Why is inbreeding illegal in America

Too much kinship can be fatal. When siblings, cousins ​​and cousins ​​have offspring, they are usually more susceptible to disease, less fertile and more often malformed. Animals and plants have therefore developed a whole range of strategies to avoid close relatives when choosing a partner. But what happens when only relatives remain as sexual partners because there are only a few specimens of a species left - and these also live in isolated groups in a sprawled landscape? What is the population size of rare species such as wolf, bison, puma or rhinoceros due to a lack of genetic diversity?

Biologists have been arguing about this for decades. The question is of great importance for nature conservation concepts, for example when it comes to the minimum size of protected areas, hunting quotas or the decision to specifically cross subspecies from different areas. But finding an answer proves surprisingly difficult. There is no question that inbreeding has its disadvantages, with a few surprising exceptions. Some animal populations survive despite extreme inbreeding.

Florida panthers recovered with the help of pumas

It is known from laboratory tests and animal breeding that inbreeding leads to more diseases and infertility. It is not without reason that zoos risk public displeasure and kill animals rather than allow close relatives to mate. The consequences are more difficult to study in the wild, but some examples are known from there as well. A clear case is the Florida panther, a subspecies of the puma. In the early 1990s there were only 20 to 25 animals left that suffered from heart problems, deformities and increasing infertility. According to projections, the subspecies would have become extinct within a decade or two. Genetic rescue brought eight female cougars of another subspecies from Texas in 1995, which were released by conservationists in Florida. The hybrids created in this way are no longer pure Florida panthers, but they are more vital and suddenly began to multiply again.

The European bison is probably also weakened by inbreeding and therefore more susceptible to disease. In the meantime he is allowed to wander freely through the Rothaargebirge again, but the population there is only twelve animals.

Things are a little more complicated with the American bighorn sheep. It is true that all populations that consisted of fewer than 50 animals became extinct in the past century. In such cases, biologists speak of an "extinction vortex", which tears a species to its ruin below certain quantities. However, the reason for this does not necessarily have to lie in the genome. It is also possible that small groups will simply be exterminated by chance events and years with harsh environmental conditions.

Chillingham cattle are almost genetically identical and healthy

And finally, biologists know examples that defy textbook knowledge. In England, for example, a herd of wild cattle has been grazing for 300 years. These Chillingham cattle are now almost genetically identical - without becoming sick or sterile. One possible explanation is that through natural selection, all disease-causing gene variants have now disappeared from the animal's genetic makeup. The genome of the cattle has been optimized - insanely, precisely because of inbreeding. This phenomenon known as "purging" is also known from laboratory mice, in which one can breed genetically identical lines through repeated inbreeding after all sick animals have been sorted out.

Now both the Chillingham cattle and the Florida panthers are spectacular isolated cases. But what does it look like beyond such extremes? To what extent can inbreeding weaken subliminally, i.e. before a species is acutely threatened? Joe Hoffman from the University of Bielefeld has just demonstrated, as part of an international team, that seals whose DNA showed signs of increased inbreeding were also more heavily infested with lungworms than animals with more genetic variations. The researchers had taken tissue samples from the animals after they had died on North Sea beaches. Similar studies have already been carried out several times, but the results were often contradicting or not significant. New to the one in the trade journal PNAS published work is that the researchers examined the seal DNA more thoroughly than before using a modern, high-throughput method. "We still know far too little about the effects of inbreeding, and our method can in principle be applied to any species," says Hoffman.

The DNA analysis takes advantage of the fact that every animal has two sets of chromosomes, each from the father and mother. Corresponding passages usually differ by tiny deviations, so that errors and weaknesses in one variant can be compensated for by those of the other parent. However, if close relatives mate, identical sequences are found in many places. Biologists refer to this as homozygosity, and this seems to have a negative effect on survivability.

Environmental conditions and genes are crucial

Hoffman's work could prove that inbreeding in the wild does more damage than is known, but the research is far from being able to prove that. Carsten Nowak heads the nature conservation genetics department at the Senckenberg Research Institute in Frankfurt and considers the problem to be overrated. He suspects that inbreeding is not the critical problem in most wild animal populations - as long as the animals still have sufficient suitable habitat available.

As an example of the resilience of decimated species, Nowak cites the beaver, hunted by humans for millennia, merged into eight isolated groups throughout Eurasia towards the end of the 19th century. Compared to earlier times, the genetic makeup of these groups is very uniform today. This was recently shown by a team led by Michael Hofreiter from the University of Potsdam based on analyzes of fossil beaver DNA. But despite the genetic impoverishment, the rodents are now thriving again. "The beaver has no enemies, and it also benefits from the mild winters and from the fact that there are fewer floods today than there used to be," says Nowak.

Whether inbreeding becomes a problem depends not only on the environmental conditions but also on the genes of the founding animals of a population. Sometimes it only takes a single vital newcomer to breathe new life into a dying population. In Scandinavia all wolves descend from only three ancestors who immigrated there after the predators in Sweden and Norway were already extinct. It all started with a couple in the early 1980s who founded a single pack. In 1991, out of nowhere, a single male wolf appeared, who had covered the almost 1000 kilometers long way from the Finnish-Russian wolf areas. Without special protective measures on the part of humans, the offspring of the male started new packs from then on. Each of the wolves living today is descended from the lonely wanderer. But there are first signs that the number of litters is decreasing and that slight abnormalities such as misaligned teeth appear.

But despite the extreme inbreeding, the Scandinavian wolves reproduce so much that they are a nuisance to many Swedes and the animals are shot down illegally. A few years ago the Swedish Parliament decided that the wolf population could be limited to 210 animals through legal hunting. However, this decision is highly controversial - also because no one can say how many specimens are needed so that the animals do not come to the brink of extinction again.