Transcript 覓食行為 ─ ( 鄭先祐(Ayo)

大學部 生態學與保育生物學學程 (必選)
2010 年 秋冬
覓食行為 (Foraging behavior)
─動物行為學 (Ethology)
鄭先祐(Ayo)
國立 臺南大學 環境與生態學院
生態科學與技術學系 教授
Ayo NUTN Web: http://myweb.nutn.edu.tw/~hycheng/
11 覓食行為 (Foraging behavior)
 Obtaining Food
 Suspension feeding
 Omnivory
 Herbivory
 Carnivory
 Adaptations for detecting prey
 Optimal foraging
 Diet selection
 The marginal value theorem
 Adding complexity and realism
 The utility of models
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Foraging (覓食)
 Foraging: finding, processing, and eating food
 Foraging decisions are based on a food item’s size
 Energetic value
 How easy it is to carry
 Its distance from cover
 And how these variables interact
 Animals have a wide range of adaptations for
acquiring food
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Suspension feeding
 Many aquatic species remove small suspended food
particles from the surrounding water


Some species sieve water and strain food
Others trap particles on sticky surfaces of mucous
 Blue whales weigh over 100 tons yet survive on tiny
shrimp-like creatures (krill)
 The annelid (環節動物) worm creates a mucous net
in its burrow

Trapping food that the worm rolls into a ball and
swallows
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 Chaetopterus, an annelid worm that lives in U-
shaped tunnels in the sand, filter feeding with a
mucous net.
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 Blue Whales can reach up
to 33 metres (108 ft) in
length and 180 metric
tons (200 short tons) or
more in weight. In volume,
it is currently the largest
animal existing or known
to have existed.
 The Blue whale (Balaenoptera musculus) is a marine
mammal belonging to the suborder of baleen whales
(called Mysticeti).
 Blue whale is one of the loudest animals, when it calls
can be heard by other whales 625miles (1,000 km) away.
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Suspension feeders
 Can be sessile (stationary) (全部接收)
 Take food that comes their way
 Others (i.e. whales) move
 Some adjust their filtration pattern to
choose particular types of particles


Dabbling (浸入水中) ducks (i.e.
northern shovelers (琵嘴鴨) ,
mallards) strain food through filters
on their bills
Feeding on different sized particles
by changing their bill’s position
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 棲息於開闊地區的湖泊河
流等處，也見於山區以及
高原上的水域，偶爾也會
在村鎮附近的污水池塘中。
琵嘴鴨在沿海也不常見，
但是在鹹水水域卻也可以
看到。
 綠頭鴨（學名：Anas
platyrhynchos），又名大頭
綠（雄）、蒲鴨（雌），
是家鴨的野生種。綠頭鴨
也是地球上飛得第五快的
鳥類，可達到每小時65公
里。
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Feeding choices
 Omnivory: animals eat both plants and animals
 Animals are omnivorous for many reasons:
 Limited amounts of a preferred food
 A need for nutritional variety
 Minimizing exposure to risks (predators or toxins)
associated with a particular food type
 Herbivory: plant eaters
 Carnivory: meat eaters
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Herbivory
 Plants - roots, leaves, stems, fruits, flowers - are
consumed by different species of animals
 The plant’s perspective: it suffers a loss of fitness if an
animal eats its roots or leaves


But benefits if its pollen is transferred to another plant
Or if its seeds are carried to a new germination site
 Challenges facing an herbivore depend on whether the
plant is marshalling a defense that must be overcome

Or encouraging an animal’s attentions
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Plants defend themselves - sometimes
 Spiny cacti (仙人掌) , poisonous hemlock (毒芹屬植物) ,
thorny roses(玫瑰), thistles (薊)
 Milkweed (馬利筋) sap is noxious to touch or taste


Milkweed beetles bite the plant
Sap leaks out the bite holes so the beetle can feed
 Fruits and flowers have evolved to attract animals
 Most fruit is red or black (apples, raspberries, blackberries)
 These colors attract frugivores (fruit eaters)
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Some plants attract herbivores
 Birds prefer black or red fruits
 They contrast well with the green background of foliage
 Flowers have evolved visual cues attractive to pollinators
 Bees, hummingbirds, bats visiting flowers transfer pollen
 Plants offer nectar or other specialized rewards
 Flower markings (nectar guides) may act as UV runway
lights
 Plants pollinated by carrion flies have a putrid odor
 Hummingbirds feed on Heliconia
 Females have a long, curved bill which matches the flowers
 The male’s short, straight bill matches its preferred flower
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 蜂鳥（學名Trochilidae）是屬於
雨燕目的蜂鳥科，體型很小，
能夠通過快速拍打翅膀（每秒
15次到80次，取決於鳥的大小）
而懸停在空中，也是唯一可以
向後飛的鳥。
 Ambrosia beetles are beetles of
the weevil subfamilies
Scolytinae and Platypodinae
(Coleoptera, Curculionidae),
which live in nutritional
symbiosis with ambrosia fungi
and probably with bacteria.
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A different approach to herbivory
 Some species cultivate some or all of
their food
 Agriculture has evolved independently
in three insect orders: ants, termites,
ambrosia beetles (仙饌甲蟲)


Leaf cutter ants cut fresh leaves and
carry the pieces back to the nest under
the ground
The fungus that grows on the leaves is
the ants’ primary food source
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 Leafcutter ants, a non-generic name, are any of 41 species of leaf-
chewing ants belonging to the two genera Atta and Acromyrmex.
These species of tropical, fungus-growing ants are all endemic to
South and Central America and parts of the southern United States.
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Species modify their own food supplies
 Limpets (帽貝) leave behind mucous trails


Mucous traps microalgae and stimulates its growth
Limpets revisit their old slime trails to harvest the crop
 Gorillas (大猩猩) are herbivorous


They rip down large plants
Resulting in a surge in the growth young, fast-growing
vegetation
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 Limpet (帽貝) is a common name for numerous different
kinds of saltwater and freshwater snails (aquatic gastropod
mollusks) that have a simple shell which is basically
conical in shape. The shells of limpets are either not
coiled, or appear not to be coiled in the adult snails.
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Carnivory
 Carnivores must capture food that never benefits from
being eaten
 Evidence of an arms race:


prey species have evolved defenses against predation
Predators have evolved to overcome those defenses
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Strategies of a successful predator: pursuit
 Species that chase their prey
 Cheetahs (獵豹) can reach a top speed of 70 mph
 Sea birds: northern gannets (塘鵝) feed by spectacular
plunge dives
 Mechanisms of pursuit
 Tracking: steering to keep the moving image in front (e.g.,
houseflies, beetles)
 Intercepting: aiming at a place front of the prey (i.e.
dragonflies 蜻蜓)
 Hunting in groups
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 Different methods of pursuing
a target.
 (a) tracking behavior, where
the predator steers to keep the
moving prey, represented by
dots, in front.
 (b) interception, where the
predator aims in front of the
target.
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Strategies of a successful predator:
stealth (偷襲)
 Pursuit is exhausting and dangerous
 Stalk-and-rush: predators approach their prey slowly until
they get close
 Predators also flush prey and pursue them over short
distances

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Predators choose hunting sites with good cover
Even if prey are fewer
 Camouflage makes the predator hard to detect
 Markings, colors, or behaviors
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Predators employ smokescreens (煙幕)
 Smokescreening: predators use environmental
disturbances to camouflage their approach


Jumping spiders prey on other spiders by climbing
into the prey’s web
Moving when the wind blew the web, pausing when the
air was still
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Aggressive mimicry
 A predator gets close to its prey because it mimics a
signal that is not avoided by the prey

And may even be attractive to it
 Specialized structures are used as lures (誘惑物)


An alligator snapping turtle’s tongue has a wormlike
outgrowth that attracts fish
Female deep-sea angler fish live in darkness and have a
luminous, fleshy appendage on the top of the head that
lures fish
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 alligator snapping turtle
 鱷龜（學名：Macrochelys
 deep-sea angler fish
temminckii）是世界最大的淡水  鮟鱇目（學名：Lophiiformes）
龜之一。
俗稱鮟鱇魚，又名琵琶魚、結
 鱷龜的口腔隱藏，舌頭形狀像
巴魚。
蠕蟲，用來捕捉魚類。牠們會
躺在水中不動，張開口才捕獵  鮟鱇魚，特別之處是有一隻由
前背鰭演化而成的發光釣竿，
獵物。牠們的舌頭會模仿蠕蟲
釣竿頂端內上百萬隻的發光菌，
的動作，吸引獵物走到牠們的
狀似小魚，會發出亮光，吸引
口中。當獵物進到口中，牠們
就會急速合口，完成埋伏。
小生物作食物。
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Some species mimic beneficial species
 The cleaning wrasse (裂唇
魚) swim into the mouths of
fish to remove parasites,
tissue, fungi, and bacteria


A blennid fish looks and
behaves like the cleaning
wrasse
But it bites the gills or other
soft parts of the fish
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Predators send signals
 Predators attract prey by sending signals that mimic the
mate of the prey species



Males fireflies fly and flash in species-specific patterns
A Photuris female signals to a Photinus male and when the
male lands the female eats him
Another firefly may have evolved to be day-active to
escape predation by Photuris
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Strategies of a successful predator: traps
 Predators trap prey
 By manipulating objects or altering their environment
 Humpback whales (座頭鯨) build bubble “nets” to
trap prey


At 15 meters deep, a whale blows bubbles from its
blowhole while swimming in an upward spiral
The bubbles form a net that traps krill (磷蝦) and small
fish
座頭鯨（學名：Megaptera novaeangliae），
又名大翅鯨、駝背鯨、巨臂鯨，屬於鬚鯨亞目
的海洋哺乳動物。
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Familiar trappers: spiders
 The “Charlotte’s web”-style orb spider (圓蛛) web
has up to seven different kinds of silk
 Bola spiders use sex pheromones to lure moths
A bolas spider, which
catches insects with a
drop of glue swung on
the end of a silk line.
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 orb spider
 圓蛛科蜘蛛種
類很多，全球
約有2500種。
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 Bolas Spiders are unusual
orb-weaver spiders that do
not spin the typical web.
Instead, they hunt by using
a sticky ‘capture blob’ of
silk on the end of a line,
known as a ‘bolas’.(兩端繫
有鉛墜的繩子)(流星錘)
 By swinging the bolas at flying male moths or moth flies nearby,
the spider may snag its prey rather like a fisherman snagging a
fish on a hook.
 Because of this, they are also called angling or fishing spider
(although the unrelated genus Dolomedes is also called fishing
spider). The prey is lured to the spider by the production of up to
three pheromone-analogues.
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Snakes have specialized senses
 Some snakes select environments that provide good
thermal contrast to hunt
 Pit vipers (rattlesnake, water moccasin, and
copperhead (銅頭蝮蛇) ), and constrictors (boa
constrictor, python, and anaconda) use a prey’s body
heat to guide their hunt
Snakes locate warm-blooded prey
even in darkness
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 Copperhead 銅頭蝮
 water moccasin
 Rattlesnake 響尾蛇
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蟒蛇
 boa constrictor
 Python
 anaconda
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Predators detect prey through touch
 Predatory sand scorpions detect a prey’s vibrations as
it runs through the sand

Slit sensilla on its leg determine the direction of prey
 The star-nosed mole finds prey by using its nose
 It searches tunnels for small prey by touching the walls
with its unusual star-shaped nose
 Touch receptors (Eimer’s organs) in its nose allow a
mole to examine 300 m of tunnel floor every day
 It can even detect odors underwater
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 sand scorpions
 星鼻鼴 （Condylura
cristata）是一種生長於北
美洲東部，在加拿大東部
及美國東北部都有發現的
小鼴。
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Some predators smell prey
 Petrels (海燕) and albatrosses (信天翁) sniff out
seafood patches scattered over vast expanses of open
ocean



Dead organisms release compounds that seabirds can
smell
Krill (磷蝦) feed on phytoplankton which releases
dimethyl sulfide (DMS)
Which leads seabirds to krill
 Odors of krill or DMS provide an immediate way to
assess the potential productivity of an area


Different species use these cues in different ways
Before they fledge, young chicks prefer particular scents
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 Giant petrel (海燕)
 Albatrosses (信天翁)
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Sharks have sensory specializations
 Sharks use many senses for detecting and tracking
prey
 Sharks hear prey

They will turn and swim toward it
 Close to its prey, its nose directs the search
 Lateral line organs detect small disturbances in the
water

Even if it cannot see prey
 At close range, the shark sees its prey
 Sharks also respond to electrical cues from prey
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Sharks and others use bioelectric cues
 The use of bioelectric cues is not restricted to sharks
 Also found in rays (魟魚), skates (魟魚),, and lungfish
 And some larval amphibians
 Australian lungfish locate their prey
 By using the electrical field generated by a living organism
in seawater
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 The results of
experiments
demonstrating that
lungfish can locate
their prey by using the
electrical field
generated by a living
organism in seawater.
 (a) the foraging
intensity and (b) the
foraging accuracy of
each treatment.
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Platypus (鴨嘴獸) predators
 A platypus has electroreceptors and
mechanoreceptors on its bill

Giving it a three-dimensional fix on prey
 A platypus hunts at night in murky streams
 Closing its eyes, ears, and nostrils
 It digs in the silt with its paws and bill to stir up small
animals
 Prey generate electrical fields, sensed by
electroreceptors
 Mechanical waves are sensed by mechanoreceptors
on its bill
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 The duck-billed platypus is among the predators
that can locate prey by detecting their electric
fields.
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Predators can generate electrical fields
 The nocturnal black ghost knifefish uses its electrical
sense to feed on insect larvae and crustaceans


Tuberous electroreceptor organs detect changes in the
electrical field the fish generates and uses for orientation
Ampullary organs detect electrical fields generated by
external sources (i.e. a prey animal)
 These receptors, and the mechanosensory lateral line
system, help the fish hunt for prey at night or in muddy
water


These fish swim backward, upside down or horizontally
Changing position affects the incoming electrical signals
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 The black ghost knifefish, Apteronotus
albifrons, is a tropical fish belonging
to the ghost knifefish family
(Apteronotidae).
 The fish is all black except for two
white rings on its tail, and a white
blaze on its nose, which can
occasionally extend into a stripe down
its back. It will grow to a maximum
length of 25 inches (60 centimeters). It
does not have scales.
 They are nocturnal, but they are
weakly electric fish and use an electric
organ and receptors distributed over
the length of their body in order to
locate insect larvae.
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Search image
 Search image: the heightened ability of a predator to detect
a target
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

After an animal gains experience with a particular species of
prey
It begins to focus its attention on it, and ignores other prey
that did not fit the search image
Obvious prey are eaten
 This hypothesis assumes that a search image is specific
 Animals improve in their ability to detect a particular kind of
prey
 Through experience, an animal learns a prey’s key
characteristics
 The predator looks for others of that type
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Blue jays create search images
 In simulated predator-prey interactions between blue
jays and underwing moths

The jays’ ability to detect prey improved with
consecutive encounters
 Experience with one type of cryptic prey improved the
predator’s ability to find that type of prey

But not other kinds of prey
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 (a) a blue jay is foraging for
cryptic prey.
 The blue jay was shown slides,
some of which contained a
cryptic moth ((b) Catocala
retecta and (c) C. relicta). If the
bird was shown a slide that
contained a moth and pecked
the appropriate key after
spotting it, the bird received a
mealworm reward.
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What comprises a search image?
 Animals look for particular features that best allow
them to discriminate prey from the background
 Practical implications for studying search image
formation


Sniffer dogs trained to detect explosives in luggage use
search images
Dogs exposed to TNT containers and given the
opportunity to form an olfactory search image were
better able to detect TNT
 This also illustrate that search images need not be
visual
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Optimality modeling
 Animals have different behavioral options (strategies)
available to them
 A model: a mathematical expression that weighs the costs
and benefits of each strategy
 Currency: measures the costs and benefits of some
indicator of fitness


Animals that make the best choice get the most fitness
They are favored by natural selection
 The animal does not need to work out complicated
solutions

Natural selection gives animals the appropriate attributes for
solving the optimality problem
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Foraging can test the optimality theory
 It is easy to fit into a modeling framework
 Foraging is divided into a series of decisions which can be
focused on one at a time
 i.e. what to eat, where to look for food, how long to search
an area, what sort of path to take through an area
 A logical currency (common measure) compares the
decisions



i.e. rate of energy gain or increased food intake over a time
period
Increased food intake increases survival and fecundity
(number of offspring)
Short-term measures of success indicate long-term fitness
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Testing the optimality theory
 Energy-based models can be measured directly
 And are a good place to start modeling
 Constraints (limitations) on foraging behavior can be
identified




i.e. an animal’s ability to gather food is limited by
Its gut capacity
Ability to detect food
The presence of predators, etc.
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Diet selection: a simple model
 Foraging animals often encounter many possible items
to eat


Should they include all types of food in their diet?
Or focus on just some of them and ignore the others?
 Considerations go into this decision
 How hard it is to gather items
 How rare or tasty they are
 Their nutritional value
 If they are dangerous to catch, etc.
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Models oversimplify nature
 Simpler models have more general conclusions that
apply to a wider range of examples

But detailed models make more precise and accurate
predictions for a given situation
 The scenario we are modeling:
 A forager is searching for food
 Two kinds of food are available
 The forager finds one piece of food at a time
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Developing the diet selection model
 Two strategies available when the forager finds food
 Eat it
 Ignore it and keep looking (find a better piece of food)
 To compare the strategies: use the currency of “rate of
energy gain”
 The model’s constraints


Handling time: the amount of time it takes to process
food
The animal cannot eat and look for food at the same time
 Search time: different food types are easier or harder to
find
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The diet selection model
 E1 = amount of energy gained by eating prey type 1 (in
calories)

E2 = energy gained by eating prey type 2 (in calories)
 For handling food:
 h1 = the time it takes to eat prey type 1 (in seconds)
 h2 = the time it tales to eat prey type 2 (in seconds)
 For searching:
 S1 = the amount of time it takes to find prey type 1 (in seconds)
 S2 = the amount of time it takes to find prey type 2 (in seconds)
 The currency = rate of energy gain (calories/second)
 The profitability of each prey = the rate of energy gain
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The profitability of each prey
E1
calories
 Profitability of prey type 1 =h1 in units of second
E2
calories
 Profitability of prey type 2 =h2 in units of second

 Prey type 1 = type with the highest profitability
 The first predictionbased on this model: the forager has just
found prey type 1
 Should it eat it?
 A forager should always eat prey type 1
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What if a forager has found prey type 2?
 Should it eat it, or should it ignore it and keep looking?
 Compare the two rates of energy gain
 The first = the rate of energy gained by eating prey type 2
 The second = the rate of energy gained by ignoring prey
type 2 and continuing to search
 The profitability of prey type 2, is E2/h2
 The rate of gain of finding and eating prey type 1 is
different

The forager must find it and then eat it
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Comparing rates of gain for 2 prey types
 Comparing the two rates: E 2  E1
h2 S1  h1
 When the rate on the left is bigger than the right
 The forager should eat prey type 2

 The predator should switch between including prey type 2
 Depending on which side of the equation gives the highest
rate of energy gained
 Another prediction: the forager should not include the
search time for prey type 2 when making this choice

If the equation says the forager should not eat prey type 2,
the forager should always ignore it
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Testing the optimal diet model
 Redshanks: shorebirds that feed on
worms

Large worms provide more energy
than small ones
 In some locations, large worms are
common (search time, S1, is short)

In other locations, they are rare (S1
is long)
 At sites where large worms were
common, the birds ignored the small
worms
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The redshank (紅腳鷸)
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 Redshank (紅腳鷸)
Tringa totanus
 Great tit (大山雀)
Parus major
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Great tits select large mealworms
 Great tits picked always picked larger mealworms off
of a conveyor belt


And ignored the small ones
Excluding small worms was gradual
 Partial preferences for high ranking prey include
discrimination errors (confusion)


And time required to learn the rates of encounter of
different prey types
Birds never totally excluded smaller prey from their
diet
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How successful is the optimal diet theory?
 The model explaining diets of foragers that consume
immobile prey (leaves, seeds, nectar, mealworms,
clams)

Did not do well when prey were active
 The optimal diet theory does not take into account
the behavior of prey



Two types of prey may be of equal abundance
But if one is better at hiding or is more likely to escape
The predator’s diet will reflect that
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Deciding when to leave a patch: the
marginal value theorem
 As an animal forages
 Food in a location may become difficult to obtain
 Prey may become increasingly rare as a predator hunts
 Because they take evasive action
 At some point it becomes advantageous for the forager
to move to a new patch

Where food is easier to find
 Marginal value theorem when should a forager stay
in a patch and when should it go?
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The marginal value theorem
 The curved line is the gain curve
 The cumulative amount of energy that a forager has
gained as it stays in a patch
 The curve flattens as the food runs out
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When should you move to another patch?
 We need to know
 How far away is the next patch?
 Travel time: the time it takes before you start getting
energy from the next patch

The marginal value: drawing a tangent from this point
to the gain curve, and dropping a line straight down,
shows the best time to leave the current patch
 If an animal leaves a patch before this time
 It will be traveling when it could still be foraging
 If it stays too long
 It wastes time searching for food in a depleted area
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Testing the marginal value theorem
 The marginal value theorem can be applied to
different questions

i.e. predicting how much food an animal should carry
in a single load back to a nest or burrow
 Eastern chipmunks carry food stuffed in their cheeks
 It gets harder to add food as their cheek pouches get
full
 The amount of time that chipmunks spend at a seed
tray (patch time) increases with the distance between
the seed tray and the burrow
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The marginal value theorem
 Animals prefer rich patches over poor ones
 Patch residence times correlate with patch quality
 Increased travel time leads to longer time in patches
 This simple version might be too simple
 Foragers consistently stay longer than predicted
 Other factors must be added to predict foraging strategies
 Energy alone is not enough
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Adding realism to the marginal value
theorem: moose
 Moose must obtain
enough energy for the
growth and maintenance
of their huge bodies

But they also have a
minimum daily
requirement for sodium
 Leaves of deciduous trees have more calories than
aquatic plants

But aquatic plants have a higher concentration of sodium
 A moose balances these needs by eating a mixture of
plants
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Adding realism to the marginal value
theorem: other species
 Stripe-tailed hummingbirds spend more time at
feeders that have vitamin tablets dissolved in the
sugar solution

Than at feeders with only sugar water
 Insects also regulate their nutrient intake
 Locusts on a diet low in protein or carbohydrate select
the food that redresses their nutritional deficiencies
 Tent caterpillars select nutritionally balanced food over
unbalanced food
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Incomplete information
 To gain knowledge about search time and patch value
 The animal must be familiar with the area
 And may have to sample other patches
 Some foragers monitor their environment
 Chipmunks feed on seeds from deciduous trees
 The supply of seeds below a tree is variable
 Chipmunks must decide how often to check the seed
supply at other trees
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Animals have to make decisions
 To determine whether it is beneficial to switch foraging
locations


Time spent checking other trees is time lost feeding, so
there is an optimal amount of sampling
Chipmunks spend more time sampling the food density
at other locations as the quality of the patch being
exploited decreased
 Other animals may or may not have time to accurately
sample their area

They assess their environment by watching conspecifics
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Rules of thumb
 Animals may get close enough to an optimal behavior
by following an approximation called a “rule of thumb”
 Northwestern crows search during low tide for whelks
(海蝸牛)

A crow will pick up a whelk (large snail) then drop it
 Crows are selective about the size of whelks they drop
 Large whelks provide more energy
 And require fewer drops to break
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Crows follow a rule of thumb
 Even after they had eaten most large whelks, they
ignored smaller ones
 Medium and small whelks require more energy to handle


And are harder to break
And cost energy to eat
 Crows make similar decisions when feeding on clams
 The largest are the most profitable and are chosen by crows
 What should crows do if offered both clams and whelks?
 Smaller clams offer more calories than larger whelks
 But crows follow the rule of thumb “take the heaviest
prey item”
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Foraging animals must avoid being eaten
 Changes in foraging behavior in the face of predation
include




Avoiding dangerous places
Avoiding dangerous times of day
Increasing vigilance when foraging
Selecting portable foods
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Animals often choose safety over food
 Species forage in less profitable
but safer sites
 Chacma baboons live in four
different habitats


Each differs in food availability
and predation risk
Baboons avoid predators by
foraging in safer areas, even
though they have less food
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Predation risk determines when an
animal forages
 Scorpions forage less on moonlit nights
 When they are more visible to predators
 An animal reduces its risk by being more vigilant
 Vigilance comes with an energetic cost – it is hard to be
both vigilant and forage
 Greater elk spend more time being vigilant and less time
feeding when wolves are present
 Predation also affects diet choice
 Foraging gray squirrels reject items with a higher
profitability (energy gained per second of handling time)
 For larger, less profitable items that easier to carry to cover
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Quantifying how an animal perceives risk
 How do animals assess how risky they perceive a
particular site to be?
 When given a choice, gerbils (沙鼠) prefer a safe plot
over a risky plot (i.e. with owls during a full moon)

When both plots held the same number of seeds
 Gerbils (沙鼠) accepted increased risk when the price
was right

If the risky plot contained enough seeds
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The cascading effect of risk avoidance
 Antipredator behavior can have a cascade of
consequences for an ecological community

The ecological consequences of antipredator behavior by
foragers are wide-ranging
 To avoid detection, grasshoppers (草蜢) decrease their
movements in the presence of spiders

Reducing the amount of plant damage by grasshoppers
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Foraging in the presence of competitors
 Producer/scrounger model: a producer is the animal that
makes the resource available

The scrounger is the one that steals it
 American crows drop walnuts to break them open
 Upward flight to drop the food item is energetically costly
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Crows adjust their foraging strategies
 American crows adjust the height from which they
drop a walnut



English walnuts break more easily than black walnuts,
so are dropped from lower heights
Height is also adjusted to account for substrate hardness
And to minimize the chance of theft
 When the risk exceeds a certain threshold
 Crows lowered the drop height
 So they can recover the nut before it is snatched by
another bird
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The role of internal state
 The simplest foraging models assume that all animals
behave the same way

However, individual foragers vary
 Some foragers might be more experienced
 Or in better condition and able to move more quickly
 Others may be closer to reproduction
 Or hungrier
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Optimal decisions of foragers differ
 There is a tradeoff between foraging success and predation risk in
colonial spiders

Especially for large females, as predators prefer them
 Colonial spider lives in large colonies with a shared frame web

A spider’s position within the colony influences foraging
success and the risk of predation or parasitism
 Individuals on the periphery are more successful
at capturing prey but are more likely to be eaten

Their eggs have an increased chance of
parasitism if unguarded
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Trade-offs between foraging and
predation
 With colonial spiders, most individuals hatch and
begin life in the central regions of the colony


Younger, smaller spiders take a chance and build their
orbs (球狀物) on the edges
They obtain more food and grow faster, increasing the
odds of reaching sexual maturity
 As spiderlings mature, the balance of risk changes
 Safety becomes more important than foraging success
 So larger spiders that have reached sexual maturity
prefer the core positions
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More sophisticated models are needed
 Where each individual’s traits influence the decisions
that are optimal
 Dynamic state-variable models: every individual is
described by a set of variables



Given the same set of ecological circumstances,
different individuals may make different decisions
The decisions an animal makes on one day influences
the decisions it makes the next day
So the system change dynamically
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Risk
 Risk: refers to variability in food abundance
 Risk-prone: some animals are gamblers and choose
the variable site
 Risk-averse animals choose reliable sites where they
are more or less guaranteed of finding at least a
mediocre amount of food
 In insects, fish, birds and mammals
 An animal’s hunger level can determine whether it is
risk prone or risk-averse
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Risk sensitivity has been widely documented
 If enough food is found at the site with a stable food
supply

There is no benefit in gambling on finding sufficient
food at the variable location
 If the stable site does not provide enough food
 The only chance for survival is to forage in the location
where the food supply is variable and hope for the best
 Animals that are full should be risk averse
 Hungry animals should be risk prone
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The utility of models
 Modeling provides a chance to clarify our
assumptions about animal behavior

And what information an animal has about its
environment
 We can generate testable and often quantitative
predictions
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Summary
 There are many ways in which an animal obtains food
 Filter feeding, herbivory and omnivory
 Carnivores often engage in arms races with their prey
 Predators may have sensory specializations that improve
their ability to detect their prey
 Search images are adaptations for finding cryptic prey
 According to optimality theory

Natural selection favors the behavioral alternative whose
benefits outweigh its costs by the greatest amount
 Several simple models make predictions about optimal
foraging
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Summary
 The diet selection model assumes that foragers attempt to





maximize their rate of energy gain
The marginal value theorem predicts when an animal should
leave a patch
Simple models may not be adequate to predict behavior
Animals may use “rules of thumb” to approximate optimal
behavior
Some animals are sensitive to rates of energy gain and
variability in food availability
Hungry animals tend to be risk-prone and choose a variable site
 Full animals tend to be risk-averse, choosing s constant site
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問題與討論
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