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Why humans are the high-energy apes

Why humans are the high-energy apes
Our metabolism runs faster than that of other apes, likely allowing us to evolve big brains

  By Ann Gibbons

We  may  not  be  raring  to  go  on  a  Monday   morning,   but   humans   are  the  Energizer  Bunnies  of  the  primate  world.  That’s  the  conclu-sion  of  a  new  study  that,  for  the  first  time,  measures  precisely  how  many  calories  humans  and  apes  burn  each  day.  Compared  with  chimpanzees  and  other  apes,  our  revved-up  internal  engines  burn  calories   27%   faster,   according   to   a  paper  in  Nature  this  week.  This  higher  metabolic  rate  equips  us  to  quickly   fuel   energy-hungry   brain   cells,  sustaining  our  bigger  brains.  And lest we run out of gas when food is short, the study also found that hu-mans are fatter than other primates, giving us energy stores to draw on inlean times. “The  brilliant  thing  here  is  show-ing  for  the  first  time  that  we  do  have  a  higher  metabolic  rate,  and  we  do  use  more  energy,”  says  paleo-anthropologist  Leslie  Aiello,  presi-dent of the Wenner-Gren Foundation for Anthropological Research in New York City. “Humans during evolution have become more and more hyper-metabolic,”  says  biological  anthro-pologist Carel van Schaik of the Uni-versity of Zurich in Switzerland. “We turned up the thermostat.” For decades, researchers assumed that  “there  weren’t  any  differences  in  the  rate  at  which  different  spe-cies burned calories,” says biological anthropologist  Herman  Pontzer  of  Hunter  College in New York City, lead author of the new  study.  Comparing  humans  and  other  primates,  they  saw  little  difference  in  basal  metabolic rate, which reflects the total calo-ries used by our organs while we are at rest. But  in  many  ways,  we’re  not  like  other  apes:  Our  brains  are  at  least  three  times  larger,   and   we   produce   more   babies   in   shorter  intervals—both  of  which  consume  more energy. “It has been an open question—how  do  we  do  all  these  expensive  things?”  Pontzer says.  For the past 2 decades, researchers looked for  an  answer  in  tradeoffs  between  the  en-ergy  demands  of  different  parts  of  the  hu-man   body.   For   example,   Aiello   and   her   colleagues  proposed  that  when  our  brains  began to expand dramatically about 1.6 mil- 

lion years  ago,  our  direct  ancestor  Homo erectus evolved a smaller gut that sucked up less  energy  (Science,  15  June  2007,  p.  1560).  Other teams suggested that humans reduced muscle mass to save energy; walked and ran more  efficiently;  or  got  extra  calories  faster  by eating a higher quality diet, cooking food to cut down on the energy spent in digestion, and sharing food. Indeed, there seemed to be no shortage of human adaptations that con-serve energy 
Then,  in  2010,  researchers  began  to  mea-sure  apes’  total  energy  expenditures  (TEEs)  accurately for the first time, rather than just their  resting  rate.  Orangutans  delivered  the  first surprise: They have an unexpectedly low metabolic rate, Pontzer says. So  he  and  primatologist  Stephen  Ross  of  the  Lincoln  Park  Zoo  in  Chicago,  Il-linois,  set  about  measuring  TEE  directly  in  as  many  apes  as  possible  at  14  zoos  and  two  ape  sanctuaries  in  the  United  States  and  Africa.  They  fed  27  chimps,  eight  bonobos,  10  gorillas,  and  11  orang-utans water labeled with certain isotopes of hydrogen and oxygen. Then they measured those two isotopes in the apes’ urine to see how the ratio changed over time. The ratio reveals  how  much  carbon  dioxide  the  ani-mal had generated, which reflects how many  
calories it had burned. The technique is the “gold  standard”  for  metabolic  studies,  and  the researchers did a “terrific job” using it to compare  the  total  calories  burned  daily  by  apes and humans, says biological anthropo-logist  William  Leonard  of  Northwestern  University in Evanston, Illinois. The   team   used   the   same   method   on   141 adults from five populations around the world.  After  taking  body  size  into  account,  they  found  that  humans  averaged  about  400   more   calories   per   day   than   chimps  and  bonobos—635  calories  more  than  gorillas  and  820  calo-ries   more   than   orangutans.   This   meant   that   humans   burned   over   27% more energy per day on average than chimps.  Although  ideally  all  data  would  come from animals in the wild, other studies  have  shown  that  TEE  rates  in  captive  and  wild  apes  are  about  the  same,  regardless  of  activity  lev-els, Van Schaik notes. To be safe, the study  matched  relatively  sedentary  humans with captive apes. The  team  also  measured  body  fat  in people and other primates by ana-lyzing  isotopes  in  the  urine,  finding  that  humans  had  significantly  more  fat   than   even   these   zoo   animals.   “If  you’re  going  to  burn  fuel  faster,  you   better   have   a   backup   tank,”   Pontzer  says.  Once  early  hominins  had  boosted  their  metabolism  and  grown bigger brains, he says, natural selection would have favored not only fatter  individuals,  but  also  smaller  guts  and  other  energy-saving  adaptations,  such as cooking and efficient walking. “What  is  fantastic  about  this  paper  is  that Herman and his colleagues have effec-tively integrated all of the earlier ideas into a  unified  theory  for  energy  and  the  evolu-tion  of  human  characteristics,”  Aiello  says.  Van  Schaik  agrees.  “There  has  to  be  more  energy  going  into  our  systems,”  he  says.  “Now, [Pontzer] has measured it, and it all fits together.”Next on Pontzer’s agenda is to try to fig-ure  out  how  and  when  human  ancestors  boosted their metabolisms above the levels of our ape ancestors, for example by analyz-ing  rates  of  bone  growth  in  fossils.  That’s  particularly  intriguing  to  Aiello.  “I’d  really  like  to  know,”  she  says,  “when  did  fossil  hominids get fat?”

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