Animals that can cheat death

Nature is teeming with examples of astonishing survival and even eternal youth: the jellyfish that resets its life cycle, the salamander that spends its entire life in a larval state, the newt that regenerates parts of its eyes. Some organisms seem to bend the rules of life, reminding us that scientific truth can be stranger than supernatural legend. Could we humans tap into our own biology to outwit death?

Dr. Claire Vergneau-Grosset (in French only), a professor of zoological medicine in the Faculty of Veterinary Medicine at Université de Montréal, helps separate myth from reality in the world of species that have seemingly supernatural powers.

For some additional examples, the jellyfish Turritopsis dohrnii can revert to the polyp stage, its “baby” form, over and over again, allowing it to rejuvenate and live forever. The axolotl, a type of salamander, retains its juvenile traits and can reproduce in its larval state, in which it has gills. Some fish can grow back their fins, and spiders can regenerate legs. Some starfish can regrow an entire central disk from just one arm plus a fragment of the central disk. Aquatic flatworms known as planarians have an unlimited capacity for regeneration, thanks to their stem cells.

What do these creatures have in common? They are all fish, amphibians or invertebrates—primitive animals, on the whole—that belong to the anamniote group of animals. Unlike mammals (amniotes), animals in this group don’t have amniotic sacs during the embryonic stage.

These species also have a simpler immune system than mammals—one that doesn’t trigger the complex inflammatory responses that limit scar tissue formation in amniotes. “It’s easier for them to regenerate a limb because, when their cells divide to reform a body part, there isn’t such an intense immune response to block it,” Dr. Vergneau-Grosset explains.

These creatures have special abilities not because they are more evolved than mammals, but because their immune systems are more permissive. For example, a spider can grow a leg back due to the simplicity of the leg’s anatomy, which consists of a chitinous exoskeleton (or shell) filled with fluid called hemolymph (the spider’s equivalent of blood). During molting, the spider produces a new leg within its exoskeleton, and the hemolymph fills the new space.

Dr. Vergneau-Grosset says when an animal grows a body part back, the new limb is rarely perfect. For example, in the axolotl, some toes may be missing on a regenerated leg. A fish’s new fin often has deformed bony rays. “They’re little monsters,” she jokes.

In geckos, which can voluntarily shed their tails to escape predators, the new tail is thicker than the original. Geckos can do this, Dr. Vergneau-Grosset explains, because they have precise break points in their vertebrae and satellite stem cells in their muscles. The new tail can’t grow back again if it breaks, but it will retain its sensitivity and mobility; the nerve regeneration that this implies is particularly intriguing for research.

Could humans benefit from—or at least take inspiration from—these fascinating animal traits? Are there genetic mechanisms at play that could be useful for human medicine? Could we, too, conquer time?

“It’s very difficult to apply these processes to humans because mammals are very complex organisms,” Dr. Vergneau-Grosset says. “To make it possible for humans to regenerate limbs, we would need to inhibit the immune system, which isn’t always desirable for the body as a whole. Otherwise, the immune system could attack the new tissue, as sometimes happens during transplant rejection.”

Some of the animals mentioned have tissues that are composed largely of water and hyaluronic acid, a molecule that promotes water retention and tissue elasticity. The cosmetics industry makes extensive use of this molecule’s hydrating capacity, especially in “anti-aging” creams.

“But for now, that’s where it ends,” says Dr. Vergneau-Grosset. “Stem cells are a burgeoning field of research, but we’re not yet regenerating limbs or implanting them, and doing so would mean venturing into territory fraught with ethical questions.”

This article was adapted and republished with permission from the Université de Montréal.

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