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Metabolism II -- Body Size, Endothermy vs. Ectothermy 12 Oct 2005
® R. B.Huey
Goals for This Lecture:
- Understand how size, temperature, and endothermy influence metabolic rate
- Understand scaling of “mass-specific” metabolic rates (i.e., per gram or per cell)
- Understand that field metabolic rates are much higher than those in the lab
I. How body size and endothermy/ectothermy influence metabolic rate (SN192-198)
A. A brief digression on terminology
1. Endotherm (birds, mammals) – animals with high metabolic rates and that maintain high
body temperature via adjustments in metabolic heat production.
2. Ectotherm (everything else!) – animals with relatively low metabolic rates and whose
body temperature depends mainly on external sources of heat
B. Because many factors influence metabolic rate (see Metabolism I), metabolic comparisons
must be based on standardized conditions. Which are:
- Basal metabolic rate (BMR) for endotherms. Animal should be fasting, resting, normal
sleep period, in thermal neutral zone (“tnz”), non-reproductive, not growing. 2. Standard
metabolic rate (SMR) for ectotherms. Animal should be fasting, resting, non-
reproductive. In reporting SMR, the ectotherm’s temperature must be reported_. Why?_
- Why does metabolic rate of endotherms increase (± linearly) as ambient temperature
decreases below the thermal neutral zone?
- Why does metabolic rate of ectotherms increase with body temperature?
C. Body mass (size) is the dominant influence on whole-animal metabolism. How does
metabolic rate change (“scale) with body size? For example, if species A is twice as heav y as
species B, is the metabolic rate of A twice that of B?
- To answer that question, we first measure the metabolic rate of several species of different
sized animals (text Table 5.8). Then we fit a “power” or “allometric” function (E
= a M
b ,
see and the Allometry “handout” ), which conveniently summarizes all these data.
- If and only if the exponent b = 1, then the answer is “yes” to the above question: in this
case, the relationship will be a straight line, as shown on the figure on the next page.
Ambient temperature Ambient temperature
Metabolic
rate
endotherm ectotherm
“tnz”
BMR
- In fact, for most organisms, b is actually close to 0.75 (or ~ ¾). This means that total
metabolic rate increases with body size, but not in a constant proportion, such that the line
isn’t straight but is curved. Thus the metabolic rate of species B isn’t twice that of A but
is something less than that.
D. Typical values for vertebrates, where E
in ml O 2
h
, M in grams
________________________________________________________________________________
Taxon a b time (h) for a 1 g animal
to use 10 ml O 2
________________________________________________________________________________
Endotherms
passerine bird (42°C) 7.5 .72 1.
placental mammal(37°C) 3.8 .75 2.
marsupial (35°C) 2.3 .75 4.
average = 4.5 .74 2.
Ectotherms
lizard (37°C) .42 .82 23.
frog (ranid) (25°C) .29 .75? calculate
fish (25°C) .20 .70 50
beetles (22-25°C) .23 .86? calculate
average = 0.4 .78 43.
_________________________________________________________________________________
( Exercises: 1) fill in the "?" values in the table using the allometric values given, 2) calculate the times
for a 100-g animal of each taxon to use 10 ml O 2
E. Key generalizations
- Exponents ("b") similar among groups, average about 3/4. Therefore, in most taxa,
metabolic rate increases with mass but not in direct proportion to mass.
- Endotherms (of a given size) have much higher metabolic rates than do ectotherms of an
equivalent size (thus larger "a" by about 10 times on average!). A huge difference.
- Multicellular ectotherms have higher E
than do unicellular ectotherms (Fig. 5.11)
Body mass
Metabolic rate
b = 1
b = 3/
not 100%. A necessary consequence is that 1 g of tissue of the small animal must have a
higher metabolic rate than 1 g of the large animal tissue. (Figs. 5.9, 5.10)
- To show this, divide an animal’s metabolic rate by its mass. sample values: .75 kJ/ (g day)
for mouse, .21 for cat, .11 for human, .05 for cow, .03 for elephant.
- One gram of elephant has metabolic rate only 4% of that of 1g of mouse! Or 1 g of mouse
is producing ~20X (20 times, not %!) heat per unit time as a gram of elephant.
- A consequence of this pattern – a shrew (smallest mammal) eats nearly its body mass in
food per day, whereas a large elephant doesn't (fortunately for zoos and for Africa).
4 Metabolic rate per gram of animal, is called mass-specific metabolic rate.
C. The scaling of mass-specific E
can be described by a power function. (Simply divide both
sides of the whole-animal metabolic equation by M. Thus, for placental mammals, E
/M =
3.8M
-.
or for lizards = 0.42M
-.
). [ Be able to graph this on arithmetic and log-log axes.]
D. This "inverse" relationship holds for all organisms. (because all b < 1).
IV. How metabolic rates of “free ranging” animals compare with those in the lab?
A. Typical metabolic data are from animals at rest in the lab & necessarily underestimate
metabolic rates of animals in nature, which are moving and digesting, at least part of the time.
By how much do lab measurements underestimate metabolism of free ranging animals?
B. Dual-isotope technique ("doubly-labeled" water, a special “handout” will be available ) is
now used to estimate metabolic rates (FMR) of free ranging animals in nature.
C. Example: Bennett and Nagy (1977) compared lizard, bird, mammal -- all about 12 g in mass.
Lizard had low T b
(= body temperature) at night. First, Bennett & Nagy calculated metabolic
rate based on lab studies and field T b
profiles, then based on isotope studies.
- Metabolic requirements in the real world are roughly 3X that in the laboratory!
- FMR of a mammal is about 26X that of lizard, bird FMR is 40X that of lizard!
- These huge increments (relative to lab differences of only 8X and 14X, respectively)
reflect greater activity birds and mammals and to the lowering of T b
by lizard at night.
D. Ecological and physiological (see below) consequences of high metabolic rates of endotherms
must be profound. They have high impact on the environment (require lots of food and O 2
and the activity of their physiological systems (respiratory, circulatory, osmoregulatory,
digestive) must be very high.
E. Study the appendix on field metabolic rates – you should understand how doubly labeled watr
works.
