Mating System

Image courtesy Brent Huffman: www.ultimateungulate.com

Basic Mating System

Mating systems come in two different but similar strategies. In one strategy, males set up territories in spring and defend fixed boundaries through the rut (breeding season). The other strategy is harem defense. The major difference in these two strategies is that males who practice harem defense allow other males to pass through the area that they scent mark. Both of these strategies are referred to as female defense polygyny. Pronghorn can shift from one type of mating behavior to another in response to environmental and demographic changes (Byers, 1997).

Courtship

During courtship, males will typically make a soft vocalization when approaching the female (Byers, 1997). Next he will display his cheek-patches by waving his head from side to side while circling the female (Min, 1997). If the female is receptive, she will stand motionless in front of the male and sniff his scent gland. Then, if she remains motionless, the male walks behind her and mounts (Byers, 1997).

Mate Choice

In polygynous species, in order to prevent male-male competition from completely determining mating success, females choose mates. Pronghorn are a model species to study in this respect because 1) females mate once per estrus, 2) forced copulations cannot occur, 3) females are not part of permanent social groups, so they can move independently, 4) males cannot completely control female movements, and 6) mating is not resource based.

Females have 3 different mate choice strategies; sampling, inciting, and quiet. Sampling females visit several male harems prior to estrus. They will always leave males that fail to keep other males away, but sometimes leave for no apparent reason. Males have to continually corral females to keep them in his harem. The more successful he is at containing the females, the more likely he is to mate. His ability to keep other males away and contain females is indicative of the quality of his genes. “Inciting” females act like sampling females until estrus. During estrus, they move away from the harem male and incite fights with neighboring males. They watch the fight and mate with the winner immediately following the fight. “Quiet” females will move to an isolated location occupied by a single male, and stay there throughout estrus. This strategy may be used when the female has low energy stores (Byers et al. 1994). Poor forage conditions, poor physical condition, and a low population density force some females to adopt the quiet strategy and remain with one group throughout the breeding season (Maher, 1997).

Another study assessing female mate sampling, conducted in Middle Park, Colorado, was one of a population where males exhibit territories up to and during a 3-4 week rut. Females in this population can move freely between male territories because males cannot pursue them into other male territories. Individual females increased their home ranges almost four times during the rut, and visited nearly 88% of the male territories. Male courtship, cheek-patch size, and scent marking behaviors were consistently correlated to female congregation size. This indicates that females were assessing males, not territories (Min, 1997).

Multiple Paternity

In all well-studied populations, females give birth to twins every year. This makes it an ideal species to examine multiple paternity through genetic testing. In a particular study performed by Carling et al. (2003), different males sired each fawn in a set of twins 44% of the time. These findings are contradictory to behavioral observations in which very few females were seen mating more than once. Forced copulations are not a likely reason for multiple paternity, because behavioral observations show females don't normally mate more than once per estrus cycle and in order for a male to mate, he must have complete cooperation from the female. However, sperm competition between males is important. Benefits of multiple paternity include fertility insurance, material benefits, genetic diversity, genetic quality, and infanticide avoidance. Only increasing genetic diversity and quality relate to pronghorns. When a female's investment in offspring is greater than the male's, they should select mates that display 'good genes'. A female should mate more than once if she encounters a genetically superior male after she has already mated (Carling et al., 2003). Sometimes females choose harems based on location, possibly to avoid the costs of mate selection which can be very high (Byers et al., 2005). Evidence suggests that females switch mating strategies from year to year, and possibly during a breeding season. One of the likely reasons for multiple paternity is the increase in genetic diversity of offspring. Fawn mortality can be very high (56%-99% from 1981-1996 at the National Bison Range), so increased genetic diversity between offspring could increase the survival probability of at least one fawn. It is also possible that the female can control fertilization by a form of postcopulatory, or cryptic, choice. Female pronghorn are polyovulators, where adults may ovulate 3-9 eggs per estrus period. Multiple ovulation is a possible way of increasing the chance that each twin will have a different father (Carling et al., 2003).

Image courtesy Brent Huffman: www.ultimateungulate.com

Image courtesy Ron Niebrugge www.wildnatureimages.com

Image courtesy Harvey Doerkson: U.S. Fish and Wildlife Service www.fws.gov