Mammals at the Cofrin Center for Biodiversity

Herps banner.

Phylum: Craniata (Chordata)
Class: Mammalia

N. Flying Squirrel (Photo by Dr. Lloyd Glenn Ingles,California Academy of Sciences) by UW Green Bay students David Marks and Todd McCoy

Mammals are fur-bearing animals that are capable of internal temperature regulation. With only a few exceptions, female mammals give birth to live young and feed their offspring milk from mammary glands. Mammals have the largest brains in relation to body size of any vertebrate. This places a severe limitation on offspring development at birth, since many females are to small to give birth to highly developed young, instead most mammals invest in long periods of parental care. Unlike reptiles, the mammal lower jaw is composed of a single bone, and the jaws contain 4 specialized types of teeth, the incisors, canines, premolars, and molars. Mammals have 3 middle-ear bones that set up a chain of vibrations that are transmitted across a membrane to nerve receptors in the fluid filled inner ear.

Mammals have been evolving and diversifying since the Triassic Period when they arose from the Synapsidae, a primitive group of amniote vertebrates. Although sometimes called the "mammal-like reptiles", synapsids were actually neither. Some advanced synapsids (therapsids) had many features in common with mammals, most notably a pelvis that allowed them to keep their legs underneath their body, rather than splayed to the sides like most reptiles. The first mammals appeared soon after the dinosaurs, about 230 million years ago. It is believed that these mammals were small mammals, the size of modern day shrews and mice, and that they were insectivores, feeding only on insects. Dinosaurs were important predators and competitors of mammals. It wasn't until about 65 million years ago, after the extinction of the dinosaurs, that larger mammals were able to evolve to take advantage of newly available habitats and food. Today mammals are found throughout the world in almost every type of habitat.

There are 4 major groups of mammals: the egg-laying monotremes, the metatheria or marsupials, the eutheria or placental mammals, and the now extinct Multituberculata . There are only 3 species of living monotremes found only in Australia, Tasmania, and New Guinea, the duck-billed platypus, and 2 echidnas, Tachyglossus aculeatus, and Zaglossus bruijni. Monotreme means "one-opening" and refers to the cloaca, the shared opening for the urinary, digestive and reproductive tracts. In marsupials the placenta that connects the fetus to the mother and provides nourishment is reabsorbed and severely limits how long embryos can survive in the womb. Females give birth to tiny, poorly developed young that then crawl into a pouch where they are fed milk and develop further. Eutherians are the most diverse group of mammals. They all have a complex placenta that grows to accommodate the growing embryos and allows much development during gestation.

Mammal Diversity

There are 4,629 described mammal species throughout the world. This number is relatively small compared with over 9,600 species of birds or 40,000 species of fish. Structurally and functionally, however, mammals are quite diverse. The class mammalia is divided into 26 orders, 133 families, and 1117 genera. Mammals can range in size from about 3 g (pygmy shrew - Sorex hoyi) to about 160,000 kg (blue whale - Balaenoptera musculus). Today, mammals occupy every major habitat. Lifestyles include subterranean, canopy dwelling, aquatic, and aerial species with many structural and functional adaptations. Mammals also provide many examples of convergent evolution, especially between marsupials and placental species.

Wisconsin Mammals

Wisconsin encompasses a wide variety of habitats, and therefore has a diverse mammal population. Twice during the Pleistocene period, Wisconsin was covered with glaciers. The second of these glacial movements extended further south, but was not extensive in southwestern Wisconsin leaving the topologically unique Driftless Area. As the glaciers scoured the land, formed lakes, and deposited debris, they created a variety of different habitats.

Mammals in Wisconsin have been observed and recorded by explorers, trappers, and missionaries as early as the middle of the seventeenth century. Jean Nicolet and Louis Joliet made descriptions of many mammals, and many were detailed enough to determine the species. Joliet made the earliest record of a cougar in Grant County in 1673. Trappers soon followed and established many trading posts. Many of these left accurate records of sales, which include species name and often place of capture. Today, there are approximately 80 Wisconsin mammal species (including threatened or endangered species) and includes seven orders and eighteen families.

Importance of Mammals

cow. The global success of humans is tied directly to the exploitation of other mammals. The earliest humans hunted or scavenged mammals for food, clothing and tools. As human populations increased the number of mammals exploited increased. It is thought by some archeologists that early Pleistocene hunters were responsible in part for the extinction of large mammals like mammoths. Humans also began capturing mammals and keeping them captive as a form of food storage. When humans began to selectively breed these wild animals based on certain features such as size or gentleness, true domestication began.

Probably the first mammal to be domesticated by humans was the dog, possibly as long as 100,000 years ago (Vila et al., 1997) . Humans began domesticating sheep and goats about 10,000 years ago as gatherers and hunters began to switch to a sedentary agriculture based society (Feldhamer et al 1999). The two main differences between all domesticated mammals and their wild counterparts are that they are bred to be more docile and better adapted to living with humans. The benefits of domesticating mammals include food from meat and milk, usable materials from bones, urine, feces, and hides, and work from transportation, pest and waste control, protection, to companionship.

Even though humans have been responsible for the decline of many wild mammal populations, mammals have had a large impact on human populations as well through the spread of diseases including plague, which killed almost 25 million humans in Europe during the 13th century, and viruses like hantavirus, influenza, and ebola.

Photos courtesy of California Academy of Sciences/Manzanita Project and CalPhotos

Biodiversity Topics: Introduction . Plants . Animals . Mammals . Birds . Reptiles & Amphibians . Arthropods . Spiders . Insects