When Five Hundred Tigers Are Not Enough

Cheetah.

In my sophomore year at Iowa State University, I sat enthralled in a dark lecture hall as my animal genetics professor described studies done on wild populations of cheetahs that had found that the animals were so genetically similar that they could almost be called clones. Past bottlenecks in the cheetah population meant that and even though population numbers had rebounded, the individuals were so genetically similar that the effective breeding population was much smaller. I was hooked. A week later I changed my major from “Pre-vet” to Genetics.

Two years after that career altering lecture, I had the incredible opportunity to spend the summer working at the Henry Doorly Zoo in Omaha Nebraska. It was 1991; a year after the world’s first test-tube tigers has been born at the zoo. While my job was mostly dealing with the public and shoveling elephant poop, I still felt like I had been given the keys to the kingdom. I got to meet some of the people who were integral to the success of those first test-tube tigers. I even got to hold some tiger cubs, although these were of the non-test-tube variety. 

Amur Tiger.

So it is not surprising that a recent article on the BBC website caught my eye. The article highlighted a newly published study focused on the genetic structure of Amur tiger populations (or Siberian tigers as they are commonly known). The authors looked at the genetic profiles of ~95 individuals from throughout the Amur tiger’s range. They found that although there are up to 500 of these beautiful cats still surviving in the wild, the genetic diversity in that population puts the effective population size closer to 35 individuals. Among other things, this leaves the population highly susceptible to stochastic events (1).

One of the things that struck me about the study was that large number of individuals sampled. There was a time when scientist had to locate an animal, tranquilize it and collect a sample to get DNA for studies of this type. Tracking down tigers would certainly be exciting, but not very efficient if the goal is to sample as large a number as possible. By contrast, the authors of this study isolated nuclear DNA from scat samples. Collecting tiger poop may not be as exciting as collecting samples from live individuals, but to me it was indicative of how different branches of science interact. The techniques used to isolate nuclear DNA from tiger scat were undoubtedly developed for a different reason. However, these techniques have given population geneticists a powerful tool to study larger sample sizes, and thereby get a better picture of the true genetic health of a population.

Although the results of this study are disheartening for those of us who have followed the course of endangered species over the years, there was also a bit of good news. Captive breeding programs like the one at the Henry Doorly Zoo have done their job, and the captive tiger population has maintained genetic variants that the wild population has lost. Carefully management may have kept hope for the species alive, but their future is far from a sure thing.

Are you interested in population genetics and captive breeding programs for endangered animals?  Here are a couple of my favorite places to find more information:

Henry Doorly Zoo – Conservation
Smithsonian National Zoological Park- Conservation and Science
San Diego Zoo’s Institute for Conservation Research

Do you have a site to recommend? Send me the link!

Reference

1. Henry P, Miquelle D, Sugimoto T, McCullough DR, Caccone A, & Russello MA (2009). In situ population structure and ex situ representation of the endangered Amur tiger. Molecular ecology, 18 (15), 3173-84 PMID: 19555412