Tiger Genetics

The Siberian tiger studbook database is maintained in a computer software program called SPARKS (Single Population Analysis Records Keeping System) produced by ISIS (International Species Information System). SPARKS automatically calculates inbreeding coefficients, which indicate how related an individual animal's parents were (technically, it is the probability that an animal will become homozygous at a given locus by descent). You can find a more complete description as well as the method for calculation in any genetics textbook. Basically, SSPs strive to avoid inbreeding, which may lead to reduced fertility, increased mortality, loss of genetic diversity, and therefore reduced options to respond to environmental variation. I have enclosed a list of the inbreeding coefficients for the Siberian tigers currently in the AZA Tiger SSP. These coefficients do not change throughout the life of the animal.

Another concern in selecting the breeding animals in a managed population is to equalize the representation of each "genetic line" in the population. An animal is referred to as a "founder" if it represents a new genetic line to the population (i.e., has no relatives in the population). Founders can be wild-caught animals, or can come from another regional captive program, such as the EEP in Europe. In an ideal situation, each founder will be equally represented; the opposite of this would be to breed only one pair of founders and never breed any other animals in the population. The challenge of the SSP Management Group is to make breeding recommendations that avoid inbreeding and also equalize founder representation to minimize the loss of genetic diversity from the captive population. This is done with the assistance of genetic analyses conducted by a software program called GENES associated with SPARKS.

Initially the AZA SSP Masterplan process and associated breeding recommendations were based upon inbreeding coefficients and the founder importance coefficient (FIC) which served to equalize relative founder contributions as well as minimize inbreeding. More recently, management has been based on mean kinship value (MK). Mean kinship is an average measure of how related an animal is to all other individuals in the population. Not only does MK allow the equalization of relative founder contributions, but MK also allows the linkage of rare and common genomes to be avoided, making future equalization of founder contributions possible. Animals with a low MK share fewer genes with the rest of the population and are therefore more valuable as potential breeders than those animals with a high MK (e.g., a founder that has never bred has a mean kinship of 0 and therefore is genetically highly valuable). Care should be taken not to breed an animal with a low MK to an animal with a high MK; otherwise, rare genes would be permanently linked with common genes and could not be increased in frequency without simultaneously increasing common lineages.

Another value which is also used in management is the kinship value (KV), which is a weighted mean kinship which considers the reproductive potential of the animal's relatives in the population. For example, if animal A and animal B had the same mean kinship, and animal A had lots of relatives that were young and could produce lots of offspring, while animal B only had old relatives that were unlikely to produce any more offspring, then animal B would be considered more valuable to breed.

When the Tiger SSP Management Group considers animals to pair for breeding recommendations, it first turns to the list of animals ranked by either mean kinship or kinship value. Animals at the top of the list are given high priority for breeding; animals near the bottom are unlikely to receive a breeding recommendation. Another major consideration is the location of each animal. The Management Group must consider which institutions have requested a breeding recommendation (there is a waiting list), which institutions want to keep their animals, and which institutions want to transfer an animal in or out of their institution. Distance from each other is also a consideration (it is better to transfer an animal from Chicago to Detroit rather than Chicago to Phoenix, both in terms of distance and climatic adjustment). It is best to minimize transfers when possible, as they are costly and involve some stress and risk for the animal.

Each potential breeding recommendation must also be separately checked for the inbreeding coefficient of any resulting offspring. For instance, the top ranked male and the top ranked female may look like a good breeding pair; however, they may be brother and sister and would produce cubs with a high inbreeding coefficient.

Also, every time one animal is born, dies, or is transferred in or out of the SSP population, the mean kinship changes for every animal in the population. For example, a brother and sister would have the same mean kinship value. If the female produces a large litter of cubs, this will lower their mean kinship (making them less genetically valuable). If the litter is made up of all females, the mother will drop more in rank relative to the other females and the male will drop less in rank relative to the other males. This is why potential breedings that look good one year may not look so good one or two years later as certain animals die and many litters are born.

Back to the Captive Management Introduction