What determines the lifespan of animals?

Aging and death are inevitable in all living organisms. However, each animal ages at a different rate. Some animal species outlive us by several years since they age much more slowly than us. Among land mammals, elephants have some of the longest lifespans, and can live for up to 86 years. Bowhead whales live the longest among mammals, lasting for up to 200 years! The rougheye rockfish, found in the northern Pacific, also has a similar lifespan. Greenland sharks only reach sexual maturity at 200 years old (enjoying a long childhood!) and can live for close to 300 years. Other animals with relatively long lifespans are giant tortoises (100+ years), macaws (100 years) and crocodiles (70 years). So what determines aging, and why do some animals live so long?

The driving factors behind aging are varied and complex. Aging results as a consequence of cell death and dysfunction. When the body is young, the cells regenerate much more quickly and efficiently. As aging occurs, this process slows down. Older cells do not repair and regenerate efficiently, speeding up aging.

Genes provide a clue

Researchers have looked at the genes of animals with a long lifespan in order to obtain clues about aging. Astonishingly, they found several genes that are responsible for conferring longevity. Every time a cell divides, there is a small chance of errors arising in the DNA, which can give rise to disorders like cancer. Some of the genes in long-living species help in quickly repairing damaged DNA, making them less susceptible to cancers and age-related disorders. Other genes help to regulate insulin and boost the immune system, providing protection. In the case of rougheye rockfish, which is found in frigid waters, metabolism plays a crucial role. These fishes have a very slow metabolism, thus burning minimal energy. This greatly increases their lifespan.

Bigger animals live longer

The body size of an animal and the environment are other powerful evolutionary processes that determine the lifespan. As a rule of the thumb, animals with greater body size have a larger lifespan. A great body size is a big advantage since there are fewer predators that have a chance of hunting down the animal. Smaller creatures, by contrast, are more prone to predators and can hardly be expected to survive for long in the wild. Large animals also enjoy a reproductive advantage – they can afford to mate multiple times over their lives after reaching sexual maturity. This ensures better care for their offspring and better chances of the offspring surviving till adulthood.

A remarkable exception to the body size rule is the naked-mole rat. While common house mice can live only around 4 years, the naked-mole rat can live up to 31 years. One reason behind the naked-mole rat’s increased longevity is its social behaviour. These rats live in colonies with one queen rat that is reproductive and several sterile “worker” rats. Another factor is the nature of the mole rat’s environment. These rats live in subterranean habitats, where there is low oxygen. Since these animals are adapted to surviving in low oxygen conditions, they do not need to burn large amounts of energy as their cousins – the house mice do. This saves their tissues from biological wear and tear, greatly extending their lifespan.

Where do humans fit into the picture? The average life expectancy today for humans is around 72 years. This has improved greatly since the 1900s, when the life expectancy was around 50 years. While we continue to search for ways to extend our lifespan, it is worth noting that a healthy lifestyle will definitely do a lot to reduce aging.

Immortal animals

A common but interesting question posed while considering age relates to immortality. Is it really possible to live forever and be immortal? Humans are certainly nowhere close to achieving immortality. However, a few invertebrates are close to this feat. The jellyfish is one such animal that can be potentially considered immortal. Jellyfishes begin their lives as larvae, and then transform into polyps. When the polyps mature and differentiate, jellyfish are formed. Under hostile environmental conditions, the jellyfish is capable of going back to the polyp stage. Since these creatures can alternate between polyp and jellyfish multiple times without showing signs of wear and tear, they can potentially live forever. Hydra is another group of invertebrates close to achieving immortality. Their bodies contain stem cells which can keep regenerating, thus eliminating aging. However, it is important to note that jellyfish and hydra both have predators and can also die due to disease, which prevents immortality.

In an interesting study carried out to understand the mechanisms of aging, researchers looked at proteins associated with aging from around 30 different mammalian species. These proteins had important functions in responding to DNA damage and initiating repair. The study found that these proteins mutated and evolved in a specific manner in longer-lived species. They also found patterns in proteins associated with metabolism that ensured that metabolic pathways were optimized for repairing molecular damage. Overall, the studies imply that the mutation and evolution of these proteins was not by accident, but rather by design. These proteins were predominantly found in longer-living animals, suggesting that these species had optimized damage to their cells and slowed aging. While there is no one way that will make us live longer, putting together evolutionary pieces of puzzles will help us better understand the process of aging.