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 non-mammals;

• Relatively few young produced but most survive to potentially reproduce

Costs of Lactation

0 18 36

Kc

al/

da

y/i

nd

ivid

ua

l

0

10

20

30

40

50Caloric Intake of Bank Voles

Source: Flowerdew (1987, Mammals: their reproductive biology and population ecology)

Breeding female

Non-breeding female

Pregnancy Lactation

Tradeoffs in Litter Size

Week of Lactation

0 1 2 3 4 5

Mu

ltip

les

of

Ma

inte

na

nc

e

0

1

2

3

4

0

1

2

3

4Lactation Costs for Cats

5 Kittens

3 Kittens

2 Kittens

Maintenance needs

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

allantois

chorion

embryo

amnion

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 absorptione.g., lemurs, perissodactyls, some artiodactyls

2) polycotyledonary: islands of villi scattered over chorione.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 elsewheree.g., carnivores

4) discoidal: regional restriction of villie.g., most mammals

discoidal

zonary

diffuse

Types of PlacentaB) 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 allantois-chorion 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 Patterns1) 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 Patterns2) 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

Fall copulation

Winter Sperm storage

Early spring ovulation

Spring-summer Embryo develops after fertilization

2) Deviations from contiuous development strategy:

a) Delayed Fertilization:

Example

Reproductive Patterns

2) Deviations from contiuous development strategy:

b) Delayed Development: blastocyst embeds into endometrium & then becomes dormant; development delayed (e.g., bats)

Reproductive Patterns2) Deviations from contiuous

development strategy:

b) Delayed Development: Late summer Blastocyst forms

Summer-Fall Blastocyst dormant

Late fall-early winter

Development begins

Early spring parturition

Example

Reproductive Patterns2) 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 PatternsSummer (Jun-Jul)

2004

Mating

March 2005 Implantation

(8-9 mo delay)

Spring

(Apr-May)

2005

Parturition

Summer (Jun-Jul)

2005

Mating (including 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 PatternsSpring-Summer (Apr-May) 2004

Mating

Spring-Summer (May-Jun) 2004

Parturition

Summer

(Jul-Aug)

2004

Mating?

Sexually Mature 2004 Females

Summer-Fall

Aug-Sep 2004

Parturition (2nd litter)

Mustela nivalis

Delayed Implantation????

* NO

(avg age at death = <1 yrs)

* gestation period = 35-37 days• 2 litter per year possible• Relation to vole cycles

Types of Breeding Seasons1) Continuous – year round breeding; no

seasonality; common to tropics

2) Restricted

a) Regular – seasonal breeding; temperate regions

b) Irregular – discontiuous breeding during rainfall, etc…

desert/arid regions

Optimal timing for:

* 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)

Fall Winter Spring Summer

Mating Birthing

Res

ourc

es

Gestation Period

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)

Fall Winter Spring Summer

Mating Birthing

Res

ourc

es

Gestation Period

Delay Major Development

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)

• 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)• breeds 21 Mar – 24 Jun• simulate photoperiod during

(22 Sep – Dec)

1) Natural light

2) Artificial light

3) Uniform 16-h daylength

4) Uniform 8-h daylength until Nov, then 13-h day

5) 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: pseudo-pregnancy 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