Transcript The 3 subclasses of mammals differ strikingly in their modes of reproduction

Prototheria (monotremes)
Metatheria (marsupials)
The 3 subclasses of
mammals differ strikingly
in their modes of
reproduction
Eutheria (placentals)
Monotremes –
1) Lay eggs
2) Cloaca
3) No nipples
Some features of monotreme reproduction
• Ovaries larger (relative to
body size) than other
mammals
• Oviducts open into
common urogenital sinus,
similar to reptile cloaca
(meaning of name
“monotreme”)
• Milk secreted by glands in
skin, licked from tufts of
fur at concentrations of
glands
• Testes abdominal, penis is
sac in floor of cloaca
• Young have “egg tooth”
like birds to help break out
of egg
• Platypus: 1-2 eggs
• Echidna: 1 egg
Typical platypus reproduction:
Egg is fertilized in fallopian tubes before entry to uterus,
eventually coated with leather mineralized shell before being laid.
But first, uterine wall secretes nutritous secretions which are
absorbed by fertilized egg, which grows in size (about 3X).
Egg at fertilization = 4 mm, when laid = 12 mm.
Egg retained in uterus about 28
days while development proceeds.
THEN shell added, egg laid, 10
more days of external incubation.
(Eggs sticky, female cradles
between stomach and tail.)
Lactation about 3-4 months.
Similar for echidnas:
Only 1 egg, incubates in a pouch where hatching occurs.
Young suckle in pouch for about 55 days, when they start to get
too prickly!
Lactation for about 4 months more.
Our friend, the opossum
1) Young tiny at birth (0.16 g)
2) Attach to nipples in pouch
(marsupium)
3) Short gestation, longer
lactation
How opossums do it:
Mating season, January –
July, 1-2 litters depending
on latitude. (2 is typical
for Illinois)
Egg fertilized in fallopian tubes, also surrounded by a shell
membrane (soft) at that time. Egg embeds in uterine wall, absorbs
nutritious secretions through membrane and grows. Membrane
retained for about 2/3 of gestation (about 8 days).
Last 1/3 of gestation (4 days), membrane shed, eggs sink into
depressions in uterine wall (has a kind of placenta, but no firm
implantation), absorbs nutrients and embryos continue to grow.
Gestation totals about 12-13 d.
Avg. 13 nipples (some
variation), about 7-9 young
make it to suckling stage.
Fixed to nipples about 50-65 d,
about 12% mortality.
Lactation continues to a total of about 95-105 days.
No continued association after weaning.
Rarely live longer than 18 months. In wild, maximum known
lifespan was 3 years.
Females often get just 1 breeding season, some early-born females
may get 2.
Some differences (from placentals)
found in marsupials
• Eggs slightly larger than in
placentals (0.12-0.28 mm vs
0.07-0.15 mm)
• Uteri separate, but females
also have paired (not single)
vaginae; new, median canal
forms at parturition
• Young never larger than
about 1 g, no matter size of
mother
• Gestation always short as or
shorter than length of estrus
cycle
• No invasive placenta
• Many have pouch, but
some only have folds of
skin, and a few don’t have
anything
• In males, penis is bifid
(forked) like female
vaginae
• Scrotum lies in front of
penis, opposite pattern in
placentals
Differences in female reproductive tracts
Gestation versus
lactation
Big difference between
marsupials and placentals
is development of young at
birth.
Marsupials have very short
gestation periods
compared to placentals of
the same size, but longer
periods of lactation.
Another way to look at this...
Time: conception to weaning
Takes marsupial longer than placental to raise young to
same size (weaning age)
So, which is “better?”
Costs: Lactation is most energetically costly period of
reproduction, and less efficient means of energy transfer to
young. Marsupials eventually raise offspring of similar
relative body sizes as placentals, but probably costs them a bit
more in terms of time and energy.
Constraints: Newborn marsupials need to be able to crawl to
pouch or teat, suckle, breathe, digest... More advanced than
placental embryo at same stage. But no wings, hooves, or
flippers!
Flexibility: Very little investment by marsupials in individual
offspring at time of birth, can adjust litter sizes, etc.
Joey the kangaroo
In marsupials, lacation shuts
down development of any other
fertilized eggs (= diapause)
Red kangaroos live in arid regions with unpredictable rainfall.
Can become baby factories during good times. Type of milk
produced by 2 teats also varies over time (% protein, fat,
carbohydrates).
Semelparity vs iteroparity
Semelparous: only 1 reproductive period per lifetime
Iteroparous: multiple reproductive events per lifetime
Semelparity is very unusual for mammals, but....
The Amazing Antechinus!
Small dasyurid marsupial
All females give birth within a
few days each year (synchrony;
Oct = Australian spring)
In late Sept, shortly after
females mate, all males die!
Every year, males go into hormonal overdrive, turn all energy
into breeding activities, form aggregations (like leks?) that are
visited by females. After 3 weeks of this, makes stressed out,
gastrointestinal ulcers, hair loss, stress-induced diseases... Die!
Females can store sperm, only receptive during mating period,
get pregnant and give birth after all males in population are
dead. (males live 1 yr, females mostly 1, 2, very rarely 3)
Most mammals are iteroparous, but iteroparity
can vary:
Monestrous: 1 reproductive event per year. Examples: large
mammal with long gestation period, such as white-tailed deer, but
also hibernators, such as ground squirrels. Iteroparous over
multiple years. Very predictable annual cycles.
Polyestrous: many estrus cycles (ovulations) per year.
Seasonally polyestrous: multiple estrus cycles, but within limited
times of year. Examples are most small mammals, such as voles
and mice. Iteroparous within a single breeding season or year.
Small rodents and insectivores can enter estrus every 4-5 days
during peak season if not mated. Gestation around 21 d, can get
pregnant again while lactating (often post-partum estrus),
weaning takes about 3-4 weeks... Young can start breeding at
about a month of age! Downside: short life. Maybe only 1 year.
Reproductive patterns of gray
and fox squirrels in Illinois
If 2 litters per year, 1 in spring and 1 in late summer. Litter size
usually 2-3. Lifespan: gray up to 6 yrs, fox around 7 years.
Capital vs Income Breeders
Capital breeder: accumulate energy reserves for breeding
Income breeder: relies on high metabolic rate, ability to obtain and
process energy currently available in environment for reproduction
Harbor seal: accumulates fat
to help support annual, intense
lactation period; predictable
White-footed mouse: litter
size and reproduction
adjusted to environmental
conditions; opportunistic
General pattern for a placental mammalian
reproductive event is:
O -> F -> B -> I -> DP -> DF -> Part
O = ovulation
Timing is important!
F = fertilization
Births should occur so that
B = formation of blastocyst
lactation, the most
I = implantation of
energetically demanding
blastocyst in uterine wall
period, coincides with the
most favorable
DP = development of
environmental conditions in
placenta
a seasonal environment
DF = development of fetus
Part = parturition (birth)
For example, red foxes
are monestrous. Females
come into estrous during
late Jan to early Feb, for
a short period of 3-4
days. Females within a
population are relatively
synchronous. (probably
to keep those mates
honest!)
Males are reproductively capable (= active spermatogenesis)
for a longer time, around Dec through March or so, that
encompasses the time of female receptivity. (probably to be
available for any opportunities!)
Gest = about 51 d. Mating in winter assures young will be
born in late March or April, right at start of “good” season.
Other species, such as ground squirrels, have short
enough gestation periods that they can wait until the
start of spring to initiate reproduction.
Hibernators, 7-8.5 months per year
underground
Males emerge 1st, mid-April, about
2 weeks before females in early
May
13-liner
Breeding occurs after females
emerge, gest = 26-28 d, young
born by late May to early June
Because young need to grow, and
adults need to fatten up, can only
have 1 litter per year
Franklin’s
It’s not smart to just reproduce whenever you
feel like it...
When particular events or
situations such as
seasonality, hibernation, or
migration constrain when
certain reproductive events
can occur...
...especially if the time
from conception to
weaning is longer than the
“good” environmental
period, or young need to
grow to meet the
challenges of the “hard”
season...
...then some mammals can
vary the order and timing
of this pattern.
Some variations on a reproductive theme
Delays in fertilization, implantation,
and development are ways used by
mammals to help coordinate births
with favorable environmental
conditions.
Mink
In delayed implantation, mating and
fertilization occurs, but development
stops at the blastocyst stage for a
time... Blastocyst floats free in uterus,
surrounded by protective zona
pelucida. Implantation occurs later.
Delayed implantation occurs in many carnivores, especially solitary
types like large mustelids, pinnipeds, bears, and a few others...
Mink in Illinois: mating extends from Jan through March...longer
in more northern populations. Gestation is variable, 40 to 75 days
depending on when implantation occurs. Births in April – May.
Bats show all sorts of patterns. Different species show delayed
fertilization, implantation, or development. Some tropical bats
are seasonally polyestrous, and show different patterns for
different reproductive cycles!
Hibernating bats have especially modified the general pattern
to their life history.
Big-eared bat
Notables:
1) Delayed fertilization
2) Sperm storage by both
males and females
3) Mating during fall, and
also during hibernation!
4) Can’t wait for reprod
activity to start in
spring, young need to
grow and prep for
hibernation, adults need
to store fat
5) Spring torpor slows
development, long
gestation (50-60 d!)
Overall fast-slow continuum
Small, short-lived mammals reproduce as continuously as
possible to balance short life span, high mortality rate. Tend to
be more opportunistic and less strictly seasonal. Larger litters,
with young being greater % of female body mass (higher massspecific metabolic rates).
Larger and longer-lived mammals are more selective about
timing of reproduction. Larger especially have longer
gestation times, which limits flexibility. Some must build up
reserves. Thus, use variety of means to vary general pattern of
events from conception to birth. Larger investment in
individual young (quality more important than number).
Precocial young:
ungulates, whales,
primates
Altricial young:
rodents, insectivores,
carnivores like canids
and bears
Altricial: can have
more young per
litter
Polyembryony in
9-banded armadillo
Record for longest gestation: 22 months
(shortest is a bandicoot, 12 days)
Enough for 1 lecture...