Laboratory 10: Mammals


Mammals are highly derived reptiles. They are distinct from other tetrapods in having hair, three ear bones and mammary glands. Also, all mammals, with the exception of monotremes, give birth to live young and provide nutrients to growing embryo via the placenta.

A. Range and Diversity:  Mammals are found throughout the world, even in extremely cold habitats. There are over 5000 extant species distributed across 26 different orders .

B. General characters shared by mammals:

The three unique traits mammals have (synapomorphies) include:

1. Three ear bones: malleus, incus and stapes. The malleus and incus are derived from jaw bones.

2. Presence of hair (dead keratin). All mammals, at one stage of their life cycle, have hair, and only mammals have true hair. Hair grows out of pits in mammalian skin called follicles, and have muscles (for erection) and glands associated with them. The glands typically lubricate the hair, while the muscles can erect hair for signaling or for thermoregulation.

3. Mammary glands for milk production. Mammary glands likely evolved from sweat glands. Both sexes typically have mammary glands, but development typically stops for males during puberty. Mammary glands produce milk, a substance rich in fats and proteins, for their young. Other organisms produce milk-like substances (like pigeons from their crops and Caecilians from their uterine wall).

Other traits mammals share but is not necessarily unique to mammals:
1. Endothermic: "warm-blooded," ability to maintain body temperature by producing metabolic heat.
2. Live bearers (exception: Monotremes)
3. Internal fertilization
5. 4-chambered heart
6. Extreme parental care -- most often maternal care

C. The Amniotic Egg

This was presented in the previous labs. However, this is an important adaptation for the Amniota clade and so I'm including this here. Key features include:

1. Shell: provides protection and prevents dessication of egg on land
2. Yolk: provides nutrients
3. Allantois: site where toxic waste from metabolic processes are stored
4. Albumin: protects embryo and provides water
5. Amnion: extra membrane that protects the embryo from dessication

Image Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

D. General Lineages

Mammals evolved from Synapsid reptiles (see Lab 8). Within the class Mammalia, mammals are further divided into three (subclasses) lineages, based primarily on characteristics of their reproduction:

1. Prototheria. The only extant class of this group are the monotremes (echidnas and platypus, see below). Prototherians retain many characters of their reptilian ancestor such as egg-laying, and presence of a cloaca.

2. Metatheria or the marsupials. Unlike Eutherians (see below), marsupials give birth to an underdeveloped young, which completes its development in a pouch. Also, marsupials tend to have a lower metabolic rate and smaller brain size than their Eutherian counterparts.

3. Eutherians or placental mammals. This group is unique in having their embryo develop internally and giving birth to more developed offspring. Eutherian mammals typically have higher metabolic rates and large brain size than Prototherias and Metatherians.


A. All Extant Mammalian Orders

Because there are 26 orders of mammals, we will not go over each one in detail. A few chosen orders are found below -- those that you will likely encounter in California. Follow this link, Mammalian Orders with Pictures to find all orders listed with examples of each. In your research you may find discrepencies in the different orders. This results from the use of different methods to classify mammals. Use the Animal Diversity Web site, linked above, as a baseline when you do your research.


B. Orders Common to the Bay Area

Subclass Metatheria

1. Didelphimorphia: Amercian marsupials/opposums. There are nearly 60 species throughout North and South America, but only one is found in the U.S.: Didelphis virginiana , the opposum. They are very common in the Bay Area. Like all marsupials, members of this group have a pouch where their young complete their development. Members of the family are omnivorous.

The opposum, Didelphis virginiana. This is the only marsupial in the US, and is a very common in San Francisco.
Opposum, Didelphis virginiana, tracks. It is often difficult to observe mammals in the wild. One of the best ways to determine that an animal has visited a specific areas is through their tracks (footprints)

Subclass Eutheria

1. Order Artiodactyla (even-toed ungulates): There are currently about 220 species across 10 families. They are referred to as even-toed animals because the plane of symmetry for their legs runs through the middle of the 3rd and 4th digits. Because artiodactyls typically feed on vegetation like grass, most artiodactyls have modified stomachs. Some, like cows, have four-chambered stomachs to facilitate digestion of tough plant matter. Further, they have a symbiotic relationship with microorganisms to decompose cellulose, which is an important part of plant tissue that  mammals cannot digest. Examples include Bovidae (cattles, sheep), Cervidae (deer), Giraffidae (giraffes) and Suidae (pigs).

The black-tailed deer, Odocoileus hemionus. A common mammal in the Bay Area.
Black-tailed deer, Odocoileus hemionus, tracks.

2. Perissodactyla (odd-toed ungulates): Like artiodactyls, members of this group eat vegetation (herbivores) and hence have long stomachs. However, unlike artiodactyls, their middle toe is larger than all other toes and the plane of symmetry of the leg passes through the middle toe (3rd toe. Modern perissodactyls are native to Africa, south and central Asia, southern North America, and northern South America. Examples include Equidae (horses and zebras), Rhinocerotidae (rhinos) and Tapiridae (tapirs). No perissodactyls are native to the Bay Area.

The horse, Equus caballus. Original populations were once found from Poland to Mongolia. Domesticated horses are now found throughout the world. They are not native to the United States. The horses pictured below is from a feral (now wild) population on Assateague Island near Virginia.


3. Carnivora (the carnivores): A group of highly diverse mammals that all eat meat (but not all meat eaters are carnivores!).  Carnivores have specialized fourth molars called carnassial teeth. These molars are specially shaped for shearing meat. Because carnivores are exclusive meat-eaters they have relatively short and simple stomachs (unlike the artiodactyls, for instance). There are about 270 species of carnivores across 11 families. Examples include Canidae (dogs, wolves, foxes), Felidae (cats, lions, tigers), Ursidae (bears), Phocidae (seals) and Mustelidae (weasels).

Canis latrans, the coyote. These animals live in packs and are active mostly at night.
Canis latrans, coyote tracks.

4. Rodentia (rodents): There are over 2000 species of rodents across 30 families -- making rodents the most diverse mammalian families.
Rodents are very diverse and can range from tiny mice to very large capybaras (140 lbs rodents of South and Central America!). Rodents have distinct teeth for gnawing: they have a single pair of upper and a single pair of lower incisors, followed by a gap ( diastema), followed by one or more molars or premolars. Their incisors grow continuously. During gnawing,  the incisors grind against each other and the dentine layer of their teeth are worn off. This leaves a sharp, chisel like edge excellent for gnawing. Examples include Muridae (mice and rats), Sciuridae (squirrels), and Geomydae (beavers).

Mus musculus, the (much-dreaded) house mouse.
Sciurus niger, the fox squirrel.

4. Lagomorpha (rabbits and hares). There are about  80 species of lagomorphs across 2 families. They are native to all continents except Australia and Antarctica. However, humans have introduced them in many areas, with catastrophic consequences. All Lagamorphs are herbivores and terrestrial. Examples include Leporidae (rabbits and hares) and Ochotonidae (picas).

Sylvilagus bachmani, the brush rabbit.

Sylvilagus bachmani, the brush rabbit tracks.

5. Insectivora (shrews and moles - insect eaters): Insectivores feed almost exclusively on invertebrates, especially insects. There are about 375 species across 6 families. Most insectivores are tiny and rely on their senses of hearing, smell, and touch rather than vision. Insectivores are found throughout the world, except Australia.

Sorex trowbridgii, Trowbridge's shrew.

Scapanus latimanus, the broad-footed mole. Very common in California, but hard to observe since they burrow underground.



Lab 10 Exercise

Task #1 - For each mammalian order listed above in section "IIB. Some Orders Common to the Bay Area" list two other species found naturalized (wild and breeding) in the Bay Area or, if only one or no other species are found here, then indicate that. Besides primates there are two mammalian orders that have species native to the Bay Area that are not listed in IIB. What are they? List three species of each that can be found here. Be sure to use the italicized binomial and, since they all have them, the English name, too.

Task #2 - Field journal. Find two mammals to write comprehensive descriptions and two drawings for each. One of these can be a domesticated or zoo mammal. Many parks, including Golden Gate Park (especially the Botanical Garden), have tame squirrels.

Task #3 - Write a one or two page research paper on one of the following topics:

Many vertebrates are threatened in the United States. Write about a pressing conservation issue about mammals (this cannot be the same subject as your poster if you are in Biology 170).

Prototherian mammals are a unique group of mammals that is not discussed in detail in this lab (due to the fact that they are not found locally). Write a thorough paper about this group of mammals.

 Describe the evolution of mammals. Include information on their role during the dinosaur age.

Due May 8.