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

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  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)
  

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  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

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  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.
  

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  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:

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   Increased and specialized jaw musculature, permitting a stronger bite
   
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   Several skeletal changes, supporting greater agility
   
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   Heterodont teeth, permitting better food processing and use of more diverse foods (Fig. 28.4)
   
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   Turbinate bones in the nasal cavity, adding retention of body heat (Fig. 28.5)
   
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   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
   
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   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.

- 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

- 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)

c. rhinoceros "horn" - not a true horn; made of horny fibers arising from dermal papillae and are cemented together

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:

- cellulose -structural carbohydrate of plants

- 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)

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  The smaller the mammal, the greater is its metabolic rate and more it must consume relative to its body size (Fig. 28.15)
  

- bats fly; "flying" squirrels and lemurs - really glide

- 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)

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  Density-independent factors – those factors that affect a population whether it is crowded or not (e.g., natural disasters like hurricane, flood, fire)
  
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  Density-dependent factors – these factors are correlated with population crowding
  

- population - members of a species that interbreed and share a particular space

- 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)

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  About 10,000 years ago, humans developed agricultural methods and also began domesticating animals (dogs were probably some of the first – came from wolves)
  

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  Cats were also probably fairly early, but they were much less social than dogs
  

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  Domestication of cattle, buffalo, sheep, and pigs probably came much later
  

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  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)
  

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  All truly domestic animals breed in captivity; many selectively bred by humans for particular traits
  

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  Rodents and other species are often considered pest species; many wild species of mammal carry diseases that can be transmitted to humans
  
  
  

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  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

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  Traditionally, the lemurs, lorises, and tarsiers have been called the prosimians, and the apes and monkeys have been called the anthropoids
  

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  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