The mystery as to why humans outlive almost all other mammals may have finally been solved.

An international team of scientists compared how much energy the average primate uses each day with the equivalent amount of energy other mammals use.

Researchers discovered, somewhat surprisingly, that primates burn half the calories of similar sized mammals, and that this slow metabolism slows down the ageing process.

To put this in perspective, a human – even someone with a very physically active lifestyle – would need to run a marathon each day just to approach the average daily energy expenditure of a mammal their size.

The study also found that primates in zoos expend as much energy as primates in the wild, suggesting physical activity may have less of an impact on the amount of calories they use than is often thought.

The majority of mammals, including rats and dogs, reach adulthood within a year or two.  In fact, many mammals reach sexual maturity in a matter of weeks.

For example, a six-month-old rat is 18 in human years, while a two-year old dog is the equivalent of an 18- to 25-year-old human.

In the wild, these mammals also reproduce much more regularly than primates and many will die in their teens, if not before.

By comparison, humans and our primate relatives, such as apes, monkeys, tarsiers, lorises, and lemurs, have long childhoods, reproduce infrequently, and live exceptionally long lives.

This slow pace of life has baffled biologists for some time because the mechanisms behind this difference were unknown.

Professor Herman Pontzer of Hunter College, New York, and colleagues, working with primates in zoos, sanctuaries, and in the wild examined the daily energy expenditure in 17 different primate species.

Using a safe and non-invasive technique known as ‘doubly labelled water,’ the researchers measured the number of calories that primates burned over a 10-day period.

This technique tracks the body’s production of carbon dioxide, which gives an indication of how much energy they are expending.

By combining these measurements with similar data from other studies, the team were able to compare this energy output to that of other mammals.

This dramatic reduction in metabolic rate accounts for their slow pace of life.

All organisms need energy to grow and reproduce, and energy expenditure can also contribute to ageing.

The slow rates of growth, reproduction, and ageing among primates match their slow rate of energy expenditure, indicating that evolution has acted on metabolic rate to shape primates’ distinctly slow lives.

‘The results were a real surprise,’ explained Professor Pontzer. ‘Humans, chimpanzees, baboons, and other primates expend only half the calories we’d expect for a mammal.

‘Understanding how human metabolism compares to our closest relatives will help us understand how our bodies evolved, and how to keep them healthy.’


Animal               Average life span (in years)

Mouse                4

Rat                     7

Cats                   14

Dogs                  16

Polar bears        18

Horses              28

Elephants           70

Humans             82

The oldest living human, according to the Guinness Book of World Records, was a French woman called Jeanne Calmen who lived to 122.