The goals of this work on pronghorn, a native North American hoofed animal, are to investigate the effects of a population bottleneck on breeding behavior. The study will complete the construction of a complete population pedigree, measure the effects of inbreeding on reproduction, and see whether females, in their mate choice decisions, are able to avoid inbreeding. In doing so, females may be able to balance inbreeding avoidance with mate choice for genetically superior males. It is possible to achieve these goals because of long-term records, as well as a recent and rare population event, for the pronghorn population at the National Bison Range in northwestern Montana.
Results from this work will be of direct use to wildlife managers, who set hunting dates, harvest levels, and target population sizes for pronghorn in 14 western states. The project will make available information on genetic markers for this species. The project will train 2 female Ph.D. students, and will involve at least 16 undergraduates, half of whom will be from under-represented groups, in field and laboratory aspects of the research. Collaborative work with the United States Fish & Wildlife Service will result in updated estimates of health trends in this important pronghorn population.
The purpose of this project was to track the fate of a closed population of pronghorn (Antilocpra americana) in the years following a population bottleneck. A bottleneck occurs when natural events cause a population’s size to decline significantly and suddenly. In this case the pronghorn population at the National Bison Range, a closed, 30 square-mile wildlife refuge in Northwestern Montana, suffered severe over-winter mortality in 2004, following the unusually hot and dry summer of 2003. About 30% of adult females, and 90% of adult males died. This natural experiment gave us the opportunity to study inbreeding in the wild. Because this was a continuation of a long-term study, in which all individuals were marked and characterized genetically, we were able to construct a complete, 6-generation population pedigree. The pedigree allowed us to calculate the exact degree of inbreeding of each pronghorn fawn born during the study period. Armed with this information, we showed, first, that there was significant inbreeding depression. This is, the more inbred a fawn was, the lower its overall condition at birth, and the lower its chances of survival to weaning at age 12 weeks. Second, we showed that pronghorn females are able, most of the time, to avoid inbreeding. In the two weeks before estrus, pronghorn females engage in a process of mate sampling, in which they visit many males and make repeated visits to some. Females have complete control over their mating decisions, and they usually mate once. We showed that the overall level of inbreeding among fawns born during the study was lower than would be expected if females mated at random. Thus, females somehow were able to avoid mating with close relatives most of the time. Finally, we observed females during the mate sampling process, to discover the behavioral mechanisms by which inbreeding avoidance is achieved. Females did not avoid visiting closely related males and they permitted these males to court them. However, courtship between a female and a related male progressed more slowly, especially in the late stages of courtship that immediately preceded copulation. Thus, females are less receptive to courtship by related males, and they apparently require close contact with the male to make this behavioral adjustment. The nature of the information that females use to discriminate between relatives and non-relatives remains unknown. Our discoveries add to a growing list of species in which deleterious effects of inbreeding have been shown in nature. Our results will be important to wildlife managers, who are charged with maintaining the stability of natural populations. Specifically, following population decline, it would be wise to introduce new individuals from other populations, to minimize inbreeding effects. Also, it is clear that the unfettered ability of females to choose mates is important. Hunting seasons should be adjusted so that hunts do not coincide with the pronghorn mating season.
