Population Genetics of Feral Horses: Implications of Behavioral Isolation

Population Genetics of Feral Horses: Implications of Behavioral Isolation Michael C. Ashley United States Department of Agriculture, Agriculture Research Journal of Mammalogy 85(4):611-617. 2004. The Garfield Flat Herd Management area in western Nevada has a large herd of feral horses, which separates into two smaller herds for most of the year. During the winter when food and water are scarce the herds combine into one for a period of time, but remain separate during the other seasons. This study tested these two herds to see if they had any genetic differences from each other, as well as another feral herd from the Granite Range Herd Management Area, in Northwestern Nevada, about 215 km (133 miles) away. They sought to assess the variety of genetics, or degree of inbreeding within and between herds, which is a concern because genetic diversity is vital to keep a population healthy. However, they predicted that behavioral tendencies would affect the spread of genetics. Feral herds can be made up of multiple males and females, but usually consist of one dominant male with a group of females. In addition, some weaker males may be in the herd, but have fewer females to mate than the dominant male does. Females typically have one foal per year, usually in the spring or summer. Foals stay in the herd until sexual maturity, at which time they may be isolated, join a different herd, overpower the dominating male or start their own herds. Females were found by Ashley to be most likely to mate with dominant males in other herds rather than bachelors if they did not mate within their own herds. This is how genetic variety occurs between herds. However, if physical surroundings or behavior patterns prevent the animals from exchanging genetics the species can be endangered. The Garfield Flat Herd Management area, (HMA), is isolated from other outside herds because of the surrounding landscape. The two herds at this location will be referred to as Garfield Flat and Garfield Hills, and at the time of the study included 192 horses while the Granite Range had 473 horses. The horses were captured, blood samples were taken and sex, age, and physical description were all recorded. Herd behavioral patterns were observed and recorded at both sites. Once collected, the blood was taken to a lab where DNA was amplified using PCR techniques, which allowed the DNA to be typed to determine parentage. Parentage allele frequencies were determined, showing high variety between the three groups. Behavior correlated with this finding, since the time of the year the groups combined was during winter, before foaling. Since most mares are rebred within 2-3 weeks after foaling, they were mostly with their own herds at this time. Although a few alleles were shared between the groups, there were 28 unique to the 3 populations. Although the two Garfield groups had a high amount of genetic difference, there was even more between these two groups and the Granite herd. Behavior does have a big influence on genetic variation. Because the two Garfield herds separated during foaling and breeding season there was a much higher diversity between the two groups, however, the overlap found was caused by the ability to travel great distances during the spring then they otherwise separate. Even when the herds combine, they separate at the end of winter into their original groups. These behaviors limit genetic overlap. During herd bottlenecking, when the breeding stock consists of less than 20 animals, the genetics are narrowed even more. Ashley also found that unequal mating of males can also limit the gene variability in a herd. According to Ashley, if endangered, keeping the groups separate would maintain genetic variety between populations, findings which can preserve wildlife of all species, not just horses. Identifying the role of behavior was also key, and how the herds maintain their genetics naturally. I found this article interesting not only because of the strong behaviorial links between genetics between the herds, but also due to the implications this has on other species. If wildlife management wants to preserve a set of genetics, they can base their practices on this study. Sarah Reeves