There are worries about those in captivity, though: > By the time that day comes, however, the wild axolotl may be gone. That worries Gardiner and Sandoval Guz

There are worries about those in captivity, though: > By the time that day comes, however, the wild axolotl may be gone. That worries Gardiner and Sandoval Guzmán because the animals that they study, like many lab animals, are highly inbred. Scientists use an ‘inbreeding coefficient’ to measure how small a gene pool is. Identical twins have a coefficient of 100%; totally unrelated individuals would score close to zero. A score above 12% indicates a population in which individuals are mostly breeding with their first cousins, and is considered a serious concern by ecologists and geneticists. The famously inbred and unhealthy Spanish Habsburg kings of the seventeenth century often had a coefficient somewhere above 20%. The average axolotl inbreeding coefficient is 35%.