Evaluation of the AZA Tiger SSP and Its Relation to the Global Siberian Tiger Program

by Kathy Traylor-Holzer, Minnesota Zoo

Historical Overview of the Tiger SSP

The Tiger Species Survival Plan (SSP) was the first cooperative management program developed for captive populations in North America and has served as a model for subsequent management programs for other species coordinated by the Association of Zoos and Aquariums (AZA). The Tiger SSP was initially developed in 1982 by Dr. Ulysses Seal, who served as the Species Coordinator for ten years. Dr. Ronald Tilson took on responsibility for managing the Siberian (Amur) tiger population in 1987, and has served as the Tiger SSP Coordinator since 1992. Today, the AZA Tiger SSP manages three tiger subspecies in North America as recommended by the Conservation Breeding Specialist Group's Tiger Global Animal Survival Plan (GASP): 154 Siberian tigers Panthera tigris altaica (coordinated by Ronald Tilson, Minnesota Zoo); 60 Sumatran tigers P.t. sumatrae (coordinated by Gerald Brady, Potter Park Zoo); and 10 Indochinese tigers P.t. corbetti (coordinated by Edward Maruska, Cincinnati Zoo).

There are currently 91 zoological institutions in North America which are official participating members of the Tiger SSP. These institutions are distributed throughout the United States and southern Canada over an area greater than that from London to Moscow, Gibraltar to St. Petersburg, and across a wide variety of climates. The Canadian tiger population is managed as a subset of the larger SSP population; transfers across the international border are minimized to avoid to lengthy permit process involved. Each SSP institution signs a Memorandum of Participation and appoints a representative to the Tiger SSP. An annual election is held among these representatives to elect three members to the nine-member management committee, each of which serve three-year terms. This management group works with the coordinators and studbook keeper to make breeding and management recommendations for the captive tiger population.

Since 1982 the Siberian tiger SSP population has been managed to produce a gradual decline in the population from about 200 to 150 animals in order to produce cage spaces for the expansion of the Sumatran and Indochinese tiger programs. In addition, no breeding has been recommended for "generic" tigers (those of unknown origin with no designated studbook number), and additional cage space for tigers has been created as these animals gradually die due to old age.

As of October 1995 there are 154 Siberian tigers maintained at SSP institutions, 100 of which comprise the managed population and the remaining 54 being designated as surplus. Historically, 15-35% of the Siberian tiger SSP population have been surplus animals. An individual is declared "surplus" if it is anticipated that it will never breed and contribute genetically to the population in the future. Selection criteria include: low genetic value (the genetic line is overrepresented and therefore the animal has a high mean kinship value); age (the animal is post-reproductive or is aging and is of low genetic value); and sterility or other condition whereby the animal is unable to reproduce. By excluding these individuals from the managed breeding population and from the genetic analysis, it is possible to get a more accurate genetic profile of the managed population. For instance, by including a genetically valuable but post-reproductive female in the genetic analysis, her offspring may not appear as particularly valuable for breeding until after her death. Also, designating an animal as surplus allows an institution more freedom in the management of the animal; for instance, zoos may neuter surplus animals or transfer them to non-SSP institutions without prior approval from the SSP.

The current age structure of the Siberian tiger population suggests a gradually declining population, with surplus animals primarily concentrated in the older age classes. Age-specific fecundity over the past 14 years is an artifact of captive management rather than a biological measure, and indicates that under SSP management conditions, females are most prolific between 5-8 years of age, while males show relatively high fecundity from 5-12 years of age. Mean litter size is 2.45 cubs per litter; most litters contain 2 or 3 cubs, but litters range from 1 to 5 cubs. Neonatal mortality is 34%. For animals surviving to one year of age, mean lifespan is about 13 years. The major causes of mortality in adults based upon necropsy reports are various forms of cancer (especially mammary adenocarcinoma in females) and degenerative joint diseases in older males.

Although the managed population has been down-sized from 200 to 100 tigers over the past 14 years, genetic population measures have improved, suggesting more efficient management. Founder representation for most of the 45 founders is approaching target levels. Recent imports of wild-caught tigers occurred in 1991 and 1993. Each time, a pair of young animals (siblings) was imported after being found orphaned in the wild. The number of founders has increased from 29 to 45 since 1982, with a corresponding increase in the number of founder genome equivalents from 6.961 to 12.664. There has also been a gradual increase in gene diversity retained from 92.8% to 96.1%, with the SSP program goal being the retention of 90% gene diversity over 100 years. Descendant mean kinship, a measure of how related an animal and its descendants are to all other animals in the population, has decreased from 0.0722 to 0.0394, indicating that animals are relatively less related to each other. Finally, the mean inbreeding coefficient has decreased substantially from 0.102 to 0.020, meaning that there have been fewer breedings between close relatives.

Management Processes and Issues

Several processes and lines of communication are used to ensure the effective operation of the Tiger SSP. Annual inventory updates are sent to each institution; these are used not only to verify the current inventory and to request missing information, but also to update any institutional requests on file. These annual updates are meant to serve as back-up confirmation of information continually communicated between zoos and the SSP. Institutions are requested to submit any births, deaths or transfers to the SSP as they occur, so that a current database can be maintained. Copies of all necropsy reports are sent to the SSP as part of an analysis of the causes of mortality in the population. All institutional requests, such as requests for replacement animals, transfers of animals out of the zoo due to lack of sufficient space, and future breeding recommendations, must be made in writing as a form of documentation. Each year after the current inventory has been verified, a new genetic analysis is performed. The SSP management committee then produces a list of breeding recommendations for the next year, a list of recommended transfers to accommodate breedings and institutional needs, and a list of animals that are newly designated as surplus. Modifications to these recommendations are made as needed throughout the rest of the year.

Compliance with SSP recommendations is very good. Institutions rarely breed tigers without a SSP breeding recommendation despite the fact that there is a waiting list and zoos generally wait several years after their request before receiving a breeding recommendation. Institutions do not transfer animals that are part of the managed population to non-SSP institutions, and do not transfer them to other SSP zoos without prior SSP approval. Generally the SSP makes about 15-20 or more transfer recommendations each year, many of which are associated with breeding recommendations. The majority of these transfers are accomplished, but many are delayed due to weather conditions (extreme cold in the north or heat in the south) or because an institution needs to transfer another animal in or out first.

When determining breeding recommendations, the Tiger SSP takes the following factors into consideration in order of importance:

1) Mean Kinship Ranking: An effort is made to breed those animals of reproductive age at or near the top of the mean kinship list, with priority given to those with the lowest mean kinship value (i.e. highest ranked and most genetically valuable). Generally, only animals within the top third of the mean kinship list are given breeding recommendations.

2) Location: Zoos are requested to breed only if they have sufficient facilities and space to have the ability to separate animals if needed. Breeding institutions are required to hold resulting offspring for up to three years before being placed by the SSP. When pairing animals at different facilities, consideration is also given to minimize transfer distance and changes in climate. For example, if genetic factors are roughly equal, it would be preferable to transfer an animal only 90 miles from Milwaukee to Chicago (both cold climates) rather than 1,700 miles from Los Angeles (hot) to Chicago (cold).

3) Institutional Requests: When possible, priority is given to institutions highest on the waiting list for breeding recommendations. Consideration is also given to requests to move out or retain specific individuals at the zoo.

4) Inbreeding Coefficient: When a potential breeding pair has been selected, the inbreeding coefficient of the resulting offspring is determined. An effort is made to minimize inbreeding and avoid inbreeding coefficients greater than 0.100.

Compliance rate is good in that there are very few unwanted litters, and most zoos attempt to transfer and breed animals according to the recommendations; however, not all recommendations result in litters being produced. About 50-70% of the breeding recommendations involve the transfer of one or more animals. As mentioned earlier, this is not always accomplished or may be delayed due to weather conditions (animals often can only be shipped in the spring and fall) or due to other reasons. Breedings are not always successful even if both animals are at the same institution. Various factors may be responsible for this: the pair may not be introduced to each other, they may not be compatible, they may breed with no resulting pregnancy, or one of the animals may be sterile, in poor health, or die before breeding. In the Tiger SSP about 42% of breeding recommendations lead to litters being produced. Often those recommendations that are unsuccessful are carried over to the next year. Over the past six years 60 breeding recommendations have been made, resulting in 62 cubs. Although results are variable from year to year, on average one cub is produced for each breeding recommendation made.

Regional and Global Tiger Populations

Table 1 provides a brief comparison of the current status of the North American SSP and European EEP Siberian tiger populations. A third large captive population of Siberian tigers is held by zoos in Japan but this population is not included in this analysis. The EEP tiger population is larger, with 219 tigers compared with 100 tigers in the SSP managed population. The EEP population is also based upon more founders (53, with a potential of 65). However, the percent of gene diversity retained and the number of founder genome equivalents is essentially the same between the two populations. The average inbreeding coefficient is higher in the EEP but is relatively low for both populations. Overall the SSP is able to manage a smaller population as effectively as the larger EEP population, but has the distinct advantage of few international borders and a common language.

The North American captive population has 17 founders that are not represented in Europe, and Europe has 36 founders not represented in North America. If these two populations are combined and managed as a single population, one might intuitively expect a noticeable genetic benefit. In reality, there is only a relative small increase in founder genome equivalents and percent of gene diversity retained. The addition of new founders has the greatest impact on a population when the total number of founders is low, but a much smaller impact as the number of founders increase. However, the potential of each of the two populations managed individually, as well as the potential when they are managed as one population, is quite high; the percent of gene diversity retained has the potential to be as high as 99.1%. To reach this potential would require "perfect" management based upon genetics without practical or political considerations and therefore is probably not achievable.

The bottom line is that overall both the SSP and EEP Siberian tiger populations are currently in relatively good shape. Complete integration of the two programs is unnecessary and impractical. If, however, animal exchanges are considered between the two regions, there are a few animals in North America that represent founders close to their target representation in North America but are not represented in Europe; likewise, there are well-represented animals in Europe that are unrepresented in North America. These animals are young or are of breeding age and would make good candidates if an exchange was made between the regional programs. Such exchanges would provide the most benefit with the least effort. It is most important, however, that both the SSP and EEP continue to manage their Siberian tigers as effectively as possible to achieve their program goals. By doing so, we can strive to maintain the relatively high quality of the global captive population of Siberian tigers.

Table 1. Comparison of the SSP and EEP managed Siberian tiger populations.

	SSP	EEP	Both	Potential
Total Population Size	154	219
Managed population	100	219
Percent gene diversity	96.1	96.0	96.8	99.1
Number of founders	45	53	70	81
Number of Founder Genome Equivalents	12.658	12.203	15.799	58.608
Number of unique founders	17	36
Descendents mean kinship	0.0398	0.0412	0.0317
Mean inbreeding coefficient	0.020	0.063	0.046

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