Class Mammalia

1. Class Mammalia • more than 5,000 species today • presence of the mammary gland – feeding of young • production of milk - balanced liquid of fats, proteins, sugars, minerals and vitamins • development of hair and a fat layer under the skin for the retention of heat • efficient respiratory and circulatory systems • larger brain • ability to learn complex tasks • differentiation of teeth • variety of shapes and sizes • adapted for chewing, ripping and grinding

2. Mammals • Concepts • 1. characteristics evolved gradually over 200 million years from the synapsid lineage • 2. two subclasses initially evolved – Prototherians and the Therians • 3. skin is thick and protective – covered with hair • 4. able to exploit a wide variety of feeding habitats • 5. efficient circulatory and respiratory systems – support a high metabolic rate • 6. greatly expanded brain with cerebral cortex • 7. metanephridic kidneys to minimize water loss • 8. complex behavioral patterns for survival • 9. most are viviparous

3. Mammals • belong to a group of amniotes known as synapsids • non-mammalian synapsids – lacked hair, had a sprawling gait and laid eggs • distinctive characteristic of synapsids – temporal fenestra • hole behind the eye socket • in humans – for the passage of jaw muscles and anchorage onto the temporal bone • synapsids evolved into large herbivores and carnivores during the Permian period • mammalian synapsids emerged by the end of the Triassic period – 200 MYA • not true mammals • development into the Prototherians and Therians • Subclass Prototheria • now only contains extinct species • Subclass Theria • made up of three infraclasses: • Ornithodelphia (monotremes) • Metatheria (marsupials) • Eutheria (placentals)

4. first true mammals arose during the Jurassic period • small – about the size of a shrew • emergence of three lineages: monotremes (egg-laying), marsupials (mammals with a pouch) and eutherians (placental mammals) • monotremes: found only in Australia and New Guinea • one species of platypus and two species of echidnas (spiny anteaters) • egg laying retained as a primitive egg-laying characteristic – unlike other mammals • have hair • produce milk – lack nipples • milk is secreted by glands on the belly of the mother • hatchling sucks the milk from the fur • possess a cloaca • marsupials: opposums, kangaroos and koalas • marsupials and eutherians share many characteristics • higher metabolic rate than monotremes • nipples provide milk • embryo develops inside the uterus • development in utero is short – female cannot produce sufficient hormones to maintain the uterine lining • born early in its development – completes development while nursing in the marsupium • eutherians: • more complex placenta than marsupials • well developed extra-embryonic membranes • longer period of pregnancy • completion of embryonic/fetal development within the uterus • made up of 12 orders including • Order Primata - primates • Order Lagomorpha – rabbits • Order Rodentia – squirrels, chipmunks, rats, mice, beavers, porcupines, woodchucks and lemmings • Order Cetecea – whales, dolphins • Order Carnivora – dogs, cats, raccoons, minks, sea lions, seals, walruses, otters • Order Proboscidea – elephants • Order Perissodactyla – ungulates (horses, rhinos, zebras) • Order Artiodactyla – hoofed (pigs, hippos, camels, antelope, deer, sheep, giraffes, cattle

5. external structure and locomotion: • like other vertebrates – skin is made up of epidermal and dermal layers • epidermis – containing glands • sebaceous, sudoriferous, scent • development of hair – specialized keratinized structure • forms in the dermis – hair follicles • many animals have different types of hair forming a pelage or coat • e.g. longer hairs protecting a coat of shorter hairs • periodically molted in many mammals • gradual in the primates (including humans!) • many mammals acquire a thicker coat in the fall and shed much in the spring • molting is frequently associated with changes in coat color • e.g. loss of white hair with snow melt • sense of touch – base of the hair is associated with sensory neurons • also can be pulled erect or flattened • controlled by the autonomic nervous system – in response to danger • insulation – air spaces between hairs warms the mammals • hair shaft also has air spaces in many mammals • pulling the hair upright can also traps more air • pigmentation – melanin content • wide variety seen in many mammals • some colorations are warnings – e.g. skunk • reduced in hotter climates – e.g. hippos, elephants, humans • e.g. naked mole rat has no pelage • claws, hooves or nails – locomotion, manipulation, defense • claw = accumulation of keratin over the distal portion of the digit – not a nail!!! • mammary glands – modified sudoriferous gland • modification in structure results in a teat (not a nipple!)

6. skull and teeth: • used to classify the mammalian class • method of jaw articulation distinguishes the mammal from the reptile • reptiles – jaw articulates at two small bones (quadrate bones) at the rear of the skull • mammals - these bones move into the middle ear along with the stapes = ossicles • have a single articulation between the mandible and the temporal bone • secondary palate seen in the reptiles extends back further into the oral cavity and forms the soft palate • almost completely separates the oral from the nasal cavity • allows mammals to breath while they eat – therefore they can chew and don’t have to swallow whole • arrangement of teeth • reptiles are homodonts • mammals are heterodonts • set into sockets in the jaw • most have two sets of teeth – deciduous and permanent dentition • adults have up to four types of teeth • shape and size of the teeth vary with diet • zoologists use a dental formula to divide mammals into their groups • # of teeth in one half of the upper and lower jaws in the order of incisor, canine, premolar and molar • e.g. human 2>1>2>3 • 2>1>2>3

7. axial skeleton • vertebral column divided into cervical, thoracic and lumbar – like birds • only the thoracic regions articulates with ribs • ribs cage is a protective structure • rib articulation with the vertebrae is very flexible – allows for twisting of the trunk • appendicular skeleton • rotation of the limbs under the body • reorientation of the pelvic and pectoral girdles • therefore the bulk of the body’s weight is borne easily by the limbs • joints usually limit the range of motion of these limbs • bones of the pelvic girdle are fused in mammals • advantageous for locomotion • problem for birth

8. digestion and nutrition: • similar to other vertebrates • specializations seen in the teeth • other specializations reflect feeding habitats • e.g. size of the intestinal tract correlates with diet

9. circulation and gas exchange: • development of a four chambered heart • similar to birds – pulmonary and systemic circuits • birth to live young required adaptations in the circulatory system of the pregnant female • fetal circulation – blood entering the right atrium can bypass the pulmonary circuit • gas exchange occurs across the placental surface • gas exchange: • high metabolic rates of mammals require increased O2 • larger snouts • separate nasal and oral cavities • highly branched respiratory passages • well-developed lungs with alveoli for gas exchange • development of a diaphragm for negative-pressure ventilation

10. temperature regulation: • ectothermy vs. endothermy • thermoregulation: maintenance of an internal temperature within a tolerable range • critical to survival • determines rates of enzymatic reactions, structures of proteins and nucleic acids etc…. • thermoregulation is an integral part of homeostasis • each species has adapted an optimal temperature range • metabolic heat is used extensively by many animals to determine internal temperature • metabolic processes generate heat as a byproduct • ectotherms: low internal body temperature • low metabolic rate – heat does not affect body temperature • rely on the environment for gains in body temperature • e.g. basking in the sun • more variable internal temp = poikilothermic • amphibians and reptiles • endotherms: high internal body temperature • higher metabolic rate • can be used to directly regulate internal body temperature • internal temp is more stable = homeothermic • leads to the development of more complicated respiratory and circulatory systems • birds and mammals • common misconception is the classification of ectotherms and endotherms • NOT based on whether they have constant or variable body temps • it is the source of heat used to maintain body temp • “cold-blooded” vs. “hot-blooded” is also correct • many ectotherms can have quite high body temps – e.g. lizards basking in the sun

11. Endothermy • many advantages • higher body temp and better CV and respiratory systems allows for vigorous activities over a sustained time point • e.g. flying, long-distance running • also solves certain thermal problems living on land enabling terrestrial animals to maintain a stable body temp during environmental fluctuations • but tolerance of large swings in external temp are better tolerated by ectotherms • but it is energetically expensive • metabolic rate = sum of all the energy-requiring biochemical reactions occurring over a given time interval • measured in calories • calorie = • the MR of a human at rest is between 1,300 to 1,800 kcal per day • but a resting ectotherm might have a MR of only 60 kcal per day • so endotherms need to consume larger amounts of food more often to keep up with their metabolic “demands”

12. Heat exchange • whether an ectotherm or an endotherm – heat is exchanged through 4 processes • 1. conduction – direct transfer of heat between objects directly in contact with each other • 2. convection – transfer of heat to the air • 3. radiation – transfer of heat from the sun • 4. evaporation – loss of heat through evaporation of liquids off the surface of the animal • in endotherms – heat is retained by the pelage • without a pelage – mammals can conserve heat by allowing the temp of surfaces to drop • well-vascularized appendages allow for the evaporation of excess heat during hotter climate periods • these are a problem is heat is to be conserved • counter-current heat exchange systems – regulates heat loss from exposed areas • arteries passing peripherally through the core of an appendage are surrounded by veins that carry blood back to the heart • heat transfers from arterial blood to the vein and vice versa • heat evaporation is not a problem in moist environments – evaporative cooling requires water loss • e.g. hot climates – jack rabbits – have large vascularized ears – heat loss through convection – no water loss

13. nervous and sensory: • active lifestyle accompanies by development of more complex nervous system and sensory structures • sense of touch becomes well developed • sensory neurons located in epidermis and dermis • proprioception becomes well developed • olfaction well developed in many mammals • larger snouts allow for more olfactory epithelium • larger areas of the brain for identifying smells • auditory senses well developed • development of a pinna and external ear canal • middle ear with three ossicles • development of a coiled cochlea for hearing • sense of balance/equilibrium becomes better developed – inner ear • vision • similar eye structure to birds • focusing through changing the shape of the lens • color vision less developed in many mammals • rods more numerous

14. excretion and osmoregulation: • like all amniotes – metanephric kidney • excretion of urea as the main product • reptile and birds – excrete uric acid which requires less water to form • urea requires conversion of ammonia through addition of CO2 • urea is highly soluble and requires relatively large amounts of water to excrete • nephron – filtration through the glomerulus • highly efficient tubular network for reabsorption of water and salts • primary adaptation to the nephron is the loop of Henle • allows for the production of concentrated urine • e.g. beaver produces urine 2X more concentrated in salts than blood • water loss can vary greatly in mammals • development of behaviors that minimize water loss

15. behavior: • to enhance survival in the terrestrial environment • visual cues – communication • habitat behaviors – minimize water loss, maximize food, thermoregulation • pheromones – species and sex recognition • attraction of opposite sex • signals reproductive state • may also induce sexual behaviors • e.g. male bull elk – urinates on his underbelly to advertise their reproductive status to the females during mating season • auditory communication and vocalization • herds stay together and remain calm through familiar sounds • also capable of warning others and scaring predators off • vocalizations in primates • variety of uses • social interactions and bonding • territoriality • marked and defended • e.g. cats rub their face – not affection, spreading of scent from facial scent glands • e.g. sea lions – 2 weeks the males engage in vocalizations, displays and fights to stake claim to favorable areas along the beach • females select the site that appeals to them for giving birth • this ensures that the male will get to mate with the female and will father next year’s offspring • development is arrested for three months after initial fertilization

16. reproduction: • development of very rigid reproductive cycles • many eggs are incapable of being fertilized or are not produced out of cycle • usually coincides with climate and resource improvements • in climates with few seasonal changes or in an environment that can be controlled – reproduce at any time of the year • most females undergo estrus = time during which the female is behaviorally and physiologically receptive to the male (fertile egg is produced) • complex series of hormonal changes that affect egg production, uterine and vaginal environments and sperm production • many mammals are monoestrus • only once a year • strictly regulated by the environment • e.g. wild dogs, bears and sea lions • e.g. domestic dogs – biestrus • other animals have cycles that can repeat on a cyclical basis • e.g. rats – every 4 to 6 days • fertilization is internal – upper 1/3 of the oviduct • in a few mammals – fertilization is delayed after coitus (delayed fertilization) • e.g. bats – coitus in fall, fertilization in spring • can store sperm in utero for up to two months!! • adaptation to winter dormancy • most mammals have direct fertilization after coitus • but development of the embryo can be arrested for one to two weeks = embryonic diapause • e.g. sea lions, some bats, bears and marsupials • may be due to presence of resources – allows mother time to feed before having to nurse • also allows young to be born during a time of good resources