The aging process is a complex series of challenges met and challenges not so well met. The overall toll of life events, such as pregnancies, illness and trauma is cumulative. We know this from our own experience -- the sprain of a joint as a child makes that joint more prone to arthritis in old age. The central concept to body functioning is homeostasis.
Homeostasis is that dynamic process whereby the various body systems function to maintain internal stability which keeps the body operating, repaired and protected. Aging is the net effect of negative changes in physiology (loss of homeostasis) that occur over time, from conception to inevitable death. Older dogs are less able to handle threats to their body systems than are dogs in their prime. Old dogs, especially males, are particularly susceptible to the harmful effects of free radicals and lipid peroxidation. Body systems can be categorized several ways and further broken down into subsystems. For the purpose of this series we will discuss aging within the framework of the following classification scheme based on function and the systems that support those functions.
The major body functions are: regulatory, nutritional, excretory, distributive, protective and reproductive. We define anything that negatively influences this dynamic and generally robust balance as stress. Thus stress beyond the self-compensating properties of the body systems results in a loss of homeostasis. Generally, there is a balance between stimulus, which excites the body systems to function, and stress, which may overload them. Thus while exercise is good for the dog, dogs used as motive power in medieval times often died early deaths from "overwork," i.e., stress.iii Stress applied to bones is necessary to keep them well-calcified.iv (Susan--should we say, "well-ossified" or "well-mineralized'? And do we need a citation for this that bones need to be mechanically stressed as a part of the calcium balance?)
Each breed of dogs has its own characteristic maximum life span, i.e. that period of time during which homeostasis is maintained. Yet within each breed, there are lines that live longer or shorter than the average. Thus in determining the dog's "physiological age," it is important to take into account not only the breed, but also the line. See Table 1 for a listing of median life spans for a representative sampling of breeds. With many Great Danes and Irish Wolfhounds, for instance, geriatric at five years and dead at seven years, the aging process is relatively rapid. Contrast this with one of the small terriers, or miniature or standard poodles, which live more than twice as long. Numerous studies have attempted to quantify the lifespan of various breeds, but more often than not, external factors such as nutrition, disease, environment, quality of data and statistical sampling techniques have interfered to make it difficult to say a great deal with much certainty. Previous work, comparing the relationship between human and dog ages, recognized that it was not a constant (1:7 ratio) over the dog's life span.
We present, for your review, Patronek et al's refinement which takes into account the non-linear aspects of canine aging. Figure 2 graphs chronological dog ages of mixed breed dogs in five different weight categories against human equivalents. Figure 1 median life spans may be inputed into the Figure 2 equation to determine breed specific human age equivalents. A study is currently ongoing at the Michigan State University to develop a longevity and mortality database of purebred dogs.v To participate in this study and to submit data on your own dogs, go to "http://18.104.22.168/Dog_Study/details.htm" on the internet. An independent study is being conducted by Dr. John Armstrong, University of Ottawa, using the CANGEN-L special interest list. For more information, go to the Diversity Project .
Size. In general, small dogs live longer than large dogs as graphed in Figure 2. One way to look at this is to examine the feral or wild dog population. These dogs run to a type very suited for survival. In some ways, feral dogs represent the ideal dog in terms of form and function as related to survival. Extreme changes from this time tested ideal usually result in animals less capable of surviving on their own, and in shorter life spans unless artificial selection for breeding included selection of the basis of life span. Bear in mind that these are generalities only. Some large mammals, horses, whales, elephants, etc. live quite a long time, others are relatively short-lived. In dogs, however, it is fairly safe to say that the smaller the dog, the longer it can be expected to live. Figure 3 provides general guidelines, consistent with Figures 1 and 2, for when to start screening for geriatric (age-related) health problems.