Essays on Non-virtual Reality: Why do animal lifespans range from minutes to centuries?
Mole came to Raven privately and said, “We haven’t talked about death very much. I’m not concerned about where I will go, but watching so many family members die, I’m wondering what happens at the point of death”?
Raven sat silently for a while, then said, “I give away my belongings.”
— Zen Master Raven, Robert Aitken
Larvae of the American sand-burrowing mayfly inhabit certain streams and rivers of the southeast. After a year as an aquatic nymph, winged forms emerge to mate and die, all in about as long as it took me to write this sentence. From the time of her emergence from the water to deposition of her fertilized eggs into the water followed by her demise, just under five minutes have elapsed.
At the other end of the insect life-span gradient, queens of the African mound-building termite have been recorded living up to 50 years during which time they produce some 30,000 eggs per day.
In fish, maximum life spans range from two months for the coral reef-dwelling pygmy goby of Southeast Asia to over 400 years for the Greenland shark of the North Atlantic and Arctic Ocean, the planet’s longest-lived vertebrate.
The longest living mammal, at over 200 years, is the bowhead whale of arctic and subarctic waters. The shortest, at just six to 12 months, belongs to the widely dispersed Muller’s giant Sunda rat of Southeast Asia.
Aging, senescence (the gradual deterioration of functional abilities), and death. Why do we get old and die? Nothing lives forever.
Actually, that last point may not be quite accurate. It rather depends on how you define “forever.”
Scientists have long been intrigued by a phenomenon referred to as biological immortality, the relatively rare situation where the rate of mortality due to senescence (rather than predation, disease, accidents, etc.) stabilizes or even decreases over time.
That is to say, it’s the condition seen in some species where the negative effects we typically associate with aging don’t seem to develop.
The eastern box turtle, for example, is one of a relatively small number of terrestrial vertebrates showing almost no signs of diminishing physical or reproductive capability with age. Given an ideal habitat (rich deciduous woodlands) it’s unclear how long they might be able to live. (They’re currently reported to have a maximum life span in the wild of 138 years.)
But in recent times, box turtles have been experiencing far less than ideal environmental conditions. In 2011, the International Union for the Conservation of Nature (IUCN) classified them as a “Vulnerable” species due to “a widespread persistent and ongoing gradual decline … that probably exceeds 32 percent over three generations.”
A lobster's life
Another example of at least near biological immortality is the American lobster. With a maximum life span (excluding lobster trap-induced mortality) of about 100 years, it seems they could live a lot longer were it not for a problem with molting.
It appears that lobsters do not lose their vitality or fertility as they age. The problem stems from the hard exoskeleton surrounding the body. In order to grow, it must be periodically shed. However, the larger the shell, the more energy is needed to complete the molt and eventually a large old-timer will hit an energy wall that exhausts its resources.
Why don’t box turtles and lobsters (along with a number of other animals like certain jellyfish, flatworms and hydra) experience the same level of senescence as most other animals?
Theories of aging include the accumulation of deleterious mutations during the course of a lifetime, damage to individual cells through exposure to highly reactive forms of oxygen (free radicals) as a bioproduct of metabolism, and replicative senescence — problems associated with the decreasing size of all the DNA in a cell every time it divides.
Species approaching biological immortality have evolved various work-arounds for each of these challenges. Unsurprisingly, researchers exploring the possibility of extending human lifespans have taken a professional interest in these species.
And finally, this: New York City is estimated to be home to some 1 to 7 million pigeons. Although the maximum life span for a Rock Dove (the most common urban pigeon around the world) is about 35 years, most seldom live longer than five years. So where are all the dead pigeons?
Well, cooper’s and red-tailed hawks, peregrine falcons, owls, stray and domestic cats certainly have a lot to say about the topic. Scavengers like turkey vultures, crows, rats, mice skunks, opossums, raccoons, dogs, foxes and coyotes are all more prevalent in urban areas than people commonly realize, and insects, bacteria and fungi quickly dispose of anything left behind.
You seldom see a dead sparrow or robin in the backyard. Insects, bacteria and fungi can finish off a small bird — feathers and all — in just three days.
Ken Baker is a retired professor of biology and environmental studies. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to firstname.lastname@example.org.