Animal lifespans reflect environmental impact

By Dr. Beth Leermakers

Why do some animals live longer than others? As I watch my 15-year-old foster dog’s health decline, I find myself wondering — like many pet parents — why our time with dogs is so short.

Animal lifespans vary widely. Many mayflies live less than 24 hours in their final winged form, with some dying within five minutes. At the other extreme, several animal species — bowhead whales, Greenland sharks, rougheye rockfish, and some mussels and clams — can survive for more than 200 years. Jellyfish may be immortal, thanks to their remarkable ability to reverse their lifecycle multiple times, turning back into polyps when they are physically threatened or starving. The polyps can later return to their jellyfish state. 

Let’s examine a few factors that determine animals’ lifespan. 

What impacts longevity? 

Environment. Extreme temperatures and exposure to an abundance of predators shorten animals’ lifespan. Climate change is altering habitats, increasing disease and causing other environmental stressors that shorten lifespan. 

Genetics. Each species has a genetically determined potential lifespan built into their DNA. Within a species, individual genetic differences can affect lifespan. Some individuals may be predisposed to certain diseases or have genes that promote longevity.

Damage to genetic material, cells and tissues that accumulates over time and cannot be repaired by the body causes the loss of function associated with aging. Long-lived species accumulate molecular damage more slowly than shorter-lived ones do. Species that live longer have more accurate ribosomes (responsible for protein synthesis), meaning that there are fewer mistakes when assembling proteins. Longer-lived species are also better at repairing damage in their DNA molecules, and their genes maintain youthful activity longer. 

Size. Large animals tend to live longer than smaller ones. Elephants live 55 or more years in the wild, while wild rabbits only live one to three years. 

Because larger animals face fewer threats from predators, they can take a long time to grow up and reproduce. In contrast, smaller animals are more likely to be eaten, so they reproduce early and age quickly. Since most small animals don’t live long enough to develop age-related diseases such as cancer, there’s little evolutionary pressure to combat aging. With fewer external threats, larger animals have a chance to evolve a longer lifespan.

There are still variations in longevity, after accounting for size. Naked mole rats live up to 35 years, while similarly sized house mice only survive for two to three years. Dogs are a notable exception, with larger dogs aging faster and having shorter lives than smaller dogs.  

Why do small dogs live longer than large dogs?

The average dog lifespan varies by size:

Small: 10-15 years

Medium: 10-13 years

Large: 10-12 years

Giant breed: 8-10 years

Genetic differences are probably responsible. The vast variation in body size across breeds stems from just a few genes, which likely influence both their larger size and shorter lifespans. 

One large study of 74 dog breeds in North America concluded that large dogs die young mainly because they age quickly. Because of their size, larger dogs may put more strain on their physiological processes, causing them to wear out more quickly.

A newer study, however, suggests that large dogs don’t necessarily age more quickly. University of Adelaide researchers examined the causes of differences in lifespan and death in 164 dog breeds, ranging in size from Chihuahuas to Great Danes. Results indicated that larger dogs were more likely to die from cancer at a younger age when compared with smaller dogs. As the breed’s average body weight increased, so did the rates of cancer. The shorter lifespans were consistent with a theory of aging known as life history optimization: if you invest most of your resources and energy into growth and reproduction, you can’t also invest them in cell repair and cancer defenses.

Why Do Cats Live 

Longer than Dogs?

The average cat lifespan is 13 to 17 years. There are several possible reasons that cats live longer than dogs:

Cats are smaller. One theory holds that growth hormone causes cancer and heart disease that lead to earlier deaths. Because they’re smaller than most dogs, cats have less growth hormone. 

Solitary lifestyle. Because many cats are independent creatures who keep to themselves, they don’t encounter as many communicable diseases as more sociable dogs do. This leads to better health and a longer lifespan. 

Survival skills. Cats’ physical abilities, such as landing on their feet when they fall, prevent life-threatening injuries. Claws and sharp teeth are terrific defensive weapons against predators. 

Indoor life. Cats who only live indoors face fewer threats from cars, predators, toxic plants, or parasites than do dogs or cats who spend time outside. 

Since our time with our beloved pets is limited, we need to make every moment count. We owe it to our cats and dogs to cherish and take great care of them. I’ll spoil 15-year-old Jinny with extra treats and daily walks, even when her pace rivals that of glacial melting.

  Featured