Lecture 34: Size And Scale

Question 1

Effects of mass

Answer 1

Surface area: proportional to square of linear dimensions-also affects surface tension
Volume: proportional to cube of linear measurement

Question 2

Growth

What are the 2 type of growth?

Answer 2

Isometric growth:

• all parts grow relatively at the same speed
• geometric similarity is retained- isometry
• unusual in living organisms, as factors of size, gravity and body part functions normally operate

Allometric growth:
-parts do not grow equally- allometry (unequal measure)
Eg human babies have big heads and small limbs and trunks

Question 3

How do we define allometry?

Answer 3

Allometry
-relative of differential growth of a part in relation to an entire organism or to a standard
-the measure and study of such growth
Refer to page 6 for visual

Question 4

Metabolism and mass

Answer 4

Many body processes involve exchange across surfaces
Eg respiration, digestion
-surface area is limiting factor-increases by square of the dimensions whereas the mass increases by the cube
Area is about equal to 2/3 V?
Large animals have to increase relative metabolic areas
Eg length of digestive tract, the area if lungs for respiration

Question 5

Consequence of body mass and energy use

Answer 5

Small animals require much more energy per unit mass than large
Eg shrew consumes 2-3 times body mass per day as they need to maintain body heat.
-heat loss is proportional to surface area- an insulating surface alone cannot suffice
While large animals need more absolute energy, larger bodies are easier to maintain
Eg guinea pig weighs 0.7kh but requires 223 calories per kg to maintain itself
-a whale weighs 150000kg but requires only 1.7calories per kg
Large animals are less likely to be preyed on

Question 6

Size and motility

Answer 6

Body mass and surface area have strong effects on the options for motility mechanisms
sperm cell can rely on a single flagellum
-strongly affected by surface tension effects
Fish need muscles
Small animals are relatively more mobile than large
-thus elephants have massive legs, can’t jump, and can’t move out if a fast walk

Question 7

What are the forces acting on the body?

Answer 7

Compressive forces:
-tend to collapse structures eg weight
Tensile forces
-tends to pull apart eg muscles on tendons
Shear forces
-twisting, sliding of one section on another
Eg rotation of spine

Question 8

How does the skeleton respond to stresses?

Answer 8

Bone subjected to continuous stress
Intermittent stress also induces bone remodelling to reinforce it
-Wolff’s law
-Antagonistic actions of osteoclasts and osteoblasts but the mechanism is unclear
-bony spicules of spongy bone reflect the lines of stress/ force

Question 9

Bone under stress

Answer 9

Bending is accompanied by compression and tension at opposite sides of a hypothetical bone
Note that stresses are greatest at the edges so bones can afford to be hollow

Question 10

What are the determinants of body size in land animals?

Two rules?

Answer 10

Bergmanns rule:

• homeothermic animals from higher altitudes and latitudes, or bred at lower temperatures, tend to be larger
• surface-to-volume ratio decreased for larger objects. Smaller objects lose heat faster as a consequence

Allen’s rule:

• the size of extremities (limbs, tail, ear, etc.) increases from cold to warmer climates in the same species or in closely related species.
• body heat is dispersed more readily by large extremities

Question 11

Climate change and body size

Answer 11

35 of 85 animal and plant species showed a documented duct ion in size over decades, including a type of Scottish sheep that is 5 per cent smaller than it was in 1985.
Polar bears which rely on sea ice during the summer, are also shrinking

Question 12

Giantism and dwarfism

Answer 12

Many vertebrates have tended to giantism

• freedom from predators (except for humans)
• wide range possible
• more efficient energetics
• low surface: volume ratio -heat

Dwarfism:

• tends to have occurred in islands or restricted environments
• restricted resources prevent large size, encourage more rapid life cycles

Question 13

Continental size sets the upper limits

Answer 13

Maximal body mass relates to the size of the landmass

• larger animals require larger territories to find food
• as more food is available to herbivores they tend to grow larger than carnivores
• ectotherms tend to be larger than endotherms (lower food requirements)
• migration from large to small territories results in loss of body mass

Question 14

Effect of human preditation

Answer 14

Humans hunted the Bison, particularly the large males

• extensive selection pressure plus climate change and use of fire resulted in marked changes to this species.
• smaller, shorter horns
• intensive herding behaviour
• mass reproduction
• colour changes emphasising male “size”
• permanent changes in genotype and phenotype

Question 15

What are the consequences of large size?

Answer 15

Reduced trunk flexibility
Eg fused lungs and rib cage in elephants
Reduced limb flexion
Skeleton modified
-compressive forces increased, shear forces reduced
-major support shifts from muscle to bone
Limb bones pillar like, vertical
-horizontal joint surfaces
-proximal portions larger than distal
Feet large, may be padded
-normally all digits retained
Safety
-size protects from most predators
-however slow reproduction means rapid extinction can be a risk