Social Structure

Image courtesy Jack Woody: U.S. Fish and Wildlife Service www.fws.gov

Social Spacing

Pronghorn are a herding species, but group sizes and stability vary between populations and throughout the year with respect to different environmental conditions. Home range sizes have been observed as small as 20 square km, and as large as 1,144 square km. Herds are usually subdivided into bands. Female bands share the same summer range and male bachelor bands form in the time between spring dispersal and the fall rut. Many populations exhibit home ranges which are usually bordered by streams, highways, and fences. While males of some populations establish mating territories, others form harem-type breeding systems where the male defends his females rather than a particular area. A literature review by Maher (1994) showed that out of 23 different populations, males occupied undefended home ranges in 12 of them while they maintained territories in the other 11. These differences in social organization are due to differences in environmental conditions, such as density and productivity (Yoakum and O'Gara, 2000).

Image courtesy Claire Dobert: U.S. Fish and Wildlife Service www.fws.gov

In a well researched pronghorn population at the National Bison Range in Montana, group size and composition shifted a great deal over the course of a year. In the winter, they form large, mixed-sex groups (at times, the whole population was found in the same group). The groups begin to break up in early spring with young males forming bachelor groups, mature males becoming solitary, and females forming smaller groups. These females individually leave their group a few days before birthing in late spring. They remain alone with their newly born fawns for about 2 weeks (the time in which the fawn remains hidden in the grass) and then join groups once again (Byers, 1997).

While some populations exhibit characteristics of a home range spacing system, others defend a territory . A study by Maher (2000) compared male territoriality in two different populations: one in the southern Black Hills of South Dakota at Wind Cave National Park and the other close to Billings, Montana at Bar Diamond Ranch. Males at Wind Cave were more territorial than those at Bar Diamond. The males at Wind Cave spent more time scent marking, vocalizing, and chasing other males than the ones at Bar Diamond. Forbs were more diverse and plentiful at Wind Cave, and had higher levels of nitrogen. Because there was lower population density and a higher abundance and quality of forbs at Wind Cave, food resources were more economically defensible there, thus males at Wind Cave were more territorial. Precipitation correlated strongly with spatial organization, suggesting that plant productivity plays a major role in determining territoriality (Maher, 2000).

Why Form Groups?

Individuals are typically found in social groups, but it is not because they have social bonds. Cooperative or “altruistic” behavior does not occur, while agnostic behavior is common and competition for food is standard. Adult pronghorn currently have no major predators, but they are selfish herding ungulates because of evolutionary pressures that are no longer present. Many years ago, numerous predators capable of preying on pronghorn existed. The physical adaptations of keen eyesight and speed are evidence of this type of selection pressure. The costs of herding (increased agnostic encounters, decreased feeding opportunities) seem to outweigh the benefits in today’s world, and suggest that the tendency for pronghorn to group together is also a result of evolutionary history (Byers, 1997).

Dominance Hierarchies and Agnostic Behavior

Social behaviors among pronghorn are mostly competitive or agnostic in nature. There is relatively no evidence of food sharing, nonoffspring nursing, or allogrooming. Adult females aggressively takeover feeding sites from other females. Male interactions are similar and include frequent sparring. There are two primary types of interactions by which pronghorn assert dominance. The first occurs while both individuals are standing. The dominant individual stares at and approaches the recipient. If the recipient fails to move, the initiator lowers its head, preparing to butt. The encounters result in one of the individuals running away. The other type of encounter is very similar, except that the initiator approaches the recipient as it is lying down. This results in a bedding displacement, and the only benefit is gaining a preferred bedding site. Both of these encounters are only used to express dominance. A feeding displacement, a third type of interaction, is very similar to a standing displacement. However this could have direct benefits to the aggressor if the individual can force the other away from a preferred feeding site (Byers, 1997).

Social dominance is prominent in female pronghorn, possibly because they don't form stable groups and matrilineal and other social bonds do not exist (Dennehy, 2001). Unlike other female ungulates, female pronghorn dominance rank is not dependent on age (Fairbanks, 1994). Females contest social rank beginning when they are one month old. Rank status attained as a fawn persists for a lifetime. One particular study, performed at the National Bison Range in Montana revealed a significant quadratic relationship in which high-ranked and low-ranked females showed higher levels of diaminopimelic acid (DAPA - indicative of quality diets) than middle-ranked females. One obvious conclusion from this study is that the highest ranking females acquire higher quality diets than lower ranking females. Hypotheses for the odd finding that the lowest ranking females have better diets than middle ranking females include: 1) low-ranking females spend more time foraging and less time contesting dominance 2) low-ranking females are often excluded from social groups, and tend to be solitary; this implies that they could acquire better diets due to the lack of competition (Dennehy, 2001).