Chapter 7. Evolution of feeding behavior. A large amount of research has focused on applying optimality theory to foraging 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. 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. 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. Reto Zach studied the crow’s behavior. Noted that crows choose only large whelks ( cm). Noted that crows choose only large whelks ( cm). Crows flew to 5m height to drop whelk Crows flew to 5m height to drop whelk Persisted in dropping until whelk broke. Persisted in dropping until whelk broke.

Optimal foraging by crows Are crows behaving optimally? 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. 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. 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. 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. 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. 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. 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. Note the curve flattens as the rate at which food is acquired slows.

Food gain curve ShortLong Arrival time in patch Food intake

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). 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. 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 ShortLong Arrival time in patch

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. 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.

Optimal site choice for food consumption Animals attempt to optimize more than just food 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 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. 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. 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? 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. 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. 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. 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. 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. 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. Often one would expect one strategy to be superior and for it to become fixed in the population. In the Roseate Tern case frequency- dependent selection appears to maintain the two strategies. In the Roseate Tern case frequency- dependent 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 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. 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. 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. 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.

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 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. 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. 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. 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. 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.

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 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

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.

Recruits trained to come to site 750m from hive. Food at 750m removed. Sites m 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 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).