Mammalian Characteristics

Chapter 20

Lactation Presence of Mammary Glands for which the class is

named. This is a unique feature of the class, Function is lactation – provide milk to the developing

young, All mammalian females lactate Mammary glands absent from males of marsupials but

present in male monotremes and eutherians Only Eutherians have nipples, others have mammary hairs,

so young ones suckle from the mother’s fur

Determinate growth Epiphyses feature of determinate

growth in mammals Growth zones of cartilage between

diaphysis and epiphysis until cartilage is used up (limited growth span)

At maturity ossify with diaphysis (shaft)

Skeletal System: Cranial Features Heterodentition

Incisors, canines, premolars & molars 2 sets of dentition 1st set = milk teeth: incisors, canines and

premolars 2nd set = second set of original teeth + later-

erupting teeth the molars Mammals are the only animals that masticate

food and swallow it as a bolus of food Therians have special molars called

tribosphenic molars

Cranial & Postcranial features Dermal bones completely enclose the brain Zygomatic arch : bowed out bones of that

formed the lower boarder of temporal opening

3-boned middle ear Dentary; single bone that forms the

mammary lower jaw Upright posture with limbs underneath the

body –now : dogs, cats, horses In opossums, semi-sprawling stance –

primitive condition

Postcranial features (fig 20.4) Unique ankle joint called

crurotarsal joint Between tibia and ankle bone called

astragalus Proximal ankle bone the calceneum

projects to form the calcaneal heel The heel is the point of insertion of

calf muscles : Gastrocnemius

Postcranial features Ilium is rod shaped, & directed

forward Pubis and ischium are short

Contrast with plate-like pubis of & ischium of reptiles

Femur has a trocanter for the attachment of gluteal muscles which are the major retractors of the hind limbs

Postcranial features All have 7 cervical vertebrae

exceptions giraffe (8); matees (7) Unique atlas-axis complex

Formed by the 1st two cervical vertebrae

Joint between head & atlas allows up & down head movement

Joint between atlas & axis allow the side to side movement

Postcranial features Ribs restricted to the thoracic

vertebrae Jaw adductor muscles

Temporalis: from head to dentary’s coronoid process

Masseter Extend from the zygomatic arch to outside

of the lower jaw Unique in mammals Enables sideways movement of jaws

Postcranial features Diagastric muscles

Jaw opening muscles in Therian mammals

Innervated by Cranial Nerves 7 (back ½) and 5 (front ½)

Facial expression muscles-unique Innervated by cranial nerve 7.

Integument Skin has epidermal, dermal and

hypodermal (with fat cells) layers Epidermis varies in thickness

Humans thickest in palms and soles Rodents – very delicate epidermis Elephants, rhinoceroses,

hippopotamuses once classified as pachyderms because, epidermis is several hundred cells thick

Integument: Hair Camouflage Communication Sensation via (vibrissae or whiskers)

Grow on muzzle, eyes, lower legs, & have touch receptors

Insulation Composed of keratin Grows from hair follicles

Integument: Glands Secretory glands that develop from the

epidermis 3 major types of skin glands Eccrine gland:

produce a watery secretion with little organic content

Function as sweat glands in humans In most animals, restricted to soles of the

feet, tail and areas that contact surfaces to improve adhesion

Integument: Glands Sebaceous glands

Over entire body Produce oily lubricant sebum that lubricates

and waterproofs the hair Apocrine

Type of gland in which the apical part of the secretory cell breaks down in the process

Secretions used in chemical communication In humans found in armpits and pubic region

Integument: Claws, Nails, Hooves, Horns

Part of the integument Claws, nails and hooves are

accumulation of keratin that protect the terminal phalanx of the digits

Horns can be keratin (e.g. rhino) or bone covered by keratin as in cows

Cardiovascular system Heart is four chambered Complete ventricular septum Single systemic arch (aorta)

Birds retained right systemic arch Mammals retained the left systemic arch as

aorta Do not have the double systemic arch seen

in reptiles Mature Red blood cells lack a nucleus

Respiratory system Large lobed lungs Finely branching bronchioles that

terminate as alveoli Diaphragm that aids in breathing

Divides the pleurocavity into the Peritoneal cavity: surround viscera Pleural cavity surround the lungs

Urogenital system Retain bladder, secretes dilute urine Cloaca replaced by separate openings for

urogenital and alimentary canal systems Testes placed in a scrotum outside of the

body Urethra common passage for urine and

sperms, hence is the penis Glans= tip of penis; single in eutherians

and forked in monotremes and marsupials os penis (baccalum) contain bone

Urogenital system Renal portal system does not

appear in mammals Have the loop of Henle which is

ideal for salt secretion Marine reptiles and birds have salt

glands in their heads that secrete excess salt

Sex determination Genetic X & Y chromosomes Gene on Y chromosome initiates

male gonadal development and female gonadal development occurs in its absence

The brain Highly developed neopallium (roof of the

forebrain): extended portion of the forebrain

tectum (visual center in lower vertebrates) reduced to corpora quadrigemina: functions mainly as a relay center for auditory information and to control visual reflexes

corpus callosum in eutherians provides additional communication

Major Lineages of Mammals Allotheria (Multituberculates)

Extinct Prototheria

Monotremes Theria

Metatheria or Marsupials or Eutheria: Placentals

Multituberculates Rodent or squirrel- like semi-arboreal

omnivores Could rotate foot backward to descend trees

headfirst. Prehensile tail (figure 20.20

Narrow pelvis: maybe gave birth to poorly developed young

Broad molars that are multicusped and specialized for grinding rather than shearing

Monotremes vs Therians Toothless bird-like beak which

contains receptors for sensing prey Reptile-like cervical ribs Reptile like pectoral girdle with

clavicle, interclavicle, precoracoid, coracoid, scapula

Large epipubic bone on pelvic girdle Lay 1 or 2 eggs, incubate 7-10 days No nipples

Monotremes vs TheriansMonotremes Lay eggs No teeth in adults No nipples No cochlea No external ear( pina) Retain sclerotic ring

around eyes Septomaxilla bone Retain ribs on cervical

vertebrae

Therians Give birth to young Tribosphenic molars Nipples Cochlea in inner ear External ear (pina) No sclerotic rings

around the eyes No septomaxilla bone No cervical ribs

Monotremes vs Therians No crurotarsal joint Reptile like pectoral

girdle with clavicle, interclavicle, coracoid, scapula (not as big)

Large epipubic bone on pelvic girdle

Crurotarsal ankle joint with complete superposition of astragalus on the calceneum (fig 20.12)

Lost interclavicle & coracoid, expanded the scapula. Clavicle lost in dogs and horses

Small epipubic bones

Eutherian vs MarsupialsEutherian Non inflected angle to

the dentary bone Rectangular shape of

nasal bones Auditory bulla (bone

elaboration around the ear to increase hearing

Post-orbital bar bone (bar of bone behind orbit in herbivores

Marsupials Inflected angle

(distinct) Flared or diamond

shaped nasal bones Lack an auditory

bulla or a very small one from a different bone

No postorbital bar

Eutherians vs Marsupials Replaces all teeth

except molars Dental formula I3/3; C1/1; P4/4; M3/3

( 44 teeth, e.g whales)

Many have fewer teeth; primitive

Humans have I2/2; C1/1; P2/2; M3/3

Lack epipubic bone

Replace only last molars

Dental formula I3/4 C1/1 P3/3 M4/4

55 teeth More incisors and

molars Epipubic bone that

project forward from the pubis

Monotremes Toothless as adults, but teeth in juveniles Egg laying mammals Have primitive anatomical features Reptile-like pectoral girdle with clavicle,

coracoid and scapula Large epipubic bone on pelvic girdle Lay 1 or 2 eggs, incubate 7-10 days

Platypus : Australia Echidna: Australia ( short nosed) Guinea (long

nosed)

Therians: Marsupials Diverse (275 spp): See Table 20.1 See also figure 20.16 Smaller brain than eutherians Short gestation length New borne crawl into pouch & attach to

the nipple Separated from Eutherian since late

mesozoic Central and South America, Australia and

New Guinea

Therians: Eutherians Placental mammals 93% of all mammals Approximately 4100 spp Chorio-allantoic placenta Longer gestation period Young are more developed Relatively short lactation length Rapid growth & development- facilitated

by high energy milk

Major orders of placental mammals

Insectivora Shrews, moles, hedgehogs, Madagascan tenrecs Chiroptera (bats) Carnivora (bears, canids, felids, cats, seals, sea

lions, walruses Rodentia: mice & squirrels Lagomorpha: pikas, rabbits, hares Perissodactyla:odd toed ungulates) Artiodactyla:even toed ungulates) Cetacea:whales, dolphins Primates: monkeys, apes, gorillas, humans

Anatomical differences between Eutherians and Marsupials Eutherian Non-inflected angle to

the dentary bone Rectangular shape of

the nasal bones Auditory bulla- bone

elaboration around the ear region

Post-orbital bar- bar of bone behind the orbit

Replace all teeth except molars

Marsupial Inflected angle to the

dentary bone Flare or diamond

shaped nasal bones Lack a bulla or it is a

very small formed from a different bone

Lacks post-orbital bar Replaces only the

last premolar

Anatomical differences between Eutherians and Marsupials

Eutherian Dental formula

I3/3 C1/1 P4/4 M 3/3

Many have fewer teeth than this e,g humans have 32, dolphins ~ 44

Lack epipubic bones

Marsupial Dental formula

I3/4 C1/1 P3/3 M4/4

Have epipubic bones that project forward from the pubis

Anatomical differences between Therians and Monotremes Therians Give birth to

young Have nipples No sclerotic rings

around the eyes Lack septomaxilla

bone in skull No ribs on cervical

vertebrae

Lay eggs No nipples Sclerotic cartilage

rings. Do not ossify to form bone

Have septomaxilla bone in skull

Ribs retained in monotremes

Anatomical differences between Therians and Monotremes

Therians More derived hinge joint

between tibia and the astralagus in the ankle

There is complete superposition of astragalus on the calceneum

Shoulder girdle more derived Coracoid & interclavicle

bones do not appear in the shoulder girdle

Clavicle lost in running eutherians (dogs & horses)

Scapula expanded by appearance of scapula spine

Monotremes Not seen in

monotremes Reptile-like

shoulder girdle Coracoid &

interclavicle present

Clavicle present