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Terror bird” jabbed like agile boxer: scientists

Aug. 19, 2010
Courtesy of Ohio University 
and World Science staff

An an­cient “ter­ror bird” known as An­dal­ga­lor­nis could­n’t fly, sci­en­tists say, but it used a huge, hard skull and hawk-like beak for a fight­ing strat­e­gy rem­i­nis­cent of box­er Mu­ham­mad Ali.

The ag­ile crea­ture re­peat­edly at­tacked and re­treated, land­ing well-targeted, hatchet-like jabs to take down its prey, ac­cord­ing to a new study pub­lished this week in the on­line re­search jour­nal P­LoS One.

The ter­ror bird An­dal­ga­lor­nis brings its pow­er­ful beak down in a hatchet-like jab to at­tack its prey, a cat-sized her­biv­o­rous mam­mal called Hemi­hege­totherium. An­dal­ga­lor­nis was an ex­tinct, 4.5-feet-tall, flight­less pred­a­to­ry bird found as 6-million-year-old fos­sils in north­west­ern Ar­gen­ti­na. (Il­lus­tra­tion by Mar­cos Cenizo, cour­te­sy of Museo de La Pla­ta)


The study is a de­tailed look at the hunt­ing style of a mem­ber of an ex­tinct group of large, flight­less birds known sci­en­tif­ic­ally as pho­rus­rhacids but pop­u­larly la­beled “ter­ror birds” be­cause of their fear­some skull and of­ten im­pos­ing size. 

Ter­ror birds evolved about 60 mil­lion years ago in isola­t­ion in South Amer­i­ca, an is­land con­ti­nent un­til the last few mil­lion years. They branched out in­to about 18 known spe­cies rang­ing in size up to the 7-foot-tall (2.1 me­ters) Ke­lenken.

Be­cause ter­ror birds have no close an­alogs among modern-day birds, their ways have been shrouded in mys­tery. Now, a mul­ti­na­tional team of sci­en­tists has per­formed what they call the most soph­is­t­icated study to date of the form, func­tion and pred­a­to­ry be­hav­ior of a ter­ror bird, us­ing soph­is­t­icated X-ray scan­ning and ad­vanced en­gi­neer­ing.

“No one has ev­er at­tempted such a com­pre­hen­sive biome­chan­i­cal anal­y­sis of a ter­ror bird,” said study lead au­thor Fe­de­ri­co De­grange of the Museo de La Pla­ta/­CON­ICET in Ar­gen­ti­na, who is con­duct­ing his doc­tor­al re­search on the ev­o­lu­tion of ter­ror birds. “We need to fig­ure out the ec­o­log­i­cal role that these amaz­ing birds played if we really want to un­der­stand how the un­usu­al ecosys­tems of South Amer­i­ca evolved over the past 60 mil­lion years.”

The bird un­der study is called An­dal­ga­lor­nis and lived in north­west­ern Ar­gen­ti­na about six mil­lion years ago. It was a mid-sized ter­ror bird, stand­ing about 4.5 feet tall (1.4 me­ters) and weigh­ing in at a fleet-foot­ed 90 pounds (40 kg). Like all ter­ror birds, its skull was rel­a­tively enor­mous (14.5 inches or 37 centime­ters) with a deep nar­row bill armed with a pow­er­ful, hawk-like hook.

Ar­ti­cle co-au­thor Law­rence Wit­mer of the Ohio Uni­vers­ity Col­lege of Os­te­o­pathic Med­i­cine ran a skull of An­dal­ga­lor­nis through a CT scan­ner, a form of X-ray scan­ner in which beams are sent sim­ul­ta­ne­oly from dif­fer­ent an­gles. This gave the team a glimpse in­to the in­ner ar­chi­tec­ture of the skull and showed that the bird had evolved a highly rig­id skull, the group said.

“Birds gen­er­ally have skulls with lots of mo­bil­ity be­tween the bones, which al­lows them to have light but strong skulls. But we found that An­dal­ga­lor­nis had turned these mo­bile joints in­to rig­id beams. This guy had a strong skull, par­tic­u­larly in the fore-aft di­rec­tion, de­spite hav­ing a cu­ri­ously hol­low beak,” said Wit­mer.

The ev­o­lu­tion of this large and rig­id bony weap­on was pre­sumably linked to the loss of flight in ter­ror birds, as well as to their some­times gi­gantic sizes, re­search­ers added. 

Afos­sil skull of the ter­ror bird An­dal­ga­lor­nis, com­pared with the skull of a modern-day gold­en ea­gle and a hu­man skull for scale. (Cour­te­sy of Ohio U.)


From the CT scans, Ste­phen Wroe of the Uni­vers­ity of New South Wales, Aus­tral­ia, as­sem­bled 3D en­gi­neer­ing mod­els of the ter­ror bird and two liv­ing spe­cies for com­par­i­son: an ea­gle, as well as the ter­ror bird’s clos­est liv­ing rel­a­tive, the ser­i­e­ma. 

Us­ing com­put­ers and soft­ware sup­plied by Wroe, De­grange and Ka­ren Moreno of the Uni­ver­sité Paul Sa­ba­tier in Tou­louse, France, ap­plied an ap­proach known as Fi­nite El­e­ment Anal­y­sis to these mod­els to sim­u­late and com­pare the biome­chan­ics of bit­ing straight down (as in a kill­ing bite), pulling back with its neck (as in dismem­bering prey) and shak­ing the skull from side to side (as in thrash­ing smaller an­i­mals or when deal­ing with larg­er strug­gling prey). 

The pro­gram gen­er­ates col­or im­ages show­ing cool-blue ar­eas where stresses are low and white-hot ar­eas where stresses get high, cre­at­ing a risk of break­age.

The en­gi­neer­ing sim­ula­t­ions sup­ported the CT-based ana­tom­i­cal re­sults, the re­search­ers said. “Rel­a­tive to the oth­er birds con­sid­ered in the stu­dy, the ter­ror bird was well-a­dapt­ed to drive the beak in and pull back with that wick­edly re­curved tip of the beak,” re­marked Wroe, “but when shak­ing its head from side to side, its skull lights up like a Christ­mas tree. It really does not han­dle that kind of stress well at all.”

A key part of the en­gi­neer­ing anal­y­sis was de­ter­min­ing how hard of a bite An­dal­ga­lor­nis could de­liv­er. To ex­am­ine bite force in birds in gen­er­al, De­grange and Tam­bussi worked with zookeep­ers at the La Pla­ta Zoo to get a ser­i­e­ma and an ea­gle to chomp down on their bite me­ter.

“Com­bin­ing all this in­forma­t­ion, we disco­vered that the bite force of An­dal­ga­lor­nis was a lit­tle low­er than we ex­pected and weaker than the bite of many car­niv­o­rous mam­mals of about the same size. An­dal­ga­lor­nis may have com­pen­sat­ed for this weaker bite by us­ing its pow­er­ful neck mus­cles to drive its strong skull in­to prey like an ax­e,” De­grange said.

Its skull, though strong ver­tic­ally, was weaker mov­ing from side to side, the scientists said. And the hol­low beak was in dan­ger of cat­a­stroph­ic frac­ture if An­dal­ga­lor­nis grap­pled too vig­or­ously with large strug­gling prey.

This means the bird was likely not a “s­lug­ger,” but rath­er had to en­gage in a more el­e­gant Ali-like style, us­ing a re­peat­ed attack-and-retreat strat­e­gy, with well-targeted, hatchet-like jabs. Once killed, the prey would have been ripped in­to bite-sized morsels by the pow­er­ful neck pulling the head straight back or, if pos­si­ble, swal­lowed whole.

Feed­ing on a di­vers­ity of strange, now-ex­tinct mam­mals and com­pet­ing with the likes of saber-tooth mar­su­pi­als, ter­ror birds be­came top preda­tors in their en­vi­ron­ment. At least one gi­gantic ter­ror bird, Ti­ta­nis, even­tu­ally in­vad­ed North Amer­i­ca about two to three mil­lion years ago, but the an­i­mals died out shortly there­af­ter.


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An ancient “terror bird” known as Andalgalornis couldn’t fly, scientists say, but it used a huge, hard skull and hawk-like beak for a fighting strategy reminiscent of boxer Muhammad Ali. The agile creature repeatedly attacked and retreated, landing well-targeted, hatchet-like jabs to take down its prey, according to a new study published this week in the online research journal PLoS ONE. The study is the first detailed look at the hunting style of a member of an extinct group of large, flightless birds known scientifically as phorusrhacids but popularly labeled “terror birds” because of their fearsome skull and often imposing size. Terror birds evolved about 60 million years ago in isolation in South America, an island continent until the last few million years. They branched out into about 18 known species ranging in size up to the 7-foot-tall (2.1 meters) Kelenken. Because terror birds have no close analogs among modern-day birds, their ways have been shrouded in mystery. Now, a multinational team of scientists has performed what they call the most sophisticated study to date of the form, function and predatory behavior of a terror bird, using sophisticated X-ray scanning and advanced engineering. “No one has ever attempted such a comprehensive biomechanical analysis of a terror bird,” said study lead author Federico Degrange of the Museo de La Plata/CONICET in Argentina, who is conducting his doctoral research on the evolution of terror birds. “We need to figure out the ecological role that these amazing birds played if we really want to understand how the unusual ecosystems of South America evolved over the past 60 million years.” The bird under study is called Andalgalornis and lived in northwestern Argentina about six million years ago. It was a mid-sized terror bird, standing about 4.5 feet tall (1.4 meters) and weighing in at a fleet-footed 90 pounds (40 kg). Like all terror birds, its skull was relatively enormous (14.5 inches or 37 centimeters) with a deep narrow bill armed with a powerful, hawk-like hook. Article co-author Lawrence Witmer of the Ohio University College of Osteopathic Medicine ran a skull of Andalgalornis through a CT scanner, a form of X-ray scanner in which beams are sent simultaneously from different angles. This gave the team a glimpse into the inner architecture of the skull and showed that the bird had evolved a highly rigid skull, the group said. “Birds generally have skulls with lots of mobility between the bones, which allows them to have light but strong skulls. But we found that Andalgalornis had turned these mobile joints into rigid beams. This guy had a strong skull, particularly in the fore-aft direction, despite having a curiously hollow beak,” said Witmer. The evolution of this large and rigid bony weapon was presumably linked to the loss of flight in terror birds, as well as to their sometimes gigantic sizes, researchers added. From the CT scans, Stephen Wroe of the University of New South Wales, Australia, assembled 3D engineering models of the terror bird and two living species for comparison: an eagle, as well as the terror bird’s closest living relative, the seriema. Using computers and software supplied by Wroe, Degrange and Karen Moreno of the Université Paul Sabatier in Toulouse, France, applied an approach known as Finite Element Analysis to these models to simulate and compare the biomechanics of biting straight down (as in a killing bite), pulling back with its neck (as in dismembering prey) and shaking the skull from side to side (as in thrashing smaller animals or when dealing with larger struggling prey). The program generates color images showing cool-blue areas where stresses are low and white-hot areas where stresses get high, creating a risk of breakage. The engineering simulations supported the CT-based anatomical results, the researchers said. “Relative to the other birds considered in the study, the terror bird was well-adapted to drive the beak in and pull back with that wickedly recurved tip of the beak,” remarked Wroe, “but when shaking its head from side to side, its skull lights up like a Christmas tree. It really does not handle that kind of stress well at all.” A key part of the engineering analysis was determining how hard of a bite Andalgalornis could deliver. To examine bite force in birds in general, Degrange and Tambussi worked with zookeepers at the La Plata Zoo to get a seriema and an eagle to chomp down on their bite meter. “Combining all this information, we discovered that the bite force of Andalgalornis was a little lower than we expected and weaker than the bite of many carnivorous mammals of about the same size. Andalgalornis may have compensated for this weaker bite by using its powerful neck muscles to drive its strong skull into prey like an axe,” Degrange said. Its skull, though strong vertically, was weaker moving from side to side, and the hollow beak was in danger of catastrophic fracture if Andalgalornis grappled too vigorously with large struggling prey. This means the bird was likely not a “slugger,” but rather had to engage in a more elegant Ali-like style, using a repeated attack-and-retreat strategy, using well-targeted, hatchet-like jabs. Once killed, the prey would have been ripped into bite-sized morsels by the powerful neck pulling the head straight back or, if possible, swallowed whole. Feeding on a diversity of strange, now-extinct mammals and competing with the likes of saber-tooth marsupials, terror birds became top predators in their environment. At least one gigantic terror bird, Titanis, eventually invaded North America about two to three million years ago, but the animals disappeared shortly after.