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Trabecula
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===Birds=== The hollow design of bird bones is multifunctional. It establishes high [[specific strength]] and supplements open airways to accommodate the [[skeletal pneumaticity]] common to many birds. The [[specific strength]] and resistance to [[buckling]] is optimized through a bone design that combines a thin, hard shell that encases a spongy core of trabeculae.<ref name="Marc Meyers 2014, p. 504-506">{{cite book|last1=Meyers|first1=M. A.|last2=Chen|first2=P.-Y.|title=Biological Materials Science|date=2014|publisher=Cambridge University Press|location=Cambridge |isbn=978-1-107-01045-1 |ref=3 |pages=504–506}}</ref> The [[allometry]] of the trabeculae allows the skeleton to tolerate loads without significantly increasing the bone mass.<ref name="ReferenceA">{{cite journal |first1=Michael |last1=Doube |first2=Michał M. |last2=Kłosowski |first3=Alexis M. |last3=Wiktorowicz-Conroy |first4=John R. |last4=Hutchinson |first5=Sandra J. |last5=Shefelbine |display-authors=1 |title=Trabecular bone scales allometrically in mammals and birds |year=2011 |journal=Proceedings of the Royal Society |volume=278 |issue=1721 |pages=3067–3073 |doi=10.1098/rspb.2011.0069 |pmid=21389033 |pmc=3158937 }}</ref> The [[red-tailed hawk]] optimizes its weight with a repeating pattern of V-shaped struts that give the bones the necessary lightweight and stiff characteristics. The inner network of trabeculae shifts mass away from the [[neutral axis]], which ultimately increases the resistance to [[buckling]].<ref name="Marc Meyers 2014, p. 504-506"/> As in humans, the distribution of trabeculae in bird species is uneven and is dependent on load conditions. The bird with the highest [[density]] of trabeculae is the [[Kiwi (bird)|kiwi]], a flightless bird.<ref name="ReferenceA"/> There is also uneven distribution of trabeculae within similar species such as the [[great spotted woodpecker]] or [[grey-headed woodpecker]]. After examining a micro[[CT scan]] of the woodpecker's forehead, temporomandibulum, and occiput it was determined that there is significantly more trabeculae in the forehead and occiput.<ref name="ReferenceB">{{cite journal |first1=Lizheng |last1=Wang |first2=Xufeng |last2=Niu |first3=Yikun |last3=Ni |first4=Peng |last4=Xu |display-authors=1 |title=Effect of Microstructure of Spongy Bone in Different Parts of Woodpecker's Skull on Resistance to Impact Injury |year=2013 |journal=Journal of Nanomaterials |volume=2013 |pages=1–6 |doi=10.1155/2013/924564 |doi-access=free |hdl=10397/31085 |hdl-access=free }}</ref> Besides the difference in distribution, the [[aspect ratio]] of the individual struts was higher in woodpeckers than in other birds of similar size such as the [[Eurasian hoopoe]]<ref name="ReferenceB"/> or the [[lark]].<ref name="ReferenceC">{{cite journal |first1=L. |last1=Wang |last2=Zhang |first2=H. |last3=Fan |first3=Y. |title=Comparative study of the mechanical properties, micro-structure, and composition of the cranial and beak bones of the great spotted woodpecker and the lark bird |journal=Science China Life Sciences |year=2011 |volume=54 |issue=11 |pages=1036–1041 |doi=10.1007/s11427-011-4242-2 |pmid=22173310 |doi-access=free }}</ref> The woodpeckers' trabeculae are more plate-like while the hawk and lark have rod-like structures networked through their bones. The decrease in strain on the woodpecker's brain has been attributed to the higher quantity of thicker plate-like struts packed more closely together than the hawk or hoopoe or the lark.<ref name="ReferenceC"/> Conversely, the thinner rod-like structures would lead to greater deformation. A destructive mechanical test with 12 samples show the woodpecker's trabeculae design has an average ultimate strength of 6.38MPa, compared to the lark's 0.55MPa.<ref name="ReferenceB"/> Woodpeckers' beaks have tiny struts supporting the shell of the beak, but to a lesser extent compared to the skull. As a result of fewer trabeculae in the beak, the beak has a higher stiffness (1.0 GPa) compared to the skull (0.31 GPa). While the beak absorbs some of the impact from pecking, most of the impact is transferred to the skull where more trabeculae are actively available to absorb the shock. The ultimate strength of woodpeckers' and larks' beaks are similar, inferring the beak has a lesser role in impact absorption.<ref name="ReferenceC"/> One measured advantage of the woodpecker's beak is the slight overbite (upper beak is 1.6mm longer than lower beak) which offers a [[bimodal distribution]] of force due to the asymmetric surface contact. The staggered timing of impact induces a lower strain on the trabeculae in the forehead, occiput, and beak.<ref name="Why Do Woodpecker's Resist Head Impact Energy">{{cite journal |first1=Lizheng |last1=Wang |first2=Jason Tak-Man |last2=Cheung |first3=Fang |last3=Pu |first4=Deyu |last4=Li |first5=Ming |last5=Zhang |first6=Yubo |last6=Fan |doi=10.1371/journal.pone.0026490 |pmid=22046293 |pmc=3202538 |journal=[[PLOS One]] |volume=6 |issue=10 |pages=e26490 |title=Why Do Woodpecker's Resist Head Impact Energy: A Biomechanical Investigation |year=2011 |doi-access=free }}</ref>
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