Why "vulnerable breeds" are vulnerable

By Carol Beuchat PhD

The UK Kennel Club has updated their table of "vulnerable" and "at watch" breeds using the registration numbers for 2016. 

Why are these breeds considered to be at risk of extinction? I'll show you using a bit of population genetics.

The graph below shows how the size of a population affects the rate of inbreeding. Let's say we have a population of 50 dogs, 50:50 male and female. If we start with 50 unrelated dogs at generation zero on the graph and let them breed randomly, the average level of inbreeding in the population will increase over the generations. By 10 generations, the average inbreeding coefficient is about 10%, and by 30 generations it is about 25%. The effects of inbreeding depression (low fertility, high puppy mortality, low disease resistance, shorter lifespan, etc) begin to be significant above about 10%. For perspective, inbreeding of 25% is what you get from crossing full siblings from unrelated parents. 
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In the graph below, I've drawn the lines to show how the average inbreeding level increases per generation for various population sizes from 10 to 500 animals, assuming breeding is random. What you can see is that small populations become inbred much faster than large ones. In the worst case here, a population of 10 unrelated animals will reach an inbreeding level of 0.4 (40%) in only 10 generations, while a population of 500 is only about 5% inbred after 60 generations.

The thing to remember here is that this dependence of inbreeding on population size is not some obscure biological principle; it is simply a matter of math. Breeders can increase the rate of inbreeding by deliberately breeding closely related animals, or they can slow it down a bit by avoiding close breedings, but it will nevertheless increase over time. 

The reason certain breeds are on the UK KC "vulnerable" list is because their numbers are so low that inbreeding will increase rapidly. With this will come increased incidence of genetic disorders caused by recessive mutations, decreased disease resistance, smaller litters, higher puppy mortality, and the many other deficits that are typical of inbreeding depression. Eventually, small populations become more and more inbred until they go extinct.

Have another look at the table at the top. The numbers are for total registrations, but you can estimate the size of the breeding population by diving registrations by some estimate of litter size (e.g., 5) and using that to estimate the number of breeding females.

For the Bloodhound, for example, 53 registrations translates to about 10 litters from 10 bitches. If we have an equal number of males, the total population size will be 20. You can see that the average level of inbreeding will increase at a rate of about 1% per generation (orange symbols and line). So, by 25 generations, the average mating will be the equivalent of a full sibling cross (25%). 

Notice that the breeds on this list have had small numbers for years, so they are already highly inbred. For instance, the average inbreeding coefficient of the bloodhound determined from DNA is already stratospheric at 43% (Dreger et al 2017). No amount of clever breeding will prevent this from continuing to increase. 

Most populations of wild animals don't survive with inbreeding levels above 25-30%. They are overtaken by small litter size, high puppy mortality, genetic disease, and physical defects. Modern veterinary care and our willingness to pay for it is sustaining many breeds with inbreeding levels even higher than this, but how much longer can they last?

If these breeds are going to survive, they need much larger population sizes, and the already breathtaking levels of inbreeding must be reduced to improve health. Otherwise, they will eventually go extinct.

Will this happen? ​Time will tell.

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