2) Foraging trade-offs

what is behaviour shaped by

evolution, under natural or sexual selection

foraging optimally, energy and time spent per resource unit is as low as possible

foraging optimally

most desirable/favourable outcome

based on cost/benefit analysis of the situation

seen in many different behaviours (mating, predator avoidance, parental care)

richest area of food, gives higher quantity and quality of food

what are optimal foraging decisions

which food types to eat (optimal diet)

which patch type to feed in (optimal patch choice)

how much time in different patches optimal patch time)

optimal patterns and speed of movements while foraging

what is the original optimal foraging theory

animals should behave as if they are tring to maximize energy gaind/time spent

maximizing fitness, not food
eat enough, not endlessly, must consider trade-offs

what are important things to consider for foraging

what offers highest return given all the factors and constraints

cost/benefit analysis. if costs>benefits, then behaviour would not evolve

trade-offs: gecko sunbathing, what could the gecko do other than sunbathing, needs to because cold-blooded, but can’t spend whole day doing it

explain oystercatcher example

they ate all the middle-sized mussels

if there are larger mussels available, they should forage the big ones, but they didn’t
because the larger mussels are harder to open, have more barnacles on them, difficult to crack open. and smaller mussels are able to be opened quicker, have less meat inside, not worth it

what are conclusions from optimal foraging theory

development of models may provide insights into behaviour that would be missed

even over-simplified or incomplete models may have heuristic value

development of OFT changed study of foraging from descriptive to predictive

what is charnov’s model

forager should leave a patch when the instantaneous rate of resource gain in a patch drops below the average rate of gain available in the environment, factoring in travel time between patches

what is example of charnov’s model

when hummingbird arrives at fresh flower patch, nectar is abundant and easy to collect, as it continues feeding, nectar becomes depleted, and rate of energy gain slows

according to MVT, it should leave the patch long before all the flowers are empty, when rate of nectar intake falls below the average rate it could achieve by visiting other patches, considering the time and energy needed to travel

what are key factors in foraging decisions

rate of gain: over time, rate of gain slows as resources deplete: diminishing returns

travel time to next patch: if next patch is far or risky to reach, animals may stay longer in current patch, while shorter travel time encourage earlier departure

average rate of gain in environment: compare current rate of gain to average rate elsewhere, if current rate drops below that average, then leave patch

predation risk and competition: high predation risk of competition may force longer stays or earlier departure

explain this graph

include travel time to patch, actual time spent in patch, and diminishing returns curve

when you arrive in a patch, all food is readily available

vertical line from the intersection between tangent line and intake curve

what are factors that will influence how many caterpillars a starling brings back?

travel time, competition, handing of food (weight, size, time taken to pickup), beak size (may have to come back anyway even if patch is good)

explain foraging starlings example

fly certain distance, could collect multiple prey items, number of prey items collected per visit and time spent at feeder were recorded

currency: rate of delivery of food for offspring. amount per unit time, time for trip = travel time and searching time

constraints: ability of starlings to find larvae, number of larvae that can be held in bill, flight speed (fixed, but travel time is influenced)

decision variable: load size/search time

what did they find with the starlings?

shorter travel time leads to not filling beaks, longer travel time leads to filling beaks more

explain this graph

optimal load of worms is 7, could potentially get 8 worms, but that gain is less than 7 worms

if starling finds a site closer to the nest, the optimal load size changes.

the optimum load is estimated by drawing a line from the travel time (negative section) towards the diminishing returns curve. find the intersection between this tangent time and the curve, then extend that straight towards the y-axis and that is the optimum load for that travel time

honeybee weight example

attached a weight to honeybees, made it more costly to forage for longer times and distances

bees with weights brought back less nectar, removed weights and bees still brought back less

in the paper, they said maybe bees had a limited lifetime fuel, less energy to forage because of the weights

explain great tit example

conveyor belt with 2 diff types of food, looked at which the bird will prefer to eat

when encounter rate of both prey items is low, birds should not be selective
hardly any food, just take all the food

when encounter rate of both pray items is high, birds should be selective
more food coming through, prefer foods with most nutrients

what are types of foraging movements

area-restricted search: when food is clumped together, reduce speed and increase turning rates after finding food

straight-line movement: use when food is sparse or evenly distributed

random walk: movement with no apparent direction of pattern

what impact does cryptic prey have

more difficult to discover

slow speed of predator increases detection of prey, but faster speed of predator increases the frequency of encounter of prey

details on vigilance for foraging

if animals can see predator, they may continue eating because they can see where they are, and gain of resources is perhaps more important in that situation

if there are danger zones, increase vigilance

sticklebacks
prefer high prey density in absence of predator, but prefer low density patches after exposure to a predator
less prey in the tank means fish are able to be more vigilant, higher density of prey, moving around more, confusing, more susceptible to predators

what is foraging behaviour influenced by (predator edition)

travel distance: closer to safety, will take more trips
handling time: takes longer to eat, will hide to eat, so less time exposed to predation
hunger state: if hungrier, will risk more when risk of predation = risk of starvation

risk averse vs risk prone

risk averse: go for consistency for more chance of survival, be risk averse if our expected 24 hour energy budget is positive (daily energy intake >> daily energy exposure)

risk prone: go for variability to have the chance of more seeds, be risk prone if your expected 24 hour energy budget is negative (daily energy expenditure >> expected daily intake)

trade-offs for great tits fat accumulation and predation

small birds must accumulate considerable amounts of body fat to survive long cold winters, but larger amount of fat increase cost of flight, and reduce ability to escape predators

increased hawk presence, average body mass decreased, but without hawks, average body mass increased

what is ideal free distribution (IFD)? what does it assume? why might this not be the case?

a group of foragers should distribute themselves between two patches in a ratio proportional to the availability of food in these patches

assumes all foragers are equal, have perfect knowledge of the environment, and are free to move between patches with negligible travel costs

if predators are around, stay in bigger groups and won’t disperse as much, though will be susceptible to competition

what is despotic distribution

as density increases, weak competitors suffer a steep decline in the realized habitat quality compared to strong competitors, they will begin migrating towards poorer habitat

at density 1, strong competitors are all present in rich habitat
for the weak competitors, the realized habitat quality has diminished to the level of fundamental habitat quality of the poor area

at density 2, the weak competitors are better off occupying the poor habitat exclusively, because the realized quality of rich area and fundamental quality of poor area are now equal

at density 3, strong competitors may begin using poor habitat, because the realized habitat density for them is now equal to the best possible version of the poor habitat

what can we do if optimal foraging models fail?

perhaps model is incorrect or incomplete

consider simpler decision rules
include additional constraints (predation risk, minimum nutrient requirements, avoidance of toxins, starvation risk avoidance)
consider currency other than profitability (such as efficiency (Egained/Espent)

what types of hoarding are there?

larder hoarding: storing food into a centralized location
hamsters storing seeds in underground chambers

scatter hoarding: storing food in many distinct locations
chickadees, grey squirrels

intermediate/multiple-central hoarding: storing food in clusters of caches

how will animals adapt hoarding strategies for different competition levels?

scatter if competition is high

larder if competition is low

details on social cognition and food caching in jays

birds are observant and steal caches from other birds

spatial memory of these birds is more developed than other birds that don’t hoard/cache

larger hippocampus, and seasonally variable, reduciton in size when not storing

what are types of social learning

social facilitation: group activity stimulates same behaviour
individual that is resting may begin grazing when nearby herd members start grazing

social learning/cultural transmission: young learning migration routes from experienced females

stimulus enhancement: animals investigating an area because other animals are seen there
soaring vulture sees other vultures gathered on the ground and investigates that location

observational learning: primates acquiring novel foraging techniques through observation, like washing potatoes

details on information centers

roosts/nesting colonies (large, unrelated individuals)

informers gain predation protection while foraging
eavesdroppers gain location of food

little cost if patch is undefendable

details on roosts

consist mainly of unpaired juveniles, which recruit to food by calling or retrieving from communal roost sites

individuals that slept together fed together

during brooding, house sparrows are in pairs, but otherwise in groups

they will only call to recruit if there’s enough food to share with others

what is cooperative hunting

two or more animals work together to capture prey, increasing individual and group success rates

can tackle larger or more dangerous prey

common in wolves, lions, dolphins, orcas

lions: take down larger prey, surprise faster prey,
individual specialization (hunts more successful when playing preferred positions)
food intake per lion is lowest when both lion and prey are in groups (gregarious)

lions go in groups for benefits of territory defences against infanticide

details on game theory of evolution

fitness of phenotype depends on their frequency in the population

optimal solution for parent is not the same as for the offspring
offspring wants to eat as much as they can, but are dependant on the parent, which needs to feed all offspring and themselves

offspring are not genetically identical to parents, which causes parent-offspring conflict and also sibling rivalry

what is ESS

evolutionarily stable strategy

only the strategies that cannot be invaded will be maintained in the population
if everyone adopts stealing food strategy, there will be no more food to to steal and will waste energy by hiding and stealing

behaviour is not only governed by optimality, but also what?

what other strategies are found in the population

the success of one animal’s chosen strategy depends on the strategies of the other individuals involved

producers vs scroungers

producers search for food on their own

scroungers steal food from producers

it’s good to be a scrounger if there’s no other scroungers around, but with increasing scroungers, there’s less to steal from, and being a producer becomes better