Essay C, Student 9, Expert Marker
Describe and discuss the influence of environmental temperature on
metabolism in poikilotherms and homeotherms.
Poikilotherms are animals whose internal body temperature varies along with variation in
the temperature of the external environment. Homeotherms, in contrast to this, are animals
that maintain a constant body temperature regardless of variation in the external
temperature. Internal body temperature effects metabolic rate as it influences the rate of
physical, chemical and biological processes within the body, such as diffusion, enzyme
activity and the nerve transmission. In homeotherms there is an added dimension to this
relationship as for homeotherms to actively raise or lower their internal temperature
requires energy and hence constitutes a rise in metabolic rate.
In should be noted that while it may seem that all homeotherms must be endotherms and
all poikilotherms must be ectotherms the relationship is in fact not so clear cut. It is
possible, for example, for ectotherms to be homeothermic by using behavioural means to
maintain a constant body temperature. Or for some poikilotherms to have a very stable
body temperature simply because the temperature of the environment they inhabit varies
very little, for example many deep sea fish inhabit waters that are always between -0.5 and
0.5°C.
Homeotherms have a thermal neutral zone, within which they can regulate their internal
temperature just by adjusting how close to the skin blood flows by either vasoconstriction
or vasodilatation of their arterioles. This process requires no additional input of energy and
insulating layers e.g. blubber or fur, can be used to aid its effectiveness. However, when the
ambient temperature is above or below the critical limits of this zone then additional
measures are required to maintain a constant body temperature and these involve the use
of additional energy and hence a rise in metabolic rate.
When the environmental temperature drops below the lower critical, this triggers a rise in
metabolic rate and the heat generated by this is used to raise body temperature. This
process is known as thermogenesis.
Thermogenesis can be either shivering (skeletal muscles utilise ATP and pull against each
other generating heat) or non-shivering. Non-shivering thermogenesis (in mammals) relies
on specialised adipose tissue called Brown Adipose Tissue or Brown Fat. Brown adipose
tissue contains many more mitochondria than white adipose tissue and these are used to
produce heat without shivering. The mitochondria in brown adipose tissue contain the
protein thermogenin (uncoupling protein 1), which gives them the ability to uncouple
proton transport from oxidative phosphorylation, allowing them to utilise substrates to
generate heat rather than ATP. Brown Adipose Tissue is mainly around the neck and large
blood vessels of the thorax and has a much higher blood supply than white adipose tissue to
allow all heat generated to be easily spread to the rest of the body. Clearly both shivering
Essay C, Student 9, Expert Marker
and non-shivering thermogenesis both require energy and so increase the bodies metabolic
rate.
Above the upper critical temperature active cooling mechanisms are deployed. This is
generally achieved through evaporative cooling i.e. panting or sweating. Evaporation of 1g
of water absorbs 580 calories of heat, so evaporative cooling can be highly effective. Its
major drawback is that active cooling is often most required in environments where water is
in short supply, hence water lost through evaporation may be hard to replace.
Homeotherms in arid environments therefore typically exhibit other additional adaptations
for heat loss, either anatomical or behavioural. However, overall, evaporative cooling is still
the main method of active cooling in homeotherms, and as a process that requires energy, it
also leads to a rise in metabolic rate.
Homeotherms therefore have raised metabolic rates when the temperature of their
external environment differs significantly enough from their ideal internal temperature as to
cause them to need to generate additional heat or to employ active cooling mechanisms.
Poikilotherms do not attempt to control their body temperature, and instead allow their
body temperature to fluctuate with the environment. Therefore, rather than more extreme
temperatures triggering a rise in energy use and metabolic rate, they instead tend to have
an exponential relationship between increasing external temperature and increasing
metabolic rate. This is caused by rising environmental temperatures (and hence rising body
temperature) allowing for faster physical, chemical and biological processes such as enzyme
catalysed reactions, which therefore allows the metabolic rate of the organism to increase.
This relationship can be described using the Temperature Quotient (Q10). The Temperature
Quotient shows how much the organism’s metabolic rate changes for a 10°C change in
environmental temperature; so a Q10 of 2 indicates a doubling of metabolic rate for a 10°C
increase in temperature, while a Q10 of 3 indicates a tripling. To give an example, the Q10
of most ectothermic fish is 1.86.
The result of these two contrasting methods for reacting to changing environmental
temperatures is that for homeotherms their metabolic rate will be lowest (basal metabolic
rate) when resting at an environmental temperature within their thermal neutral zone, as
they will then not have to expend energy on temperature regulation. Above and below this
point metabolic rate steadily increases until the environmental temperature becomes too
extreme for the animal to cope. When this point is reached varies between species.
For poikilotherms, on the other hand, their lowest metabolic rate will correspond with low
external temperature, as low external temperature slows their body processes and
therefore acts directly as a limiting factor on their metabolic rate. As external temperature
increases, their metabolic rate will increase. While this relationship is always exponential,
the exact rate of increase varies between species.
Essay C, Student 9, Expert Marker
Overall, perhaps the most significant difference is that homeotherms always have far higher
metabolic rates than poikilotherms and hence have a far higher demand for food. Even
when at rest within the animal’s thermal neutral zone homeotherms have a far higher basal
metabolic rate than the metabolic rate of a similar-sized poikilotherm at any temperature.
Reference List:
CAMPBELL, N. A. & Reece J. B. (2008) Biology, 8th Ed. Pearson.
CANNON, B (et al). 2004. Brown adipose tissue: function and physiological significance. PubMed, 84,
(1), 277-359.
PURVES, W. K. (et al). (2002) Life, the Science of Biology (7th ed.) pg 785-790.
RANDALL D. (et al). (2001) Ekert Animal Physiology. Mechanisms and Adaptations (5th ed.)
Freeman, pg 704-711.