Hybrid vigour... fact or fiction?

Illustration: Kevin Brockbank

So which is healthier -  posh pedigree or humble mutt? It’s this question more than any other that makes breeders of the purebred dog hyperventilate. And you can understand why. Surely, after years of honing their beloved breeds, their dogs are vastly superior in terms of health than their crossbreed cousins – especially, God forbid, than those awful designer dogs.

The breeders on the show-focused Champdogs website certainly think they know the answer. They recently had a big whinge about the prices of Labradoodles, Cockapoos and other money-earning crosses and, over 100 posts later, they emerged having convinced themselves that their purebred dogs had to be healthier. Phew!

“There’s no such thing as hybrid vigour in dogs,” was one comment, oft-repeated. “You only get hybrid vigour if you mate two species together and all dog breeds are the same species.”

Well no. While it is true that a lion/tiger cross would produce a hybrid animal, the term hybrid vigour is used more usually in science and agriculture to describe the rude health/better yield you get by breeding animals or plants of different varieties. Indeed, we enjoy the benefits of it every day in our food. Your morning toast? Very likely to be bread made with hybrid wheat. Your steak for supper? It may say Aberdeen Angus on the packet, but it only has to be 60 per cent purebred to be allowed to use the moniker. Same goes for eggs, chickens, corn-on-the cob and many other foodstuffs. The reason farmers use crosses is because the wheat grows stronger, the maize grows sweeter, the poultry thrives better and the cattle grow bigger. Indeed, the hybrid vigour so dismissed by dog breeders is one of the things keeping our farming industry afloat (if barely). That’s not to say that farmers don’t maintain purebred lines too – they do. But it’s often a more expensive business because the yield is less.

In fairness, one informed poster acknowledged this. She wrote: “You can get hybrid vigour when crossing two separate 'types' which have been kept pure in their own gene pools for some time. The theory is that over time, 'inbreeding depression' sets in with any closely-bred/line-bred strain, which is immediately lost when crossed to a different strain.” This is exactly right. But she then added:  “Of course if you cross two breeds that get epilepsy, or two breeds prone to hip dysplasia, or PRA (progressive retinal atrophy) - all you're going to get is epilepsy, HD or PRA. No amount of hybrid vigour can wipe away recessives present on both sides.”

This post goes hand-in-hand with another one that’s repeated with depressing regularity by breeders on many websites – and that is: “If you mate two different breeds the puppies will be cursed with all the problems of both breeds.”

Again, it’s not the case – although there is some truth in this.  Many conditions in dogs are recessive. That means that both parents must carry and pass down the genetic mutation that causes that condition for the pups to be affected. If they’re different breeds, this is much less likely, as confirmed by the Animal Health Trust: “Because of the small gene pool in purebred dogs, inherited diseases resulting from single gene mutations are more likely to occur than in their cross bred cousins,” said the AHT in a recent press release.

Of course if both breeds suffer from the same condition, the pups could be affected. There’s very good evidence, for instance, that Labradoodles  are just as likely to suffer hip dysplasia as their purebred parent breeds.  The breed mean hip score for Labradors is 15 and for Standard Poodles it is 14 while the Labradoodle’s average hip score is 14.  Over 400 have now been tested  via the KC/BVA scheme– more than many Kennel Club breeds, including the Boxer and Dalmatians – and clearly the cross doesn’t bring any benefits in terms of joint disease. This is as you might expect, but it isn’t a given. Both Springer Spaniels and Irish Setters suffer from the eye condition PRA, for instance, but it’s caused by different mutations. The Springer x Setter pups, therefore, will not go blind.

There’s another point here, too: dogs from genetically-different parents are likely to have stronger immune systems. This may not help with avoiding genetic disease, but it most certainly helps in terms of withstanding infections, parasites, viruses, auto-immune diseases and allergies. So here again, crossbreeds tend to have the advantage.

Next up on Champdogs was this old chestnut: “All my pedigree dogs are perfectly healthy but my mongrel has lots of problems, including epilepsy.”  No doubt this is the case – but I could chip in with evidence that all my mutts enjoy rude health whereas my purebreds are more sickly. It doesn’t prove a thing. To know which is more representative you have to compare a lot of crossbreeds with a lot of purebred dogs. Has that ever been done? Champdogs didn’t think so: “The claims about crossbreeds being healthier than pedigrees are based on unproved statistics” wrote one poster and no one chipped in to point out that they were wrong.

And they are wrong. There are several scientific studies which have explored the issue and found either no difference or that crossbreeds are healthier overall [see below].  However, “overall” means comparing all purebreds with all mutts and if you compare all mutts with individual breeds, it’s clear that some of the smaller, hardier breeds are healthier.

The same goes for longevity, too. Overall, mutts live – on average – a little longer. In fact, they’re likely to be at our feet for up to two years longer according to one key study. But this doesn’t mean that mutts outlive every breed – they don’t. That’s because small dogs tend to live longer than large ones and that endures across the purebred/crossbreed divide. Some terriers, miniature poodles and whippets live longer than the average cross.  So breeders of these breeds really could justifiably claim that their dogs are healthier than most mutts. But it’s not a very fair comparison because if you split the crosses into size categories, the crossbreed benefit manifests once again. In other words, small crossbreeds live longer than small pedigree dogs (and the same is true for medium and large dogs too).

Insurance company data also confirms the crossbreed health benefit. Most charge lower premiums for crosses and mixed breeds. This isn’t because they’re “anti-pedigree” – an accusation often levied at anyone who dares sing the health-merits of the average mutt. Nope, it’s bottom-line not opinion that cuts ice with the actuaries. They’ve calculated the risks by looking at their data (ie claim history) and priced their premiums accordingly.

For all these reasons, we felt able to say in Pedigree Dogs Exposed that crossbreeds are – overall – healthier than their purebred cousins. Of course this isn’t to say that every mutt enjoys rude good health – they don’t. I wrote recently about losing a lovely collie cross to lymphoma at the age of two and you don’t have to look very far to find crosses suffering from many of the same conditions that blight our purebred dogs. It’s just that, statistically, they’re less likely to be sick. In fact, as I’ve mentioned before, one recent Italian study found that purebred dogs were twice as likely to suffer from cancer than crossbreeds.

“Ah but,” cry responsible breeders, “studies comparing mongrels and purebreds lump all the purebreds together. If you just compared the dogs we breed – carefully selected and fully health-tested – you’d find that our dogs are healthier.”

What the breeders are claiming here is that the disease statistics are being skewed by all those dodgy purebred dogs being produced by backyard breeders and puppy farms.  This sounds plausible, but is there any evidence that it’s true?

No, there isn’t. Actually, there’s next to no evidence to suggest that it isn’t true, either, so the jury is out on this one.  I’ve only found one survey that did the comparison – of Scottish Terriers in America – and it disproves the breeders’ claim. In 2005, Great Scots Magazine did the biggest-ever health survey of Scotties (785 dogs) and found absolutely no difference in disease-rates or longevity between “well-bred” Scotties and others.

“I expected to find statistically verified greater health in the well-bred group,” says Joseph Harvill, publisher of Great Scots Magazine. “That has been the classic claim of show breeders. But I found no significant difference in health benefit between the two groups and it raises real questions to me about the validity of the show breeders' health claims.” In fact, more recent studies by the Scottish Terrier Club of America's Health Trust Fund looking at cancers  - especially bladder cancer from which Scotties are 30 times more like to suffer than other breeds – support the findings. “There is no safety among show dogs,” insists Harvill.

It’s only one study though and you can’t apply the findings across all breeds. It may well be that, in other breeds, show-bred dogs are indeed healthier than the rest of the population. But the show-breeders can’t simply assume this without data to support the claim.

So there we are.  On the evidence we have to date, pedigree dogs can fairly boast predictability of type, looks, character traits and temperament. But in terms of health and average longevity, the humble mutt rules – with the possible exception of working dogs that have been carefully and sometimes ruthlessly bred for fitness and function – a process that in many ways mimics natural selection. Racing greyhounds, for instance, do not suffer from hip dysplasia.

It is the case that not all mixed breeds are created equal. If we breed crosses in the same way as we breed pedigree dogs (as certainly happens with some designer dogs) it’s asking for trouble.  One reason why randomly-bred dogs enjoy a health benefit is that they get to choose their own mates (a process by which nature ensures that only the fittest get to breed). Further, if the individuals dogs are in poor health (genetically or otherwise) and we then go on and inbreed the offspring, we’re on a hiding to nothing.  In other words, just being a crossbreed or mutt is no guarantee of superior health and if a breeder of Labradoodles, Goldenoodles, Cockapoos, Puggles or any other cute mix tries to claim otherwise, vote with your feet and walk away - particularly if they insist there is no need to do any health-tests because their pups will be the automatic beneficiary of hybrid vigour.  It ain’t true.

References (researched and collated by Dr Hellmuth Wachtel):

• B.N. Bonnett, A. Egenvall, P. Olson, A. Hedhammar, Mortality in Swedish dogs: rates and causes of death in various breeds, The Veterinary Record, 1997. ("Mongrels were consistently in the low risk category")

•  P.D. McGreevy & W.F. Nicholas, Some Practical Solutions to Welfare Problems in Pedigree Dog Breeding, Animal Welfare, 1999. ("Hybrids have a far lower chance of exhibiting the disorders that are common with the parental breeds. Their genetic health will be substantially higher.")

• A. Egenvall, B.N. Bonnett, P. Olson, A. Hedhammar. Gender, age, breed and distribution of morbidity and mortality in insured dogs in Sweden during 1995 and 1996, The Veterinary Record, 2000. ("Mongrel dogs are less prone to many diseases then the average purebred dog.")

•  A. R. Michell, Longevity of British breeds of dog and its relationship with sex, size, cardiovascular variables and disease, Veterinary Record, 1999. ("There was a significant correlation between body weight and longevity. Crossbreeds lived longer than average but several pure breeds lived longer than cross breeds, notably Jack Russell, miniature poodles and whippets”.)

• G.J. Patronek, D.J. Walters, L.T. Glickman, Comparative Longevity of Pet Dogs and Humans: Implications for Gerontology Research, Journal of Gerontology, Biological Sciences, 1997. ("The median age at death was 8.5 years for all mixed breed dogs and 6.7 years for all pure breed dogs For each weight group, the age at death of pure breed dogs was significantly less than for mixed breed dogs.")

• H.F. Proschofsky et al, Mortality of purebred and mixed breed dogs in Denmark, Preventive Veterinary Medicine, 2003. (Higher average longevity of mixed breed dogs. Age at death when split into three age bands: mixed breeds 8,11,13, purebreds 6, 10, 12)

• Marta Vascellar et al. Animal tumour registry of two provinces in northern Italy: incidence of spontaneous tumours in dogs and cats. BMC Veterinary Research 2009. (“In both dogs and cats, purebreds had an almost 2-fold higher incidence of malignant tumours than mixed breeds.”)

• Agneta Egenvall et al. Mortality in over 350,000 Insured Swedish Dogs from 1995–2000; Breed-Specific Age and Survival Patterns and Relative Risk for Causes of Death. Acta Veterinaria Scandinavica, 2005. (No difference overall, but mongrels low-risk for locomotor problems and heart disease.)

This article first appeared in the October 2010 issue of Dogs Today Magazine.