Pronghorn (Antilocapra americana) Fact Sheet: Reproduction & Development

Courtship

• 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

Breeding Systems

• 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

• 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

Juvenile

• Reach full weight by 4.5 years, females reach this faster than males

Adult

• 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.

Wild populations

• 9 years, on average (data from Byers 1997 [unhunted pronghorn in Montana], discussed by O'Gara and Yoakum 2004)
• Females live longer than males (Byers 1997)

Female pronghorn usually give birth to twins, but subordinate females sometimes have one fawn.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Byers (1997)
O'Gara (1978)
O’Gara & Yoakum (2004)