Posts Tagged Inbreeding
I was watching yet another presentation about the possibility of reviving extinct life forms (http://www.ted.com/talks/hendrik_poinar_bring_back_the_woolly_mammoth.html). Now that we appear to have the technology, at least in theory, to extract DNA from long dead animals and place it in currently extant species there seem to be more and more articles about how terrific it would be to bring them back from oblivion. This particular talk concerned reviving the woolly mammoth but I have seen similar ones suggesting restoring the thylacine, or Tasmanian tiger, to life.
Technology has been astonishingly uninspiring when it comes to saving the world’s current long list of endangered species. While there have been a few cases of successful artificial insemination or an embryo from an endangered species being reared to term by a surrogate (http://www.scientificamerican.com/article.cfm?id=cloning-endangered-animals), the majority of effective conservation programs have relied on natural breeding to create enough animals to sustain a population, along with addressing the causes why the species became endangered in the first place. There are too many unknowns when it comes to artificially breeding wild animals: how best to harvest the eggs and collect sperm, how to freeze gametes and embryos, synchronising reproductive cycles, carrying a foreign species to term, providing appropriate milk, etc, etc. All these difficulties, and more, apply to the resurrection of extinct species.
If we are able to successfully impregnate a surrogate that takes the pregnancy to term, then what? We have a single individual being reared by an individual of a different species. If this works what do we then do with our mammoth or thylacine? Apparently appropriate mammoth habitat exists in Siberia. But what would our solitary mammoth do in such a place? We would need to produce at least 50 mammoths (probably more like 500: http://www.eoearth.org/article/Minimum_viable_population_size?topic=58074), to create a self-sustaining population. Given the paucity of genetic material available to play with, these mammoths would be virtual clones with very little genetic diversity. All conservation programs seek to maximise genetic diversity to avoid the problems that now occur in inbred populations e.g. Tasmanian devils and their contagious cancer.
What would be the point of bringing back one species? The mammoth will be no more than a curio, devoid of any real value, unless you bring back its entire ecosystem. This ecosystem would need to include not just all the extinct animals but also all the plants it shared its former existence with.
There is more logic in restoring the thylacine as it only became extinct in the 1930s, not 4000 years ago, but even there the Tasmanian ecosystem has changed in the past 70 years. Would it not make more sense to invest all that time, money and expertise into preventing other species from heading down the same extinction path, instead of wasting it on frivolous projects whose only purpose seems to be to let scientists marvel at their own cleverness?
Dr. F. Bunny
And so we bid a fond farewell to another Spring Racing Carnival. Good riddance, I say. Flat races are at least somewhat less lethal than steeplechases which see an average of six horse deaths for every 1000 that take part (six deaths per 439 horses between 2000 and 2010 for the Grand National (http://en.wikipedia.org/wiki/Grand_National)). Still 1.5 dead horses out of every 1000 that start a flat race in the US (http://en.wikipedia.org/wiki/Horse_racing) is nothing to be sneezed at.
Personally I can’t derive any excitement from watching a bunch of horses running around in a circle. The excitement must come from the betting, I suppose. I also fail to understand how normally sane people come out of the closet at Melbourne Cup time every year (the race that allegedly stops the nation) just to throw money away betting on something they know absolutely nothing about.
However, it is the horse welfare aspect that concerns me the most. Race horses are like elite human athletes and, although human athletes don’t die with the same frequency (0.75 per 100,000 for male athletes and 0.13 for female athletes: http://www.diet-blog.com/07/why_do_healthy_athletes_die_of_heart_attacks.php), they both suffer elite athlete injuries: shin splints, fractures, bone chips, strained tendons and ligaments, arthritis, etc. The trouble is that racehorses are all inbred and have been selected artificially to run faster than is physiologically sustainable (See “Do I Hear Banjos?” for more information on inbreeding). As with most things we have tampered with we are not happy unless we’ve taken things beyond the extreme.
Today’s racehorses are extremely large, 450-500 kg, by equine standards. If you look at wild equids, such as Przewalski horses and zebras, they weigh around 350 kg. Horses run on their toes. That hoof you see at the end of their leg is actually their third toe. All that weight as they come thundering down the straight is borne on four toes. And, because horses are generally raced before they are mature, that equine skeleton has not finished developing, which further predisposes them to injury. Horses, like most athletes, are pushed to the very limit of what they can physically do, so it should come as no surprise that virtually every horse suffers from exercise induced pulmonary haemorrhage following a race, basically bleeding into the lungs.
When you take that artificially selected enormous amount of weight, support it on four tiny limbs and push it further than nature intended it is no wonder that as many horses break down as they do. What does come as a surprise is that they don’t all crumble into a heap of broken muscles and tendons. But, with so much money at stake and people taking such a perverse delight in seeing animals running around in a circle with people on their backs, it seems unlikely to change any time soon.
Dr. F. Bunny
Zoos spend a lot of time trying to find the right match between males and females, not to pair those individuals that like each other, but to maximise genetic diversity in what is often a very small population. This is important to minimise the effects of any deleterious genes on the population as a whole. Many physical characteristics, behavioural traits and diseases are genetically determined. There is a condition in humans called sickle cell anaemia. Affected people have abnormal red blood cells. To develop the disease the person needs to have received a sickle cell gene from both their father and their mother. If the father carries the gene and has children with one of his relatives, chances are that relative will also carry the gene and so any children will be much more likely to receive both genes and develop the disease. If he had children with someone else those children may carry one of the genes but probably not both and will live and be well. Interestingly people with one sickle cell gene have increased resistance to malaria compared with people that have no sickle cell genes, which is probably why the gene has not disappeared from the human population.
Similar problems can occur in zoo animals once the genetic pool is too small. This can happen naturally in the wild if species pass through severe population bottlenecks, as occurred with cheetahs, or live on islands where there is minimal influx of new genes, such as the Chatham Island robin. This reduced genetic diversity is why the Tasmanian devil is in so much trouble with its facial tumour. Because they are all so genetically similar if one devil is susceptible to the tumour then they all are. If individuals were more genetically diverse there would be more chance of encountering resistant individuals.
How perverse then is our desire for purebred dogs, cats, cattle, horses, etc. These animals have been created by doing exactly those things that zoos desperately try to avoid: brother/sister matings, father/daughter matings, etc. because misguided breeders are trying desperately to select for certain specific traits they deem important. The trouble is when one trait is selected other traits tend to fall by the wayside. Dairy cattle are selectively bred to produce huge volumes of milk to maximise profits. What they are not selected for is an ability to deliver calves naturally resulting in more and more veterinary interventions. Belgian Blue cattle are selected for an ability to lay down mountains of muscle, so much so that they cannot give birth naturally any more. Every birth needs a caesarean.
This lunacy has reached its pinnacle when it comes to dog breeds. Every breed of dog has its own specific inherited genetic abnormalities caused by this inbreeding. Bulldogs can’t breathe because they have been selected for pushed in faces. Shar Peis develop horrible skin diseases because they have been bred to have insanely wrinkly skin. A large number of Dalmatians are deaf and develop kidney stones. Many Dobermans suffer from a bleeding disorder called von Willebrand’s disease. Up to 25% of Bedlington terriers develop chronic hepatitis because they cannot metabolise copper. The list goes on to the point where it is almost impossible to buy a dog that does not have some form of genetic defect (For a complete list of inherited dog diseases check out http://www.upei.ca/~cidd/intro.htm). Your best chance of getting something reasonably healthy is to stick as closely as possible to the original wolf model. After all this was developed over centuries of natural selection for hardiness, ease of breeding and no genetic problems. If you are a wolf that can’t breathe, gets a skin condition or bleeds to death if someone bites you then you are not going to last long enough to pass that trait on to your offspring.
Unfortunately all these ridiculous dog shows, like Crufts, just perpetuate this sort of nonsense. It seems ironic that, this time, zoos are actually trying to do the right thing by encouraging outbreeding and maximising genetic diversity. It’s a pity that our domestic animal breeders refuse to do the same thing.
Dr. F. Bunny