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Introduction
 Many
animals spend a considerable amount of time and energy hunting for food, and
the time and energy expended while hunting cannot be used for other important
activities such as mating and rearing young. Thus, we might expect that
natural selection would favour those animals that
forage in the most efficient manner possible. This is the foundation of
"Optimal Foraging Theory". Optimal foraging theory predicts that
predators should select those prey types that result in the highest food
value (usually expressed in energy units) per unit of time spent hunting. In
this lab we will test whether a fish predator (the guppy, Poecilia
reticulata) preferentially selects the size
range of Daphnia pulex (a small fresh-water
crustacean) that results in the highest food value per unit of time spent
hunting.
Methods
Profitability of prey
Students should work in pairs. Each pair will
receive fish belonging to several different size categories and Daphnia
that have been sieved through three different sizes of mesh. Please be
careful to avoid disturbing the fish as little as possible - they will not
feed if frightened.
Working with three fish of one size class at
a time, use a pipette to add three Daphnia of the same size class to
each jar. Use a stopwatch to record the "handling time," i.e. the
time from the first bite to the last swallow, for up to 3 handling times in
10 minutes. Include any time the prey is spit out by the fish, as long as the
prey is not subsequently ignored. Notice that the fish will often have to
shift the prey around before swallowing. Record the handling time, the size
of the individual Daphnia subject and the size of each fish. Repeat
this procedure until each fish size-class has handled each size class of
prey. Report the mean handling time for each prey size by fish size category
on the board. The food value of the prey can be estimated from their weight.
Table 1. The relationship between size class and mass for the Daphnia
prey used in this experiment.
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Large Daphnia
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Medium Daphnia
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Small Daphnia
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Mesh Size (mm)
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1.0
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0.8
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0.5
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Mass (μg)
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108
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37
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18
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Selection of Prey
Place 20 prey of each size in a jar that
already contains a fish of a known size. Remove the fish after it has eaten
about 25 Daphnia. Be careful not to let the fish eat more than half of
the prey. After removing the fish (be careful not to remove Daphnia
with the fish) count the number of prey of each size that is left. Each table
will do part of the experiment with one size class of fish. Repeat the
experiment for another size class of fish if time permits.
Analyses and Interpretation
The class data will allow us to determine the
relative foraging efficiency of fish predators feeding on different sizes of
prey. It will also allow us to test whether fish preferentially select the
size range of Daphnia that results in the highest feeding efficiency.
You should find references from the primary
literature to support the Introduction and Discussion sections in the full
write-up for this lab (you can refer to Dave
Nykamp’s “Bio Lab write-up guide"
for some hints on preparing a lab report). An abstract is not required.
Data should be displayed by:
1) Plotting the mean handling time as a function of prey size for each size
class of fish.
2) Plotting the foraging efficiency (weight/handling time) as a function of
prey size for each size class of fish.
3) Plotting the mean number of consumed prey in each size class for each fish
size.
Consider the following questions when
interpreting your results. You should not address these points in a numbered
format, but instead work them into the discussion if they are relevant.
1) How does handling time relate to prey size and fish size?
2) Did the fish prefer prey with the shortest handling time or prey with the
greatest mass/handling time?
3) Is it possible that the fish simply selected those prey that were the most
obvious?
4) How might the results differ if the fish was hungrier? Would you expect
the observed preference and our prediction to differ? If yes, how would they
differ?
5) What characteristics other than mass and handling time of prey may affect
a predator’s choice of prey?
6) What environmental factors might affect the choosiness of the fish?
Marking Scheme
Introduction (10 marks)
Results (10 marks)
Discussion (10 marks)
References (5 marks)
Data Display (10 marks)
Total (45 marks)
BIO318Y
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