>On 12 Nov 97 at 12:48, Eirik Korsell wrote:
>>> why does small animals have a higer metabolic rate per mass unit than
>> large ones?
>
Everything that has been said in response to this quetion I more or less
agree with.
BUT there is something odd about the question (even though it is a very old
and many people have asked it before).
The metabolic rate of large animals is actually greater (substantially
greater) than that of small animals.
So why aren't we asking why large animals have a higher metabolic rate
than small animals?
We must be expecting animals to get bigger (or smaller) in a very particular
way: that is for them to scale everything in proportion.
But if animals were to get bigger in this particular way, they would be
presented with many problems which have been noted before, including
the disposal of the extra heat, the relative stresses on bones, and the
aquisition
of proportionally more food from the environment.
This being so, why don't animals get bigger (or smaller) by scaling their
metabolically inactive tissues, such as bones, skin and fat, without changing
their active tissues?
Well to large extent this is in fact what animals do.
Most (about 2/3) of the change in metabolic rate per mass unit with body
size is due to
the fact that larger animals have relatively more metabolically inactive mass
(bones, skin, and fat) than small animals. However some (about 1/3) of the
change
is due to a decrease in the metabolic activity of the cells. This decrease in
metabolic activity of the cells is not due to any one particular component of
metabolism, most activities decrease, although not neccessarily in proportion.
Of course working with larger bones and body is likely to require more muscles,
etc. but there is no reason to expect that a proportional increase is required.
We need to think about why an animal would want (or rather evolve by selection)
to be bigger (or smaller) before we can predict how we expect things to scale.
There is no obvious reason why everything should scale in proportion.
Some of the metabolic issues are covered in our recent review:
Cellular energy utilisation and the molecular origin of standard metabolic rate
in mammals" D.F.S. Rolfe & G.C. Brown (1997) Physiol. Rev. 77, 731-758.
Dr Guy C. Brown
Department of Biochemistry,
University of Cambridge,
Tennis Court Road,
Cambridge CB2 1QW,
UK
Tel: +44 (0)1223 333342
Fax: +44 (0) 1223 333345
