Modern aging science is focused on extended human health and lifespan. It’s instructive to explore the limits of animal longevity when thinking about aging in humans. Animal species show a wide variety of lifecycles and longevities. Here, I will present snapshots of long-lived animals and how they inform human healthspan and longevity research.
Hydra is a very successful genus of freshwater cnidarians (think anemone and jellyfish). They have high regenerative capacity, replacing all of their cells approximately seven times a year. In 1998, Daniel Martinez (Pomona College) published a study following cohorts of hydra for four years. He found that, unlike many other animals, asexually reproducing hydra did not experience increased mortality with age. From this observation he inferred that hydras do not senesce (senescence is the gradual deterioration of cells and tissues as they age). While few debate that hydra exhibit remarkable longevity, there is controversy over whether hydra truly age because their asexual reproductive rate does decline gradually over time.
The lesson from the humble hydra might be that innate regenerative capacity drives tissue and organismal longevity. This would argue for promoting adult stem cell health in human tissue.
#2. Ocean Quahog
“Ming the clam” is the 507-year-old reigning longevity champion among ocean quahogs (Arctica islandica). Ming got its name because it was born during the Ming dynasty in 1499. Quahogs have pronounced annual growth bands on their shells that can be counted like rings in a tree trunk to determine their age. Recently researchers have found that ocean quahogs have better resistance to oxidative stress and more unsaturated lipids in their membranes than related short-lived species.
These observations support the “membrane pacemaker” theory of aging, which states that reactive oxygen species throughout life damage fatty acids in cellular membranes, which causes tissue damage and aging. Perhaps human lifespan could be extended by modifying fatty acid content in our diet to avoid oxidative damage in our cells.
There are many reports of long-lived tortoises from different species. One extreme documented example is a radiated tortoise (Astrochelys radiate) originally from Madagascar named “Tu’I Malila”. The tortoise was given to the king of Tonga by the explorer James Cook in 1777 and lived in the care of the royal family for 188 years until his death in 1965. Perhaps the prevalence of tortoise longevity is due to their long reproductive lifecycle. Tortoises’ and turtles’ reproductive capacity peaks relatively late in life and does not drop off appreciably past 60 years of age. Therefore, unlike many organisms, they experience a selective advantage for long life.
How does the human reproductive lifecycle influence our longevity? Human reproductive age is undergoing a subtle shift as the age of childbirth gradually rises in the developed world. In the U.S., the average age of first-time mothers has gradually risen from 21.4 years in 1970 to 25.0 years in 2006. Similar shifts have happened in every state in America. I speculate that these demographic shifts might produce adaptive extensions in human healthspan because selective pressure will act on older individuals than previously in our history.
Several whale and dolphin species have very long lifespans. Bowhead whales can live to 211 years old, and are occasionally found still stuck with ancient stone harpoon points from native whalers. Orcas (Orcinus orca) in the wild live up to 90 years. A Science brevia from 2012 suggests that long-lived post-reproductive females benefit the health and longevity of their offspring. Orca mothers and offspring may live together for more than 70 years.
Perhaps orcas and humans owe their longevity to a similarity in our reproductive lifestyle. Orcas, short finned whales (Globicephala macrorynchus) and humans are the only animals that undergo menopause. The survival advantage of orca post-reproductive longevity is consistent with the “grandmother hypothesis”. That is, that human longevity emerged from our ancestors’ relatively shorter lifespans in order to maximize the evolutionary benefits of grandparents’ protection and care of grandchildren. This theory is somewhat controversial because it is unclear that humans often lived long enough to become grandmothers early in our history.
#5. Three-way tie
“Immortal” jellyfish– Adult Turritopsis dohrnii can de-differentiate back into a juvenile polyp form under harsh conditions (such as temperature change, reduction of salinity, or mechanical damage). They can repeat this transition indefinitely through a process called transdifferentiation where one adult tissue can turn directly into another tissue. There are emerging limited examples of inducible transdifferentiation in humans (including pancreatic cells switching cell-type), but none that make people immortal yet.
Tuatara – Sphenodon punctatus. A 111-year-old tuatara became a new father recently. These lizard-like New Zealanders are the sole remaining members of a primitive group of reptiles. Similar to tortoises and orcas, its slow growth and late reproductive maturity may contribute to its overall longevity. It gets extra points for having a vestigial third eye on the top of its head.
Naked mole rats– Heterocephalus glaber. These rodents from east Africa are amazing. In captive lab colonies there has been no evidence of cancer in any necropsied animals so far. Mole rats also have a recorded maximum lifespan of 31 years. That’s 5 times longer than the average expected from a mammal of its size. Stochastic theories of aging (that accumulated mutations and mechanical damage produce aging) might predict that aging rates for a given organismal size and complexity should be relatively similar. Comparing mole rats’ longevity and lab rats’ longevity (4 years is old for a Rattus norvegicus lab rat) supports the idea that longevity is “programmed” independently of stochastic processes. Discovering and manipulating this program is one of the dreams of aging research.
For reviews on the comparative biology of aging see: