L6 Flashcards
What is optimality theory?
- In order to maximise individual fitness animals should forage as efficiently as possible
-Animals should balance benefits of caloric intake and costs eg time and energy expense- Usually graphical models
- Can predict how and animal should go about optimising its or its offspring’s food intake
Example: North Western Crows
- Feed on whelks
- Drops welk at height and eats soft body of whelk
- Which welk should it choose, height of dropping it, how many times should it drop it before it gets another?
Observations of Whelk feeding
- Crows select large whelks
- Drop from 5m height
-Don’t move onto another welk after it doesn’t break
- Drop from 5m height
Predictions
- Large whelks should be more likely to break at 5m than smaller whelks
- <5m results in fewer breakages
- > 5m breakage plateaus
- Each time a whelk is dropped it has the same chance of cracking
- Large whelks take fewer drops to crack than smaller sizes
- This amounts to a large reduction in cost
- Larger also provide more calories
- 5m was the optimum drop height, going higher doesn’t increase breakage chance that much more
- Also takes longer to retrieve whelks at higher height so more chance another animal may steal it
- Dropping it x amount of times is independent
What causes a mismatch between cost benefit logic - why might an experiment prove an animal’s foraging not to be optimal? (4 reasons)
- The animal may not have been well ‘designed’ by selection
- Environment may have changed an evolution hasn’t caught up
- Observations weren’t appropriate
- Experimental design needs to be reassessed - An important factor may have been omitted from the model
- The assumptions may not have been valid
Example: Oystercatchers feeding on mussels
- Smaller mussels were chosen than it was predicted
-Birds behaviour did not match model as large mussels were impossible to open- Largest mussels should theoretically be taken by oystercatchers
- Size chosen is far less than theoretically optimal as they cannot open them
- When mussels they can open is considered then their foraging is close to optimal
- Food availability has placed a constraint on an individuals ability to forage optimally
Nutrient quality is more important to herbivores than carnivores: Moose
- Moose feed in two habitats
- Deciduous forest leaves which are high in energy but low in sodium
- Aquatic vegetation which is high in sodium but low in energy
- Herbivore digestive system can only process so much as it is hard to digest
- Stomach space is an additional constraint
What combination of aquatic and terrestrial vegetation should moose eat for optimality?
- Large amount for aquatic and small terrestrial or vice versa
- Have to take in enough of both
- Minimum sodium intake means a certain amount of aquatic vegetation
- Digestive bottleneck means they cannot take in too much food
- Moose cant exist just on aquatic plants as its rumen is too small
- Maximising sodium intake requires more aquatic material
- Maximising energy intake requires more terrestrial vegetation
- Moose maximise energy intake whilst meeting sodium requirements
-There is an optimal diet to be above sodium constraint, below rumen constraint and above the energy constraint
Charnov’s marginal value theorem
-Do animals balance cost benefits?
-Foraging environments tend to be patchy with food distributed in clumps
- Eg a swarm of krill
- Patch feeding requires knowledge of how long to spend in patch before moving on
- As time goes on in patch rate of food intake decreases
- Loading curve
- Curve of diminishing returns
- Returns diminish as eg carrying weight, less food resource
When to give up on a feeding patch?
- If it gives up too early on a patch there is too much travel time travelling and there is little reward
-Too late wastes time foraging ineffectively- Optimal patch time
-Travel time to patch needs to be least as possible for greatest resource size - Maximum benefit means travel time has to hit loading curve at greatest intake of energy
- A longer travel time means longer in patch to maximise energy gain
- Bigger shop for greater travel time
- Optimal patch time
What is the loading curve in Charnov’s marginal value theorem?
Loading curve
- Curve of diminishing returns - Returns diminish as eg carrying weight, less food resource - as longer time is spent in patch the fitness diminishes to a plateau
Do animals behave according to charnov’s theorem? : Starlings
- Starlings with chicks have to fly long distances
- How many prey items should be collected on each trip to maximise food delivery?
- It is harder to find food whilst carrying prey so they get diminishing returns
- Shorter travel time means patch time should be shorter and vice versa so less food collected
Experiment
- Trained starlings to collect mealworms from a tray at which they could be dropped at a certain rate giving birds diminishing returns
- Load size increases with increasing distance from the nest
- Close correspondence between observed load size and predicted loads
- Starlings have to be concerned about predation, self feeding etc
- Charnov is accurate and a robust model
The four assumptions of the MVT
1.Travel time between patches is known
2.Travel costs = patch time costs
3.Patch profitability is known
4.No predation
Travel time between patches is known
- Assumption is true if going backwards and forwards to feed, but if searching for eg a patch of krill distance is often unknown
