46
“Dense compact bone bears bending or torsional stresses on the periphery of the
shaft. Less dense internal supportive tissues lend secondary support, or are lacking
entirely, with a marrow-fi lled cavity lending haematopoietic function,” (Bostwick
et al. 2012 ).
Wings contain some of the longest and strongest bones. Flying animals appear to
have denser bones than terrestrial counterparts of similar size. In bones, differences
in density generally mean differences in mineral content. Higher mineral contents
are linked to greater stiffness and strength. Dumont ( 2010 ) thinks that such material
properties may be central to understanding why bone density is relatively high in
birds. Along with fl ight, birds have also evolved small body size; and one way to
design a small skeleton while preserving its strength is to make its bones denser.
Investigating links between the material properties of bone and the evolution of
fl ight and small body size in birds is going to require much more work – as we
would need to obtain bone density measurements from transitional forms in the
fossil record like Archaeopteryx (Hone et al. 2008 ).
The skeletons of birds are described in detail (see for review Shufeldt 1890 ;
Baumel et al. 1993 ; Kaiser 2007 ; Hospitaleche et al. 2009 ). These constructs are
examples of selection for minimizing the energy required for the bird’s fl ight. “From
a functional perspective, the weight (mass) of an animal relative to its lift- generating
surfaces is a key determinant in the metabolic cost of fl ight. The evolution of birds
has been characterized by many weight-saving adaptations that are refl ected in bone
shape, many of which strengthen and stiffen the skeleton” (Dumont 2010 ).
According to this author “the round and thin-walled humeral shafts of birds are an
optimal shape for resisting both torsion and bending, and fused skeletal elements
have been interpreted as increasing force resistance (i.e. stiffness; Buhler 1992 ),”
(Dumont 2010 ). It was observed that fl ightless and diving birds possess thickerwalled bones which may serve as reinforcement or ballast. In diving seabirds,
the rib cages and bones must be stronger and heavier, respectively, to recover from the
increased pressure during a dive. During this process, the rib cage collapses inwards
but springs back when the dive is achieved (see for review Norberg 1985 ).
The evolution of fl ighted birds can be characterized by the expansion of
pneumatized spaces within some bones as well as by gradual reduction and fusion of
many skeletal elements. Correspondingly their bones are typically lighter, but denser
than that in mammal skeletons. Also bird’s skeletons must be stronger and stiffer
because of their ability to withstand the mechanical forces encountered during fl ight.
It is little wonder that most seabirds have partially hollow or hollow bones
(Fig. 1.10 ). The morphological properties of seabird bones that allow fl ight (Casinos
and Cubo 2001 ; Simons 2010 ; Simons et al. 2011 ), as well as their biomechanics
(Pennycuick 1967 ; Kirkpatrick 1994 ; Cubo and Casinos 1998 , 2000 ; Mi et al. 2005 ;
de Margerie et al. 2005 , 2006 ), are still the subjects of numerous investigations.
Aeronautical engineers mimic seabirds skeletal structures “by designing load- bearing
structures with shapes that confer strength, and by using materials that have high
strength-to-weight and stiffness-to-weight ratios,” (Dumont 2010 ).
The important role of the shape in the mechanical properties of bones is well
accepted in the literature. For example, “it is well known that given a tube and rod
1 Introduction
“Dense compact bone bears bending or torsional stresses on the periphery of the
shaft. Less dense internal supportive tissues lend secondary support, or are lacking
entirely, with a marrow-fi lled cavity lending haematopoietic function,” (Bostwick
et al. 2012 ).
Wings contain some of the longest and strongest bones. Flying animals appear to
have denser bones than terrestrial counterparts of similar size. In bones, differences
in density generally mean differences in mineral content. Higher mineral contents
are linked to greater stiffness and strength. Dumont ( 2010 ) thinks that such material
properties may be central to understanding why bone density is relatively high in
birds. Along with fl ight, birds have also evolved small body size; and one way to
design a small skeleton while preserving its strength is to make its bones denser.
Investigating links between the material properties of bone and the evolution of
fl ight and small body size in birds is going to require much more work – as we
would need to obtain bone density measurements from transitional forms in the
fossil record like Archaeopteryx (Hone et al. 2008 ).
The skeletons of birds are described in detail (see for review Shufeldt 1890 ;
Baumel et al. 1993 ; Kaiser 2007 ; Hospitaleche et al. 2009 ). These constructs are
examples of selection for minimizing the energy required for the bird’s fl ight. “From
a functional perspective, the weight (mass) of an animal relative to its lift- generating
surfaces is a key determinant in the metabolic cost of fl ight. The evolution of birds
has been characterized by many weight-saving adaptations that are refl ected in bone
shape, many of which strengthen and stiffen the skeleton” (Dumont 2010 ).
According to this author “the round and thin-walled humeral shafts of birds are an
optimal shape for resisting both torsion and bending, and fused skeletal elements
have been interpreted as increasing force resistance (i.e. stiffness; Buhler 1992 ),”
(Dumont 2010 ). It was observed that fl ightless and diving birds possess thickerwalled bones which may serve as reinforcement or ballast. In diving seabirds,
the rib cages and bones must be stronger and heavier, respectively, to recover from the
increased pressure during a dive. During this process, the rib cage collapses inwards
but springs back when the dive is achieved (see for review Norberg 1985 ).
The evolution of fl ighted birds can be characterized by the expansion of
pneumatized spaces within some bones as well as by gradual reduction and fusion of
many skeletal elements. Correspondingly their bones are typically lighter, but denser
than that in mammal skeletons. Also bird’s skeletons must be stronger and stiffer
because of their ability to withstand the mechanical forces encountered during fl ight.
It is little wonder that most seabirds have partially hollow or hollow bones
(Fig. 1.10 ). The morphological properties of seabird bones that allow fl ight (Casinos
and Cubo 2001 ; Simons 2010 ; Simons et al. 2011 ), as well as their biomechanics
(Pennycuick 1967 ; Kirkpatrick 1994 ; Cubo and Casinos 1998 , 2000 ; Mi et al. 2005 ;
de Margerie et al. 2005 , 2006 ), are still the subjects of numerous investigations.
Aeronautical engineers mimic seabirds skeletal structures “by designing load- bearing
structures with shapes that confer strength, and by using materials that have high
strength-to-weight and stiffness-to-weight ratios,” (Dumont 2010 ).
The important role of the shape in the mechanical properties of bones is well
accepted in the literature. For example, “it is well known that given a tube and rod
1 Introduction