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Overview of the Reproductive System

By Harry W. Momont, DVM, PhD, DACT, Clinical Associate Professor, Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison

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The reproductive system provides the mechanism for the recombination of genetic material that allows for change and adaptation. Manipulation of this system in breeding management programs allows the rapid and dramatic alteration of the conformation and productivity of domestic animals. Theriogenology is the veterinary clinical specialty that deals with reproduction. The reproductive system is incredibly complex in both its anatomy and physiology, and all aspects must be considered when resolving reproductive problems. The differences in the reproductive system between the sexes and among species are extensive. Both sexes have primary sex organs and primary regulatory centers. Gonads and function-adapted, tubular genital organs constitute primary sex organs in both sexes. The pituitary gland and the hypothalamus are the primary regulatory centers; thus, the regulatory function is, in part, neuroendocrine in nature. In pregnant females, the fetoplacental unit has a significant role in maintaining and terminating pregnancy.

For the temporal and physiologic features of the reproductive cycles of selected species, see Table: Approximate Gestation Periods and Approximate Incubation Periods and Features of the Reproductive Cycle.

Approximate Gestation Periods

Domestic Animals

Days

Wild Animals

Days

Cat

65

Bear (Black)

210

Cattlea

Bison

280

Angus

281

Camel

365-400

Ayrshire

279

Chimpanzee

236

Brahman

292

Coyote

63

Brown Swiss

290

Deer (Mule and White-tailed)

200

Charolais

289

Elephant

660

Guernsey

283

Elk, Wapiti

255

Hereford

285

Giraffe

425

Holstein

279

Gorilla

270

Jersey

279

Hare

36

Limousin

289

Hippopotamus

240

Shorthorn

282

Leopard

95

Simmental

289

Lion

108

Dog

62–64b

Marmoset

150

Donkey

365

Monkey (Macaque)

180

Goat

150

Moose

235

Horsec

335–342

Muskox

255

Llama, Alpacac

335–365

Opossum

12

Pig

114

Panther

90

Sheep

150

Porcupine

210

Fur Animals

Days

Pronghorn

250

Chinchilla

111

Raccoon

63

Ferret

42

Reindeer

225

Fox

52

Rhinoceros (African)

480

Mink

Seal

330

European

41

Shrew

20

American

40–75

Skunk

63

Muskrat

29

Squirrel (Gray)

40

Nutria, Coypu

130

Tapir

390

Otter

270–300d

Tiger

103

Rabbit

31

Walrus

450

Wolf

63

Whale (Sperm)

450

Woodchuck

31

a Individuals may range ±7–10 days from these averages.

b Gestation period is 58–72 days from breeding at unknown stage of estrus; from day of ovulation (which can be determined by progesterone or LH monitoring), gestation period is 62–64 days.

c Individuals may range 20 days from these averages.

d 180+ days due to delayed implantation

Approximate Incubation Periods

Domestic Birds

Days

Caged and Game Birds

Days

Chicken

21

Budgerigar

18

Duck

28

Finch

14

Muscovy duck

35

Parrot

26

Goose

28

Pheasant

24

Guinea fowl

28

Pigeon

18

Turkey

28

Quail

16

Swan

35

Features of the Reproductive Cycle

Species

Age at Puberty

Cycle Type

Cycle Length

Duration of Estrus

Best Time to Breed

First Estrus After Parturition

Comments

Cattle

10–12 mo, usually first bred ~14-15 mo

Polyestrous all year

21 days (18–24)

18 hr (6–24)

Insemination from midestrus until 6 hr after end of estrus

20–60 days

Ovulation 10–12 hr after end of estrus.

Sheep

6–9 mo

Seasonally polyestrous, early fall to winter

17 days (14–20)

24–36 hr

18–20 hr after onset of estrus

Next fall

Ovulation near end of estrus.

Goat

5–7 mo

Seasonally polyestrous, early fall to late winter

21 days

24–48 hr

Daily during estrus

Next fall

Many intersexes born in hornless strains.

Pig

6–7 mo

Polyestrous all year

21 days (19–23)

40–60 hr

Daily during estrus

4–10 days after weaning

Ovulation usually ~40 hr after beginning of estrus.

Horse

10–24 mo

Seasonally polyestrous, early spring through summer

~21 days (19–23)

5–7 days

Last few days of estrus, just before ovulation; should be bred at 2-day intervals

4–14 (9) days

Ovulation usually 1–2 days before end of estrus. Double ovulation occurs in ~20% of estrous periods, but twins rarely progress to term.

Alpaca

12–18 mo

Polyestrous all year in North America

Not applicable

Up to 36 days

When a large, viable follicle is present

Fertile within 15–20 days

Alpacas are induced ovulators.

Dog

6–24 mo; earlier in smaller breeds, later in larger breeds

Monestrous all year

6–7 mo

9 days (3–21)

Second day after ovulation

4–5 mo

Proestrous bleeding 7–10 days. Ovulation usually 1–3 days after onset of estrus. Ova shed before first polar body has been extruded (primary oocyte).

Cat

4–12 mo

Seasonally polyestrous, spring through early fall

14–21 days

6–7 days

Daily from day 2 of estrus

4–6 wk

Induced ovulation 24–48 hr after breeding. Pseudopregnancy lasts 40 days. Infertile matings delay onset of next cycle ~45 days.

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