that occur in pairs in both the upper and lower jaw. These incisors are kept chisel-like because the tips of the upper incisors wear away the tips of the lower ones and vice versa. This keeps them sharp and much like the shape of chisel blade. As primary consumers they are low on the food chain. They also provide many meals for predators and thus have a short life. Only a high reproductive potential overcomes their high mortality. Many rodents have large litters and reproduce up to several times each year. Some rodents, even though they are primarily vegetarians, are also good predators. Many feed on a variety of invertebrate prey, especially insects. Some, such as ground squirrels can be so abundant that they consume crop plants to the excess. Others, such as pocket gophers, may create problems for farmers and ranchers. Overall, though, they are ecologically beneficial and important in most Idaho ecosystem.

Order: Carnivora (Carnivores)

Order: Artiodactyla (Hoofed Mammals)

Order: Chiroptera (Bats)
Of the 26 orders of mammals in the Class Mammalia, the order Chiroptera, which means winged hand, is graced with an amazing diversity of 925 recognized species. In fact, bats are one of the most diverse groups of mammals, achieving second place to the largest group, the rodents. Many people think of bats as flying rodents, but bats are really more closely related to primates.

Although the familial diversity of bats is especially high in the tropics, only one group, the family Vespertilionidae, is known to occur in Idaho. It is likely that one additional species, Tadarida brasiliensis, the Mexican free-tailed bat, a member of the Family Molossidae, will be found in the extreme southwestern corner of Idaho as our collecting effort expands into less accessible habitats. A Idaho echolocation recording does exist for this species in that area. Additionally, I suspect that Lasiurus blossevillii, the Western red bat, a member of the family vespertilionidae may occur in Idaho. Fourteen species of vespertilionids are confirmed with museum voucher specimens. All Idaho bats feed on insects, two are obligate tree roosters and one appears to be restricted to cracks in desert canyons containing cliffs. The remaining species are found in multiple roost situations.

Three distinct characteristics that separate Idaho bats from other Idaho mammals include the ability to fly, echolocate, and the rotation of the upper leg bones. Rotation places the knee joints on the opposite side of the leg. The leg position aids wing support and permits bats to hang upside down, a condition enhancing rapid flight from a resting state and enabling watchful vigilance if they are not hibernating. Some bats hibernate in Idaho during winter whereas others migrate to warmers regions.

Order: Lagomorpha (Pikas, Rabbits and Hares)
Legendary for their ability to reproduce, members of the order Lagomorpha are found on every continent. The order includes two families: Ochotonidae, the Pikas, and Leporidae, the rabbits and hares.