Population Management: How to Recover an Endangered Species

In order for the studbook keeper and the captive community that house Mexican gray wolves to be successful with their charge of preserving an endangered species, they need a plan.  This plan is referred to as the Master Plan within the SSP community and the zoo world.  The Master Plan is meant to set the goals for survival of the species by preserving genetic diversity and determining the best method for succeeding in achieving that goal. 

The overall goal set by the 1994 Mexican Gray Wolf SSP Master Plan is to preserve 75% of the gene diversity in captivity for 50 years. The master plan projects a need for 240 wolves in captivity to achieve the genetic goals. The calculated 2003 gene diversity of the captive population was 81.13% (the amount of genetic variability or heterozygosity within the population) and the founder genome equivalents were 2.65. The founder genome/population is a small number of individuals, originating from a large population, that form the basis of a new, independent (isolated) population of that species. These are slight increases over the previous year. Additionally, the mean inbreeding coefficient (0.1764) is a slight increase from the previous year. Inbreeding occurs when two related individuals breed with each other ("consanguineous breeding"). Inbreeding results in an increase of homozygosity "by descent". The probability of becoming homozygous by descent is called the inbreeding coefficient. The closer the common ancestors are to the parents in the pedigree, the higher the inbreeding coefficient, and the higher the proportion of loci at which the offspring will become homozygous by descent. These improvements in the genetic picture of the captive population are primarily the result of careful merging of the three Mexican wolf lineages over the past eight years.

In order to maintain gene diversity, the Master Plan projects a need for a captive carrying capacity of at least 240 wolves. However, the lack of sufficient captive space continues to be of concern for the program, especially considering the carrying capacity does not include wolves surplused for the reestablishment effort. As a means of creating more captive space, the SSP© and its counterpart in Mexico (Mexican Wolf Technical Advisory Committee) continue to aggressively promote participation by North American Zoos and their like in the captive breeding program. Another way to maintain or even increase the gene diversity is by increasing the number of founders. However, finding new founders seems unlikely: except for the reintroduced population, the presence of wolves in the wild has not been confirmed for more than fifteen years. Although 81% of the genetic diversity of the population has been retained, it can be increased by managing the captive population better. One management practice is to increase the generation time, the longer the generation time, the smaller the loss of genetic diversity. Semen collection and cryopreservation is one way to increase generation time, assuming artificial insemination techniques are perfected.

Once a year, the studbook keeper, the genetic advisor, management committee and the individual facilities all have some level of input into how to best achieve the highest level of genetic diversity and work together towards a common goal of survivorship of the species.  The primary management strategies that are used in the Mexican Gray Wolf SSP are as follows:

1. The first priority is to breed individuals of the lowest Mean Kinship (MK) which are underrepresented and, therefore, possess the rarest alleles in the population.
2. Among individuals with low MK, the second priority is to breed those individuals whose alleles may be lost soon. Priorities should be determined by the manager's knowledge of an individual's age, health, and/or reproductive condition. In the absence of other information low Kinship Value (KV), printed on the Master Plan report, can be used.

In order to be successful with achieving the highest level of genetic diversity and to determine which animals will be bred during the year, there are five criteria that are considered for the captive community (in order of priority):

• Mean Kinship Value: Mate individuals with roughly similar MK to avoid combining rare and common alleles in offspring that reduces long term gene diversity.
• Inbreeding Coefficient: Mate individuals whose offspring will have low inbreeding         coefficients (F), for the best probability of viable, healthy offspring.
• Biology of Individual: Maximize mating success based on the species' biology, including suitable age of individuals, mate choice, social structure, reproductive history, etc.
• General Logistics: Minimize logistical difficulties of moves (e.g. distance, cost, quarantine).
• Politics: Maximize inter-institutional harmony and minimize political conflicts.

It has also been recommended that special attention be given to the behavioral characteristic of the individual given how much this criterion weighs on the success of the release candidate. Wolves that are potential candidates for release to the wild are evaluated based on a number of behavioral and physiological criteria including genetic makeup, age, reproductive performance, proven parental skills and appropriate social behavior, and aversion to humans.

The heavy task of preserving an endangered species is one that cannot be taken lightly.  In the zoo world, the survival of the Mexican Gray Wolf not only relies on their existence in captivity but the group also has the task of returning them to select areas in their historic range-a task that the SSP takes very seriously.