Most large dog owners know they are setting themselves up for heartache because their beloved pets will die much sooner than smaller breeds.
The correlation between size and lifespan in dogs is well documented, but scientists are still unclear about the reasons behind it. Why, for example, does a 150-pound Great Dane only live for about 7 years, while the average lifespan of a 9-pound toy poodle is 14 years?
"This tradeoff has been known about for a long time, but nobody has yet investigated the underlying demographic mechanism," said Cornelia Kraus, an evolutionary biologist at the University of Göttingen in Germany.
For example, veterinarians recommend starting geriatric checkups for small dogs around age 11, for medium-sized dogs at around age 9, and large dogs around age 7.
One might assume from this that large dogs age faster, Kraus said, but it could also be that they just started aging earlier, and thus develop age-related problems sooner.
To distinguish between these two hypotheses, Kraus and her team analyzed demographic data, including age and cause of death, for more than 50,000 dogs from 74 breeds taken from the Veterinary Medical Database, a compilation of pet-health data from North American veterinary teaching hospitals.
The researchers also considered a third possibility: large dogs may just have an increased mortality risk throughout their lives, regardless of their age. In other words, their "baseline" mortality rate is higher than that of smaller breeds.
Each of these three hypotheses generates different so-called "mortality curves" -- a chart that results when mortality risk is plotted against age on a graph.
When the different dog breed data from the database was graphed, its curve most closely matched the one predicted by the faster-aging hypothesis.
"That's where we see a strong correlation," Kraus said.
The analysis also indicates that large dog's age at an accelerated pace, such that "their adult life unwinds in fast motion," the authors write in a new study that will be published in an upcoming issue of the journal The American Naturalist.