Changes

==='''Allometry and scaling'''===

==='''Allometry and scaling'''===

Allometry is important mechanically if we recognise that developmental patterns in the musculoskeletal system arise because of scaling.  Does a newborn animal differ in musculoskeletal proportions from an adult because of its size?  Is a species of small size inevitably different in musculoskeletal proportions from a large maturing species?   

Allometry is important mechanically if we recognise that developmental patterns in the musculoskeletal system arise because of scaling.  Does a newborn animal differ in musculoskeletal proportions from an adult because of its size?  Is a species of small size inevitably different in musculoskeletal proportions from a large maturing species?   

The scaling problem that has interested investigators since Galileo (Fig. 7.5) is that while body weight scales as f3, the structures supporting it scale as f2.  Many have followed Galileo to the conclusion that larger animals compensate by developing disproportionately more massive limbs. However, Fig. 7.6 shows that a million-fold increase in body weight is insufficient to reveal a consistent change in the proportions of forelimb bones of quadrupeds.  Neither is the change in shape of the bones of the pig during a nearly one hundredfold increase in body weight from birth to maturity consistent with the idea of more massive skeletons in heavier animals (Fig. 7.7).  

The scaling problem that has interested investigators since Galileo (Fig. 7.5) is that while body weight scales as f3, the structures supporting it scale as f2.  Many have followed Galileo to the conclusion that larger animals compensate by developing disproportionately more massive limbs. However, Fig. 7.6 shows that a million-fold increase in body weight is insufficient to reveal a consistent change in the proportions of forelimb bones of quadrupeds.  Neither is the change in shape of the bones of the pig during a nearly one hundredfold increase in body weight from birth to maturity consistent with the idea of more massive skeletons in heavier animals (Fig. 7.7).  

Postnatally, the proportion of bone in the body declines (Figs. 7.4 and 7.8).  Apparently, an immature animal needs a greater proportion of its body as bone because growing bone is less mineralised.  Bone mineral, however, maintains a constant proportion of carcass weight (Fig. 7.8).  Muscle tissue, forming approximately 40% of the body mass, is too large a proportion of body mass to adapt by increasing this proportion.  To scale with the same static muscle strength as a mouse, an elephant would need to be almost 100% muscle!  In fact real mammals of different size, within and between species, have approximately the same muscularity (Figs. 7.4, 7.8 and 7.9).   

Postnatally, the proportion of bone in the body declines (Figs. 7.4 and 7.8).  Apparently, an immature animal needs a greater proportion of its body as bone because growing bone is less mineralised.  Bone mineral, however, maintains a constant proportion of carcass weight (Fig. 7.8).  Muscle tissue, forming approximately 40% of the body mass, is too large a proportion of body mass to adapt by increasing this proportion.  To scale with the same static muscle strength as a mouse, an elephant would need to be almost 100% muscle!  In fact real mammals of different size, within and between species, have approximately the same muscularity (Figs. 7.4, 7.8 and 7.9).   

:::::'''Fig 7.6 Bone proportions in terrestrial quadrupeds'''  

 

 

 

 

:::::'''Fig 7.6 Bone proportions in terrestrial quadrupeds'''  

:::::Animals can vary in size with no apparent necessity for changes in skeletal proportions.   

:::::Animals can vary in size with no apparent necessity for changes in skeletal proportions.   

:::::'''Fig 7.7 Postnatal change in shape of the radius and ulna of the pig'''

 

 

 

 

 

 

 

:::::'''Fig 7.7 Postnatal change in shape of the radius and ulna of the pig'''

:::::The bones are drawn to the same length.  Unossifiied regions of the immature bones are shown blue-grey.  There is no apparent requirement for the bone in the adult to be more massive than that in the neonate.

:::::The bones are drawn to the same length.  Unossifiied regions of the immature bones are shown blue-grey.  There is no apparent requirement for the bone in the adult to be more massive than that in the neonate.

:::::'''Fig 7.8 Postnatal growth of muscle and bone in the pig'''  

 

 

 

 

 

 

 

 

 

 

 

:::::'''Fig 7.8 Postnatal growth of muscle and bone in the pig'''  

:::::During a hundred-fold increase in carcass weight, from which the variable effect of adipose tissue has been removed, total muscle and the mineral content of the limb bones grow proportionately. "Bone", which here includes periosteal, cartilage and medullary tissues, comprises a greater proportion of the carcass in the immature animal than in the adult.   

:::::During a hundred-fold increase in carcass weight, from which the variable effect of adipose tissue has been removed, total muscle and the mineral content of the limb bones grow proportionately. "Bone", which here includes periosteal, cartilage and medullary tissues, comprises a greater proportion of the carcass in the immature animal than in the adult.   

:::::'''Fig 7.9 Muscle and bone proportions in mature animals of different size'''  

 

 

 

 

 

 

 

 

 

 

 

 

:::::'''Fig 7.9 Muscle and bone proportions in mature animals of different size'''  

:::::There is no apparent need for animals differing in size as much as do dogs and horses, to support and propel themselves using a different proportion of muscle and bone in their bodies.   

:::::There is no apparent need for animals differing in size as much as do dogs and horses, to support and propel themselves using a different proportion of muscle and bone in their bodies.   

:::::'''Fig 7.10 The effect of body size on histochemical fibre type populations'''  

 

 

 

 

 

 

 

 

 

 

 

 

 

:::::'''Fig 7.10 The effect of body size on histochemical fibre type populations'''  

:::::A schematic representation of the relative density of the myosin in ATPase low fibre type of fibre, as observed in transverse sections of the semitendinosus muscle of seven species of mammals. The density in the deep part of the muscle, as well as the extent throughout the muscle, of myosin ATPase low fibres increases with increasing size of the animal.

:::::A schematic representation of the relative density of the myosin in ATPase low fibre type of fibre, as observed in transverse sections of the semitendinosus muscle of seven species of mammals. The density in the deep part of the muscle, as well as the extent throughout the muscle, of myosin ATPase low fibres increases with increasing size of the animal.

=='''The design of limbs'''==  

=='''The design of limbs'''==