Mammals

Mammals
Occupy almost every environment on earth that can support life
- Not a large group (5,700 species)
- But one of the most biologically differentiated
- Terrestrial forms
- Aquatic forms
- Flying forms
- Diverse in size
- 2g bat
- 170 ton blue whale

Mammals
Of great importance to humans
- Domestication
- Food and fiber
- Beasts of burden
- Pets
- Medical research

Origin and Evolution of
Mammals
One of two living clades of amniotes
- Reptilia is the other
Well documented fossil transition
- Most data from skulls and teeth
Mammals belong to the synapsid
lineage of amniotes
- Single temporal opening on the
side of the skull

Origin and Evolution of Mammals
Earliest synapsids gave rise to pelycosaurs
- Superficially similar to lizards, but not closely related
- Synapsid vs. diapsid
Pelycosaurs gave rise to the therapsids
- Therapsids had limbs placed under the body rather than on the sides of the
body
- Less stable than the ancestral form, so required an expansion of the brain
- Fossil evidence shows enlargement of the cranium

Therapsids gave rise to the cynodonts
- Powerful jaws
- Heterodont dentition
- More diverse foods
- Turbinate bones in the nasal cavity
- Heat retention
- Secondary palate
- Allows breathing with prey in mouth
- Allows breathing during suckeling

Earliest mammals were small
- Mouse or shrew in size
- Possessed diphyodont dentition
- Two sets of teeth in lifetime
- Possessed three middle ear ossicles
- Simplified jaw articulation
- Most likely endothermic
- Had hair
- Likely also glands to support hair

Despite evolving in the later Triassic, mammals
would not diversify for another 150my
- Niches and habitats occupied by synapsids
- Once vacated, mammals could diversify

Mammals divided into two clades
- Monotremes
- Therians
- Marsupials (metatherians)
- Placentals (eutherians)

Structural and Functional Adaptations of Mammals
Integument and its derivatives
- Thicker than in other amniotes
- Epidermis and dermis
- Epidermis is generally thin where
covered with hair
- Where not covered by hair or
where wear is great, epidermis is
thickened and strengthened with
keratin

- Hair
- Characteristic of mammals
- Grows from a follicle that is epidermal in origin, but sunk in
the dermis
- Growth is continuous
- New cells pushed away from source of nourishment
die and get filled with keratin
- Two kinds of hair form pelage
- Underhair - insulation
- Guard hair - protection and coloration

- Hair
- Mammals will molt their hair
- Most have two annual molts
- Summer coats always thinner than winter coats
- May involve a change in color too

- Hair
- May be modified for several purposes
- Vibrissae - “whiskers” used for sensory purposes
- Quills - modifications of hair by some mammals for
defense

- Horns and antlers
- Horns - hollow sheaths of keratinized epidermis that surround a core of bone
- Not shed
- Not branched
- Grow continuously
- Occur in both sexes
- Antlers - solid bone when mature
- Branched
- Shed
- Generally occur only in males

- Glands
- Mammals have glands that fall into one of four classes
- Sweat -
- Eccrine - evaporative cooling
- Apocrine - not cooling, but associated with reproduction
- Scent - locations vary, but all help in communication
- Sebaceous - associated with hair and secrete oil to condition hairs
- Mammary - occur on all females. Secretions used to nourish young

Food and Feeding
- Mammals exploit a wide variety of foods
- Teeth are likely the greatest indicator of diet
- Heterodont dentition
- Incisors
- Canines
- Premolars
- Molars

Food and Feeding
- Feeding specializations
- Insectivores - insects
- Herbivores
- Browsers and grazers
- Gnawers
- Ruminants - four chambered stomach
- Utilized bacteria for fermentation
- Carnivores
- Omnivores

Reproduction
- Most mammals have defined reproductive seasons
- Males may copulate at any time
- Female only receptive during estrus (heat)
- Monoestrous vs. polyestrous
- Number of receptive periods within breeding season
- Primates have a different cycle
- Menstrual cycle

Reproduction
- Monotremes
- Egg-laying
- Spend 10-12 days in uterus developing
- Nourished by yolk
- Thin, leathery shell applied
- Eggs laid

Reproduction
- Marsupials
- Do possess a transient placenta
- After “hatching” from a shell membrane, embryo floats in
uterine fluid but does not implant into wall of uterus
- Gestation is brief and young are born at an early stage of
development
- Climb from vagina to marsupium
- Implant on a nipple and finish development

Reproduction
- Placentals
- Embryo remains in uterus until gestation is complete
- Born at a advanced stage
- Some young are precocial
- Some young are altricial

Mammals

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Mammals