Body Size and Scaling

Question 1

Scaling

Answer 1

the study of body size effects

Question 2

What are the two types of scaling?

Answer 2

• isometric

- allometric

Question 3

Isometric scaling

Answer 3

• proportions do not change
• the baby is an exact mini version of an adult
• ex: Salamander

Question 4

Allometric scaling

Answer 4

• proportion change, and relationships are exponential
• ex: human head size vs. total body length; at first the baby’s head is the same size as body, but then the head becomes exponentially smaller compared to body
• on a log scale relationship looks linear

Question 5

Why does scaling matter?

Answer 5

• May be constraints on adaptation (Ex: theory that insect can’t become too large because of the way they breathe (tube trachea with holes at end instead of lungs). The bigger the insects are, the farther oxygen needs to travel and the more difficult it is to reach all cells…therefore if too big, the oxygen won’t make it all the way around the body with their current organ system – in Paleozoic period, there were higher oxygen levels, allowing insects to grow larger)
• Affects metabolic rate, locomotion, home range size, population density and more…

Question 6

Scaling of metabolic rate

Answer 6

• metabolic rate (R); energy used/time (ex:KJ/day)
• Basal metabolic rate (BMR); energy used/resting time (KJ/hour)
• Differences in body size (mass) account for >90% of variation in metabolic rate

Question 7

Kleiber’s law

Answer 7

• R αMb^0.75 (R is metabolic rate, and M is body mass)

- Animal’s metabolic rate is 3/4 it’s mass

Question 8

Variation in Kleiber’s Law - exponent

Answer 8

• exponent changes: 0.7-0.75 for birds and mammals, 0.6-0.6 for other animals (0.66-0.72 for many invertebrates)
• R = 70Mb^0.75

Question 9

Variation in Kleiber’s Law - Intercept

Answer 9

• Intercept: unicellular organism

Question 10

Relationship between animal body size, energy use and metabolic rate

Answer 10

• Larger animals use overall more energy, but have a lower relative metabolic rate (allometric relationship)
• Why? not sure, theories about geometry (surface area and volume)

Question 11

Relationship between body mass and rate of oxygen consumption

Answer 11

• Body mass increases, oxygen consumption decreases
• Allometric; why?
  • Small animals have more surface area, more surface area in which to lose oxygen, so maybe consumption has to stay high to compensate
  •Cells are the same size, but there are more in bigger animals, so is it more efficient?

Question 12

Rate of metabolism and oxygen consumption decrease as body mass increases; what are consequences for small organisms?

Answer 12

• Need to consume more food
• Breathe faster
• Have more blood per unit tissue
• Have faster heartbeat
• Shorter lifespans

Question 13

Example of when scaling of metabolic rate is NOT allometric?

Answer 13

Daphnia’s respiration rate increases proportionately with body size

Question 14

Scaling of Locomotion

Answer 14

• Measured in net transport cost

- Differences in body size (mass) account for >85% of variation in net transport cost

Question 15

What is net transport cost?

Answer 15

• additional energy (over BMR) used to move 1 kg of body mass/unit distance (J/m/kg)

Question 16

Relationship between body size and net transport cost

Answer 16

• Larger animals have lower net transport cost
• Larger animals use less energy to move a certain until/distance/mass
• Allometric relationship; because it is exponential (about -0.28; not 1:1 increase/decrease)
• Cost of movement is greater for smaller animals
• Less relative cost of movement for larger animals (i.e: relative to body mass), therefore greater efficiency
• Larger animals have more efficient muscles because
  • Slower metabolic rate
  • They have to take fewer strides, make fewer wingbeats etc. to cover same distance
• Maximum speed may also increase exponentially with increasing body mass, especially for swimming and running (allometric)
• Note: flying animals have the lowest transport costs, followed by swimmers and then runners
• Consequences: foraging areas generally increase with size

Question 17

Constraints on size

Answer 17

• Inheritance
• Basic physiological design (ex: circulation; insect size)
• Basic mechanical design (ex: endoskeleton vs. exoskeleton)
• Habitat (ex: similar animals in aquatic vs. terrestrial habitats may be larger because of self-weight)

Question 18

Constraints on lower size

Answer 18

• Aquatic: size of offspring because of cost of thermoregulation (surface area vs. volume - lose heat)
• Terrestrial: metabolic cost and energy availability (would have to eat all the time)

Question 19

Constraints on upper limit

Answer 19

• Bone density and structure (how to hold up own weight)
• Heart size and circulation
• Risk of overheating (lower surface area to volume ratio?)

Question 20

Body size in mammals in late quaternary period

Answer 20

• frequency decreases in increasing size (fewer large mammals)
• Unimodal distribution; why more small mammals? habitats support more smaller and few large, few large carnivores can be adequately fed by many small animals, speciation more likely among small species that are less able to cross geographical barriers
• ** However, across continent and time periods this unimodal relationship has been seen**

Question 21

How does temperature affect body size?

Answer 21

• cold climates –> large animals; why?
• surface to area ratio: in cold climates animals need to keep heat in, so reduce surface to area ratio; in warm it’s opposite