Pronghorn Group Size and Social Organization

We analyzed locations of groups with radio-collared, adult, female pronghorn during 1999–2005 to evaluate changes in the size and composition of groups over time. Radio-collared pronghorn were located 2–22 times per month, and the number and sex of pronghorn in each group (>1 animal) were recorded. We compared average group size and the percentage of groups with males among months. We also calculated correlations between group sizes on successive locations >5 days apart for a given radio-collared pronghorn during both summer (excluding the June parturition period) and winter in 2005. If group sizes changed substantially among successive locations, then females were likely mixing and switching associations with other animals frequently rather than staying with the same group. Lastly, we assessed apparent changes in the breeding system and migration tendencies of Yellowstone pronghorn since the early 1990s. Groups of pronghorn were relatively small (mean = 4.8) in May and June, when does scattered to isolated sites to give birth and hide their young fawns. Does coalesced into significantly larger nursery groups (mean = 10.0) during July–September after their fawns were 3–6 weeks old and capable of rapid, sustained running. Group sizes remained relatively large (mean = 11.9) through the winter during November–April. Mixed groups of females and males were observed during all seasons, but males comprised a relatively low percentage of groups during May and June (mean = 8.9%), when does dispersed to give birth and bucks became solitary or formed small bachelor groups. Males comprised a higher percentage of groups during July–September (mean = 10.6%), when mature bucks began defending groups of does from other bucks as the rut approached and breeding took place. We did not observe males defending territories. A higher percentage of groups consisted of males during October–March (mean = 21.1%), when all pronghorn congregated on the winter range. Similar seasonal trends were observed for the percentage of groups containing males. There was fluidity in group cohesion, with female associations changing within and among days. The behavioral plasticity of pronghorn makes them resilient to changes in environmental conditions, but often makes it difficult to predict the consequences of conservation actions to control disease, protect or restore key habitat, regulate harvests, and limit adverse effects of development and recreation.