chapter 7. evolution of feeding behavior. a large amount of research has focused on applying...

Chapter 7. Evolution of feeding Chapter 7. Evolution of feeding behavior.behavior.

A large amount of research has focused A large amount of research has focused on applying optimality theory to foraging on applying optimality theory to foraging behavior. behavior.

Costs and benefits can be translated into Costs and benefits can be translated into energy and so can be evaluated quite energy and so can be evaluated quite easily.easily.

Optimal foraging by crowsOptimal foraging by crows

Northwestern crows commonly eat whelks Northwestern crows commonly eat whelks and other shellfish and usually open them and other shellfish and usually open them by flying up and dropping them onto a by flying up and dropping them onto a hard surface.hard surface.


Reto Zach studied the crow’s behavior. Reto Zach studied the crow’s behavior. Noted that crows choose only large whelks Noted that crows choose only large whelks

(3.5-4.4 cm).(3.5-4.4 cm).Crows flew to 5m height to drop whelkCrows flew to 5m height to drop whelkPersisted in dropping until whelk broke.Persisted in dropping until whelk broke.


Are crows behaving optimally?Are crows behaving optimally?

If so, large whelks should be more likely to If so, large whelks should be more likely to break than small ones, 5m drops should break than small ones, 5m drops should yield best chance of breaking whelk, and yield best chance of breaking whelk, and the likelihood of a whelk breaking should the likelihood of a whelk breaking should not depend on the number of previous not depend on the number of previous drops. drops.


Zach experimentally dropped different size Zach experimentally dropped different size whelks from different heights and whelks from different heights and confirmed the three predictions.confirmed the three predictions.

Fig 7.1


Zach also calculated the caloric yields of Zach also calculated the caloric yields of different size whelks. different size whelks.

He found that when the costs of opening a He found that when the costs of opening a whelk were deducted from the energy whelk were deducted from the energy gained, large whelks yielded by far the gained, large whelks yielded by far the highest energy return.highest energy return.

Optimal prey choice by young Optimal prey choice by young Dark-eyed JuncosDark-eyed Juncos

Young juncos clumsy at handling large prey, but can eat small items.

Adults can handle larger prey.

Different abilities result in different optimalchoices for age classes.

Young birds choose small prey. Adultsselect 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 MVTSolving problem with MVT

To solve the problem graphically you first To solve the problem graphically you first plot the cumulative gain curve which is the plot the cumulative gain curve which is the rate at which the bird gains food.rate at which the bird gains food.

The X-axis is time and the Y-axis is food The X-axis is time and the Y-axis is food intake.intake.

Note the curve flattens as the rate at which Note the curve flattens as the rate at which food is acquired slows.food is acquired slows.

Food gain curve

Short Long

Arrival time in patch

Food intake


To identify the optimal number of food items to To identify the optimal number of food items to take and the optimal time to spend in the patch take and the optimal time to spend in the patch draw a straight line from the travel time that draw a straight line from the travel time that intersects the gain curve at one point only (i.e. is intersects the gain curve at one point only (i.e. is a tangent). a tangent).

From this intersection point drop straight lines to From this intersection point drop straight lines to the X and Y axes to figure out the optimal time to the X and Y axes to figure out the optimal time to spend in the patch and the optimal number of spend in the patch and the optimal number of food items to consume respectively.food items to consume respectively.

Food gain curve

Short Long

Arrival time in patch


As travel time to the patch increases it is As travel time to the patch increases it is predicted that the forager will stay longer predicted that the forager will stay longer in the patch and consume fewer items.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.

Optimal site choice for food consumption

Animals attempt to optimize more than justfood intake.

Food intake may be traded off against survival.

Optimizing things other than food.

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 perceivedrisk 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 Risk avoidance by foraging leaf cutter antscutter ants

Leaf cutter ants harvest leaves that they then Leaf cutter ants harvest leaves that they then use to grow fungi, which they then eat.use to grow fungi, which they then eat.

The ants do most of their foraging for leaves at The ants do most of their foraging for leaves at night and only small inefficient ants search for night and only small inefficient ants search for leaves during the day. At night the larger, most leaves during the day. At night the larger, most efficient ants forage for leaves. efficient ants forage for leaves.

Why do the large ants not forage during the Why do the large ants not forage during the day?day?

Fig 7.7

Risk avoidance by foraging leaf Risk avoidance by foraging leaf cutter antscutter ants

Ants with head widths of 1.8mm or more Ants with head widths of 1.8mm or more are parasitized by a parasitic fly that lays are parasitized by a parasitic fly that lays its eggs in the ants head with lethal its eggs in the ants head with lethal consequences for the ant. consequences for the ant.

These flies are active only during the day, These flies are active only during the day, so large ants avoid them by foraging at so large ants avoid them by foraging at night. Smaller ants are not parasitized night. Smaller ants are not parasitized and so can forage during daylight.and so can forage during daylight.

Risk avoidance by skinksRisk avoidance by skinks

In a similar fashion garden skinks (a lizard) In a similar fashion garden skinks (a lizard) that were reared in experimental that were reared in experimental enclosures that contained the scent of a enclosures that contained the scent of a predatory snake moved around less and predatory snake moved around less and avoided open areas more than skinks avoided open areas more than skinks reared in similar, but scent-free reared in similar, but scent-free enclosures.enclosures.

Fig. 7.6

Game theory and foraging behaviorGame theory and foraging behavior

Game theory examines situations in which Game theory examines situations in which individuals play different strategies.individuals play different strategies.

For example, roseate terns catch fish by For example, roseate terns catch fish by diving for them, but an alternative diving for them, but an alternative approach is to steal fish from successful approach is to steal fish from successful birds.birds.

Foraging Roseate TernsForaging Roseate Terns

Often one would expect one strategy to be Often one would expect one strategy to be superior and for it to become fixed in the superior and for it to become fixed in the population.population.

In the Roseate Tern case frequency-In the Roseate Tern case frequency-dependent selection appears to maintain dependent selection appears to maintain the two strategies.the two strategies.


The fish stealing phenotype is going to be most The fish stealing phenotype is going to be most successful when rare and least successful when successful when rare and least successful when common (too much competition and too few fish common (too much competition and too few fish being caught).being caught).

The fish hunting phenotype will be most The fish hunting phenotype will be most successful when common (few fish being lost to successful when common (few fish being lost to thieves) and least successful when rare. thieves) and least successful when rare.

Fig 7.9


As a result, the fitness curves for the two As a result, the fitness curves for the two strategies will intersect and this will be an strategies will intersect and this will be an equilibrium point at which the payoffs to equilibrium point at which the payoffs to the two strategies will be the same.the two strategies will be the same.

Any deviation from this optimal ratio of Any deviation from this optimal ratio of hunters to thieves will result in a lower hunters to thieves will result in a lower payoff and the system should return to the payoff and the system should return to the equilibrium point.equilibrium point.

Perissodus microlepis in Lake Tanganyika has an unusual foraging technique.

Another game theory example

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 behavioralphenotype -- attack left flank or attackright flank.

Genes for both probably closely linked onchromosome.

These strategies are fixed and their successdepends on their relative frequency in thePopulation.

Phenotypic frequencies fluctuate around 50%each.

Rarer phenotype has an advantage in attackingprey. It becomes more common, and then theadvantage switches.

This is example of frequency-dependent selection.

Frequency-dependent selection occurs whena phenotype’s success is affected by its frequency in the population.

Conditional strategiesConditional strategies

Sometimes as in the case of Sometimes as in the case of PerissodusPerissodus an individual is locked into one strategy.an individual is locked into one strategy.

However, in other cases an individuals However, in other cases an individuals strategy is contingent on what its strategy is contingent on what its circumstances are.circumstances are.

Conditional strategiesConditional strategies

For example, turnstones (a small wading For example, turnstones (a small wading bird) foraging in flocks on beaches use bird) foraging in flocks on beaches use different techniques and parts of the beach different techniques and parts of the beach depending on their status in the flock. depending on their status in the flock.

Dominant birds forage in patches of Dominant birds forage in patches of seaweed which contain lots of seaweed which contain lots of invertebrates, but subordinates instead invertebrates, but subordinates instead probe in mud or sand for food.probe in mud or sand for food.

Getting assistance from others when hunting

Hunting in Groups

Prey benefit from grouping. Predators alsocan benefit by cooperating to attack prey.

Lions, hyenas, African hunting dogs, wolvesall 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 recruitother 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”. Sitefar away from food where information is exchanged about location of food

Adult ravens discover moose carcass on their territory.

Marked immature raven also discoverscarcass but driven off by adults.

Markedimmature returns tocommunal roost andnext morningleads otherbirds to food.

Large group overwhelms defenses of adults and gains access to food.

Black Vultures and Turkey Vultures alsoroost 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.

Black Vulture

Turkey Vulture

Main reason for difference: Black Vultures are more aggressive.

BVs drive TVs away from large long-lasting carcasses.

TVs depend on small carcasses and BVs onlarge 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 informationcommonly 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 directionosprey came from.

Ospreys that see neighbors returning with fish catch alewifes quicker than

those that don’t.

Best example of an information center isin honeybees.

Honeybees “dance” to convey information.

Karl von Frischpioneered thework on dancing bees.

A honeybee that has found food dances topass information about food location toother bees in hive.

If food close to hive (< 50m) bee performsround dance.

Round dance

Round dance

If food further away bee performs “waggle” dance

Bee performs dance on path that is roughlyfigure 8 shaped.

Bee travels in straight line while wagglingher body.

Then turns left or right to circle back tobeginning of path.

If bee outside hive, direction ofwaggle dance points directly atsource of food.

Inside hive, bee performs dance in darknesson vertical surface.

Vertical indicates direction of sun.

Angle of dance relative to vertical indicatesdirection 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 in15s, 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.

Recruits trained to come to site 750m fromhive.

Food at 750m removed.

Sites 200-2500m from hive established.

To give Distance Information.

Most bees occurred at site 800m from hive.

Most bees occurred 800 m from hive

Adaptive value of dances.

Enables colony to exploit food sourcesmore 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 Apisrelatives.

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 symbolicleading to show direction (leading to partialleading to pointing).

chapter 7. evolution of feeding behavior. a large amount of research has focused on applying...

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