The "small animal, short life" phenomenon

The question of why we get old is a very hot topic in life science research right now. (Of course, it’s been a hot topic of human interest for hundreds of years! Consider the story of Ponce de Leon and the quest for the fountain of youth.) But only relatively recently has science delved deeply into what makes us age. There have been lots of theories put forth over the years to try and explain aging. Many years ago, scientists studying aging discovered something interesting, which is this: the predicted life span of a mammal generally correlates with its body size. To explain this, let’s take a look at some examples, and I’ll show you how it works.

Your standard pet gerbil, weighing in at around 0.3 pounds, will live somewhere between 2 and 4 years. Holland Lop rabbits, a bit heftier at 5 pounds, can be expected to last 7 to 12 years. Golden retrievers, around an average of 70 pounds, usually survive 12 to 17 years. Let’s move away from domesticated pets and head on up mammal size chart. How about camels? Weighing in at 1,500 pounds, they hang around an average of 40 years. Going even larger still, elephants, which weigh anywhere from 6,000 to 16,000 pounds, typically live somewhere around 70 years. And what about the largest mammal of all, the blue whale, being a hefty 100 tons? Since whales don’t have teeth, which are typically used to determine age, it’s a little difficult to say exactly. But some scientists believe that blue whales can survive upwards of a hundred years. In fact, if you were to pick a mammal at random from the entirety of the natural world, you’d be pretty safe betting how long it could be expected to live based on its average body mass.

This brings up an interesting question - why do big mammals live longer than little ones? Perhaps it’s because bigger mammals are harder to eat than smaller ones, and are less likely to turn into someone else’s dinner! I’m just kidding. The average life spans I’ve listed above are the length of time until a “natural” death, not death from predation or disease. Actually, scientists really don’t know why small animals live short lives. It might have something to do with metabolism and metabolic rate, or it might be related to the fact that small mammals produce babies extremely rapidly. The question is still up for debate.

But now that I’ve said all that, here’s what I find really the most interesting. There are some very notable exceptions to this “small mammal, short life; large mammal, long life” phenomenon. And just as scientists don’t know why small body size usually means short life span, they also don’t know why some animals are completely off the charts.

One animal I find particularly intriguing is the bat. Even though many people don’t like them, bats have a ton of interesting features that I might discuss in a future entry. For today’s purposes, suffice it to say that bats live much, much longer than you would expect for their size. On average, though they have a high infant mortality, once they reach adulthood, they live between 10 and 30 years. The oldest bat ever recorded was a wild caught banded little brown bat, 34 years old. Amazing, considering the average banded little brown bat weighs in at 8 grams – or 0.01 pounds. Comparing different animals, in fact, bats live longer per ounce of body weight than any other mammal on earth. The only mammal capable of true flight, with the ability to echolocate and an amazing anatomy, is also the longest-lived per ounce. Who would have thought that?