Mammals

Mammals are endothermic vertebrates with hair, strong maternal care, and teeth

Evolution of Mammals

• Evolution of vertebrate skull

  • anapsid - primitive amniotes and turtles

  • Synapsid

  • Diapsid

• Arose from ancient synapsids

• Ancient reptilian-like ancestor was probably partially endothermic with rudimentary hair and adaptations that allowed for the development of mammary glands, facilitating the nurturing of young.

• Therapsids ad modifications suggesting increased metabolic rate and perhaps hair- both of which may have allowed them to outcompete pelycosaurs

• most therapids went extinct, but one derived group, the cynodonts, let to modern mammals

• The evolution of cynodonts including a reduction in size from large dog down to shrew-size, and lots of other modification toward a more mammal-like morphology

  • Cynodont (photo)

The First Mammals

• Morganucodon - one of the oldest mammals

• tiny - shrew sized

• mammal like jaw and skull modifications which allowed for more efficient chewing and a better diet adaptation, setting the stage for the evolution of more advanced mammalian features.

• Teeth - heterodont (different types) - like us have different teeth throughout the mouth

• Ability to maciste (chew)

• Hairy coat - endothermy selected due to size

  • body size and surface area/volume ratio (SA/V)

Evolution of Lactation

• Appears with origin of mammals

• Primitive state: secretion of pheromones to maintain bond with offspring?

• Original “milk” mainly to protect eggs in a nest against microorganisms (immune function)

• Once such a secretion evolved, any evolutionary change to a more copious, nutritive secretion accidentally consumed by the young would provide advantages

• Lactation supplements egg yolk - then replaces it - allowing multiple advantages

• This transition marks a significant evolutionary step, as it enables the young to receive essential nutrients and antibodies, enhancing their survival chances during the early stages of life.

Advantages of Lactation

• production of offspring separated from seasonal food supply (unlike birds)

  • mammals can store food as fat and convert to milk later

• females not dependent on paternal care of offspring

• Viviparity made less strenuous as young can be born at a relatively underdeveloped stage and cared for outside of the uterus

Diversification of mammals

• Mammals did not diversify into larger bodied forms with more varied diets until extinction of non-avian dinosaurs

• Monotremes: One hole for reproductive and excretory functions, they lay eggs rather than giving birth to live young, which distinguishes them from most other mammals.

  • duck-billed platypus and echidna

  • most primitive extinct mammals

  • reptilian characteristics:

    • lay eggs

    • cloaca

  • No nipples; young lap milk from teatless mammary gland onto tufts of hair

  • all monotremes are in Australia and New Guinea

• Marsupials: Viviparous mammals characterized by giving birth to relatively undeveloped young that complete their development inside a pouch.

  • give birth to “premature“ young

  • well developed legs used to move to nipple, where they complete their development

  • Approx. 50% of marsupials have a marsupium (pouch) to protect the young

  • Most development outside of mother’s body

  • once distributed worldwide, now restricted because of competition of Eutherians

  • dominant mammal in Australia

  • only one species in North America, which is the virginia opossum

• Pacentals:

  • most diverse and successful group of mammals

  • Placenta: structure in which maternal and fetal blood streams in close contact

    • acts as lung, kidney, and intestine

    • Aids development of young

    • much less developed in marsupials

    • give birth to “mini adults”

    • many functional groups

• Insectivores: most primitive eutherians, small, primarily insect easters

• Flying Mammals: (bats) pectoral appendages modified into wings via a skin membrane. closely related to insectivores

• Toothless Mammals: specialized for eating ants and termites. Anteaters.

• Carnivores: flesh-eating. large changes in tooth and jaw morphology. weasels, dogs, raccoons, bears, cats, seals, sealine, walrus

• Ungulates: specialized for a plant diet. jaws and teeth modified for nipping and grinding plants. horses, deer, antelope

• Rodents and Lagomorphs: Small herbivores that gnaw food. Enlarged pair of chisel-like teeth

  • Rodents: mice, squirrels, gophers, beaver

  • Lagomorphs: rabbits and hares with an additional set of upper incisors, allowing them to effectively feed on grasses and other vegetation.

• Whales: highly specialized marine mammals. Lots of tooth modification; conical in fish eaters, comb-like in plankton eaters. Highly intelligent.

• Primates: Adapted to arboreal life

  • Flexible limbs and grasping hands

  • Feeth modified for eating fruits, seeds, and insects

  • Binocular vision, but poor sense of smell

  • Good coordination led to enlarged brains

• Order Primates:

• General Characteristics of Primates:

  • related to arboreal (tree dwelling) life

  • Rolling shoulder joint

  • five digits on fore- and hind limbs

  • Mobility of digits

  • claws modified to flat nails

  • Sensitive pads at the end of digits

  • Reduced snout → binocular vision

  • Simple molar cusp patterns

  • complex visual perception

  • most herbivorous or omnivorous

  • complex social groups

Convergent Evolution of Marsupials and Placentals

• picture on phone

Learning Mammals

• size and shape

• Shape of Legs

• Shape of head and ears

General Characteristics

• Thermoregulation

  • endothermic

  • hair = modified skin (compare to bird feathers)

    • insulation

    • regrow after shedding

    • secretions exude oil, produce glossy coat

    • Water resistance in some species

  • Other uses for hair

    • Protection

      • eye lashes protect eyes, …

    • camouflage

    • communication

      • cats and others modify hairs to their behavior

      • cats arch back and fluff back hair

    • sensation (whiskers)

  • Sweat glands

    • rid solutes and evaporate to cool skin

  • Carnivorous mammals have few sweat glands, cool by panting

• Living with winter

  • main problems with winters for all animals

    • cold temperatures

    • low prey availability

• Three main ways to deal with winter

  • hibernation: metabolism slows, temperature approaches ambient

    • arctic and northern temperate species

    • small species; increased SA/V ratio

    • Bats are a good example; huddle up to minimize SA/V ratio and still hibernate

  • Sleep: many mammals do not truly hibernate, but sleep much of the winter

    • like bears and squirrels: they spend more time sleeping and will occasionally come out

  • Migrate: some mammals move to areas with warmer temperatures, better prey

    • not moving as far as birds but they will move

Morphology

• High metabolism = efficient at gathering and consuming food

• teeth vary based on food habits

• several different kinds for different purposes (tearing, grinding, chewing, ect. ) = heterodont

• teeth are useful for identifying skulls

• Brain - large, mammals learn faster than most other species

• Sensory systems - varied

  • olfaction: most important sense is majority of mammals (nocturnal evolution)

    • scent marking

    • young and kin

  • Vision - visual sensitivity (forming images at low light) more imporantant than acuity (sharpness) for nocturnal animals - moer rods for light sensitivity

  • Cones = color vision, but most mammals can perceive some color

  • hearing - greater hearing than other tetrapods

    • pinna (external ear) - unique to mammals

  • Exception: primates, which have excellent vision and a generally poor sense of smell

Morphological diversity

• Feet and legs

  • moles have large front hands for digging

  • lynx have big feet acting as snow shoes

Reproduction and Parental care

• all sexual, all have mammary glands

  • mostly viviparous, some oviparous

• Precocial versus Altricial

  • Altricial - small mammals and large predators, born naked with closed eyes

    • mice, …

  • Precocial - large herbivores, can run and care for self (generally) very quickly

    • deer, elephants, and horses.

• Depends on ecology of species:

  • small rodents -(altricial) have hidden nests; small body size means trade-off between development and number of offspring;

• Large predators - (altricial) few predators to worry about

• Large herbivores - (precocial) usually only produce one young; need to outrun predators very soon after birth

• Marine mammals - (precocial) often give birth to single young that are able to swim almost immediately; this adaptation helps them evade threats in aquatic environments.

Conservation of Mammals

• all of the major conservation issues we have talked about with other groups are also found in mammals

  • over exploitation

  • habitat loss, degradation, fragmentation

  • disease

    • CWD being spread by deer farms through the feces

      • predators will prey on the weaker deer and when the predators eat them digest the disease better so in their feces there is less of the disease put back into the earth

    • White nose syndrome spread from the north east to the upper midwest all the way over to the rockies and pacific north west, impacting bat populations significantly along its path. This fungal disease thrives in cold, humid environments, and has led to steep declines in several bat species.

  • environmental pollution

  • climate change

    • alpine chipmunks in Yosemite National Park - moved upward toward the mountain to higher elevations

  • invasive species

    • new zealand

    • the introduction of invasive mammals, such as stoats and rats, has led to significant declines in native bird populations, highlighting the complex relationship between climate change and species competition.