New research shows that humans and other primates burn 50% fewer
calories each day than other mammals.
The study, published January 13 in
the Proceedings of the National Academy of Sciences, suggests that
these remarkably slow metabolisms explain why humans and other primates
grow up so slowly and live such long lives. The study also reports that
primates in zoos expend as much energy as those in the wild, suggesting
that physical activity may have less of an impact on daily energy
expenditure than is often thought.
Primates: Now with only half the calories
Male silverback Gorilla in SF zoo
Most mammals, like the family dog or pet hamster, live a fast-paced
life, reaching adulthood in a matter of months, reproducing prodigiously
(if we let them), and dying in their teens if not well before. By
comparison, humans and our primate relatives (apes, monkeys, tarsiers,
lorises, and lemurs) have long childhoods, reproduce infrequently, and
live exceptionally long lives. Primates' slow pace of life has long
puzzled biologists because the mechanisms underlying it were unknown.
An international team of scientists working with primates in zoos,
sanctuaries, and in the wild examined daily energy expenditure in 17
primate species, from gorillas to mouse lemurs, to test whether
primates' slow pace of life results from a slow metabolism.
Using a safe
and non-invasive technique known as "doubly labeled water," which
tracks the body's production of carbon dioxide, the researchers measured
the number of calories that primates burned over a 10 day period.
Combining these measurements with similar data from other studies, the
team compared daily energy expenditure among primates to that of other
mammals.
"The results were a real surprise," said Herman Pontzer, an
anthropologist at Hunter College in New York and the lead author of the
study. "Humans, chimpanzees, baboons, and other primates expend only
half the calories we'd expect for a mammal. To put that 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."
This dramatic reduction in metabolic rate, previously unknown for
primates, accounts for their slow pace of life. All organisms need
energy to grow and reproduce, and energy expenditure can also contribute
to aging. The slow rates of growth, reproduction, and aging among
primates match their slow rate of energy expenditure, indicating that
evolution has acted on metabolic rate to shape primates' distinctly slow
lives.
"The environmental conditions favoring reduced energy expenditures may
hold a key to understanding why primates, including humans, evolved this
slower pace of life," said David Raichlen, an anthropologist at the
University of Arizona and a coauthor of the study.
Perhaps just as surprising, the team's measurements show that primates
in captivity expend as many calories each day as their wild
counterparts. These results speak to the health and well-being of
primates in world-class zoos and sanctuaries, and they also suggest that
physical activity may contribute less to total energy expenditure than
is often thought.
"The completion of this non-invasive study of primate metabolism in zoos
and sanctuaries demonstrates the depth of research potential for these
settings. It also sheds light on the fact that zoo-housed primates are
relatively active, with the same daily energy expenditures as wild
primates," said coauthor Steve Ross, Director of the Lester E. Fisher
Center for the Study and Conservation of Apes at Chicago's Lincoln Park
Zoo. "Dynamic accredited zoo and sanctuary environments represent an
alternative to traditional laboratory-based investigations and emphasize
the importance of studying animals in more naturalistic conditions."
Results from this study hold intriguing implications for understanding
health and longevity in humans. Linking the rate of growth,
reproduction, and aging to daily energy expenditure may shed light on
the processes by which our bodies develop and age. And unraveling the
surprisingly complex relationship between physical activity and daily
energy expenditure may improve our understanding of obesity and other
metabolic diseases.
More detailed study of energy expenditure, activity, and aging among
humans and apes is already underway. "Humans live longer than other
apes, and tend to carry more body fat," said Pontzer. "Understanding how
human metabolism compares to our closest relatives will help us
understand how our bodies evolved, and how to keep them healthy."