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Comparison of the “mammal machine” and the “reptile
machine”: energy use and thyroid activity
Summary of Journal Article
• Experimenters:
• Measured oxygen consumption of Amphibolurus nuchalis
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(Lizard) and Mus musculus (Mouse) by the amount of
energy used at the cellular level while in oxygenated
Ringer solution.
Inhibited the Na+-K+-ATPase of each animal’s tissues
with ouabain and then compared the results to
oxygenated ringer solution results (exp 1 and exp 2)
Assessed the thyroid activity of each animal by
measuring the release of a radioiodine.
Measured growth rates by measuring snout-vent length.
Oxygen Consumption: Experimental
Procedure and Results
Materials and Methods
• Experimenters sacrificed the animals by
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decapitation and within 30 minutes extracted the
liver, kidney and brain of each animal.
Oxygen consumption was then measured every
30 minutes for 2 hours at 37 degrees Celsius.
Weighed wet values and dry values of each
animal’s tissues.
Calculated total organ metabolism values from
combined respective oxygen consumption
values.
Results
• In vitro metabolism dry weight :
• Kidney and liver oxygen consumption was
5X greater in the mouse tissue.
• Brain oxygen consumption was 2X greater
in the mouse.
DIAGRAMMATIC REPRESENTATION OF ENERGY
CYCLE SHOWING PARAMETERS MEASURED
Sodium Transport Metabolism:
Results
• The mouse liver had 9X the metabolic rate of
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sodium-potassium transfer than the lizard.
The mouse kidney had 3.5X the metabolic rate
of sodium-potassium transfer than the lizard.
The mouse brain had 7X the metabolic rate of
sodium-potassium transfer than the lizard.
Thyroid Gland Activity: Experimental
Procedure and Results
Materials and Methods
• Experimenters injected radioiodine into the thyroid tissue of each
animal because the radioiodine would be used by the tissue to
secrete the growth hormone thyroxine.
• After the animals were sacrificed blood was collected and
centrifuged in order to separate out the blood serum. The serum
was then stored in liquid nitrogen for 4 months. The amount of
thyroxine in the blood serum was then tested using a thyroxine
radioimmunoassay kit.
Results
• Results were inconclusive due to the fact that the experimenters
had no way of knowing how much iodine was taken up into the
thyroid in order to make growth hormones.
Growth Rates: Experimental
Procedure and Results
Materials and Methods
• Measurements from the snout to vent of baby mice were taken daily
and captured juvenile lizards were measured weekly under constant
temperature of 25 degrees Celsius.
• The final part of the growth experiment had the lizards kept at a
increased temperature of 37 degrees Celsius.
Results
• The juvenile lizard’s growth rates were influenced by their body
temperature but they’re growth rate was still significantly less than
that of the mice.
Theories and Discrepancies
• The results obtained for the three tissues
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represents the oxygen consumption of the
cellular processes needed to sustain itself. In the
living animals oxygen consumption can be many
times higher when they are performing functions
for each organ.
Ideally metabolic rate studies should be
performed on mature animals and not juveniles
which was not the case in this study.
• The results gathered from the study of growth between
ectotherms and endotherms suggest an increased ability
for rapid growth that exists due to the higher metabolic
rate of endotherms.
• Experimenters suggest that increased temperatures lead
to increased thyroid gland activity and as a result leads
to an overall increase in metabolic rate as well as growth
rate. There is much debate as too how the thyroid
hormone actually does this.
Theories explaining high energy consumption of
Na+-K+-ATPase in mammals
i) mammalian sodium pump may be less efficient than that of
the lizard
ii) the mammalian tissue may need to transport significantly
higher amounts of sodium and potassium across it’s cellular
membranes. If this is the case then:
a) the amount of cellular membrane per unit volume of
tissue may be greater than that of the lizard
b) the passive permeability of the mammal membrane
may be much higher than that of the lizard
c) the concentration gradients of sodium and
potassium maintained across the membrane may be
greater than that of the lizard
d) any combination of these could be possible
Very little research has been done in this area.
Evolutionary Implications of Endothermy
•The switch from the reptilian metabolic “machinery” to the
mammalian metabolic “machinery” was an evolution of the
metabolic ability to achieve endothermy.
•This benefited the transition animals in many ways:
–It allowed animals to live in colder environments
–It allowed much higher levels of sustained activity
–It allowed for faster growth and maturity
–Another intriguing possibility is that it also may have increased
the rate of evolution
•It also has one big disadvantage in that these animals
require much more food than other organisms of similar
size.