Chapter 7. Evolution of feeding
behavior.

A large amount of research has focused
on applying optimality theory to foraging
behavior.

Costs and benefits can be translated into
energy and so can be evaluated quite
easily.
Optimal foraging by crows

Northwestern crows commonly eat whelks
and other shellfish and usually open them
by flying up and dropping them onto a
hard surface.
Optimal foraging by crows

Reto Zach studied the crow’s behavior.
Noted that crows choose only large whelks
(3.5-4.4 cm).
 Crows flew to 5m height to drop whelk
 Persisted in dropping until whelk broke.

Optimal foraging by crows

Are crows behaving optimally?

If so, large whelks should be more likely to
break than small ones, 5m drops should
yield best chance of breaking whelk, and
the likelihood of a whelk breaking should
not depend on the number of previous
drops.
Optimal foraging by crows

Zach experimentally dropped different size
whelks from different heights and
confirmed the three predictions.
Fig 7.1
Optimal foraging by crows

Zach also calculated the caloric yields of
different size whelks.

He found that when the costs of opening a
whelk were deducted from the energy
gained, large whelks yielded by far the
highest energy return.
Optimal prey choice by young
Dark-eyed Juncos
Young juncos clumsy at handling large
prey, but can eat small items.
Adults can handle larger prey.
Different abilities result in different optimal
choices for age classes.
Young birds choose small prey. Adults
select larger items
Optimal prey delivery.
Birds feeding young have to deliver food
Items to their nestlings.
Must travel to food patch and feed. How
many food items should be brought back?
What factors affect decision?
Declining ability to catch food as bill fills up.
Prey in patch becomes depleted.
Costs of travel to patch.
Marginal Value Theorem (MVT) can be used
to analyze when it is optimal to leave patch.
At what point does it not pay to search for
one more item?
Marginal value is a central idea in
Economics. It is the amount you will pay for
one more of a particular item.
Value of one more item to you declines the
more items you have.
This explains why you pay a lower price
for more of a good.
Can use the MVT to solve the bird’s
problem.
Solving problem with MVT

To solve the problem graphically you first
plot the cumulative gain curve which is the
rate at which the bird gains food.

The X-axis is time and the Y-axis is food
intake.

Note the curve flattens as the rate at which
food is acquired slows.
Food intake
Food gain
curve
Short
Arrival time in patch
Long
Solving problem with MVT


To identify the optimal number of food items to
take and the optimal time to spend in the patch
draw a straight line from the travel time that
intersects the gain curve at one point only (i.e. is
a tangent).
From this intersection point drop straight lines to
the X and Y axes to figure out the optimal time to
spend in the patch and the optimal number of
food items to consume respectively.
Food gain
curve
Short
Arrival time in patch
Long
Solving problem with MVT

As travel time to the patch increases it is
predicted that the forager will stay longer
in the patch and consume fewer items.
Alejandro Kajelnik trained starlings to visit a
feeder where mealworms were dispensed.
Varied distance of feeder from nest.
Recorded load sizes.
Load size increased with distance to nest.
Optimizing
thanconsumption
food.
Optimal sitethings
choiceother
for food
Animals attempt to optimize more than just
food intake.
Food intake may be traded off against
survival.
Chickadees generally carry items to cover
to eat them in safety.
A chickadee’s decision whether to carry an
item to cover is affected by its distance to
cover (energetic costs) and its perceived
risk of predation.
Steve Lima observed feeding behavior of
chickadees at sites 2m, 10m, and 18m
from cover.
Chickadees were less likely to carry items to
cover as distance increased.
However, when a “predator” was flown
overhead the probability of carrying food to
cover increased.
Predator present
No predator present
Risk avoidance by foraging leaf
cutter ants

Leaf cutter ants harvest leaves that they then
use to grow fungi, which they then eat.

The ants do most of their foraging for leaves at
night and only small inefficient ants search for
leaves during the day. At night the larger, most
efficient ants forage for leaves.
Why do the large ants not forage during the
day?

Fig 7.7
Risk avoidance by foraging leaf
cutter ants

Ants with head widths of 1.8mm or more
are parasitized by a parasitic fly that lays
its eggs in the ants head with lethal
consequences for the ant.

These flies are active only during the day,
so large ants avoid them by foraging at
night. Smaller ants are not parasitized
and so can forage during daylight.
Risk avoidance by skinks

In a similar fashion garden skinks (a lizard)
that were reared in experimental
enclosures that contained the scent of a
predatory snake moved around less and
avoided open areas more than skinks
reared in similar, but scent-free
enclosures.
Fig. 7.6
Game theory and foraging behavior

Game theory examines situations in which
individuals play different strategies.

For example, roseate terns catch fish by
diving for them, but an alternative
approach is to steal fish from successful
birds.
Foraging Roseate Terns

Often one would expect one strategy to be
superior and for it to become fixed in the
population.

In the Roseate Tern case frequencydependent selection appears to maintain
the two strategies.
Foraging Roseate Terns

The fish stealing phenotype is going to be most
successful when rare and least successful when
common (too much competition and too few fish
being caught).

The fish hunting phenotype will be most
successful when common (few fish being lost to
thieves) and least successful when rare.
Fig 7.9
Foraging Roseate Terns

As a result, the fitness curves for the two
strategies will intersect and this will be an
equilibrium point at which the payoffs to
the two strategies will be the same.

Any deviation from this optimal ratio of
hunters to thieves will result in a lower
payoff and the system should return to the
equilibrium point.
Another game theory example
Perissodus microlepis in Lake Tanganyika has
an unusual foraging technique.
It feeds by biting scales off other fish.
Population divided into two phenotypes
whose jaws are angled left or right.
Jaw orientation heritable, as is behavioral
phenotype -- attack left flank or attack
right flank.
Genes for both probably closely linked on
chromosome.
These strategies are fixed and their success
depends on their relative frequency in the
Population.
Phenotypic frequencies fluctuate around 50%
each.
Rarer phenotype has an advantage in attacking
prey. It becomes more common, and then the
advantage switches.
This is example of frequency-dependent
selection.
Frequency-dependent selection occurs when
a phenotype’s success is affected by
its frequency in the population.
Conditional strategies

Sometimes as in the case of Perissodus
an individual is locked into one strategy.

However, in other cases an individuals
strategy is contingent on what its
circumstances are.
Conditional strategies

For example, turnstones (a small wading
bird) foraging in flocks on beaches use
different techniques and parts of the beach
depending on their status in the flock.

Dominant birds forage in patches of
seaweed which contain lots of
invertebrates, but subordinates instead
probe in mud or sand for food.
Getting assistance from others when hunting
Hunting in Groups
Prey benefit from grouping. Predators also
can benefit by cooperating to attack prey.
Lions, hyenas, African hunting dogs, wolves
all hunt cooperatively.
Main advantages of cooperative hunting:
1. Hunting success rate is increased.
2. Larger prey can be tackled.
Some birds also hunt cooperatively.
Pelicans cooperate to herd schools of fish.
Harris Hawks hunt rabbits and other game in
groups.
Main disadvantage of group hunting is that
prey has to be shared.
Not all individuals have equal access to food.
Information sharing among foragers.
Foragers sometimes can get information
about food from other individuals.
Bernd Heinrich’s ravens
Ravens use
(i) Local enhancement. Yell to recruit
other birds.
Local enhancement information is
transferred at the location of the food.
Other examples of local enhancement.
(i) Vultures descending to feed on carrion.
(ii) Seabirds diving on a school of fish.
Ravens also use
(ii) Information centers.
Roost acts as an “information center”. Site
far away from food where information
is exchanged about location of food
Adult ravens discover moose
carcass on their territory.
Marked immature raven also discovers
carcass but driven off by adults.
Marked
immature
returns to
communal
roost and
next morning
leads other
birds to food.
Large group overwhelms defenses of
adults and gains access to food.
Black Vultures and Turkey Vultures also
roost communally. Do their roosts act as
information centers?
Dr. B.’s dissertation research was on this topic.
Dr. B. tagging a
Black Vulture.
Dr. B with Turkey Vulture outside walk-in trap.
Turns out Black Vultures roosts do
sometimes serve as information centers,
but Turkey Vulture roosts don’t.
Main reason for difference: Black Vultures
are more aggressive.
BVs drive TVs away from large long-lasting
carcasses.
Black
Vulture
Turkey Vulture
TVs depend on small carcasses and BVs on
large carcasses.
TVs use their sense of smell to locate
carcasses first.
Note large nostril and bulge (olfactory bulbs)
before eye.
Difference in behavior between vultures is
a consequence of their different
food-finding abilities and aggressiveness.
Local enhancement information
commonly used by birds.
However, only a few studies have provided
strong support for the information center
Hypothesis (ICH).
One of these is Greene’s work on ospreys.
ICH foraging in ospreys.
Ospreys fish-eating birds.
Sometimes breed in loose colonies.
An osprey returning to nest carrying an
alewife (schooling fish) causes others in
colony to search for food in direction
osprey came from.
Ospreys that see neighbors returning
with fish catch alewifes quicker than
those that don’t.
Best example of an information center is
in honeybees.
Honeybees “dance” to convey information.
Karl von Frisch
pioneered the
work on dancing
bees.
A honeybee that has found food dances to
pass information about food location to
other bees in hive.
If food close to hive (< 50m) bee performs
round dance.
Round dance
Round dance
If food further away bee performs
“waggle” dance
Bee performs dance on path that is roughly
figure 8 shaped.
Bee travels in straight line while waggling
her body.
Then turns left or right to circle back to
beginning of path.
If bee outside hive, direction of
waggle dance points directly at
source of food.
Inside hive, bee performs dance in darkness
on vertical surface.
Vertical indicates direction of sun.
Angle of dance relative to vertical indicates
direction of food relative to sun.
Length of waggle portion indicates
approximate distance of food.
Vertical
orientation in hive
Waggle dance.
Length of waggle portion indicates
approximate distance of food.
The fewer dance circuits the bee performs in
15s, the further away the food is located.
Tests of “waggle dance” effectiveness.
To convey information on food location
need to convey both distance and
directional information.
Conveying directional information.
Fan test. Recruits trained to come to site F.
Compared arrivals at F and at six other
sites equidistant from hive but in different
directions.
Site F much higher visitation rate.
To give Distance Information.
Recruits trained to come to site 750m from
hive.
Food at 750m removed.
Sites 200-2500m from hive established.
Most bees occurred 800 m from hive
Most bees occurred at site 800m from hive.
Adaptive value of dances.
Enables colony to exploit food sources
more efficiently.
Evolution of bee’s dances.
Honeybee is Apis mellifera.
Other Apis perform dances too.
A. florea dances on horizontal comb
built in open. Dancer points directly at food.
Possible intermediate stages in various Apis
relatives.
Trigona bees hum and move excitedly.
Other Trigona smell bee and search
for that food.
Some Trigona make scent trails to food.
Melipona bees make sound pulses.
Longer pulses imply food further away.
Discoverer makes several short flights in
direction of food, then leads others to it.
Overall, evolution of dance probably
involved standardization of “excited
behavior” to indicate amount and distance
of food.
Also, switch from actual to symbolic
leading to show direction (leading to partial
leading to pointing).