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When it comes to burning energy, humans are slacker mammals

Steve Johnson

Tribune reporter

January 14, 2014

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You may think you're working hard. But relative to other mammals, you're kind of a slacker.

Primates, including humans, expend about half the energy in daily activities compared to similarly sized mammals, according to new research conducted at Lincoln Park Zoo and elsewhere.

The study, published Monday in the Proceedings of the National Academy of Sciences, sheds new light on the question of why primates grow, reproduce and age so slowly.

"It presents a neat alternative hypothesis to our understanding of why primates have such extended life histories," said Steve Ross, an author of the study and director of the Chicago zoo's Lester E. Fisher Center for the Study and Conservation of Apes. "That might change the way we think about why primates have evolved this slow process of maturing."

"When we started getting the data in, we were just totally blown away by how different the primates were from anybody else," said lead author Herman Pontzer, associate professor of anthropology at Hunter College in New York. "That's a whole new trait for primates …. It opens the door to a lot of new kinds of questions."

Groups of animals at zoos and in the wild, including five gorillas, an Allen’s swamp monkey and four chimpanzees at Lincoln Park, were given doses of "doubly labeled water" containing biological markers — basically additional molecules. A rough analogy would be to human medical procedures that involve injecting a dye to reveal internal processes, the scientists said.

The animals' urine, in most cases, was then analyzed, and measurement of the marker molecules revealed how much energy the animals had spent. (In the case of chimps at the Tchimpounga Sanctuary in the Congo, where Lincoln Park Zoo staff assisted in the research, saliva was analyzed instead.)

In addition to wild and captive animals, primates of very different sizes were included in the study, and the lowered energy expenditure was found to be true across the range of the group.

The dozens of non-primate mammals whose energy expenditure was used in the comparison included the pygmy gerbil, the gray wolf and the Arabian oryx.

"When we finally measure the total energy budget of primates, it's 50 percent less than everybody else," said Pontzer.

Current understanding, says his paper, entitled "Primate energy expenditure and life history," is that "primates' slow life history schedules are generally thought to reflect an evolved strategy of allocating energy away from growth and reproduction and toward somatic (bodily) investment, particularly to the development and maintenance of large brains."

The new information suggests that slow metabolic rates also play a key role in our slow maturation and long lifespans.

"People are very interested in what makes us age," Pontzer said. "Now that we have a nice connection here to total metabolic rate, that gives us some idea of where to look next."

sajohnson@tribune.com

Twitter @StevenKJohnson