3: Body size

Question 1

Describe Blackburn & Gaston’s (1994) graph on bird body size

Answer 1

Log normal distribution
Skewed to the left
= more small species than big

Question 2

Why can mammals get much larger than birds?

(reminder: largest mammals are aquatic)

Answer 2

Heat dissipation/over-heating
- Water has 4.23 specific heat capacity of air
- So aquatic endotherms can get bigger than terrestrial endotherms without risking overheating

Mass bearing/buoyancy of water
- Buoyancy higher for denser materials

Question 3

Why can terrestrial mammals get larger than birds?

Answer 3

Mammals have 4 limbs, bipedalism of birds constrains their ability to have a large body mass

Question 4

Describe how body size controls energy use

Answer 4

→ energy use determines how many individuals can live in an area e.g pop density/size
→ pop. density/size determines lots of key ecological attributes:
Extinction risk
Competition risk
Predation risk

Question 5

Describe the Metabolic Theory of Ecology (MTE)

Answer 5

→ extends relationship between mass and metabolic rate to temperature
= warmer individuals (warmer climates or endothermy) have higher metabolic rate

Argues that metabolic rate is the primary constraint that determines all biological processes (by determining rate and timing)

Question 6

Describe how MTE works on an individual level

Answer 6

Small animals - grow fast, breed early, die young
→ MTE life history traits constrained by metabolism
- Small animals have higher metabolic rate relative to a given tissue mass
- Metabolic rate produced trade-offs e.g free radical production causes cellular damage accelerates senescence and ultimately death
- Selection favours organisms which best propagate given these constraints
- So, smaller shorter lived organisms reproduce earlier

Question 7

Describe why there are more small species than large and more species in the tropics

Answer 7

• Rate of molecular evolution scales with metabolic rate (free radicals again - cause mutations)
• Faster molecular evolution = faster speciation
• Smaller species and those in warmer areas (tropics) have faster speciation rates so more species

Question 8

Smaller bodied species will respond to the environ at ______ spatial scales

Answer 8

smaller

Question 9

Describe the relationship between body size and extinction risk

Answer 9

• Larger bodied species will generally have smaller pop. size = more likely to be a risk
• Larger bodied species have much slower reproductive rates (life history correlates) = less able to recover quickly
• Persecution/hunting

= true for all groups apart from amphibians (smaller bodies have a greater extinction risk)
- Due to deforestation changing forest structure
- Chytrid fungus, exposure may be higher for smaller bodies due to SA:V

Question 10

Describe Bergmann’s rule

Answer 10

→ Observed that body size increases at high latitudes in mammals (o.g Moose in N. America)

= Tendency for positive association between the body mass of species in a monophyletic higher taxon and the latitude inhabited by those species
- General rule for species that occupy large range in latitude

Question 11

Describe how Bergmann’s rule applies to higher taxonomic groups

Answer 11

→ becomes less strong, largely disappears at family level
= driven by differences in species rather than taxonomic groups

Question 12

Describe Allen’s rule (1877)

Answer 12

→ Observed the length of appendages in closely related endothermic vertebrates increased in hotter environments