|Class Mammalia Revised from 2009 Book: Revised 13 March 2009
Characteristics of Mammals: see handout - p. 200 (616) - know
Mammals = about 4,800 species
Birds = about 9,700 species
Fishes = about 28,000 species
Insects = about 1, 100,000 species !
In 2006, 510 species of mammals were listed as “critically endangered” or “endangered” by the International Union for the Conservation of Nature and Natural Resources (IUCN), including most cetaceans, cats (except domestic cats), otters, and primates (except humans)
Origin & Evolution of Mammals & Relationships
There are 3 major groups of amniotes that diverged in the Carboniferous Period of the Paleozoic Era - see Fig. 28.1, p. 197 (613):
1. synapsids (includes mammals and their ancestors); skulls of animals in this group have 1 pair of temporal openings (1 on each side of the skull behind the orbits) in the skull associated with attachment of jaw muscles; synapsids were the first amniote group to radiate widely into terrestrial habitats
2. anapsids – skulls of animals in this group are solid (no additional openings for muscle attachment -- only openings for the orbits); this group includes some of the earliest amniotes; NOTE: turtles have a skull with anapsid morphology, but phylogenetic analysis places them in the diapsid clade, suggesting their skull morphology evolved independently
3. diapsids –skulls of animals in this group have 2 pairs of temporal openings in the skull; this group contains the dinosaurs, lizards, snakes, crocodilians, birds, and their ancestors
The earliest synapsids radiated extensively into diverse herbivorous and carnivorous forms often collectively called pelycosaurs – See Figs. 28.2, 28.3, and 28.4, pp. 198-200 (614-616). These early synapsids were the most common and largest amniotes in the early Permian.
From one group of early carnivorous synapsids arose the therapsids (Fig. 28.3), the only synapsid group to survive beyond the Paleozoic.
In therapsids, we see some changes:
1. lateral-extending reptile-like legs were replaced by straight legs held close to body
2. higher from ground; muscular coordination center of the brain (i.e., cerebellum) had an expanded role
3. modifications in the morphology of the therapsid skull and mandibular adductor muscles increased feeding efficiency.
NOTE: Previously, pelycosaurs and therasids have been called “mammal-like reptiles”, but use of this term is inappropriate because they are NOT part of the clade Retilia.
One therapsid group to survive into the Mesozoic Era was the cynodonts. Cynodonts evolved several features that supported a high metabolic rate:
Increased and specialized jaw musculature, permitting a stronger bite
Several skeletal changes, supporting greater agility
Heterodont teeth, permitting better food processing and use of more diverse foods (Fig. 28.4)
Turbinate bones in the nasal cavity, adding retention of body heat (Fig. 28.5)
Secondary bony palate (Fig. 28.5), enabling an animal to breath while holding prey in its mouth or chewing food (secondary palate would be important to subsequent mammalian evolution by permitting the young to breathe while sucking
Loss of lumbar ribs in cynodonts is correlated with the evolution of a diaphragm and also may have provided greater dorsovental flexibility of the spinal column
Within the diverse cynodont clade (Fig. 28.3), a small carnivorous group called trithelodontids most closely resembles the mammals, sharing with them several derived features of the skull and teeth.
The earliest mammals of the late Triassic Period were small mouse- or shrew-sized animals with enlarged crania, redesigned jaws, and a new type of dentition called diphyodont (where deciduous teeth are replaced only once by permanent teeth).
A very important transformation involved the 3 middle ear bones, the malleus, incus, and stapes, which function to transmit sound vibrations in mammals. The malleus and incus originated from the articular and the quadrate, respectively, 2 bones that previously served as the jaw joint but became reduced in size (better to transmit sound vibrations) and relocated to the middle ear (Fig. 28.4).
A new jaw joint formed between the dentary and squamosal (temporal) bones. This dentary-squamosal joint is the defining characteristic for fossil mammals (Fig. 28.4).
Early mammals were endothermic and somewhere acquired hair, mammary glands.
Early mammals of the mid-Triassic, having developed nearly all novel attributes of modern mammals, had to wait another 150 million years before they could achieve their great diversity.
When dinosaurs became diverse and abundant, all nonmammalian synapsid groups became extinct, but mammals survived (first as shrew-like, probably nocturnal, creatures).
In the Cretaceous Period, but especially during the Eocene Epoch that began about 58 million years ago, modern mammals began to diversify rapidly – probably in part due to numerous habitats being vacated by the extinction of many amniote groups (including dinosaurs) at the end of the Cretaceous.
- "Age of Mammals" - Eocene and Oligocene Epochs of Tertiary Period (55-30 million years ago)
Classification: 2 main subclasses; see book pp. 221-224 (637-640) (know below groups)
Infraclass Ornithodelphia – monotreme mammals
Order Monotremata - monotremes; egg-laying (oviparous); "one hole" - spiny anteater (=echidna), duck-billed platypus
Infraclass Metatheria – (Marsupialia) – marsupial mammals (=pouched mammals) (many species found in Australia, also quite a few in South and Central America)
Order Didelphimorphia – American opossums; many species found in South and Central America; one species, the Virginia opossum, is widespread in North America(66 species)
Note: there are 6 more orders of marsupials, but you do not have to know those names; none of them occur in North America
Infraclass Eutheria - viviparous, placental mammals
Note: there are MANY more orders of mammals, but you only have to know the ones listed here. See you book for a more detailed classification.
Order Insectivora – insect-eating mammals: shrews, hedgehogs, tenrecs, moles (440 species)
Order Lagomorpha - rabbits, hares, pikas (81 species)
Order Rodentia – gnawing mammals – rodents squirrels, ratrs, mice woodchucks (2,052 species)
Order Chiroptera – bats (977 species)
Order Primata - primates (including humans); prosimians, monkeys, apes (279 species)
Order Carnivora – carnivores: dogs, cats, bears, weasels, seals, sea lions, walruses (280 species)
Family Canidae - dogs, wolves, etc.
Family Felidae - cats
Order Perissodactyla - "odd-toed" hoofed mammals (have old number of toes, i.e., 1 or 3; horses, asses, tapirs, zebras, tapirs, rhinoceroses (ungulate - is another term for hoofed mammal) (17 species)
Order Artiodactyla - "even-toed" hoofed mammals; cattle, sheep, goats, bison, camels, pigs, deer and their allies, giraffes, hippopotamuses, antelopes (221 species)
Order Cetacea – whales, dolphins, porpoises (78 species)
Structural and Functional Adaptations of Mammals
A. Integument and Derivatives:
- skin is thicker than in other vertebrates; have epidermis and dermis
1. Hair - see Fig. 28.6 on p. 202 (618)
- follicle in dermis
- fur = pelage; composed of:
a. underhair (underfur) - often soft, fuzzy; insulation
b. guard hair - outer, coarser; protection against wear and to provide coloration
- "flag" of white-tailed deer - warning, other meanings
- pronghorn antelope - rump patch with long, white hairs erected by special muscles; hairs can catch sun like a mirror and signal danger to others
- spines, bristles, vibrissae (= whiskers) - special hairs for sense of touch
2. Horns and Antlers - Fig. 28.10 on p. 203 (619)
a. true horns - in ruminants (4-chambered stomachs; in sheep, cattle, etc.)
- hollow sheaths of keratinized epidermis with bony core arising from skull
- not (usually) shed; not branched; on both sexes
- pronghorns - are a special case; new sheath grows and old sheds (retains bony core)
b. antlers - in deer family - entirely bone; velvet on outside while antler is growing; shed yearly; usually only on males
c. rhinoceros "horn" - not a true horn; made of horny fibers arising from dermal papillae and are cemented together
3. glands -
a. sweat glands
1) eccrine glands - watery secretions; temperature regulation
2) apocrine glands - open into hair follicles (develop at puberty; in humans, in axillae, mons pubis, breast, external auditory canals, prepuce, scrotum, etc.
- secretions are milky, white or yellowish; correlated with sex cycle
b. scent glands - orbital, metatarsal, interdigital regions
(region depends on species); pheromones??
c. sebaceous glands - associated with follicle, but some open to surface; produce sebum (oily substance)
d. mammary glands - produce milk; probably modified apocrine glands (some think sebaceous glands)
B. Foods and Feeding
- teeth reveal lifestyle
- diphyodont - means they have deciduous & permanent teeth
- heterodont - teeth differentiated for tearing, grinding, etc.; (versus homodont - all teeth the same)
- types of teeth in mammals:
- incisors - biting, nipping
- canines - piercing
- premolars - shearing, slicing
- molars - crushing, mastication;only permanent molars
- feeding specialists- herbivores (canines suppressed; molars high) different kinds of herbivores:
- browsers (wood)
- grazers (grass, forbs)
- gnawers, nibblers - rodents, rabbits (rodents have chisel-like, ever-growing incisors)
- cellulose -structural carbohydrate of plants
- herbivores have microflora (of anaerobic bacteria) that can digest cellulose
- horse, rabbit, elephant, many rodents - have sidepocket or diverticulum called a cecum that is a fermentation chamber and absorptive area that allows cellulose digestion
- coprophagy - the practice of eating feces; hares, rabbits, and some rodents eat feces; they think maybe they get vitamins produced in large intestine (which are too far along the tract to absorb); from other sources I have also heard that sometimes the babies eat the feces so as to pick up the correct microflora for their gut so they can digest cellulose; maybe they also get a few more nutrients by running it through again
-ruminants - have 4-chambered stomach (e.g., cattle, bison, buffalo, goats, antelope, sheep, deer, giraffes, okapis)
- pathway of food--goes into esophagus--to rumen--back to mouth as cud and rechewed--to rumen (partly digested)--to reticulum (fermentation continues)--to omasum (water, soluable food, and microbial products are absorbed)--to abomasum (true stomach, proteolytic enzymes are secreted and normal digestion takes place in acid environment)
- carnivores - digestive tract is shorter than in herbivores; carnivores often eat herbivores; cecum is small or absent; they lead more active lives that herbivores; natural selection favors traits for intelligence, hunting skill (herbivores- selection favors keen senses and agility)
- omnivores - eat meat and vegetation; pigs, raccoons, rats, bears, most primates (including humans)
- insectivorous - moles, shrews, anteater, most bats
- many rodents make caches of food for later
The smaller the mammal, the greater is its metabolic rate and more it must consume relative to its body size (Fig. 28.15)
Migration - more rare than in birds
- e.g., - barren-ground caribou, plains bison, seals, whales
- most bats hibernate; some migrate
Flight and Echolocation-
- bats fly; "flying" squirrels and lemurs - really glide
- echolocation - bats and some marine mammals use it; dolphin “melon” (picture on p. 210 (626); bats (Fig. 28.20)
- bats are noctural and usually insectivorous; they emit ultrasonic sound waves and then they pick up (with ears??) the echo to detect food or avoid obstacles
- estrus = heat; female is sexually receptive to male
- Monestrous – only 1 estrus during their breeding season (dogs, foxes, bats)
- Polyestrous – having a recurrence of estrus during their breeding season (field mice, squirrels, many tropical species)
- Menstruation – experienced by Old World monkeys and humans
3 patterns of reproduction:
a. egg-laying monotremes - e.g., duck-billed platypus;
1 breeding season/year; usually 2 ovulated eggs; fertilization in the oviduct; get albumin and shell; egg laid (it is about the size of robin egg); young are fed milk after hatching (they lick milk, they don't suckle)
b. marsupials - (pouched mammals) - born in undeveloped state; then they go to pouch and attach to teat (red kangaroo has only a 33 day gestation); mother red kangaroo then gets pregnant again immediately; but suckling arrests development of embryo; embryo goes into diapause (sort of a state of suspended animation) which lasts about 235 days during which the first joey grows in pouch; she canhave 3 generations going at once (an embryo, joey in pouch, joey on foot which returns to pouch occasionally to suckle)
c. placental mammals = eutherians; young are attached to inside of mother and they develop there; gestation varies and state of maturity at birth varis; birth rate varies
Territory and Home Range -
- territory - is a defended area of the larger home range
- home range - area that is used on a regular basis (too big to defend it all)
- territory is often scent-marked (urine, feces, scent glands)
Mammal Populations -
Density-independent factors – those factors that affect a population whether it is crowded or not (e.g., natural disasters like hurricane, flood, fire)
Density-dependent factors – these factors are correlated with population crowding
- population - members of a species that interbreed and share a particular space
- Hudson Bay Company - p. 215 (631) - read;
- talks about lynx and snowshoe hare cycles and what causes them; some of it is controversial now (they now think it is a combination of predator/prey relations PLUS habitat changes/crashes all interact to influence the cycles)
Humans and Mammals – read pp. 215-216 (631-632)
About 10,000 years ago, humans developed agricultural methods and also began domesticating animals (dogs were probably some of the first – came from wolves)
Cats were also probably fairly early, but they were much less social than dogs
Domestication of cattle, buffalo, sheep, and pigs probably came much later
Some domestic species no longer exist in the wild (e.g., one-humped dromedary camels of North African and llamas and alpacas of South America)
All truly domestic animals breed in captivity; many selectively bred by humans for particular traits
Human Evolution – read pp. 216-224 (632-640)
The earliest primate was probably a small, nocturnal animal similar in appearance to tree shrews; this ancestral primate lineage split into 2 lineages:
a. one of which gave rise to lemurs and lorises, with wet noses (Strepsirhini)
b. the other to tarsiers (Fig. 28.31), monkeys, and apes
Traditionally, the lemurs, lorises, and tarsiers have been called the prosimians, and the apes and monkeys have been called the anthropoids
We now recognize 3 simian clades:
a. New World monkeys of Central and South America (ceboids, Fig. 28.32A), including howler monkeys, spiders monkeys, and tamarins
b. Old World mondeys (cercopithecoids), including baboons (Fig 28.32B), mandrills, and colobus monkeys
Humans, orangutans, gorillas, and chimpanzees are now recognized to belong to a single family, Hominidae, and are refereed to here as hominids.
All fossil hominid species that are phylogenetically placed closer to living humans than to chimpanzees are referred to in your book as humans.
Early Humans – read this section thoroughly, page special attention to Fig. 28.34
Know about “Lucy”, Australopithecus, Homo habilis, Neandertals, Homo erectus, and Homo sapiens