Foraging behaviors – many decisions
 Where is food available
 Is it really food (non-toxic, high energy
content)
 Is food A a better choice than food B
 Strategy to obtain food (can include social
interactions)
1
Some Feeding Strategies:
Detritivores
Sit and wait predators
Active foragers
 Solitary
 Communal
Cultivators
2
food comes to you
Trade-offs…
Search for food: Can spend energy at a high
rate, but spend little time.
Sit-and-wait: Can spend little energy (low
rate), but spend much time.
3
Orb weaving spiders
– ultimate and classic sit and wait
predator.
Spider video
Ant cultivation: Gardening for food
Ants – Central American species, e.g., Atta colombica and
Atta cephalotes….
• Cut and stash leaf fragments.
• Chew up leaf fragments, regurgitate pulp on which grows
the fungus Leucoagaricus gongylophorus.
• Maintain colonies of bacteria (Enterobacteriaceae) that
help break down leaf pulp cellulose and produce sugars
and amino acids.
• Maintain other colonies of bacteria (Actinomycetes) that
live on the ant’s abdomen and produce antibiotics that
block growth of unwanted fungi.
White powder on ant is collection of bacteria
Ants aren’t the only cultivators: 3-toed sloths
are amazing!
- Live in trees
- Eat low grade plant material (leaves)
- Has low body weight so it can stay in the
canopy  can’t store lots of food like
ruminants
- Low metabolic rate
To supplement diet sloths maintain an ecosystem in
their fur of algae, fungi and arthropods.
When the sloths groom their fur, they eat the algae,
fungi and arthropods.
Moths living in sloth fur.
High N2 content. Sloth
needs N2.
Moths die at
some rate.
Dead moths
decomposed
by fungi.
Sloth grooms
out algae &
digests.
Fungi fed
upon by algae
Unlike other sloths, the 3-toed sloth slowly climbs
down to the ground once every week and
defecates. Very risky re: predators!
Why does the sloth do this?
Female moths will only lay eggs in sloth feces,
and only when the feces are deposited on the
ground.
Moth larvae pupate in the feces – The adults then
fly up to the canopy and look for a sloth.
So to maintain a constant moth supply, sloth has
to poop on the ground.
Optimal foraging: Which item to select?
It is also necessary to consider other needs & dangers
Optimize return
(marginal value
theorem)
Risk assessment
Other needs
(sleep, mating,
thirst, etc)
The factors for optimizing return are…
• Energy value of prey
• Encounter rate (is it rare to find this food?)
• Handling time (time to dispatch prey & eat it)
Encounter rate gets to the notion of efficiency.
The more rare the food, the more efficient a
predator you must be.
If food is common, you can be “sloppy” or
inefficient about capture.
The term “optimal” is not the best. It implies
some animals or groups make bad choices if they
are not optimizing their food selection.
PLUS: Food specialists are typically more efficient.
16
Eat anything
that vaguely
resembles
food.
Continuous Range
Very
selective
about what
to grab, eat.
Each strategy across range can be equally
“optimal”. Being “efficient” as a predator is not
always the best way to leave more genes in
the next generation.
Profitability =
E
Handling Time (H)
food 1
Eating larger prey is not an
optimal strategy if time to
locate larger prey is more
food 1
Net Food Obtained
H
Energy Content (E)
Dotted line: smaller
prey, less time to find.
Time
E
food 2
H
food 2
Solid line  dashed
line as larger prey
become harder to
find.
Great tit feeding experiment.
Different densities of mealworms and mealworm
pieces pass by the test bird.
Researchers know:
Encounter rate,
handling time,
energy value.
Great tit feeding experiment.
Large prey more profitable,
when the density of large
prey is high.
Small prey more profitable, when the density of large prey is low.
Classic optimal foraging experiment
with crows & whelks.
Crows grab whelks in intertidal zone.
Fly above rocks and drop whelk (multiple times) to
break open shell.
The higher the drop, the more likely the shell will
break…
… but it takes more energy to fly higher…
… but the alternative is to drop the shell more times
= more flights.
What is the best strategy?
Complication:
• Small whelks more abundant, but less energy
in each.
• Small whelks less massive – must be dropped
from a greater height to break, but less energy
required to carry a small whelk.
• The higher the drop, the fewer drops required,
but it takes more energy to fly higher.
If you don’t fly very high, it
takes many flights/drops
What the crows do – the
optimal.
Still need as many drops;
higher height not buying much
•
•
•
Large whelk: 10 drops from 3 meters (30 dm).
Medium whelk: 26 drops from 3 meters (78 dm) or 18 drops from 4 meters (60
dm).
Small whelk: 55 drops from 4 meters (220 dm) or 18 drops from 5 meters (90 dm).
What about flying off to find more whelks… or
whatever … in another location?
If food distribution is patchy, again a calculation
for best strategy.
The mathematics: “marginal value theorem”.
As food declines in a patch, travel to a new patch?
Short answer: Depends on rate of food exhaustion and
effort (time/travel distance) to go to a new patch.
Food gain curve
Tangent lines on food gain curve:
Stay longer = more travel time to
new patch.
Optimal time to
stay in patch.
Food return declining in
current patch.
Average
travel time to
new food.
Time to
abandon
current
patch.
Great tits seem to have read about the marginal value
theorem!
Stay in patch (food dish) longer if it is more trouble to
get to the next patch.
(Increased flight
distance faked by
making different
food dish lids
harder to
remove)
BUT…
What if there is a predator at the next patch?
This changes the equations.
Foraging trade-off with predator avoidance
Flock of sparrows feeding in a
large shed. No need for
vigilance against predators.
Feeding rate same for most
flock sizes, even small ones.
Flock of sparrows feeding
in an open field.
Larger flock requires less
vigilance, so feeding rate
depends on flock size.
31
Tropical ant colony – predator adjusted foraging
Of the colony members, the most efficient
foraging is by ants with head sizes 2.2-2.6
mm.
BUT  these larger ants are more likely to be
preyed upon by parasitic flies….
BUT  the flies are only active during the day!
So colony sends out smaller foragers during
day, and “optimal” larger foragers at night.
Dealing with variability in food supply.
One in the hand vs two in the bush…
If you are well fed, little
motivation to go after a
“risky” food source where
you might score big, or
not at all.
Risk prone
Risk
averse
If you are
hungry, take
the chance!
Juncos tested with two
types of food trays (they
can tell them apart):
Pick tray 1: Always the
same number of food
items appears.
Pick tray 2: Sometimes a
lot of food, sometimes
little or none.
Behavior you would
expect (and got!) for
hungry birds. They take a
risk and pick the variable
dish.
Well fed birds
did this:
Selected the
fixed-food tray.
Well fed birds did this:
Selected the fixed-food
tray (risk averse).
A reward of 8 has a smaller
value for well fed birds
A reward of 8 has a value >1.5
for hungry birds
Behavior you would expect
(and got!) for hungry birds.
They take a risk and pick
the variable dish.
The total amount of food received over time is the
same, but the birds opting for the variable try hoped
for an early bonanza.
Group foraging & cooperative hunting
Mammals with large brains –
There are many examples of
cooperative hunting.
Humpback whales bubble feeding near Juneau, AK
Water buffalo & lion video
Water buffalo & lion video
Well some fish have pretty good brains too!
Malawi electric fish.
Mormyrops anguilloides: Mormyrid weakly electric fish.
Eats cichlids in Lake Malawi (Africa)
Study technique:
1. SCUBA videos of fish, identify individual
Mormyrops by markings.
2. EOD has distinctive waveshape.
3. Track and observe prey capture.
[Study by Arnegard & Carlson,
Proc. Royal Soc. 2005)]
Plot showing how individual fish can be
discriminated in a recording based on their EOD
“signatures”.
Experimental advantages:
Lake Malawi has extremely clear water.
The electric fish do not pay much attention to
their visual system, so SCUBA lights (2 35-watt
halogen lamps) do not change their behavior.
Mormyrops are large and so easy to find, track
at night.
Individuals hide in solitary shelters during the
day.
At night they assemble into hunting packs of
2-10 individuals.
Same adults assemble across multiple nights
(3 week duration of study).
It works
Capture rate of cichlids
Solitary Hunters: 1.90 per hour of
hunting
Pack Hunters: 2.09 per hour of
hunting
Bluegill sunfish also group
hunt.
Group flushes more prey
than can single
individuals.
Public Information
 Foragers need to keep track of a number of
environmental variables.
 Predators, food availability, etc.
 Environmental quality is estimated by tracking
time spent in a patch and how much is eaten.
 Individuals of social species may use the
foraging success of others to estimate patch
quality.
 Solitary foragers don’t have this luxury
 Social foragers spend less time in crappy patches
than solitary foragers
Testing Public Information with starlings:
30 cups filled were presented to starlings, some had
food while others were empty.
Testing Public Information with starlings.
 Birds who had been exposed to the cups
were paired with birds who had not.
 Birds who were paired with individuals with
knowledge of the placement of full and empty
cups spent less time foraging in the empty
cups than birds who did not have a partner.
 Birds with partners who sampled all of the
cups had better success than birds with
partners who only sampled a few of the cups.
Whomever has found food is the place
to look for food!
pigeons
Observed bird feeding / foraging innovations
Example of innovation: Harrier & Goose
Example of innovation: Harrier & Goose
Example of innovation: Raven
Example of innovation: Raven
Not surprisingly, innovation ability  more brain power!