title: Mechanical analysis of feeding behavior in the extinct "terror bird" Andalgalornis steulleti (Gruiformes: Phorusrhacidae).
abstract: The South American phorusrhacid bird radiation comprised at least 18 species of small to gigantic terrestrial predators for which there are no close modern analogs. Here we perform functional analyses of the skull of the medium-sized (approximately 40 kg) patagornithine phorusrhacid Andalgalornis steulleti (upper Miocene-lower Pliocene, Andalgalá Formation, Catamarca, Argentina) to assess its mechanical performance in a comparative context. Based on computed tomographic (CT) scanning and morphological analysis, the skull of Andalgalornis steulleti is interpreted as showing features reflecting loss of intracranial immobility. Discrete anatomical attributes permitting such cranial kinesis are widespread phorusrhacids outgroups, but this is the first clear evidence of loss of cranial kinesis in a gruiform bird and may be among the best documented cases among all birds. This apomorphic loss is interpreted as an adaptation for enhanced craniofacial rigidity, particularly with regard to sagittal loading. We apply a Finite Element approach to a three-dimensional (3D) model of the skull. Based on regression analysis we estimate the bite force of Andalgalornis at the bill tip to be 133 N. Relative to results obtained from Finite Element Analysis of one of its closest living relatives (seriema) and a large predatory bird (eagle), the phorusrhacid's skull shows relatively high stress under lateral loadings, but low stress where force is applied dorsoventrally (sagittally) and in "pullback" simulations. Given the relative weakness of the skull mediolaterally, it seems unlikely that Andalgalornis engaged in potentially risky behaviors that involved subduing large, struggling prey with its beak. We suggest that it either consumed smaller prey that could be killed and consumed more safely (e.g., swallowed whole) or that it used multiple well-targeted sagittal strikes with the beak in a repetitive attack-and-retreat strategy.
authors.affiliation: CONICET-División Paleontología Vertebrados, Facultad de Ciencias Naturales y Museo, Museo de La Plata, Universidad Nacional de La Plata, La Plata, Argentina. fjdegrange@fcnym.unlp.edu.ar
authors.forename: Federico J
authors.initials: FJ
authors.lastname: Degrange
authors.forename: Claudia P
authors.initials: CP
authors.lastname: Tambussi
authors.forename: Karen
authors.initials: K
authors.lastname: Moreno
authors.forename: Lawrence M
authors.initials: LM
authors.lastname: Witmer
authors.forename: Stephen
authors.initials: S
authors.lastname: Wroe
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D001717:Birds; D001732:Bite Force; D053476:Extinction, Biological; D005247:Feeding Behavior; D020342:Finite Element Analysis; D007698:Kinesis; D012886:Skull
references.citation: Anat Rec A Discov Mol Cell Evol Biol. 2005 Apr;283(2):259-74
references.pmid: 15747355
references.citation: Nature. 2001 Feb 22;409(6823):1033-7
references.pmid: 11234010
references.citation: Proc Biol Sci. 2004 Jul 22;271(1547):1451-9
references.pmid: 15306316
references.citation: J Morphol. 2008 Mar;269(3):283-301
references.pmid: 17960801
references.citation: J Anat. 2008 Jun;212(6):736-46
references.pmid: 18510503
references.citation: Anat Rec (Hoboken). 2007 Oct;290(10):1248-55
references.pmid: 17847056
references.citation: Physiol Meas. 2005 Apr;26(2):S251-61
references.pmid: 15798238
references.citation: Proc Biol Sci. 2005 Mar 22;272(1563):619-25
references.pmid: 15817436
references.citation: Anat Rec A Discov Mol Cell Evol Biol. 2006 Aug;288(8):827-49
references.pmid: 16835925
references.citation: Proc Natl Acad Sci U S A. 2007 Oct 9;104(41):16010-5
references.pmid: 17911253
references.citation: J Exp Biol. 2004 Jul;207(Pt 16):2745-53
references.pmid: 15235003
references.citation: Proc Biol Sci. 2009 Jan 7;276(1654):39-46
references.pmid: 18765341
references.citation: Anat Rec A Discov Mol Cell Evol Biol. 2005 Apr;283(2):349-65
references.pmid: 15751029
references.citation: J Exp Biol. 2001 May;204(Pt 10):1735-44
references.pmid: 11316494
references.citation: Ecology. 2007 Feb;88(2):347-58
references.pmid: 17479753
references.citation: J Evol Biol. 2005 May;18(3):669-75
references.pmid: 15842496
references.citation: Proc Biol Sci. 2005 Sep 7;272(1574):1769-73
references.pmid: 16096087
references.citation: Proc Biol Sci. 2004 Jun 7;271(1544):1203-11
references.pmid: 15306371
references.citation: Comp Biochem Physiol A Mol Integr Physiol. 2001 Dec;131(1):197-205
references.pmid: 11733177
references.citation: Nature. 2006 Oct 26;443(7114):929
references.pmid: 17066027
references.citation: Science. 1932 Jun 10;75(1954):607
references.pmid: 17747113
references.citation: Proc Natl Acad Sci U S A. 2009 May 19;106(20):8273-7
references.pmid: 19416822
references.citation: J Exp Biol. 2006 Sep;209(Pt 17):3329-35
references.pmid: 16916969
references.citation: Gegenbaurs Morphol Jahrb. 1984;130(2):235-62
references.pmid: 6724280
references.citation: J Exp Biol. 2005 Sep;208(Pt 17):3409-19
references.pmid: 16109900