Great question. Lots of people would like to know that. Humans live a long time compared with most animals, but lots of people would like to live longer and to avoid some of the problems of aging.

So let’s start off by looking at what old age is. Our cells divide to make new cells. Some of our cells are dividing fast all the time. The cells of our skin and the cells of our digestive tract divide a lot. Other cells divide slowly, like our liver cells. Some cells, like our brain and muscle cells, do little or no dividing after we’re toddlers.

Almost every cell in your body has a nucleus with DNA in it. The DNA tells the cell how to do its job. Every time a cell divides, the DNA has to be copied and divided so that the new cells each get a full set of DNA. When the DNA divides, a part of it at the end called the telomere gets shorter. When a cell’s telomere gets too short, it will no longer divide. This puts a limit on cell division. So after a while, the cells stop dividing.

Another thing that causes aging is damage to cells from sunlight, radiation, chemicals, disease, use, and just random events. When we’re young, damaged cells (except for muscle and brain cells) can be replaced. With fewer and fewer cells dividing, replacement takes longer, then finally stops.

The powerhouses of our cells, the mitochondria, may also stop working as well as we age, so less of the energy we take in as food winds up as energy we can actually use.

So what do tortoises have that we don’t? Like us, they have cells with DNA and telomeres. Unlike us, have a VERY slow metabolism. Metabolism refers to all the chemical reactions that our bodies do. Everything is slower in a tortoise. This means that all of the wastes that metabolism makes are going to be made very slowly, reducing damage to the cells.

If we look at the other end of metabolic rate for vertebrates (animals with skulls and backbones), we have the shrew. It looks a lot like a tiny mouse, but it’s an insectivore. It has a super-fast metabolism. Its heart beats so fast that it may be impossible for the heart muscle to work faster. They can starve to death in few hours. They are also dead of old age in about 2 years. Maybe their cells don’t divide any more or less than a tortoise’s cells, but they do it a whole lot faster. Their wastes from metabolism would also be produced a lot faster.

There’s probably a lot more to learn before we understand the whole picture, but it looks like if you live fast you die young.

A lot of animals that live a long time are big (elephants, whales, us). A lot of animals that live a short time are small. Why do you think that is?

If you are interested in questions like this, you may want to study animal physiology.