Friday, September 01, 2006

ENV: What we get . . .

Bird attacks influenced human evolution, researchers say
By Holly Wagner/Ohio State University and World Science staff, Aug. 29, 2006

Pre­his­tor­ic birds of prey may have tar­geted our an­ces­tors for meals so of­ten that the threat of them helped drive hu­man ev­o­lu­tion, re­searchers say based on a study.

An­a­ly­z­ing hun­d­reds of mo­d­ern mon­key bones ga­th­ered be­low Af­ri­can ea­g­les’ nests, the sci­en­t­ists found the birds are a se­vere me­nace to some of our pri­mate cou­sins.

They al­so con­c­lu­d­ed that the re­sult­ing bone in­ju­ries are sus­pi­cious­ly ve­ry like those on the skull of an ape-like child of hu­man an­ces­tors, found de­cades ago. It ap­pa­r­ent­ly has clawed-out eye­sock­ets, they sa­id.

“It seems that rap­tors have been a se­lec­tive force in pri­ma­te ev­o­lu­tion for a long time,” said W. Scott Mc­Graw of Ohi­o State uni­ver­si­ty, the stu­dy’s lead au­thor.

“Be­fore this stu­dy I thought that ea­g­les would­n’t con­t­ri­b­ute that much to the mor­tal­i­ty rate of pri­mates in the fo­r­est. I could­n’t have been more wrong.”

The ide­a that birds ate ear­ly hu­mans is­n’t new. A 1995 stu­dy sug­gested that the pre­his­tor­ic 3½-year-old “Taung Child,” whose skull turned up in a South Af­ri­can ca­ve in 1924, might have been a vic­tim of such an in­ci­dent. But sci­en­tists saw the ev­i­dence as in­con­clu­sive.

McGraw, though, ar­gued that punc­ture marks on mon­key skulls close­ly re­sem­ble those found on the Taung skull, of the spe­cies Aus­t­ra­lo­pi­the­cus af­ri­ca­nus. “Ea­gles leave ve­ry dis­tinc­tive beak and tal­on punc­tures around the face and in the eye sock­et­s,” he ar­gued. “The skull of the Taung child has these same kinds of punc­ture mark­s.”



An African crowned ea­gle. (Pho­to by N. My­burg, cour­te­sy Ro­berts Mul­ti­me­dia Birds)


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“This fos­sil is pro­b­a­bly the most writ­ten-a­bout, stu­d­ied and han­dled ho­m­i­nid skull ev­er,” he added. “But al­most no one had real­ly bo­th­ered to look at skulls dis­carded from ea­gle nests” to ce­ment the case against birds.

Such re­mains of­fer the best way to learn about ea­gles’ prey, Mc­Graw ar­gued.

“Ea­gles are am­bush pre­da­tors—they go in for the kill quick­ly,” he said. “So the chance of ac­tu­al­ly see­ing an ea­gle at­tack a mon­key is ex­treme­ly slim.”

However, rap­tors, or birds of prey, “are kind en­ough to leave all the bones a­round af­ter­wards. That means we can work back­wards and con­s­truct a prey pro­file based on what’s left over.”

His team col­lect­ed some 1,200 an­i­mal bones dis­carded from 16 nests of Af­ri­can crowned ea­gles at the Ivo­ry Coast’s Tai Rainfor­est. The birds, about as big as Amer­i­can Bald Ea­gles, weigh 10 to 12 pounds (4.5 to 5.5 kg) as adults.

Slight­ly more than half of the bones, 669, came from pri­mates, the re­searchers found. The stu­dy is to ap­pear in the Oc­to­ber is­sue of the Amer­i­can Jour­nal of Phys­i­cal An­thro­pol­o­gy.

Most of the bones came from smaller mon­keys, but some origina­ted from ones weigh­ing up to 24 pounds (11 kg), they added. The majority were from mangabeys, the for­est’s larg­est mon­key. It seems the ea­gle “specif­i­cally tar­gets these large, rel­a­tively rare mon­keys,” Mc­Graw said.

The find­ing sug­gests birds of this size could suc­cess­ful­ly take on a young hom­i­nid, he added; ar­chae­ol­o­gists estima­te that the Taung tod­dler weighed around 26 pounds (12 kg).

“Many peo­ple thought that an ea­gle of this size would­n’t have enough strength to lift a pri­ma­te the size of the Taung child,” Mc­Graw said. “That’s a non-is­sue, be­cause ea­gles don’t hunt and proc­ess their kills that way. They typ­i­cal­ly dis­mem­ber their prey ve­ry quick­ly, and then take pieces of the car­cass back to the nest.”

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