L5: Metabolic Rate & Body Size

define scaling

the study of structural and functional consequences of changes in size or scale among otherwise similar organisms

define principle of similarity

objects with the same shape but different sizes are geometrically similar, maintaining constant ratios of lengths, areas, and volumes

define isometric scaling

when all dimensions change proportionally

smaller bodies have _________surface areas relative to their volumes than larger objects of the same shape.

larger

Rubner's surface rule

small animals have larger body surface relative to body mass than a larger animal

how does heat retention work in Rubner's surface rule?

heat is lost through the animals skin ->rate of heat loss is proportional to animals surface area -> heat must be replaced

define allometric scaling

how characteristics of animals disproportionally change with size

why are logs used in allometric equations?

- easier comparisons across wide ranges

- transforms curvilinear into linear form

the value of b (slope) provides information on:

how the variable of interest changes with body size

b =1.0 -> isometric (direct proportion)

b=0 -> no relation

0>b<1.0 -> Y increases slower than x

b>1 -> Y increases faster than x

do allometric equations represent laws?

no, just descriptive

Kleiber relation or mouse to elephant curve

BMRs plotted against body mass

what can affect differences in BMR?

taxonomy

life history traits

0.75 law

metabolic rates always show a mass exponent 0.75 in relation to body mass

(unsure why- maybe Rubners surface law)

* NOT a universal exponent

specific metabolic rate

metabolic rate per unit body mass; decreases with increasing body size

how is oxygen consumption used to scale metabolic rate?

specific oxygen consumption of mammals decreases regularly with increasing body size

why does the rate of oxygen consumption per gram of tissue decrease consistently as body weight increases?

possible factors of lower mass specific metabolic rates of larger species:

1. lower contents of expensive tissues

2. slower ion leak across membranes (lower ion pump)

3. lower mass specific costs of larger species during locomotion are due to lower costs for lower frequency muscle activity with slower myosin + mor elastic energy

how is MMR scaled?

metabolic scope -> ratio of resting and maximum rates of metabolism

in mammals, max mass-specific oxygen consumption scales to body mass to the power of:

-0.25

mass specific enzymes in heart and skeletal muscle scales to body mass to the power of:

-0.21

if specific oxygen consumption increases with decreasing body size, does the density of muscle mitochondria also increase systematically with decreasing body size?

the fraction of muscle fiber volume that's occupied by mitochondria also scales with a slope of -0.21

what is the upper limit of mitochondria volume density in hummingbirds?

50-55%

what are the environmental implications of the scaling of metabolic rate?

dictate how organisms interact with their environment, influencing food web dynamics, population resilience to climate change, and ecosystem-level energy flow. Smaller organisms have higher mass-specific metabolic rates, necessitating more food per unit of body mass and leaving them more vulnerable to environmental shifts, while larger, more efficient animals dominate under different resource conditions