Optimality and Economic Decisions Flashcards
Optimality theory
- Posits that animals have evolved to make decisions that maximize their overall fitness
- Fitness ~ Reproductive Success Conferred by Behavior
- Optimization: Striking a delicate balance between the costs and benefits of different behavioral strategies
Costs and benefits in behavior?
Costs:
- Energy expenditure
- Predation risk
- Risk of injury
- Missed opportunities/time costs
Benefits:
- Acquiring resources
- Securing a mate
- Social status
- Avoiding danger
Optimality models
- Provide a formal framework for quantifying trade-offs inherent in decision-making
- Mathematically model the costs and benefits of different behavioral choices
- Predicts what strategies animals should adopt under specific circumstances
- Can account for factors like resource availability, competition, predation risk, social dynamics
- Generates a testable hypothesis about behavior
- Note: Optimality models assume nature is optimal!
The optimal clutch size for Great Tits is ~9. However, we see that birds usually lay fewer eggs than predicted. Why?
Two hypotheses:
1) Success per brood vs. lifetime success:
* Selectionshouldfavorstrategiesthat maximize lifetime RS
* Increasedbroodsizesarecostlytoadult survival (chances of future reproduction) * Clutchsizewhichmaximizeslifetime
RS < clutch size that maximizes RS per reproductive season
* There is a trade-off between current and future reproductive effort
Some costs are unaccounted for in this model
* Here,we considered the costs of being given extra eggs and rearing extra young * But, some costs were unmeasured, such
as the cost of producing extra eggs or incubating extra eggs
* Perhaps if females were forced to pay the ‘full cost’ of laying and incubating extra
eggs, the predicted optimal brood size would decrease
Constraints of optimality models
- We often cannot directly measure total fitness, which is the “real currency” of selection
- Instead, we use “proxies” like energy intake, energy efficiency, or risk avoidance
- Optimality models rely on being
able to compare costs and benefits
directly so we can determine ≠ whether costs or benefits are higher - But costs and benefits are not always measured in the same “currency” (e.g. securing a mate vs. energy intake)
Economics of foraging: starlings
- Starlings feed their young leatherjacket larvae
- Parents make up to 400 round trips from the nest to feeding sites every day
- How many leatherjackets should the parent bring home on each trip?
- Load size determines rate of food delivery which determines chick survival
- Horizontal axis = time (travel + searching)
- Vertical axis = load of leatherjackets
- Red curve = # leatherjackets found relative to search time
- ‘loading’ or ‘gain’ curve: rises steeply and flattens off
- The first couple leatherjackets found quickly and easily
- Once there are a few leatherjackets in the beak, it takes longer and longer to find additional prey
- One hypothesis is that the best option = providing the maximum net rate of food delivery to chicks
- Slope of this line is y/x and represents Rate of Delivery of food =
(load/[travel + foraging time]) - The point where the tangent line intersects the red curve gives us optimal searching time and optimal load
The Economics of Foraging: Starlings
Load - What if the nest is closer to the meadow?
- When travel time is shorter, the load size that maximizes the delivery rate decreases
Prediction vs. reality - starling foraging
When starlings were trained to bring back mealworms from feeders at different distances, they brought back bigger loads from greater distances (as predicted)
Copulation in dung flies
- The longer a second male mates, the more eggs he fertilizes
- But the returns for extra copulation time diminish rapidly
- AND there is a cost if you spend too long copulating (what might this cost be?)
- Males miss the chance to go and search for a new female
- In dung flies, the analogue of travel time is the time the male dung fly must spend guarding the present female until she has laid her eggs + search time for a new female
- The same model can be used to predict optimal copulation time in dung flies as travel time in starlings
Starlings: Central Place Foraging Theory
- For animals that must return to a central location (the central “place”) to process or store resources (like a starling returning to a nest)
- Predicts that animals should:
- preferentially forage in patches where resource densities are higher or
more accessible (maximize resource intake per unit time) - Allocate time between foraging and traveling in a way that maximizes overall energy intake
- Compensate for needing to travel further by carrying an increased load
- Spend more time in patches with higher resource density or that are closer to the central place
Marginal Value Theory
- The Marginal Value Theorem (MVT) was developed by Eric Charnov in 1976
- It deals with situations where animals must choose whether to leave a resource patch, or how much time to spend in a patch before moving on
- These can be foraging decisions, but can also be reproductive and other decisions (like our dung flies)
The economics of prey choice: crabs
- Shore crabs forage on mussels of different sizes
- In choosing which mussels to eat, crabs must consider the benefit (Energy Yield, E) and the cost, the time it takes to break a mussel open (Handling Time, h)
- Large mussels: high E, high h
- Small mussels: low E, low h
- Mussels vary in their profitability by size
- There IS a most profitable mussel size, which maximizes profitability
- But crabs eat a range of sizes aside from the most profitable.
- Why?
Assume a predator has encountered a prey item—should it eat it?
1) If it encounters prey type 1, it should always eat it, because it is more
profitable.
2) If it encounters prey 2, it should eat it if:
- The gain it gets from prey 2 > gain from rejecting it and continuing to search for prey type 1
- E2/h2 >E1 /(S1 +h1)
- S is the searching time (S1 + h1 is total ‘foraging’ time)
- We can rearrange the above to show that the predator should eat prey 2 if: S1 >(E1*h2/E2)–h1
- This shows us that the choice to eat the less profitable prey depends on how much time it would take to search for a more profitable prey item (abundance of prey type 1)
- Models like this lead to general predictions about prey choice:
- Top quality prey should always be taken if encountered
- Lower quality prey are taken only if the higher quality prey is
not too common - The switch to specializing on prey 1 to also eating prey 2 should be sudden and occur only when this equation is true:
S1 >(E1h2/E2)–h1 - Only when the two sides of the equation are equal will it make no difference to the predator whether it eats one or both types of prey
Starvation
- Foraging animals care about food intake rates and energy, but also the risk of starvation
- Especially in unpredictable environments
- Imagine the bird has to choose between two foraging options
- 1) Provides one unit with probability 1
- 2) Provides 2 units with probability 1⁄2, 0 units with probability 1⁄2
- The average payoff is the same, but option 2 has greater variance (is riskier)
Risk beyond starvation: predation
- Foragers change their behavior when they sense higher predation risk
- E.g., garden skinks (Lampropholis guichenoti) forage in the open less when predator cues are present
