Lecture 24 Mammals Flashcards
Describe mammals
- ~ 5,000 species
- 3 unique features
1) hair: insulation, camouflage, sensory & defence
2) sweat glands
3) mammary glands
– milk production
Note: hair and feathers evolved independently.
Phylogeny between reptile and mammal.
Describe mammal evolution
- true mammals evolved ~225 mya (Triassic)
– Early mammals were small, arboreal, shrew-like insectivores, nocturnal
– “improvements” to middle ear - ~195 mya (Jurassic)
- found in China
- size: 32mm, 2g
- significantly larger brain for its size
- has middle ear bones derived from jaw
Describe mammal radiation
Mammals did not radiate until extinction of the “dinosaurs”
Coexisted with “saurs”
– became dominant group upon “saur” extinction (~65 mya)
– diversified
– filled empty terrestrial & marine niches (large herbivores & carnivores)
– return to oceans
Describe origins of extant marine mammals
Ambulocetus
* ancestor to whales
* ~50 mya
* ~ 3 meters
1) Ear similar to whales, can hear well underwater
2) Similar teeth to cetaceans
From when is the age of mammals
Cenzoic (66 mya to now)
Describe megatherium
*~2 mya to 8,000 yrs
* herbivorous
*elephant-sized, giant ground sloth
Describe Smilodon
- saber-toothed cat
- 1.5 mya to 11,000 yrs (Pleistocene)
- contemporary with humans, mammoths & giant ground sloths
Both smilodon and megatherium could go extinct because of hunting by humans or climate + human-caused fires
Describe mammals other features
- Endothermic, 4 chambered heart
- “advanced” nervous system
- internal fertilization
- Unlike reptiles and fish, mammals also show heterodonty – different teeth specialized for different tasks
- specialization in teeth among species reflects varied diets
Describe three mammal groups
- Prototherians (Monotremes)
–egg laying
– egg incubation until hatching - Marsupials – pouched
– early birth, fetus completes development in pouch - Eutherians – placental
– embryo retained in female reproductive tract
Describe Prototherians
- lay shelled eggs (oviparous)
– females lack placenta
– incubated by parents - have mammary glands
– no nipples, young suck milk from fur - platypus, echidna
– Australia, New Guinea
Platypus
* Eastern Australia
* use bill to dig for crustaceans & worms
* build nests in river banks
* lay 2-4 eggs, hatch ~8 days
* nurse 5 months
* venomous “spur” on males
Echidna
* short & long nose “spiny anteaters”
* insectivores
* sticky tongue
* temporary pouch for egg
* hatch ~8 days
* in pouch, spines form (~3 weeks)
* live ~50 years
Describe marsupials
- do not lay eggs
– Viviparous, placenta - short gestation, long nursing period
- birth after short internal development
- newborns crawl over mother’s body to pouch
– attach to nipple in pouch
– complete development - ~350 species
– kangaroos, koalas, opossums - N & S America, Australia
Tasmanian devil (transmissable cancers)
* largest extant marsupial carnivore (size of small dog)
* scavengers
*extinct on mainland due to hunting & dingos
Tasmanian Tiger/Wolf (Thylacine) - extinct
Describe marupial evolution
- diverged from early placentals ~100 mya
- most of NA marsupials are extinct
ex: opossum
Describe eutherians
- 94% of species
- viviparous
– no pouch or shelled egg - amniotic egg retained in female reproductive tract
– embryo nourished by mother via placenta
Eutherians often referred to as placental animals, but some marsupials have placentas
The most speciose group of mammals being rodents
Describe placenta
Organ formed by the embryo & mother after implantation
* formed by extraembryonic membranes & uterus lining of mother
* site of gas, nutrient, & waste exchange between mother and embryo
* produces hormones necessary to maintain pregnanc
- substances pass between mother and fetus by diffusion
- two separate blood systems
- no mixing of blood cells or plasma
Note: Very different circulation patterns in fetus
