Mammal Reproduction Monotremes Therians Eutherians Marsupials ...

Mammal Reproduction
• All lacatate and care for young
• Only monotremes lay eggs
– Likely the primitive condition because egg-laying is primitive for
amniotes in general and once the uterine glands that produced egg
shells disappeared, it seems unlikely they’d be back
• Initial embryonic blastocyst forms the embryo and
trophoblast which is tissue specialized for getting nutrients
and producing hormones
• All have glandular uterine epithelium which produces
materials to nourish embryos in utero
• All have the hormone-secreting corpus luteum in the ovary
which controls pregnancy process
Therians
• All have extra-embryonic membranes
including an initial placenta developed from
the yolk sac
• A second placenta develops
later in the eutherians
• Embryonic diapause
Marsupials
• Retain evidence of oviparous
ancestry
• Neonates are altricial except for well developed
forelimbs, lungs, jaw, secondary palate, facial
muscles and tongue
• Not all marsupials have a pouch
• Offspring generally climb to the pouch unaided
and spend longer there than gestation lasted
• Unlike eutherian milk, marsupial milk varies over
the course of lactation and kangaroos can actually
produce two kinds at once
Monotremes
• Primitive condition
• 2 oviducts meet to form the urogenital sinus
• Small amount of yolk compared to birds or
reptiles so eggs are also sustained by maternal
secretions in utero and have leathery shells
• 1-2 eggs hatch in 2 weeks
• Young are almost embryonic at hatching and
brooding by mom continues for 16 weeks
• They have a pouch for this, but its not
homologous to the marsupial pouch
Eutherians
• Rodents and insectivores are born in a highly
altricial state in which they are only slightly more
developed than marsupial young
• Even the highly precocial ungulates still require
lactation for transfer of antibodies
• Larger-bodied species tend to have fewer
offspring per litter, but the lifetime total is
probably about the same
– Large carnivores have several altricial young per litter
– Ungulates have one or two precocial--escape vs. diet?
Primitive Therian Condition
• Altricial young
– Advantageous because parental brooding keeps
offspring warm so their energy can go into growth
instead
• Short life span
• Short gestation
• Either lots of small litters or 1-2 large
• Nipple attachment may be primitive but pouches
are derived
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Naked Mole
Rats
• Eusocial like bees with:
– Queen: suppresses
breeding by other females
– Frequent workers: burrowing and feeding
– Infrequent workers: same but 75% less
– Non-workers: larger, care for young
• Males and females equally
represented in the castes and all
males produce sperm
Eutherians vs. Marsupials
• It wasn’t that eutherians outcompeted
marsupials
• Marsupials invest their energy into their
offspring during the extended post-gestation
lactation while eutherians invest it during
intrauterine development
So is there an advantage to either?
So what’s the downside?
• Marsupials probably couldn’t have
developed a fully aquatic existence
• Marsupials couldn’t have had hooves
• It does appear that the littler eutherians do
reproduce more quickly than similar
marsupials---that’s one reason that feral
eutherians in Australia are outcompeting the
natives…
Spotted Hyenas
• These are the only hyenas that hunt in packs
• Females have masculinized genitalia
– Side effect of very high levels of testosterone
which is advantageous because it makes the
females more aggressive and so their offspring
get more food during pack meals?
Possible marsupial advantages:
• Ability to dump offspring
• More gradual offspring growth means less
stress for mom, and maybe less energy lost
if the baby dies somewhere along the way,
and maybe mom can conceive again more
quickly
Based on extant mammals though, and things
were different……
Rabbit Populations
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• Incisors-biting, chisels
• Canines-stabbing, larger in males than
females
• Premolars-pierce and slice
• Molars-grinding
Hindgut Pros and Cons
• Con: nutrient uptake is less efficient
– Cellulose in the cell walls does not get digested
until it reaches the cecum and large intestine
where nutrient uptake is less efficient
• Pro: the process is fast
– Food passes through the system in 30-45 hours
vs. 70-100 for a ruminant, so while it’s not
efficient, the animal can eat more, more quickly
Herbivore vs.
Carnivore
• Hypsodont teeth
(horses)
• Deep jaws,
cheeks
• Eyes far back
• Hypselodont
teeth (rodents)
• One molar at a
time (elephants)
How to Digest Plants
Foregut Pros and Cons
• Pro: highly efficient at nutrient extraction
– Cellulose gets broken down prior to “real”
digestion in the gut so microbes can work on
everything
– Microbes keep some nutrients but produce
essential amino acids, so since they’re digested
as well, the animal can eat anything
• Con: slow
– High fiber diets don’t work--the animals can
actually starve with full stomachs
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Ecological Consequences
• Hindgut fermenters can live on low quality food as
long as there’s a lot of it--they’re limited by the
amount
• Ruminants need high quality food but not as much--
-they’re limited by the type
– And they don’t need to drink as much because they don’t
need to excrete as much urea since it gets used up by the
microbes
Types of Digging
• Rapid Scratch Diggers
– Sort of swim through the sand
• Rotation Thrust Diggers
– More compact soil so they rotate their forelimb
to dig and push soil upward to compact it
• Chisel-Tooth Diggers
– Dig with their teeth and push
soil out as they excavate
Why the design for speed?
• Predator-Prey arms race? Not.
– Long legged ungulates show up early Miocene
while their pursuit predators don’t appear until
the Pliocene, nearly 20my later
– Maybe more efficient in the new grasslands?
• Why are ungulates so much more
specialized for speed?
– Horses aren’t much faster than dogs, so why
bother with all the re-engineering?
– Probably relates to digestion: ungulate gut and
gut contents make up 40% of its mass so it
can’t fit much muscle---had to do something
else
• Monotremes
• Marsupials
• Insectivora
• Rodentia
• Carnivora
• Artiodactyla
Aquatic Mammals
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The Only Completely Aquatic
Mammal Groups
• Cetacea
• Sirenia
• Pinnipeda
• (sea otters?)
They don’t use their limbs for swimming, but have
adopted the whole body undulations of a fish
except dorsoventrally rather than laterally
All use blubber for insulation rather than hair
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