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Reproduction
The Mammalian Strategy:
•
•
•
Relatively few intrauterine
young (higher survival rate)
Nourish neonates with milk
(high survival early; bonding)
Young remains with mother
(or parents) at minimum until
weaned (parental protection;
learned behaviors)
Reproduction
The Mammalian Strategy:
•
•
Amount of energy
invested per young is
lower than nonmammals;
Relatively few young
produced but most
survive to potentially
reproduce
Costs of Lactation
K c a l/d a y /in d iv id u a l
C a lo ric In ta k e o f B a n k V o le s
50
40
B re e d in g fe m a le
30
20
N o n -b re e d in g fe m a le
10
S o u rc e : F lo w e rd e w (1 9 8 7 , M a m m a ls : th e ir
re p ro d u c tiv e b io lo g y a n d p o p u la tio n e c o lo g y )
0
0
18
P re g n a n c y
36
L a c ta tio n
M u ltip le s o f M a in te n a n c e
Tradeoffs in Litter Size
L a c t a t io n C o s t s f o r C a t s
4
4
5 K it t e n s
3
3
3 K it t e n s
2
2
2 K it t e n s
1
1
M a in t e n a n c e n e e d s
0
0
0
1
2
3
W e e k o f L a c t a t io n
4
5
Reproductive Endocrinology
“Crash Course”
* Feedback mechanisms (environmental stimuli; hormone secretions)
Reproductive Endocrinology
“Crash Course”
Ovarian Cycle Influenced by:
1) Follicle stimulating hormone (FSH)
and luteinizing hormone (LH)
secreted by pituitary
• follicle growth which triggers
ovary to secrete estrogen
Reproductive Endocrinology
“Crash Course”
Ovarian Cycle Influenced by:
2) Estrogen secretion feeds-back to
hypothalamus-pituitary; more LH
secreted & less FSH
• Ovulation & corpus luteum
formation (spongy body which
forms in place of ruptured
follicle)
• Corpus luteum secretes
progesterone for uterine wall
preparation
Reproductive Endocrinology
“Crash Course”
Ovarian Cycle Influenced by:
3) No fertilization
• Corpus luteum recedes to
Corpus albicans
• Progesterone & estrogen level
drop
• Begin again in cycle
Reproductive Endocrinology
“Crash Course”
Ovarian Cycle Influenced by:
3) If fertilization occurs…
• Corpus luteum continues to
produce progesterone for
maintaining pregnancy
• Placenta soon assumes
estrogen & progesterone
secretion
amnion
chorion
embryo
allantois
Four Major Parts of
Embryonic Membranes
1) yolk sac: part of primitive intestine
lying external to embryo; forms from
endoderm
• No nutritional value
• Portion of placenta in some
cases (e.g., marsupials)
Four Major Parts of
Embryonic Membranes
2) amnion: forms from ectoderm &
mesoderm around the embryo
• Filled with serous fluid =
prevent dessication/shock
3) allantois: out-pocket from hindgut of
embryo
• Movement of nutrients & O2
• Forms blood vessels for
placenta
Four Major Parts of
Embryonic Membranes
4) chorion: outer embryonic layer
(ectoderm); envelopes entire
assemblage
• villi
• contact with uterine wall
placenta: includes embryonic membranes
& lining of uterine wall
(endometerium)
Types of Placenta
A) Placenta types based on villi distribution on chorion:
1) diffuse: villi scattered over entire surface of chorion =
increased SA for absorption
e.g., lemurs, perissodactyls, some artiodactyls
2) polycotyledonary: islands of villi scattered over chorion
e.g., other artiodactyls such as bovids
Types of Placenta
A) Placenta types based on villi distribution on chorion:
3) zonary: band of villi encircle center of blastocyst;
lacking villi elsewhere
e.g., carnivores
4) discoidal: regional restriction of villi
e.g., most mammals
discoidal
zonary
diffuse
Types of Placenta
B) Placenta type based on connection between villi &
endometrium:
1) nondeciduate: loose fitting of villi with endometrium;
villi pull free without disrupting endometrium during
parturition
(whales, ungulates)
2) deciduate: close fitting of villi-endometrium; villi pull
free & cause erosion of endometrium during parturition
(rodents, carnivores)
Types of Placenta
C) Placenta type based on
degree of intimacy between
embryonic & maternal parts:
1) choriovitelline: blastocyst
lies in endometrium
depression; does not
embed
2) chorioallantoic: villi;
blastocyst rests against
endometrium at allantoischorion contact point
Types of Placenta
C) Chorioallantoic Placenta Types:
1) epitheliochorial – lemurs, cetaceans, equids, suids
- epithelial cells of chorion in contact with epithelial cells
of uterus; villi in pockets in endometrium
2) syndesmochorial – artiodactyls
- lacking uterine epithelial barrier; contact uterine tissue
Types of Placenta
C) Chorioallantoic Placenta Types:
3) endotheliochorial – carnivores
- epithelial cells of chorion in contact lining of uterine
capillaries
4) hemochorial – insectivores, bats, higher primates
- villi in direct contact with maternal blood
Types of Placenta
C) Chorioallantoic Placenta Types:
5) hemoendothelial – lagomorphs, some rodents
- lining of villi blood vessels only barrier to maternal
blood
Reproductive Patterns
1) Continuous embryonic development (“typical”)
a) ova fertilized in oviduct
b) zygote begins mitosis - descends towards uterus
c) zygote reaches uterus – mitosis ongoing – reaches blastocyst
stage as implanting into endometrium
d) placental connection: uterus to embryo
e) continual development until parturition
Reproductive Physiology
- Implantation of
embryo in uterine
wall for varying
lengths of time
- Embryo supplied with
nutrients via the
placenta
Reproductive Patterns
2) Deviations from contiuous development
strategy:
a) Delayed Fertilization: ovulation &
fertilization delayed until an
extended time after copulation
•
•
•
Viable sperm retained in female
Ovulation occurs ~months after
copulation
Common to many temperate bats
(vespertilionids)
Reproductive Patterns
Example
Fall
copulation
Winter
Sperm storage
2) Deviations from contiuous
development strategy:
a) Delayed Fertilization:
Early spring
ovulation
Spring-summer Embryo
develops after
fertilization
Reproductive Patterns
2) Deviations from contiuous development strategy:
b) Delayed Development: blastocyst embeds into endometrium &
then becomes dormant; development delayed (e.g., bats)
Reproductive Patterns
2) Deviations from contiuous
development strategy:
Example
Late summer
Summer-Fall
Blastocyst
forms
Blastocyst
dormant
Late fallearly winter
Development
begins
Early spring
parturition
b) Delayed Development:
Reproductive Patterns
2) Deviations from contiuous
development strategy:
c) Delayed Implantation: obligate &
facultative examples
e.g., weasels, seals, bears
• Blastocyst forms but does not
embed & ceases to develop
• Floating blastocyst remains
dormant 2 weeks to 1 year
Reproductive Patterns
Summer
Mating
(Jun-Jul)
2004
March 2005 Implantation
(8-9 mo
delay)
Spring
Parturition
(Apr-May)
2005
Summer
Mating
(Jun-Jul)
(including
2005
2005 females
2) Deviations from contiuous
development strategy:
c) Delayed Implantation:
e.g., Mustela erminea
(avg age at death = 1.5 to
2 yrs)
*gestation period = 9-10
months
Reproductive Patterns
Spring-Summer
(Apr-May) 2004
Spring-Summer
(May-Jun) 2004
Summer
(Jul-Aug)
2004
Summer-Fall
Aug-Sep 2004
Mating
Parturition
Mating?
Sexually
Mature
2004
Females
Parturition
(2nd litter)
Mustela nivalis
Delayed Implantation????
* NO
(avg age at death = <1 yrs)
* gestation period = 3537 days
• 2 litter per year
possible
• Relation to vole cycles
Types of Breeding Seasons
1) Continuous – year round breeding;
no seasonality; common to
tropics
2) Restricted
Optimal timing for:
a) Regular – seasonal breeding;
temperate regions
b) Irregular – discontiuous
breeding during rainfall, etc…
desert/arid regions
* mating (time with best
availability of mates)
* birth (time with
abundant
resources
Seasonality to Mating & Parturition based on resource
availability (i.e, mates or food)
Resources
Gestation Period
Mating
Fall
Birthing
Winter
Spring
Summer
Body size relation to length of gestation period….What if
mammal could “extend” the gestation period to birth in a more
favorable time and/or insure mating opportunities? (e.g.,
weasels)
Gestation Period
Resources
Delay
Mating
Fall
Major Development
Birthing
Winter
Spring
Summer
Reproduction
Sexual Maturity (puberty) – age when capable of producing gametes
influence onset/cessation (restricted)
*environmental factors
efficiency of reproduction (continuous)
Influences on Puberty &
Reproduction
1) Light (photoperiod)
Rattus norvegicus
(continuous breeder)
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•
•
normal light
continuous light = 6 days
earlier than normal (FSH)
Constant dark = 16 days
later than cont. light
Influences on Puberty &
Reproduction
1) Light (photoperiod)
Microtus arvalis
(seasonal breeder)
•
•
1)
2)
3)
4)
5)
breeds 21 Mar – 24 Jun
simulate photoperiod during
(22 Sep – Dec)
Natural light
Artificial light
Uniform 16-h daylength
Uniform 8-h daylength until
Nov, then 13-h day
Control (“out of season”)
Results….
•
#1-4 = reached puberty
•
>60% females =
pregnant
•
Control = no
reproduction/puberty
*Light (photoperiod)
linked to
reproductive
development
Influences on Puberty &
Reproduction
2) Temperature
rodents
Temp
Puberty
1st Estrus
Experimental
Animals
-3oC
33 days
61 days
Control
21oC
26 days
38 days
**Growth rates lowered due indirectly to low temps. Thus,
results directly in delayed puberty
Influences on Puberty &
Reproduction
3) Nutrition – under-nutrition delays puberty in both females and
males
4) Precipitation – deer in Texas (Knowlton)
- “high” rainfall lead to shorter breeding season, more
synchronous breeding & fawning
- lower predation rates (functional response of coyotes)
# prey
consumed
Prey density
Influences on Puberty &
Reproduction
5) Social Effects/Density
(examples from captive
mice)
Lee-Boot Effect: pseudopregnancy induced among
crowded females; may go
anestrus
Whitten Effect: synchronized
estrus cycles when male
introduced into population
of females
Bruce Effect: implantation
blocked, pregnancy aborted
if females exposed to
strange, new male
* Male urine stimulates FSH &
LH secretion (pheromones)
Readings
•
Reproductive Cycles & Life-History Strategies, pp. 354-356
•
Litter Size & Reproductive “Seasons”, pp. 356-357
•
Lactation and Postnatal Growth, pp. 359-363