The size and width of a dog's head accurately predict the strength and running ability of the canine, according to a paper in the latest issue of the journal Behavioural Processes.
The finding likely applies to other animals, including humans, and suggests that physical performance is constrained by trade-offs between structure and physiological features. It's therefore very rare to have a dog, person or other animal that is both super strong and speedy.
"In the real world, it would be hard to be both fast and efficient at running, and to be extremely strong in combat at the same time," author William Helton told Discovery News. "Nature does not allow unlimited budgets and the trade-offs are often physical constraints."
Helton, a senior lecturer in the University of Canterbury's Department of Psychology, studied how well 217 dogs performed during International Weight Pulling Association sporting events for canines.
Brachycephalic, or broad-headed, dogs that participated included American Pit-Bull Terriers, American Bulldogs and Bernese Mountain Dogs. Dolichocephalic, or more narrow-headed, breeds consisted of Samoyeds, Siberian Huskies and Alaskan Malamutes.
None of the studied breeds included the extremes of each condition. Pugs, for example, have incredibly broad heads, while Borzois are the polar opposite.
Even so, all of the more broad-headed dogs in the study were able to pull substantially higher weights than their narrow-headed competitors. Other studies indicate dolichocelphalic breeds are faster and more efficient runners than the brachycelphalic dogs.
Just looking at the size, width and basic shape of a dog's head can then inform the viewer about the dog's ability to fight, pull and run.
Based on this determination, "my guess is the absolute strongest dog would be one of the giant mastiff breeds (Saint Bernard, Bernese Mountain Dog, for example)," Helton said. "The strongest dog for its overall size in my sample was the American Pit Bull Terrier."
"The overall fastest dog is undoubtedly a sight-hound, most likely a Greyhound," he added. "Other dogs are also very adapted to endurance running, such as huskies."
Conditioning, nutrition, personality and other factors can also affect the health and physical capabilities of an individual.
"The narrower (or wider) head is simply a skeletal trait that is an indicator of other skeletal traits," he explained.
Lisa Peterson, a spokesperson for the American Kennel Club, agrees with the study's conclusions.
"They make total sense, especially as breeders over the years have looked to enhance certain traits, such as speed and endurance," she said. "The same holds true for horses. Race horses do have more narrow-shaped heads versus those of draft horses."
A separate study conducted on humans demonstrated that people across cultures are able to predict, with a fair degree of accuracy, how much a man can bench press just by looking at a picture of his face. This indicates we may assess the strength of other people using this visual information more often than we may realize.
Since humans have narrower heads than other great apes, running speed and endurance were probably important for our ancient hominid ancestors, who must have often engaged in persistence hunting, meaning they were literally running prey to death.
"While we are not as fast as some quadrupeds, we are definitely well adapted to running for very long durations under extreme thermal stress, as marathon runners attest," Helton said. "This has definitely come at a cost of strength. Anyone I have known who has worked with adult great apes has immense respect for their relative advantage in strength contests."