Gene Pool Management (Organization of the entirety of genes contributing to one population)

• Two to several year breeding stop by transferring stallions from their harem group to stallion groups
  
• Sterilization or castration of breeding stallions provided that their genes are sufficiently presented in the population
  
• Birth regulation by using contraceptives in mares which gave birth to four or more offspring. This guarantees that 90% genetic variability of parent generations is preserved in the following generations. Breeding of mares which have not reproduced yet and are of valuable contribution to the gene pool
  
• Establishment of mare herds in order to avoid early reproduction and increase generation succession
  
• Establishment of stallion herds in order to monitor development of individuals and in order to choose more breeding stallions for shorter periods of time
  
• Rotation of breeding stop and breeding recommendations for retention of genetic diversity

Population growth in Przewalski's horses		Age distribution

Breeding Programmes: Przewalski’s Horse

Historic background

The current population of Przewalski’s horses descends mainly from their wild ancestors which once inhabited the Dzungarian Gobi. Nomads reported that Przewalski’s stallions sometimes stole mares from domestic livestock. Livestock traditionally grazed without attendance in this region. It is uncertain if hybrids were capable of surviving in the wild, but there is evidence (mtDNA, mitochondrial DNA) that only mares were able to do so. Male hybrids were possibly inferior to their wild conspecifics (Seal et al. 1990).

Only 12 of 56 individuals brought to western zoo collections between 1899 and 1902 contributed to today’s population. The phenotype of mare #18 Bijsk 8 which is the ancestor of every living Przewalski’s horse reveals domestic horse influence (Dolan 1982). Through the years cross-breeding of one domestic mare in Halle (Germany) and Askania Nova (Ukraine) respectively occurred (Bowling et al. 2003). Thirty-one Przewalski’s horses existed after World War II, but only 11 individuals were reproductive (Zimmermann 1997). In 1947 mare # 231 Orlitza III, as the last Przewalski’s horse caught in the wild, entered the captive population (ex situ population).

These genetic drifts and both dramatic decreases in the captive population (bottle-necks) have resulted in the loss of genetic diversity represented by the original founders. In addition there had also been an artificial selection for a certain phenotype which did not follow the diversity of the preserved specimens and hides of wild individuals exhibited in museums. This procedure biased the genetic contribution of each founder.

Under these circumstances inbreeding had been inevitable. Studies on the effects of inbreeding depression revealed a correlation between the degree of inbreeding and juvenile mortality, decreased life span and sterility in stallions, respectively.

The effects of outbreeding depression on the Przewalski’s horse population (breeding of unrelated stocks or individuals, e.g. #231 Orlitza III) were also analyzed in order to reveal the benefits of bringing new founders into an inbred population (Ballou 1994). In 1959 Erna Mohr published The International Studbook for the Przewalski’s Horse which contained entries for all 238 individuals kept between 1899 and 1958. Subsequently, annual updates followed by Prague Zoo. Yet it is not possible to manage animal populations following genetic and demographic aspects simply by means of studbooks. Therefore global breeding programmes for endangered animal species were launched. In 1979 the North American Species Survival Plan (SSP) and in 1986 the European Endangered Species Programme (EEP) were established for the Przewalski’s horse. A corresponding programme was initiated in 1990 by the Australasian Regional Association of Zoological Parks and Aquaria (ARAZPA). These three programmes cooperate for many years and unite 63 institutions to a global breeding programme. Together they manage a population of about 600 Przewalski’s horses considering scientific viewpoints. Cologne Zoo (Germany), Front Royal (Smithsonian Institution, USA) and Western Plains Zoo (Australia) are mutually responsible for coordination and they decide on the exchange of individuals and select reintroduction projects.

Playing foal

Since 1990 Przewalski’s horses are kept in semi-reserves which are increasingly established in nature preserves. In these preserves horses are often associated with cattle in order to keep areas open and prevent bush encroachment. Przewalski’s horses living under semi-natural conditions are important subjects to research, since the gained findings are valuable for the difficult reintroduction into their original habitat.

Ex situ populations (zoological institutions and semi reserves) serve as metapopulations for the exchange of individuals. Management of these populations is supposed to aim at

• maintaining 79% of heterozygozity
  
• avoiding loss of alleles of underrepresented individuals
  
• maintaining the present genetic diversity
  
• careful selection referring to phenotype
  

In situ (wild) populations are established and maintained in the original habitat of Przewalski’s horses. Criteria for the choice of area considered for reintroduction are dependant on habitat structure and the quality of cooperation with local and government authorities. Individuals thought for reintroduction are chosen referring to genetic criteria and phenotype. Last but not least, the success of reintroduction programs is based on long term financial support.