Captive Breeding Criteria

You said: Select breeding pairs based on their genes.

That's just what zoos do! They want to ensure that captive tigers remain strong, healthy animals for generations to come. They don't mate whichever tigers are convenient. That would create tigers with mixed or unknown lineages. Knowing the genetic history of all the animals in the breeding program helps prevent mistakes (like breeding animals that are closely related) that would be disastrous to the long-term survival of the gene pool, and tigers in general.

So how do zoos decide which animals to breed together? The mission of the Tiger Survival Species Plan (SSP) is to increase the genetic diversity of tigers in captivity. It defines this genetic diversity in two ways:

Gene Diversity compares gene diversity retained in the captive population with the original amount of gene diversity in the founders. (Founders are tigers that were born in the wild and later captured and brought to a zoo. They have no known relatives among captive tigers.) The goal of the Tiger SSP is to retain 90% of captive tiger gene diversity over the next 100 years. Currently it ranges from 92% for Sumatran tigers to 96% for Siberian tigers. Without new genes from wild tigers, gene diversity will slowly decline.

Founder Genome Equivalents indicates how many founders (unrelated wild tigers), if perfectly bred in captivity, would have been required for the current level of gene diversity. For example, the Siberian SSP has 45 founders, so if these animals were bred perfectly the Founder Genome Equivalent would be 45. The actual Founder Genome Equivalent of the Siberian SSP is 12.6 because the captive tigers have not been bred perfectly.

A number of factors, including fertility, the age of the tiger and its location, are considered in addition to the founder genome equivalent when making breeding recommendations. The "best" choice for a breeding recommendation may not always be the one that is statistacally perfect.

To equalize the representation of each founder to the captive gene pool, the SSP management group bases its breeding recommendations on two genetic measures: Mean Kinship Value and Genetic Diversity.

Mean Kinship Value (MK) is an average measure of how related an animal is to all other animals in the population. If a tiger has many brothers and sisters, aunts and uncles, and nieces and nephews, then the Mean Kinship Value is very high. If a tiger has few relatives, the Mean Kinship Value is very low. The lower the MK value, the fewer relatives it has among captive tigers. That makes it a rare animal with valuable genes. A tiger such as Ruslan (below) would have a high MK, because it has so many relatives (mostly descendents) in zoos.

Family tree for "Ruslan," Studbook (SD) Number 1306

First Generation (Ruslan's offspring): 16 cubs--8 male, 8 female
Second generation: 55 cubs--17 male, 28 female, 10 unknown*
Third generation: 15 cubs--5 male, 8 female, 2 unknown*

*"Unknown" means cubs died before their sex was determined

If a tiger has a Mean Kinship Value of 0.0, it is a "founder"--it has no relatives in that population. In that case, it represents "new blood"--and more importantly for long-term conservation efforts, "new genes." Tigers with a low MK should not be bred with tigers with a high MK; otherwise, its rare genes would forever be linked with common genes. By breeding tigers with low MKs (and few relatives) together, zoos try to make sure that each founder is equally represented in the gene pool.

Inbreeding Coefficient reveals how closely related the tiger's parents were. The higher the coefficient, the more closely related the two mates are. Parent-offspring or brother-sister pairs have a coefficient of 0.25. In comparison, first cousins among humans would have a coefficient of 0.0625. Inbreeding is best avoided, since it can cause reduced fertility, increased mortality and birth defects. Also, a loss of genetic diversity reduces the ability of tigers to adapt to environmental change.

Other Considerations. After the genetic analysis, the SSP must also consider several other aspects of the potential mating.

Location: Moving a tiger from one zoo to another is very stressful for the tiger and may affect its ability to breed. If all else is equal, it's better to select a tiger from a nearby zoo than one from a distant zoo. It's also a good idea to select tiger that live in similar climates. For example, it's better to transfer a tiger from Chicago to Detroit rather than Chicago to Phoenix, for both distance and climatic reasons.

Age: The SSP must look at several implications of a tiger's age. Male tigers are ready to mate by age three or four and remain fertile until fourteen or older. Females become fecund (ready to breed) by age two or three and remain so until eleven or twelve.

However, tiger mortality begins to rise at age twelve. The SSP must balance age and fecundity considerations with an eye on the animals' genetic value. Gene diversity is lost with each generation, so it's best to make generation length as long as possible--that is, to breed the animals when they are older rather than younger. With this strategy, however, the tigers may die before they are bred.

Institutional requests. When possible, the SSP gives priority to zoos highest on the waiting list for breeding recommendations. Many zoos want tiger cubs, since that helps them fulfill their dual mission: to be a zoo ark for endangered species, and to educate and entertain the public. Zoos often wait several years before they receive permission from the SSP to breed.

More about studying and managing tiger genetic diversity.

Back to the Captive Management Introduction