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 Apocrine 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 Therians 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 Marsupials 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 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 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 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 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