Bigger is not always better

Did you know that no common mouse has made it beyond its fourth birthday while Asian elephants can live beyond 80? That’s because large mammals usually live longer than small ones.

Humans (actually all primates) are a bit of an anomaly - our maximum lifespan is longer than expected for our size and more than many bigger animals.  Frenchwoman Jeanne Calment, the oldest known human, managed 122 years and 164 days.

But oddest of all, perhaps, are dogs. And that’s because the rules are completely reversed: smaller ones live longer than big ones and the difference is extremely marked.

In the 2004 Kennel Club breed health survey, there wasn’t a single large or giant breed in the Top 14 of the longest-living breeds. At the other end of the scale, eight of the 11 breeds that lived less than 8 years were either giant or large breeds.

The giant breed with the highest median age-of death was the Newfoundland at 9.67, while the Great Dane could muster only 6.5yrs, the Dogue de Bordeaux only  3.83 yrs and – worst of all with an average lifespan of just  2.33yrs  - was the Neapolitan Mastiff (although this was based on very few survey returns – 90 were sent out and only 9  were returned, almost certainly skewing the data).

The longest living breeds included the Lakeland Terrier (15.46 yrs),  the Toy Poodle (14.63yrs)  and the Tibetan Spaniel (14.42 yrs) – all small breeds.  The oldest dog in the survey – at 22 years old – was a Border Terrier (a breed with a median age of death of 14).

It is not completely linear - quite a few large-ish (if not giant) breeds do muster a respectable lifespan. Border collies, Dalmatians and most spaniels often make it to 12.5yrs. And there are some exceptions in the small-dog category:  the Japanese Chin averaged just 9.25yrs, the Pomeranian 9.67yrs and the Cesky Terrier just 8.42 yrs - half that of many other terrier breeds (and probably reflecting the very high level of inbreeding in the Cesky).

The KC survey had its limitations: it wasn’t random (so may not be representative of the breed as a whole) and some breeds returned very few survey forms. The oldest Leonberger in the KC survey for instance, was 12.67 (from a sample of around 200 dogs), whereas the excellent International Leonberger Database now has data on more than 3000 Leos and the eldest recorded died at 17.

But, overall, the evidence is pretty strong and every other survey has found much the same:  large dogs die younger than small ones. Given the reports of some very big dogs living well into their teens, it’s not that they’re incapable of living longer. It’s that something causes them to die prematurely.

But what? There are two main theories. The first is that the bigger and heavier the dogs are, the greater the strain there is on the skeleton and vital organs such as the heart that have to work that much harder. In other words, bigger breeds simply wear out more quickly.

And if that’s not true of elephants, it’s because elephants have been honed for millennia by natural selection to be the size they are.  In contrast, many dog breeds are only 200 years old or less. They may have evolved so rapidly thanks to human selection that it’s possible some body systems have had a hard time keeping up.  There is some evidence of this in the mismatch between skull and brain size that is thought responsible for the neurological problems in Cavalier King Charles Spaniels.

Elephants also grow much more slowly than dogs. And this ties in with the second theory – which is that giant dog breeds die younger because they grow too fast. Clearly, small breeds mature a bit quicker than large ones but the difference is not that great given the huge difference between a Chihauhua and a Great Dane (the equivalent, incidentally in human terms of some adult humans being just 2ft tall and others 31ft!)

Giant breeds suffer more than their fair share of problems associated with both this rapid growth and from being so darn heavy. These include joint problems such a hip dysplasia and bone cancer. The bone cancer often seen in big dogs is the same type that hits human adolescents and it is strongly associated with the teenage growth-spurt. Many breeders of large and giant dogs are aware of the problem – and try to limit it by feeding a lower-protein food and limiting exercise, but there may be more they should be doing.

In a 2007 paper, Dutch scientist Frietson Galis asked the question: “Do large dogs die young?” (abstract here)The answer was yes – overall. I asked her if she thought we should be breeding smaller dogs. “We think that health could improve by selecting for slower growth, she says. “The problem is not so much the size, it’s that they have selected for the fastest growing individuals.”

Surprisingly, Galis found from one large database (although not another) that smaller individuals within a breed lived very slightly less long than larger ones – the reverse of what one might expect. given what we know about dogs. But I still think we should be selecting for smaller big dogs.

Frietson Galis worked out that Great Danes increase in weight 100-fold in their first year, compared to 60-fold in captive wolves , 20-fold in Poodles and just 3-fold in humans.  Clearly, if Great Danes weren’t so big, they wouldn’t have so much fast-growing to do.

Some breeds still doing the job for which they were bred – such as livestock guardian breeds – do need to be a certain size. But in others there really is no good reason, beyond human whim, to breed bigger and bigger dogs.

The problem is that bigger is often seen as better in the dog world, particularly in the show ring.

I grew up with Great Danes – always big dogs, but the Danes in the ring today are much bigger than the dogs I knew and loved in the Sixties and Seventies. The same goes for many other breeds.  

The Berne Natural History Museum in Switzerland has extensive data on the St Bernard dating back to the 1800s. The UK breed standard for the St. Bernard now specifies a minimum shoulder height of 75cm (30in) for dogs and 70cm (27.5in) for bitches and they weigh 65-85kg (143-187lb). But a typical 19th century dog was approximately 60 cm (20in) high and weighed less than 50 kg (110lb) . It is a huge increase.

“Breeds like the Bullmastiff, the Dogue de Bordeaux, the Neapolitan Mastiff and the Mastiff were never intended to be admired and valued because of their size but for what they could do,” says canine historian David Hancock, who has a particular passion for the Mastiff. “No breed of dog benefits from being too big; that is a human desire for dogs to suit their concept rather than the dog's best interests.”

David is dismayed by the fashion for “good bone” as it’s called.  “Strength, power and endurance do not reside in heavy bone, as any dog-sledder will tell you. To breed dogs with bone heavier than nature intended is asking for trouble, as the statistics on hip and elbow dysplasia, cervical vertebral malformation and osteochondrosis sadly demonstrate.”

David recalls the following boast made by a Mastiff breeder advertising his stock in the dog press a fear years ago: 'I am pleased to say that we have now bred the largest and heaviest dog in Britain'. The dog in question was 89cm (35in) tall and weighed over 127kg (275lb).

Unfortunately, some breed standards have encouraged excessive size too, with minimums but not maximums set for some of the giant breeds.  Until the revision of breed standards that followed Pedigree Dogs Exposed, the breed standard for the St Bernard demanded: “Taller the better”.  It has now been changed to: “Size is desirable but only if combined with quality, correct balance and absolute soundness.” Again, it stipulates a minimum height, but not a maximum.

The breed standard for the Deerhound, too, sets a minimum desirable height but no maximum, while the standard for the Great Dane sets both a minimum height and a minimum weight that the dog must have achieved by the age of 18 months (a stipulation that positively encourages rapid early growth).

“As recently as 100 years ago, some accounts of Newfoundlands state their average weight at around 45kg (99lb),” says canine historian Scottie Westfall, who writes the Retrieverman blog. “It was a large dog, but not today's giant Newfoundland that can exceed 70kg (154lb).” In fact, the largest Newfie on record weighed a whopping 120kg (264lb).

The breed standard for the Irish Wolfhound even includes this:  “Great size, including height of shoulder and proportionate length of body is to be aimed at, and it is desired to firmly establish a breed that shall average from 81-86 cms (32-34 ins) in dogs.”

“It takes a big, tough dog to kill a wolf in a battle,” explains Scottie Westfall. “In proportion to their body size, dogs have smaller heads and teeth than wolves do, so it was necessary to breed really big dogs if someone wanted them to fight wolves. This is probably the main selection pressure that drove the Irish wolfhound to its great size. If we are to believe saga and ancient texts, Irish wolfhounds were originally larger dogs than they are today, and their fighting prowess was renowned.”

Taking the old texts as gospel, then, the breeders have decided to try to breed an even bigger Wolfhound. Given what we now know about the cost, perhaps it is time to re-think this.

Over the years, some breed standards have even upped the minimums, too, as the dogs have got bigger and bigger (the breed standard for the Clumber Spaniel has seen no less that three weight revisions  - all upwards).

Scientists now know that a surprisingly few number of genes govern the huge differences in different breeds of dog. In April 2007, researchers announced that just one gene – called IGF1 – determines a breed’s average size (although other genes and environmental factors account for why one German Shepherd will be a bit bigger than another).

More recently, another gene - HMGA2 – has been found to control a dog’s height with different variants of this gene dictating whether a dog is short, medium or tall.

There are many other things beside size that impact on longevity – including diet, environment , gender and also whether or not dogs have been neutered.  A fascinating study of Rottweillers published in 2009 found that female rotties were more likely than males to be exceptionally long-lived – but not if they had been spayed in their first four years of life.

Clearly, there is much more to unravel.  But given what we know about size and longevity, I believe we should be cautious about growing bigger and bigger dogs. It would be good to see Kennel Club and breed clubs to get together to bring in more limits. In dogs, size really does matter – and bigger is not always better.

This article appears in the April  2011 issue of Dogs Today magazine.  Dogs Today is now available internationally for iPad and iPhone for the bargain price of 59p for the app, which includes one edition free.