Mammals

Question 1

Cenozoic Radiation of Mammals

Answer 1

• Shrew-like, nocturnal
  Radiated after the K-T extinction
• 66 mya (end of Cretaceous)
• Massive asteroid
• Only <25 kg tetrapods survived (except sea turtles and crocodiles)
• New habitats
  >5400 living spp.
• Many characteristics made them so successful!

Question 2

Mammal Evolution

Answer 2

• Evolution of mammals from earliest amniote ancestors is well documented
  Over last 150 my:
• From small, ectothermic, hairless ancestors… to..
• endothermic, furry mammals
  Skull structures, especially teeth, provide abundant evidence of evolutionary descent

Question 3

Cynodont

Answer 3

The reptile-mammal connection (260 mya)
- Laid eggs
- More mammal-like skull
- Secondary Plate

Question 4

Characteristics of Mammals

Answer 4

1. Hair/fur/bristles/wool
2. Integumentary glands: Sweat glands, sebaceous glands, scent glands, mammary glands
3. Skull and Teeth

Question 5

Hair/Fur/Bristles/Wool

Answer 5

Hair Follicle:
- epidermal structure
- found in dermis of skin

A hair grows continuously by rapid proliferation of cell in the follicle

Cells in hair shaft are shifted upward away from their source of nourishment, accumulate keratin, and die

Numerous modifications for diverse uses
- Primarily thermal insulation

Question 6

Eccrine Sweat Glands

Answer 6

• Controlled by sympathetic nervous system
• Regulates body temperature by secreting watery fluid that draws heat away from the skin surface
• Water with some electrolytes (e.g., salt) derived from blood plasma

Question 7

Apocrine Sweat Glands

Answer 7

• Open into hair follicles
• Secrete a fatty sweat
• Do not function in temperature regulation and are correlated with reproductive & chemical communication functions
• Emotional stress, hormonal changes (e.g., human puberty)

Question 8

Sebaceous Glands

Answer 8

• Most associated with hair follicles
• Glandular cells produce an oily secretion, sebum
  
  • lubricates skin and hair

Question 9

Scent Glands

Answer 9

• Modified from sebaceous or apocrine glands
• Allow for communication: mark territory, warning and defense signals

Question 10

Mammary Glands

Answer 10

• Modified apocrine glands
• Rudimentary in males and occur on all females
• Females lactate/produce milk to feed young
• Epidermis thickens to form a milk line along each side of the abdomen -> nipples/teats
  
  • Monotremes (echidnas and platypuses) lack nipples; lap milk from hairs

Question 11

3. Skull and Teeth

Answer 11

Diphyodonts
- Teeth replaced only once from milk teeth to permanent teeth

Bony secondary palate

Single lower jaw bone (dentary bone)

Heterodont teeth
- differentiation of teeth
- vs homodont (uniform) teeth in reptiles

Structure of teeth reveal mammal lifestyle

Question 12

Reproduction

Answer 12

• Mating seasons: often coincide with most favourable time to give birth and rear young
• Females usually restrict mating to a fertile period during the estrus (also commonly called heat) cycle
• Mostly viviparous, some oviparous (e.g., platypuses, echidnas)

Question 13

Human Importance of Mammals

Answer 13

• Domestication
• Biomedical research
• 783 spp. endangered or critically endangered (IUCN Red List, Dec 2023)
• 27% of all assessed endangered spp.

Question 14

Divergence of Mammals

Answer 14

1. Monotremes
2. Marsupials
3. Placentals
   -All 3 groups of living mammals derived from the same lineage
   - early synapsid (“pelycosaur”) -> early therapsid -> early cynodont -> early therian
   - Divergence most obvious in their reproductive differences

Question 15

Class Mammalia -> Subclass Prototheria -> Order Monotremata (Monotremes)

Answer 15

Monotremes
- “one + hole” (cloaca)
- 4 spp. of Echidna
- 1 sp. of duck-billed Platypus

Oviparous
- lay eggs with one breeding season per year
- After hatching, young nourished by milk lapped off mother’s skin/hair near mammary glands (no nipplets/treats)
- Australia and New Guinea
- Lineage mostly extinct!

Question 16

Class Mammalia -> Subclass Theria -> Clade Metatheria (Marsupials)

Answer 16

• “resembling pouch”
  >330 spp.
• Pouched, viviparous
• Two major divisions:
  1. Americas
  2. Australia, New Guinea

Question 17

Marsupials Development

Answer 17

• Embryo is first encapsulated by shell membrane & floats free in uterus for several days bf attaching
• Yolk sac placenta (yolk + chorion) to nourish embryo less complex than Placentals
• Brief gestation & give birth to tiny embryonic (altricial) young
• Front pouch contains multiple nipples

Question 18

Class Mammalia -> Subclass Theria -> Clade Eutheria (Placentals)

Answer 18

• “true + beast”
• 21 orders
  Examples:
  1. Rodentia >2200+ spp.
  2. Chiroptera; >1400 spp.
  3. Primates; ~500 spp.
  4. Carnivora; > 290 spp.
  5. Artiodactyla; > 280 spp.

Question 19

Placental Pelvis

Answer 19

• Wider pelvic opening
• Loss of epipubic bones

Question 20

The Eutherian Placenta

Answer 20

Fetal membranes:
- Yolk sac
- Chorion -> Chorioallentoic
- Allantois -> Placenta
- Amnion

Question 21

Limbs - Walking Types

Answer 21

Plantigrade: Walking with feet flat on the ground (primates, carnivores (bears, racoons..), rodents..)

Digitigrade: Walking on toes (raised heel/wrist) (cats, dogs)

Unguligrade: Walking on nail (digits and remainder of foot raised) (Horse, deer, antelope..)

Question 22

Marine Mammals

Answer 22

Return to the ocean were separate events
- Order Sirenia (Manatees and dugongs)
- Order Artiodactyla (Whales, dolphins)
- Order Carnivora (Walruses, seals, sea lions)

Question 23

Primate Evolution

Answer 23

• From small, tree-shrew-like nocturnal animal

Mostly arboreal
- long arms w. large range of motion

• Acute binocular vision
• Large cerebral cortex
  Synapomorphies:
• Opposable thumbs/big toes
• Nails instead of claws

Question 24

The First Humans (Hominins) & Bipedalism

Answer 24

• Foreman magnum under braincase = bipedalism
• less robust jaws = omnivores
• At least 22 spp.
• Homo sapiens (us)
  ~ 200 000 - 300 000 yrs
• only extant humans (hominins)