Frozen Sperm Helps the Ferret
The global population of black-footed ferrets hit an all time low of just 18 individuals, but there is hope for the long-term future of this species, as frozen sperm has been used to inject some much needed genetic diversity into the population.
The black-footed ferret is a species familiar to students and followers of conservation biology. It is a classic success story, showing how conservation can help to save a critically endangered species with just a few individuals left in the wild, using cutting edge techniques.
The black-footed ferret is the only ferret species native to North America. It once roamed across the western plains, from southern Canada right down to northern Mexico. Sadly, its population plummeted, and for many years was thought to be extinct.
Its life is inextricably linked with its prey, the prairie dog. The ferrets are dependent on them for food, shelter and for raising their young – one ferret may eat over 100 prairie dogs in a year; the dogs make up 90% of their diet. Black-footed ferrets also spend most of their time in prairie dog burrows, where they eat, sleep, give birth and escape from predators and harsh weather. But prairie dogs have lost much of their habitat, to agriculture, livestock and expanding urban development, and fell victim to a plague brought to the USA from China via rats transported on ships in the early 1900s. So when their numbers declined, so did the black-footed ferrets’.
So much so that it was thought that they had declined to the point of extinction, until a small population was discovered in 1981. The 24 individuals that were found were the last left in the wild, and were immediately taken into captivity. Six of them died from either plague or canine distemper, leaving a grand total of just 18 black-footed ferrets in the world.
These last 18 animals were held at a breeding facility in Wyoming, where they produced 7 offspring. From one of these 7 animals, sperm was taken and frozen in liquid nitrogen. Twenty years later, this frozen sperm was used to inseminate female black-footed ferrets, initially producing 8 kits.
Artificial insemination had been used, so far it has produced 139 kits. But this species has been through what is known as a genetic bottleneck, meaning that it had experienced a sharp reduction in the size of its population and making survival more difficult. The genetic health of the population is hard to maintain when get numbers as low as the black-footed ferrets’ did. It is vital to introduce new genes, and the frozen sperm was one way to do that.
One of the main challenges was that the freezing and thawing process causes the quality of the sperm to decline over time. However, the success of the procedure means that it can be applied for many other species, aiding captive breeding efforts because it avoids having to transport the animals themselves between facilities.
The ultimate goal of black-footed ferret conservation is to maintain their genetic health and produce enough of them to release them back into the wild. So far, more than 7,000 kits have been born and more than 2,600 have been released. There are currently around 370 in the wild today and around 280 in breeding facilities. Clearly threats to the ferrets remain – a lack of suitable habitat, diseases and the continued decline of prairie dogs – but slowly but surely, numbers are growing.
Given that all black-footed ferrets in the wild today are descended from those 18 animals that were captured in the 1980s, the use of frozen sperm has been a major boost to their genetic diversity, and keeps hopes alive that North America’s only ferret will be roaming the plains for many years to come.