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- Females direct mate choice, using three strategies:
- Sampling: females visit several harem-holding males, staying with each a few days before switching, usually returning to a male they visited within a week of estrus for mating
- Inciting: use sampling behavior until estrus, then run away from the male, inciting chasing and aggressive displays and fights among competing males. Female watches competition and mates with the winner
- Quiet: a month before estrus, female moves to an isolated, peripheral location occupied by a single male and stays with him throughout estrus and mating
- Breeding sequence:
- Male approaches female with a high-pitched whine
- Receptive female raises her tail and stands
- Male approaches waving his head from side to side in a pronounced gesture, making low sucking sounds, then mounts
- Copulation is brief, one forceful pelvic thrust
- Females may be harassed by bachelor males, with increasing aggression as estrus approaches; they often seek refuge with a territorial male, which chases off the bachelor
- Male pronghorn utilize different breeding systems within the same population, influenced by resources, population density, and sex ratio. (Maher, 2000)
- Territorial: When resources vary in quality or are clumped in defined locations, breeding males compete for the best territories (containing water and succulent vegetation) then defend a band of females within that territory
- Dominance: When resources are more uniform, bands with females and multiple males form; the males maintain a dominance hierarchy among them and the most dominant male conducts most of the breeding
- Harems: When resources are widely dispersed, population levels are low, or the sex ratio is skewed (1 male to 10 or more females), individual males defend a harem without regard to a defined territory
- Sexual maturity: females – 16 to 17 months; males – 1 year, although most will not breed until they are 3 to 4 years old and can compete to defend a territory or harem
- Breeding season:
- Northern populations: mid to late September, three-week rut is typical
- Southern populations: September to October, although breeding behavior is seen as early as July
- Synchronous birthing, most fawns born within a 10-day period
Gestation and Birth
- 245 to 255 days
- After fertilization, ova can develop for nearly a month before implantation.
- Birthing season
- Northern populations: typically May to June
- Southern populations: typically June to July
- Birthing season is influenced by habitat and environmental conditions
- Litter size: Usually twins; subordinate females sometimes have one fawn
- Size at birth: 7 to 9 pounds, 20 to 29 inches
- Fawns are large; combined weight of twins at birth is about 18% of maternal weight
Infant (< 1 year old)
- Age at weaning: 4 to 5 months
- Reach full weight by 4.5 years, females reach this faster than males
- Sexual maturity for males by one year, but at this age they don't usually have an opportunity to breed.
- Females are sexually mature around 16 months, but have been known to conceive at 5 months.
Female pronghorn usually give birth to twins, but subordinate females sometimes have one fawn.
Image credit: © San Diego Zoo Global. All rights reserved.
O’Gara & Yoakum (2004)