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Lecture 2.1_Mammalian Reproduction

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welcome to the first lecture in module

two entitled mammalian reproduction so

in this video I'm going to be covering

the highlights just the highlights from

chapter nine which is entitled

reproduction so our overarching

objective today is to analyze the anatomical

anatomical

physiological and developmental

differences between the order mono

tremata the infra class

metatheria specifically the marsupials

as well as the emperor class Syria the

placentals on the left we have a jelly

bean sized newborn kangaroo Joey which

is in the mom's marsupium suckling on a

teat and then on the right we have a

lion cub that is in utero and being

nourished by mother's placenta so with

that let's dive in so let's begin by

recognizing that the mammals are going

to vary greatly in the structure of

their reproductive systems so first we

have the monotreams or the prototheria

like the duck-billed platypus here on

the bottom right and the spiny echidnas

we've got a species here hatching on the

bottom left

these guys are going to lay eggs and

incubate those eggs and as in the case

of most reptiles and birds the

monotreams have a cloaca that is to say

just a single

urogenital opening so a single opening

for both their urinary and reproductive tracts

tracts

additionally as in the burns the

monotreams only have one functional

ovary only the left ovary functions in

the monotremes they do have mammary

glands but no nipples

the marsupials or the pouched mammals

like the monotreams have a cloaca that

is to say a single urogenital opening

the marsupials do indeed have a placenta

but relative to the euthyrian mammals

their placenta is very inefficient so

this means the marsupials only have a

very brief gestation period when they're

in utero and then they're born altricial

meaning they're born very underdeveloped

they're going to emerge from the cloaca

very very tiny and then they're going to

crawl around to the marsupium or the

pouch where they're going to be

nourished by milk during a very long

lactational period on the bottom left we

have a newborn wallaby you can see it's

very very small and underdeveloped and

then on the right we have one that's

considerably more developed and it's

just hanging out half outside of mom's

marsupium and lastly of course we have

the U Theory and mammals like you and I

so these are commonly called the

placental mammals so they're called this

because they have this highly

sophisticated placenta that facilitates

very efficient respiratory and excretory

exchange between the maternal and Fetal

circulatory systems while the fetus is

in utero so unlike the marsupials the

placental mammals are going to have a

relatively very long gestational period

when they're inside of the uterus and a

relatively shorter period of nursing or

lactation so it's flipped from the

marsupials furthermore at Birth the

young placentals are considerably more

developed they're more precocial uh at

least relative to the marsupials your

textbook brings up a very good point

early on in the chapter that I think is

worth noting here and that is

it is incorrect to assume that lineages

that have a relatively more primitive

reproductive system so I.E the

monotremes and the marsupials are

somehow less successful or less

evolutionarily fit than they're more

derived placental cousins

so I want you to consider the Virginia

opossum so this species originated in

South America and yet in a very short

time the opossum has spread from South

America throughout Central America

throughout North America all the way up

to Ontario Canada so it's very successful

successful

the opossum has an unusually fast life

history Tempo so it's going to live fast

breed early and die young

so both the female and the male opossum

are going to become sexually mature

during their first year

following partition you're going to have

4 to 25 pink skinned hairless and blind

young that are the size of honeybees

emerge from the cloaca and move into the

marsupium to begin lactation so these

young are really small they weigh

0.13 grams on average and are only about

13 millimeters long

most opossums survive to breed for only

one year

their average litter size is about seven

and they have two breeding seasons in a

year so this results in a lifetime

reproductive output of up to 14 young

per female so this reproductive

potential High reproductive potential

yields a very high per capita growth

rate a very high Lambda which has

allowed this species to thrive and

spread this strategy works okay so

before we get into the nitty-gritty

Anatomy really quickly I just want to

take a moment and highlight some of the

truly just mind-blowing variation that

we see in mammals with respect to

reproductive strategies

so gestation the period of time that the

fetus is in utero is going to range from

zero days in the monotreams because of

course they lay eggs

to a mere 10 days and the dazzy urid

marsupials it's a family of marsupials

also known as the marsupial mice

although of course they're not truly

mice very small marsupials just a mere

10 days

to up to 22 months in elephants

similarly the lactation period is also

highly variable so the hooded seal here

on the bottom left this little pop is

only going to nurse for four days

whereas the chimpanzee here on the

bottom right she will nurse her young

for two and a half years litter sizes

typically vary between 1 and 15 young in

the vast majority of mammal species But

Metal voles are incredibly fecund

capable of producing nine litters of

five to eight young per litter which

equates to 72 pups per female per year

but the record litter size is our uh

beloved naked mole rat pictured here

it's a highly unusual lineage the

colonies consist of sterile workers and

only one fertile Queen who can produce

litters of up to 28 Offspring at a time

let's begin with some basic anatomy with

the male reproductive system so the

testes are the male reproductive organs

they produce the male gametes the sperm

as well as the primary Androgen

testosterone the testes are housed in

the scrotum like in our fish ancestor

the testes develop in the chest cavity

next to the heart actually however

during fetal development the testes are

going to migrate

posteriorly and eventually and then

after birth they're going to drop down

into the scrotum in primates carnivores

like you see here in these African lions

as well as in the hooved mammals the

ungulates this is likely because the

abdominal cavity is simply too warm for

the production of sperm for spermatogenesis

spermatogenesis

in some bats and rodents however the

testes will descend each breeding season

as needed and then at the end of the

breathing breeding season they're

retracted back up into the aguinal

cavity and then finally in the

monotremes some insectivores the

anteaters tree sloths armadillos the

sirenia those are the manatees and the

dugongs as well as the seals whales

elephants and hyraxes the testes are

always in the abdominal cavity

so if you think about it some of these

species have relatively lower metabolic

rates so think sloths and ant eaters

While others like the seals the whales

uh the manatees the dugongs the

duck-billed platypuses they live in

cooler aquatic environments

spermatogenesis occurs via meiosis

within the seminiferous tubules inside

of the testes the sperm is then stored

in this coil tube called the epididymis

from here sperm moves into this part of

the tube where it straightens out the

vas deferens

sperm is then emptied into the urethra

where it's combined with other

secretions that are generated by the

single prostate gland here as well as

the seminal vesicles here and the

Calpers glands here

some species have a a coagulating gland which

produces a substance that creates a

copulatory plug which can actually last

for up to two days and this copulatory

plug is going to ensure the retention of

the sperm within the reproductive tract

as well as block entry from other

competitors sperm so natural selection

really begins at the cellular level

mammals unlike bony fish and amphibians

have internal fertilization where the

male is going to directly deliver the

sperm to the females reproductive tract

internally via the penis so

interestingly the penis may include a

complex bony structure the Oz penis or

the baculum so on the bottom left here

are photos of various baculum from the

short-tailed weasel here all the way up

to the impressive uh raccoon here we

have the baculum on a red fox and then

check these crazy morphologies out these

are this bone the Oz penis in um on the

top left here these are various ground squirrels

squirrels

uh these are rice rats and kind of like

tridents and then these two are bears

and that's a seal next up we have the

anatomy of the female reproductive

system which consists of a pair of

ovaries the ovaries produce the egg it's

then released into these oviducts also

called the Fallopian tubes the Ed

travels down the fallopian tube and is

then released into one uterus or two

uteri the uterus is this Large Hollow

muscular chamber and this is the site of

development of fetal development

and then we have the cervix the cervix

is going to connect the uterus to the

vagina and lastly the vagina is the

opening to the outside of the body the

primary reproductive organ of females

are the ovaries these are a pair of

small oval bodies that lie slightly

posterior to the kidneys and they

produce the female gametes the eggs the

eggs are called the OVA which is the

plural the singular is ovum

so immediately under the surface of the

ovary is a thick layer of spherically

grouped cells which are called follicles

the follicles here

each one of these follicles encloses a

single egg so at Birth large numbers of

follicles are already present in the

female mammal so there's about two

million follicles that are present in

the ovaries at Birth in humans but that

number is going to steadily decrease

with age so ladies your eggs were

already within your follicles at Birth

and those eggs they are precious unlike

sperm which is energetically cheap

so ovulation is induced by high levels

of luteinizing hormone which is going to

cause that follicle to burst open and

Liberate the ovum into the fallopian

tube the oviducts the ruptured follicle

is next going to fill with yellow

follicular cells and become one of the

yellow bodies in the ovary also called

the corpus luteum so the corpus luteum

is really important because it's going

to promote the production of the hormone

progesterone and progesterone is what's

going to cause the thickening of the

uterine lining and the development of

the endometrium the endometrium is going

to allow for the implantation of the

fertilized egg progesterone is also

going to stimulate the growth of those

mammary glands it's important to

recognize that reproduction is truly a

carefully coordinated dance and it's

coordinated by hormones hormones are the

chemical messengers of the endocrine

system so to summarize here are the

ovaries the primary female reproductive

organ the ovaries are going to produce

the hormone estrogen

estrogen is going to stimulate the

hypothalamus located in the brain to produce

produce

gonadotropin releasing hormone that's

going to signal to the pituitary gland

to release both luteinizing and follicle

stimulating hormones and then we're

going to enlarge the ovary here

those two hormones are going to promote

the growth and maturation of the follicles

follicles

the follicle is going to burst and

release that single enclosed ovum the

egg into the fallopian tube and we call

this ovulation it's actually within the

fallopian tube that the sperm and the

Egg fuse and then we have the diploid

state which is called a zygote

the zygote is going to travel down the

fallopian tube it's going to undergo

cell division mitosis and then the

blastocyst is going to travel to the

uterus this thick Hollow muscular

chamber the corpus luteum which is the

follicle that is now filled with these

yellow follicular cells the corpus

luteum is going to promote the

production of progesterone progesterone

is going to cause the thickening of the

uterine wall and the development of the

highly vascularized

endometrium which is going to allow for

the implantation of the blastocyst most

mammals with the exception of the Apes

have a breeding season

which often results in a subsequent

partition window a birthing period that

aligns with some environmental phonology

that maximizes the probability of

survival for The Offspring so for

example the elk are going to rut in the

fall this is followed by an

approximately 250 day gestational period

and then the elk calves are going to hit

the ground in late spring right when the

Greenup is occurring

so the brief period during which time

females are receptive the few days

before and after ovulation this is

called estrus or heat so most mammals

are going to ovulate spontaneously as

controlled by hormones as we talked

about in the last slide

but there are some species mained wolves

Island foxes and Bush dogs that are

going to require

copulation they're going to require sex

in order to induce ovulation

in desert rodents and montane voles

ovulation is actually induced by Greenup

so environmental factors following rains

or snowmelt

many carnivores like wolves uh Bears

they are monstrous meaning they have

just one uh breeding Cycle One estrous

cycle per year whereas other species

like rodents and Lego morphs these are

smaller bodied species with a much

faster life Tempo they can have multiple

estrous cycles per year so they're what

we call polyestris your text has an

insert on the Big Bang breeders like the

Australian dazzy urid the genus is

anti-kindness but this is such an

unusual species that I want you to do

more than just read the insert I want

you to put me on pause right now and

please take the two minutes and seven

seconds and watch the video that's in

embedded in canvas that's entitled

mating to death the tough life of

anti-kindness implantation in the uterus

usually occurs when the embryo is still

a blastocyst meaning it's composed of a

mere 32 to 64 cells so as previously

mentioned the blastocyst is going to

implant into the endometrium which is

highly vascularized

upon implantation the blastocyst is

going to differentiate into the embryo

as well as the trophoblast

the trophoblast is then going to send

down these finger-like projections which

are going to merge with the female blood

vessels in the endometrium and this

merger is what's going to eventually

form the placenta be truly remarkable

placenta is going to Anchor the fetus to

the uterus and then perhaps most

importantly it's going to transport all

of the nutrients that that fetus needs

so glucose oxygen from Mother's

circulatory system to that of the fetus

further it's going to receive all of the

metabolites all of the nitrogenous waste

from this fetus and then lastly the

placenta is going to produce an entire

Suite of hormones that are going to

regulate the organs of both mother and

fetus the crazy thing is that the

mother's immune system doesn't reject

the placenta and the embryo because

after all the zygote is the product of

sexual recombination the zygote and the

placenta contain approximately 50

percent of Dad's DNA and you have to

remember the primary function of the

immune system is to seek out and

identify foreign Alien Invaders and to attack

attack

but it turns out that the placenta is

integral in suppressing mom's immune

response against it and the embryo which

brings us to this week's primary

literature summary

um this is actually a really amazing

article so it was published in 2018

and the researchers have highlighted the

importance of endogenous

retroviruses an endogenous retrovirus is

a virus that's inserted itself into the

host genome but the host has actually

co-opted uh that DNA in regulating its

own genes

so these endogenous retroviruses are in

euphyrian mammal DNA

and they're actually responsible for

allowing the placenta to mediate cell to

Cell fusion suppress maternal immunity

and protect the fetus from exogenous viruses

viruses

so after covid viruses get such a bad

rap but these retroviruses they may have

been crucial in the evolution of the

eutherian placenta before moving on from

the placenta I do want to remind you

that marsupials the metatheria they do

indeed have a placenta however it

differs because the marsupial placenta

is derived from the yolk Sac whereas the

eutherian placenta is derived from the

extra embryonic membranes in the

amniotic egg the Korean and the Allen toas

toas

so the comparatively the marsupial

placenta is very inefficient it's going

to form a much weaker connection with

the uterine wall because it doesn't have

all those extensive projections

extending into the endometrium moving on

to gestation that's the time period

between fertilization and partition or

birth gestation is going to vary

dramatically across the major mammalian

lineages from no intrauterine gestation

in the egg Lane monotremes to the very

short period of gestation in the

marsupials because as we've covered

they've got that relatively inefficient

placenta to the relatively long

gestational period in the euthyrian

mammals or the placental mammals so

beyond taxonomic group gestation is also

highly correlated with body mass so

larger animals like elephants and Elk

they're going to have longer gestational

times than say rodents also the degree

of development that the young is born

with is going to influence gestational

times so ungulates here on the right

they're young are born relatively

precocial relatively well developed so

they have a longer gestational period

than say primates so primates are born

relatively altricial meaning

underdeveloped less developed so they

have relative to their body mass a

shorter gestational period next I want

to summarize some of the reproductive

variations that we observe when we look

across mammalian taxa so the normal

normal reproductive sequence I guess I

should say the reproductive sequence

that's most often observed and is

exhibited by this primate goes like this

so it begins with spontaneous genius

ovulation the follicle bursts and it

releases that ovum we know that

spontaneous ovulation ovulation is

carefully coordinated by the endocrine

system by the hormones

following the release of the ovum we

have copulation which is the sexual act

this results in fertilization which is

the fusion of the male gamete the sperm

with the female gamete the ovum

uh following fertilization the

blastocyst is going to undergo mitosis

cell division

it's going to travel down the Fallopian

tubes into the uterus where it is going

to implant on the uterine wall

specifically on the endometrium

okay it's here that the fetus will

develop in a period known as gestation

and then The Offspring is born and

partition and then it enters into a

period of lactation where it receives

milk as nourishment

that's the most common sequence that we

see in mammals in the felids like this

Bobcat here uh letter B uh it's slightly

different so uh the felids have induced

ovulation so the felids are going to

require copulation first in order to

have that follicle burst and release

that ovum

so uh copulation then ovulation and then

fertilization occurs in the Fallopian

tubes and the sequence Remains the Same

implantation gestation partition and

then lactation

and then we also observe a couple of

patterns where different lineages uh

have delays in this process so the first

one is uh delayed ovulation or delayed

fertilization like we see in some

species of bats we're going to talk

about this in the next slide but in

short uh some bat species will copulate

and then the females will actually store

the sperm and then following seasonal

hibernation when they awaken then the

follicle bursts then we have ovulation

and then the sperm become motile again

and uh fertilization occurs

uh we also observed uh cases of delayed

implantation uh in the ursids like this

sun bear and my beloved black bears and

we'll come back to this uh but

essentially the blastocyst is going to

remain uh in suspended animation inside

of the uterus there's a delay uh before

the blastocyst is implanted with delayed

fertilization or delayed ovulation

insectivorous bats will mate in the fall

and then they're going to store that

Immortal sperm until spring when the

females emerge from hibernation in

Spring then their follicles will release

the eggs the sperm becomes motile and

fertilization occurs so delayed

fertilization or delayed ovulation

in numerous forgiverous bat species

fruit eating bat species

fertilization occurs the blastocyst

implants but then development occurs

very very slowly so for example in the

Jamaican fruit eating bat right here the

gestation length is seven months which

is very very slow for an animal with

such a small body mass because there is

this delay in the development of the

blastocyst black bears are a wonderful

Exemplar of delayed implantation so in

black bears females are going to go into

estrus in mid-summer during which time

copulation occurs and fertilization

however the blastocyst is going to

remain in suspended animation within the

uterus of the female until the

hyperphasia period the hyperphasia

period that occurs during the fall and

it's this critical period in the life

cycle of bears during which time Bears

have to accumulate enough fat to live on

during their upcoming hibernation and

it's only after they've met their

energetic demands their individual inter

energetic demands that the blastocysts

will implant if the Mast crop is poor

meaning Acorn production is really poor

in the Northeast then those suspended

blastocysts will actually abort and not

implant and this makes sense because of

the females in really poor nutritional

condition the Cubs are unlikely uh to be

born and if they are born they're

unlikely to survive because she's likely

not to be able to make enough milk to

support those Offspring marsupials have

independently evolved a similar trick to

delayed implantation called embryonic

diapause during embryonic diapause the

blastocyst is going to enter a state of

dormancy during which time cell division

all but stops so embryonic diapause is

reported to occur in almost all

kangaroos wallabies rat kangaroos pygmy

possums feather tail gliders and honey

possums what's so cool about this is

that at a given time a red kangaroo a

female red kangaroo like we see here

bottom left can simultaneously nourish

three Offspring at once so she may have

one weaned Joey that runs at the heel of

the mother and occasionally is going to

suckle from an elongated teat outside of

the pouch

the second young is a nursing pouched

Joey that's attached to another teat

within her marsupium and then finally

she'll have a tiny blastocyst uh that is

uh in this state of embryonic diapause

within one of the two uteri in a red

kangaroo females at the end of gestation

the adrenal glands in the fetus actually

are going to secrete adrenocortical

hormones like cortisol which are going

to initiate this whole hormonal Cascade

which is going to induce parturition or birth

birth

so next the placenta is going to begin

to secrete more estrogen and less

progesterone and then hormones called

prostaglandins which are going to

initiate the contractions of the

muscular uterus

as the baby's head presses against that

cervix the nerves of course are going to

carry that impulse to Mom's brain a

mom's brain is going to stimulate the

pituitary gland to produce the hormone

oxytocin the oxytocin carried in the

blood back to the uterine muscles and

then the contractions are going to start

in Earnest which is going to further

push that baby's head down and through

the cervix which is going to cause this

positive feedback loop the oxytocin is

also going to stimulate the letdown of

the milk so that it's available for that

newborn infant and then finally there's

a hormone called relaxin it's produced

by those corpora ludia the yellow bodies

and the ovaries which were once the

follicles it's going they're going to

produce relaxin and those are going to

soften the ligaments of mom's pelvis so

that it can spread and allow for the

passage of the fetus through the birth canal

canal

the vagina dilates and then rhythmic

contractions of the uterus gradually

force that fetus through the vagina and

to the outside world and the baby is born

born

if the amniotic sac is not ruptured

during the birth process then mother is

going to tear that away from the young

allowing those lungs to begin to take in

oxygen allowing that newborn to breathe

and then mother is going to sever that

umbilical cord and then consume that

nutritious placenta two terms that I've

previously mentioned but I just want to

take a moment and Define those because

they will show up on this week's

assessment wink wink

altricial young altricial young are born relatively

relatively

underdeveloped less developed

so they're born hairless uh blind

meaning their eyes are not open they're

essentially helpless so examples include

these adorable baby rabbits here on the

bottom left

carnivores are often altricial as well

as rodents like you see these altricial

social young are born relatively well

developed they're born fully haired

Their Eyes Are Open they're ready to get

up and walk around shortly after birth

so examples of pre-kosho young include

the hairs many grazing animals the

ungulates like the zebra here

citations the dolphins and whales

hyraxes some rodents and some primates

so if you think about some of these

lineages it kind of makes sense from an

evolutionary perspective if you're a

zebra calf born on the Serengeti Plains

with lions and leopards and cheetahs and

hyenas you need to move around

relatively quickly if you're a dolphin

and you're born into an aquatic

environment you need to be able to swim

relatively quickly so you can get up to

the surface and take that first breath

milk production by the mammary glands is

the quintessential feature of the class

manalia and it's namesake so mammal

comes from the Latin word Mammoth which

means breasts lactation is a crucial

element in the life history of all

mammals be it monotremes marsupials or

euthyria milk is an incredibly

nutritious substance containing lots of

fat and proteins like casein and Whey

lactose which is a milk sugar salts vitamins

vitamins

further milk is going to support the

growth of the microbiota in the

offspring's intestines this microbiota

is an incredibly important mutualistic

relationship between mammals and

microbes the first product actually

produced by the mammary glands following

birth is called cholesterol colostrum is

a protein-rich fluid that's going to

Vector antibodies that's going to confer

mom's immunity to her Offspring as you

would predict mammals living in harsh

Northern climates have milk that's very

high in fat and protein to allow their

offspring to accumulate insulative fat

as quickly as possible so that they can

thermoregulate whale and seal milk is

comprised of between 40 and 61 percent

fat so that's about 12 times the fat

content that's in whole cow's milk that

you and I may drink also it's got 11 to

12 percent protein which is four times

higher than cow's milk

so uh different properties of milk

depending upon uh what latitude these

mammals are living at as I mentioned at

the beginning of this admittedly very

long lecture

monotreams do not have true nipples

instead they're going to secrete milk

through pores onto the skin and then

it's going to travel down Tufts of hair

on the Platypus abdomen to be lapped up

by Young or in the pouch of the Echidna

marsupials have a circular arrangement

of nipples within their pouch like we

see on this gray short-tailed possum the

number of nipples on all mammals is

going to vary among groups and it's

related to mean litter size last slide I

really like this figure a lot it's a

figure 9.16 in your textbook we've got

two species of small mammals the

southern grasshopper mouse is a

placental and then the marsupial Mouse

on the bottom right but you can really

see the strategies of the marsupials

relative to the placentals here the

marsupials have a relatively longer

lactational period relative to the

placentals and the placentals have a

relatively longer gestational period

which is in black on the bar graph same

thing holds for large bodied placentals

versus marsupials this is a Thompson's

gazelle this is a Wallaroo here and you

can see this really long lactational

period in the Wallaroo relative to the

Thompson's gazelle and then this long

intrauterine gestation period in the gazelle

gazelle

and with that a round of applause for

you a pat on the back I very much

appreciate your time and your attention

and I hope you find these helpful thank you

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