and advance testing of source and target herds. Below is an outline of some factors to consider in selecting animals for translocation for genetic management.
Genetic Factors. Because the goal of translocation is to supplement the genetic diversity in a herd and reduce the probability of inbreeding, it is advisable to select animals that are unrelated to the target herd. In most cases, pedigree information on free-ranging horse and burro populations will not be available, so absolute genetic relationships among individual animals will be unknown. The use of genetic information, however, will make it possible to choose individual animals that have moderate levels of differentiation from the target population.
The term outbreeding depression is used to describe a decrease in fitness due to hybridization between individuals from populations that have differentially adapted genomes (Frankham et al., 2011). Frankham et al. (2011) used empirical data and modeling to develop a decision tree for predicting the probability of outbreeding depression. Their tree proved robust when crosses that had known outcomes were used, and it suggested that outbreeding depression is likely when the populations being crossed are of different species, exhibit fixed chromosomal variants, have not exchanged genes in 500 years, or inhabit different environments. None of those risk factors seems to apply to free-ranging horses and burros in HMAs. Environments may differ between HMAs, but Frankham et al. (2011) suggested that environmental differences need to be substantial enough to select for different traits among populations. They recommended paying particular attention to the needs and resources to which a species is most sensitive and to the range of variation in important features of the environments under consideration. The adaptability of the horse and its associated ability to live in various environments appears to lessen the concern about environmental differences between possible translocation sites.
By using the genetic data generated for the evaluation of level of genetic diversity, it is possible to estimate the level of differentiation among HMAs. The fixation index (Fst) is a measure of genetic distance, or population differentiation, that is based on genetic polymorphisms (Wright, 1931). Polymorphic microsatellite loci constitute a powerful tool for predicting which populations are so similar (low Fst value) that translocating animals will probably not be successful in supplementing genetic diversity and which are so different (high Fst value) that genetic compatibility between individuals may not be optimal and may reduce the probability of successful translocation. Matrices of pair-wise Fst values for horses and burros based on genetic data from BLM herds are in Appendix F and could be used to identify the mixtures that might be most successful because they exhibit moderate Fst values.
New genetic variation needed by an HMA does not necessarily need to come from another HMA. Mares in long-term holding facilities could also be used as sources of genetic diversity if necessary, assuming that they present no novel disease risk for free-ranging horses and burros. The genetic tools described above can be used to identify free-ranging horses and burros on other public lands, in private sanctuaries, and in long-term holding facilities that could be used to infuse new genetic variation into an HMA.
Behavior and Social Factors. Given the harem social structure of free-ranging horses and the fact that this structure means that more sexually mature females than sexually mature males are breeding at any one time, it appears that the most rapid way to infuse new genetic material into an HMA via translocation would be to move young, sexually mature mares between HMAs. Young mares new to an HMA are likely to be courted by bachelor males and to be open to forming consortships with them. Older mares would also probably be bred relatively quickly, but they may be more selective in forming consortships with