Human relationships with animals have experienced great changes since our hominid ancestors first used fire to keep predators at bay. Once aggressors, then helpers, animals only recently have enjoyed the companion roles we bestow on pets. As pets become more emotionally intertwined with human life, the issue of their life spans -- considerably shorter than ours in most cases -- have become a more troubling issue.
Changing Roles, Shifting Expectations
Animals once served mostly utilitarian roles in human life. In more agrarian times, farm cats kept rodent populations in check, and ship's cats were essential to seafaring vessels otherwise in danger of crippling damage by rats. Modern dog breeds can be traced to jobs the animals performed: German shepherds and Great Pyrenees for their livestock skills, for example, or terriers for their rodent catching instincts. In modern times, animals perform more like family members, so their life spans, which present humans with the near certainty of losing a dear friend, now weigh heavier on the decision of pet ownership.
Animals Experience Varying Life Spans
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Every species, as well as subspecies, has its own expected life span. Members of the human species, according to the Centers for Disease Control, can expect to live to 78.8 years old. While it's not fully understood why animal life spans vary, with Great Danes getting only 7 years to humans' 70-plus, theories abound. Brain size in relation to body size provides some insight, as animals with larger brains live longer, reach sexual maturity slower and reproduce in smaller numbers. Another theory states that metabolism, and its resultant expenditure of an animal's tissue, is to blame for aging.
Dog Size Affects Expected Life Span
Domestic dogs, with their wide spectrum of subspecies, provide an interesting area of study, as it's recognized that larger dogs live far shorter lives than their smaller counterparts. Findings from study at the University of Göttingen in Göttingen, Germany, suggest that larger dogs do, in fact, age at a faster rate, as opposed to starting aging earlier or suffering increased chances of mortality throughout life. Researchers pointed to increased levels of a hormone called IGF-1 in larger dogs as a possible reason. Brain-to-body ratios and metabolism differences also function in exploring differences in rate of aging between large and small dogs.
Some Animals Outlive Humans
Though humans judge animal longevity by species most commonly encountered, the animal kingdom offers plenty of organisms with wildly varying life spans. A box turtle, for instance, can live 123 years, and a swan's likely to reach 102. Turkey buzzards live 118 years, carp 100 years and macaw parrots, a good choice for pet seekers who never want to say goodbye, can live for more than a century.