Finding food Flashcards
why forage?
basic requirement for life
- cell metabolism
improve body quality
- status ornaments
feed breeding partner
feed offspring
economics of foraging
maximise profits and minimise loss
time budget equation
time budget
= searching time + handling time
handling time equation
handling time
= no. of prey taken in time T
x time spent handling 1 prey item
number of prey taken in time T equation
number taken
= area searched
x prey density
x time searching
searching time equation
searching time
= number of prey taken in time T/
(area searched x prey density)
complex time budget equation
time budget
= (Ha/ a H) + (Ha*Th)
assumptions of budget time equation
all animals are identical
- same handling time
no variation in any of the parameters in space/time
no interactions between animals
- no prey excluding others
how is the environment patchy?
vary in:
food density
predator no.s
size + type of prey
- may exclude each other
how many items should a parent collect?
- best type of foraging
- too few
- too many
= optimal foraging
wasted time + energy on flying to food patch
-> not delivering much profit
lose prey items whilst foraging
+ increased risk of predation
marginal value theorem (MVT)
considers economic costs of altering the value of another factor (e.g. energy costs)
MVT in animal behaviour
- 3 axes on graph
loading curve
= no. of items taken to offspring
searching time
= time spent in food patch
travel time
= time spent between nest + food patch
MVT in animal behaviour
- optimal searching time
- longer travelling time means…?
- longer searching time means…?
time that delivers highest profit at lowest travelling time
more time spent gaining nothing
results in some gain
-> gain increases with searching time
marginal value equation in animals
marginal value
= food intake/
(travel time + searching time)
how can the optimum time be altered?
change travel time
- longer travel time means longer optimum searching time + need to take more prey
change shape of gain curve
when will optimal foragers spend longer in a patch of resources?
more profitable patches
as distance between patches increases
when whole environment in less profitable
optimality models
- similar to
- key components
quantitative economic cost/benefit analyses
>constraints >behavioural options >'currency' - may be maximised e.g. food intake - may be minimised e.g. risk of predation
optimality models
- pros
make testable + quantitative predictions
use explicit assumptions
general
- may be applicable to different organisms + behavioural situations
optimality models
- cons
selection of constraints, behavioural options + ‘currencies’ may be incomplete/ hard to quantify
- e.g. predation risk often uses surrogate measures
if observed behaviour different to expected, how can discrepancy be explained?
- wrong model
OR
- animals behave non-optimally?
-> come up with new model with new constraint + hypothesis
testing support for competing hypotheses
e. g. bird + mussels
- hypotheses
predicted:
prey on big mussels as most profitable
observed:
not many big mussels were taken
so rejected hypothesis
testing support for competing hypotheses
e. g. bird + mussels
- what happened next?
made additional observations:
large mussels potentially excluded as too big + hard to open (encrusted with barnacles)
trade off between starvation and predation
animals need to feed BUT also avoid being eaten
trade off between starvation and predation
e.g. prey selection in 3-spined sticklebacks
presence of predator:
fish attack prey less frequently even if high prey density
2 competing factors from fish POV:
> probability of seeing prey again
> if they make an attack on prey, predator will see and eat them
trade off between starvation and predation
e. g. body mass in birds
- benefits of energy reserves
fuel for foraging breaks
buffer against unfavourable foraging conditions
insulate body from environment
trade off between starvation and predation
e. g. body mass in birds
- costs of energy reserves
acquisition
(time, energy, risk of injury)
maintenance (mass-dependent costs)
metabolic expenditure
pathological conditions
e.g. pulmonary stress
decreases locomotor performance
- > fly slower + less agile
- > increases predation risk
trade off between starvation and predation
e. g. body mass in birds
- study of small passerine birds + sparrow hawks
absent hawks
= high body mass of small birds
increasing hawks
= decreasing body mass
established hawks
= low body mass
how can we combine the various measures of performance into one?
what does this link?
reproductive success
= fitness
e.g. no. of offspring produced over life-time
foraging behaviour
predation risk
mating success
reproduction
