Overview of Reproductive System: Reproductive System Introduction: Merck Veterinary Manual

Approximate Gestation Periods ^a
Domestic Animals	Days	Wild Animals	Days
Cat	65	Bear (Black)	210
Cattle^b		Bison	270
Angus	281	Camel	410
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	62–64^c	Marmoset	150
Donkey	365	Monkey (Macaque)	180
Goat	150	Moose	240
Horse^d	335–342	Muskox	255
Llama, Alpaca^d	330–365	Opossum	12
Pig	114	Panther	90
Sheep	150	Porcupine	210


Fur Animals	Days	Pronghorn	230
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	270–300^e	Tiger	103
Rabbit	31	Walrus	450
Wolf	63	Whale (Sperm)	450
		Woodchuck	31
^a See also selected physiologic data of laboratory animals, see Laboratory Animals: Selected Physiologic Data of Laboratory Animals a.
^b Individuals may range ±7–10 days from these averages
^c 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.
^d Individuals may range 20 days from these averages
^e 180+ days due to delayed implantation

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 ~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	4–9 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 (18) mo	Seasonally polyestrous, early spring through summer	~21 days (19–23)	5–7 days	Last few days of estrus, just prior to 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)	2nd 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 coitus. Pseudopregnancy lasts 40 days. Infertile matings delay onset of next cycle ~45 days.
Fox	10 mo	Monestrous December to March but mostly late January to February		2–4 days		Next winter	Ovulation usually on first or second day of receptivity. Ova shed before first polar body has been extruded. No proestrous bleeding.
Mink	10 mo	Induced ovulation, seasonally polyestrous mid- February to early April	Waves of follicles at intervals of 7–10 days	2 days	Induced ovulator	Next spring	Ovulation begins 36–48 hr after coitus, which must last ≥30 min.
Chinchilla	6–8½; mo (400–600 g)	Polyestrous, intense November to May	30–50 days (41)	Vagina perforated ½;–6 days during estrus; mate at night	Mate on second night, rarely on third night	2–48 hr, ovulation on second night
Nutria	5–8 mo	Polyestrous	24–29 days	2–4 days		48 hr
Rabbit	5–9 mo; range 4–12 mo for most breeds	Induced ovulation; breed all year, more or less; may show seasonal anestrus	No regular estrous cycles	Up to 1 mo	When vulva is enlarged and hyperemic	Immediately, but blastocysts die if doe suckles large litter	In USA, do not breed well in summer. Ovulation 101/2 hr after coitus. Pseudopregnancy lasts 14–16 days.
Rhesus monkey	3 yr	Polyestrous all year; tendency to anovulatory cycles in summer in USA	27–28 days (23–33)	~3 days	Near ovulation, day 10–13 of cycle	After weaning of previous young	Menstruation lasts 4–6 days. Ovulation usually ~13 days after onset.
Rat	37–67 days, varies with strain; body length at puberty 148–150 mm	Polyestrous all year	4–5 days	~14 hr (12–18), usually begins ~7 pm	Near ovulation	Within 24 hr	Ovulation soon after midnight. Stimulation of the cervix causes pseudopregnancy lasting 12–14 days.
Mouse	35 days (28–49)	Polyestrous all year	Usually 4 or 5 days	A few hours beginning in evening	Most receptive during first 3 hr	Within 24 hr	Ovulation soon after midnight. Stimulation of cervix causes pseudopregnancy lasting 10–12 days.
Guinea pig	55–70 days	Polyestrous all year	16½; days	6–11 hr, usually begins in evening	Midestrus on	Usually immediately	Ovulation ~10 hr after onset of estrus.
Hamster	4–6 wk	Polyestrous all year; few pregnancies in winter	4–5 days	12 hr, one night	Midestrus	After weaning	Ovulation 8–12 hr after onset of estrus. Pseudopregnancy lasts 7–13 days.
Gerbil (Mongolian)	9–12 wk	Polyestrous	4–6 days	12–15 hr	Midestrus	1–3 days	Ovulation spontaneous 6–10 hr after mating.

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 ~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	4–9 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 (18) mo	Seasonally polyestrous, early spring through summer	~21 days (19–23)	5–7 days	Last few days of estrus, just prior to 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)	2nd 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 coitus. Pseudopregnancy lasts 40 days. Infertile matings delay onset of next cycle ~45 days.
Fox	10 mo	Monestrous December to March but mostly late January to February		2–4 days		Next winter	Ovulation usually on first or second day of receptivity. Ova shed before first polar body has been extruded. No proestrous bleeding.
Mink	10 mo	Induced ovulation, seasonally polyestrous mid- February to early April	Waves of follicles at intervals of 7–10 days	2 days	Induced ovulator	Next spring	Ovulation begins 36–48 hr after coitus, which must last ≥30 min.
Chinchilla	6–8½; mo (400–600 g)	Polyestrous, intense November to May	30–50 days (41)	Vagina perforated ½;–6 days during estrus; mate at night	Mate on second night, rarely on third night	2–48 hr, ovulation on second night
Nutria	5–8 mo	Polyestrous	24–29 days	2–4 days		48 hr
Rabbit	5–9 mo; range 4–12 mo for most breeds	Induced ovulation; breed all year, more or less; may show seasonal anestrus	No regular estrous cycles	Up to 1 mo	When vulva is enlarged and hyperemic	Immediately, but blastocysts die if doe suckles large litter	In USA, do not breed well in summer. Ovulation 101/2 hr after coitus. Pseudopregnancy lasts 14–16 days.
Rhesus monkey	3 yr	Polyestrous all year; tendency to anovulatory cycles in summer in USA	27–28 days (23–33)	~3 days	Near ovulation, day 10–13 of cycle	After weaning of previous young	Menstruation lasts 4–6 days. Ovulation usually ~13 days after onset.
Rat	37–67 days, varies with strain; body length at puberty 148–150 mm	Polyestrous all year	4–5 days	~14 hr (12–18), usually begins ~7 pm	Near ovulation	Within 24 hr	Ovulation soon after midnight. Stimulation of the cervix causes pseudopregnancy lasting 12–14 days.
Mouse	35 days (28–49)	Polyestrous all year	Usually 4 or 5 days	A few hours beginning in evening	Most receptive during first 3 hr	Within 24 hr	Ovulation soon after midnight. Stimulation of cervix causes pseudopregnancy lasting 10–12 days.
Guinea pig	55–70 days	Polyestrous all year	16½; days	6–11 hr, usually begins in evening	Midestrus on	Usually immediately	Ovulation ~10 hr after onset of estrus.
Hamster	4–6 wk	Polyestrous all year; few pregnancies in winter	4–5 days	12 hr, one night	Midestrus	After weaning	Ovulation 8–12 hr after onset of estrus. Pseudopregnancy lasts 7–13 days.
Gerbil (Mongolian)	9–12 wk	Polyestrous	4–6 days	12–15 hr	Midestrus	1–3 days	Ovulation spontaneous 6–10 hr after mating.