When looking at mammals in general, being big correlates quite well with living long lives (barring disease, being eaten and things of that nature, of course). But, within species, it seems that the bigger individuals live less long than their smaller conspecifics.
What could be going on here?
And what better species to study this in than the domestic dog? Due to the existence of many different breeds, Canis lupus familiaris provides us with a large range of body sizes and life spans.
(Meet Zeus, the aptly named Great Dane that is the current Guinness World Record holder for ‘largest dog’.)
A recent study, published in The American Naturalist, collected data from 74 dog breeds (and at least 120 individuals per breed) with the aim of shining some light on this riddle.
Based on the data, the authors identified several possible survival costs of large size in dogs: higher early mortality, early onset of senescence, elevated baseline mortality, or an increased rate of aging.
Through the use of several statistical models (so-called mortality hazard models), the researchers were able to assert that the main culprit of earlier death in big dogs is an increased rate of aging (the baseline hazard was also slightly elevated). As the authors write:
Large dogs die young because they age quickly.
Some proposed reasons for this phenomenon include:
- A later onset of age-related diseases in smaller dogs.
- Neoplastic processes (abnormal tissue development, for example a tumor) are more common in large dogs. Perhaps because selection for larger size leads to higher rates of cell division?
- Rapid growth could lead to more developmental errors.
- Large dogs populations have been observed to be more heterogeneous than those consisting out of small ones. Perhaps the ‘frailer’ individuals among smaller dogs have already been ‘selected out’?
And maybe some, or all, of these proposed mechanisms work together in their relentless hunt for large bodied victims.
But, all you tall people out there, don’t despair. Being tall does have its advantages (not that I would know really…).
Kraus, C., Pavard, S., & Promislow, D. (2013). The Size–Life Span Trade-Off Decomposed: Why Large Dogs Die Young The American Naturalist, 181 (4) DOI: 10.1086/669665