Animal behaviour L20-25

• Potential cues for orientation
• - Polarity
• - Intensity (strongest at poles)
• - Inclination of lines of force (most animals that use magnetic cues use this one)

• Polarity: used by small number of species e.g. salamanders, salmon fry, mole rats, bats
• Intensity: not known for any species
• Inclination of lines of force: implicated as critical aspect of magnetic stimulation in most studies of magnetic orientation

• Initially discovered in birds - European robins, homing pigeons
• Controversial topic during 1950s and 1960s - results of empirical studies apparently contradictory

• Can experimentally alter magnetic field
• - Reverse polarity
• - Alter inclination of field

• Never any effect of polarity
• Effects of inclination varied among experiments

Via meta-analysis, Keeton identified cloud cover as the factor determining whether inclination change affected orientation

• Inclination compass only critical for navigation on overcast days
• When sun available, European robins and homing pigeons use sun compass
• - Ignore information from altered inclination compass
• Redundancy of systems provides backup

• Earth's magnetic field weak (ca. 50μT)
• difficult to resolve in biological systems
• 2 traditional candidate mechanisms
• - Lateral line system (limited to fish and amphibians)
• - Ferromagnetic particles (e.g. magnetite)
• -> Have been found in association with nerve endings in upper beak of birds
• -> Ablation of those nerves doesn't affect compass orientation
• An alternative mechanism must underlie avian magnetic compass orientation

• in newts, fruit flies, and birds, experiments have consistently demonstrated the orientation responses to magnetic cues are dependent upon wavelength of light
• Under short wavelength (blue-green)/high energy light, orientation fine
• Under long wavelength (yellow-red)/low energy light, unable to orient

Compass requires light energy

• Wiltschko et al. 2002. Lateralization of magnetic compass orientation in a migratory bird
• Bird's ability to orient is eye dependent
• - If left eye covered, orientation unaffected
• - If right eye covered, unable to orient

Except for garden warblers. They use both eyes.

How does high energy light striking the right eye allow magnetic orientation?

• biophysicist with background in quantum physics of bacterial photosynthesis
• became interested in light-dependence of avian orientation and collaborated with Wiltschko

• Pigment molecules called "cryptochromes" located in avian retina
• Light energy excites electrons, influencing electron spin states of "radical pairs" 
• ---> Closely opposed groups of atoms, each with an odd number of electrons
• Radical pairs exist in either a single or triplet conversion state (depending on electron spin)

• Ratio of states is affected by magnetic fields, changing predictably with angle of animal relative to inclination of field
• Found that orientation disrupted by low freq. RF energy (0.1-10 MHz)
• RF of such wavelengths known to affect transition states of radical pairs, but not operation of magnetite receptors

• Bird's magnetic compass is in effect a "chemical compass"
• Mechanism of transduction from ratio of radical-pair transition states to neural stimulation as yet unknown

Suggests parallel arrangement of cryptochrome membranes in receptor may allow radical-pairs to form and influence membrane permeability and thus produce angle-dependent firing rates

• Cryptochrome-based receptors give rise to axons that ultimately project to centers where visual and gravitational input integrated
• Suggests birds may perceive field as a "shadow" in the visual domain, with intensity contingent on angle of orientation
• While left and right eye would "see" mirror images, eye-dependence results suggest that central mechanisms filter information producing right-eye dependence.

Builds upon earlier finding that cryptochromes and area of forebrain (the "N cluster") show increased activity in the context of compass orientation.

• Worked with migratory garden warblers
• - Used neuronal tracing to identify how cryptochromes and "N cluster" interact
• Identified a functional neural connection between retinal neurons and "N cluster" via the visual thalamus
• Magnetic information transduced by the cryptochromes is part of the ascending visual processing system
• Suggests birds "see" compass information provided by earth's magnetic field

Ability to maintain reference to a goal without the use of landmarks

• Requires compass and map
• Map may be -
• Magnetic:
• - Birds become lost in areas of geomagnetic anomalies on sunny days
• - Whale strandings common in areas of such anomalies

• Olfactory
• implicated in navigation by some fish

• Auditory
• e.g. Homing pigeons - Atmospheric progation modeling indicates homing pigeons use loft-specific infrasonic 'map' cues

• Commonly expressed in animals that migrate
• - Periodic movements between one area & climate and another area & climate

Methods employed in study of migration have improved dramatically over the years

• Marked individuals - bands
• Radiotelemetry
• Satellite telemetry
• Geolocators

• Light electronic devices that periodically record ambient light intensity over time to determine location
• - day length (dawn-dusk) determines latitude
• - mid time between dawn and dusk gives longitude
• Far less costly than satellite telemetry if archival

• Purple martins did not migrate faster or arrive earlier at breeding sites in an early spring
• Possible fitness costs if they missed peak food for young

• Chemical composition of organism varies among geographical locations
• quantification of that variation allows identification of origin

• e.g. Wassenaar ad Hobson. Natal origins of migratory monarch butterflies at wintering colonies in Mexico: New isotopic evidence
• - They migrate up 3500km

• Examined hydrogen isotope composition of monarchs from wintering grounds in Mexico
• - Concentrations of deuterium in rain water varies predictably across continent (More deuterium in coastal than inland regions)
• Found each overwintering group consisted of individuals from across the monarch's breeding range
• Good news for monarch breeding populations given deforestation in Mexico

• Merlin, Gegear and Reppert
• Antennal circadian coordinates sun compass orientation in migratory
• - We show that the antennae are necessary for proper time-compensated sun compass orientation

• Spiny lobsters migrate to deeper waters as winter approaches
• Form single-file lines
• Rely on magnetic cues to orient migration (Lohmann 1984)

NOT swift current direction or swell

• European eels have 2.5 year journey to Sargasso sea (as larvae)
• Larvae return to freshwater and mature

• Salmon hatch in freshwater rivers or lakes
• Descend from natal streams into the ocean
• Spend 1-5 years (depending on species) at sea before attaining breeding condition
• At maturity leave feeding grounds and return to same river in which they hatched to spawn
• Rely on different cues for orientation at different stages of their journey.

Returning to shoreline: magnetism, sun compass, moon compass, polarized light 

• Returning to home stream: Olfactory cues
• Determined experimentally 
• - Blinding doesn't impede return to home stream
• - Olfactory impairment (zinc sulfate or nose plugs) significantly reduces homing

• Reared groups of young salmon in water of three possible types:
• -Containing morpholine
• -Containing phenethyl alcohol (PEA)
• -Control water (no chemical added)

• Fish marked as to group of origin and released into lake equidistant from streams containing test chemicals
• - Along 200 km of shoreline, 19 streams with one containing morpholine and one with PEA
• - Trapped fish entering all 19 streams
• - Over 90% of the fish caught in the morpholine and PEA treated streams had been reared in water containing those chemicals as smolts
• Morpholine and PEA fish never caught in opposite chemical-treated stream
• Young fish imprint to their odour of their home stream
• Stream odour derives from mixtures of chemicals from rocks, soil, and plants plus those of mucus, feces and pheromones in the stream in nature

• Individual fish orient directly to the shoreline
• At shoreline, individuals "cruise" the shore
• Individuals sample the chemical effluent of each stream until they arrive at the one matching the chemical bouquet of their home stream
• Engage in both positive rheotaxis and positive chemotaxis at home stream (if odour lost, fish will float downstream until odour re-acquired

• 1800 km to lay eggs on shore of Ascension island 
• -Use chemical cues
• - Mostly magnetic cues- Geomagnetic map used in sea-turtle navigation (Lohmann et al. 2004)
• (Steeper or shallower inclination depending on latitude)

• Return to traditional calving grounds
• In calving grounds, predators are easier seen
• On coast/calving grounds, wind from coast, keeps flies away

• 10,000 km migration from above arctic circle to calving grounds off Baja
• (Warmer for calves to survive because they don't have enough blubber) greater survivorship of young

• Arctic tern nest within 10 degrees of north pole, but overwinter in the Antarctic 
• Annually circumnavigate globe (travel ca. 44,000 km/yr)
• They follow maximized daylight

• Tracking of Arctic terns reveals longest animal migration
• - 82,000 km/yr

• 1. Remain within limits of survival: may be unable to withstand climate
• 2. Reproductive benefits: certain areas impart greater success
• 3. Energetic payoffs: cost of living in harsh environment exceeds cost of migration
• (All relate to abiotic environment, but biotic environment important as well)
• 4. Reduced competition: move to areas of reduced population density or to areas with more resources. Explains return to temperate zone to breed.
• 5. Reduced predation (possibly): move to areas with fewer predators
• 6. Reduced parasitism (flawed. only theoretically, not observed) 
• - Prevent close adaptation of parasite to host

Reasons not mutually exclusive

• 1. Glacial retreat hypothesis
• 2. Radiation hypothesis
• 3. Resource distribution hypothesis
• 4. Continental drift hypothesis

• Wisconsinan glaciation - ca. 80 - 10kybp
• Temperate animals forced south
• Glacial retreat - animals return to ancestral breeding grounds
• Some current routes follow path of glaciers
• Why not stay south?

• Vast majority of species evolved in tropics
• Expanded range to temperate regions:
• - Reduced competition
• - Exploitation of resources
• Return to warmer ancestral area for winter

• Makes no assumption about area of origin
• Migrate to exploit ephemeral or moving resources
• e.g. monarchs moving to exploit milkweed (They sequester the toxin)

• Earth about 4.5 billion years old
• Pangea - supercontinent (225 mybp drifted apart)
• Migration evolved as spp. followed drift
• Continents in present position 65 mybp
• - Many migrants evolved after continental drift
• - Can explain only some spp. migration
• e.g. Green sea turtles

• - a place to live
• Also allow orientation relative to other elements of the environment

• --> Food to eat - foraging
• --> Predators to avoid - antipredator behaviour

Methods employed in locating and consuming food

• All living things require energy 
• Animals obtain energy by finding and consuming food

• 1. Filter feeding
• 2. Herbivory
• 3. Carnivory

- Not mutually exclusive, but convenient in terms of behaviour

• Filter food particles from water by moving water through gills
• Not just in invertebrates
• - Adult paddlefish
• - Baleen whales
• - Flamingos
• - Mallards, shoverlers

• Animals decide:
• When to filter 
• Where to filter (orient to areas of abundant prey)

• Consumption of material produced by plants
• May involve simple behavioural decisions:
• e.g. simplest form of herbivory - grazing
• - When to graze
• - Where to graze
• - Discern edible from inedible items (what)
• May also involve complex behaviours

• Honey bees normally avoid pollen foraging from alfalfa 
• - Alfalfa has spring-loaded anther that strikes bee
• If we force honey bees to forage from alfalfa:
• - Bees adopt two strategies

• 1. Recognized and visit only "tripped" flowers (only spring once)
• 2. Chew through side of flowers, avoiding anther

Fostering growth of another species for consumptive purposes

• e.g. Black-tailed prairie dogs
• Pull non-preferred grasses from areas around burrows
• Culling ensures proliferation of preferred plant species
• May have evolved to preclude growth of tall-growing grasses that act as visual obstruction

• Fostering growth of another species for consumptive purposes
• Rip down large plants or parts of small trees
• Results in surge of growth of young, succulent vegetation
• Individuals return to areas they modify to feed on new growth (but also potential display function to other competing males)

• Cut fresh leaves and return those to the nest
• Don't feed directly upon the leaves
• Actively prepare leaves to be used as substrate to grow fungus
• Inoculate leaf material with hyphae and grow fungus to eat
• This fungus only grows in these colonies

• Move aphids from plant to plant, milking them for "honeydew"
• Ants provide protection and foraging opportunities for aphids
• Ant/aphid mutualism

Where members of one species assume control of the feeding, breeding, and general care of members of another species

e.g. Aphid tending

• Consumption of animals
• May involve simple behavioural decisions
• e.g. simplest forms of carnivory - like grazing: gleaners/scavengers
• - When to eat
• - Where to eat
• - Discern edible from inedible items

e.g. scavengers

*Not saying that other behaviours are simple for scavengers (e.g. communication)

May also involve complex behaiours

• Pursuit
• Ambush
• Trap use
• Tool use

• Involve active chasing of prey
• e.g. Cheetah
• Stalks to within a few hundred meters of prospective prey
• Charges prey at speed up to 115 kph
• Averages nearer 60 kph
• Bursts of speed typically sufficient to capture prey

• Waiting motionless until prey draws close enough to strike
• e.g. cats, frogs, and toads, snakes, spiders

• Key to success is avoiding detection 
• - Use of cover
• - Crypsis
• - Aggressive mimicry

• Moray eels (hide underneath rocks and corals)
• California trapdoor spider:
• Crounches behind moveable cover of underground burrow
• When prey detected above ground, spider pops out and grabs it

Blending with background via evolutionary modification of colouration, markings, morphology, and behaviour

e.g. Spotted coats of leopards and jaguars

• Insectivorous mantid with body parts and colours resembling flowers of orchid on which it rests
• Walks unevenly, mimicking petals blowing in wind

Where a species mimics the appearance or behaviour of a harmless or beneficial species for the purpose of agression

• e.g. False cleaner wrasse ("blenny") mimics appearance and behaviour of actual cleaning wrasses
• Cleaner wrasses remove external parasites, fungi, and bacteria from others
• Blennys take a bite out of "unsuspecting" hosts

• e.g. Angler fish
• - Have modified spine of dorsal fin on tip of snout
• Tip fleshy or endowed with tuft of fibres resembling prey item
• Animated and thus attracts smaller fish

• Body otherwise cryptic
• Lure brings potential prey close enough to ambush

• e.g. Firefly "femme fatales" Lloyd 1975
• Females attract males with flashes of light
• Signals and counter signals are species specific
• Female Photuris versicolor "femme fatale"
• - Mimics signals of several different Photinus spp.
• - Attract Photinus males
• - Eat Photinus  males gaining energy and defensive chemicals (lucibufagins)

• Spiders spin silken web with proteinaceous glue --> captures prey
• Often incorporates thickened cross-like structure
• Stabilimentum 

• Tension and stabilize web
• Attract prey (by reflecting UV light)
• Warn vertebrates not to collide with web

• Caught more insects with stabilimentum
• Experiment of using half webs with either stabilimentum not present, present on other half, and present on the interested half

• Compared damage to decorated versus undecorated
• 45% less damage to decorated webs
• - Stabilimentum attracts prey
• - Stabilimentum warns off vertebrates

BUT stabilimentum also imposes a cost

Attracts predators

worm lions dig holes in sand near an object. Obstacle encourages ants to walk along edge of hole, and sides of hole begin to collasping, bring the ant towards the worm lion

• Corral krill and small fish using air bubbles
• Swim through middle with open mouth consuming prey

• Beat silt with tails to create rings
• Fish try to jump out of "mud" ring and get eaten by waiting dolphins

• Inanimate objects that are not of internal manufacture, that are used to alter the position or form of some other object in the environment
• - Originally thought to be limited to humans
• Many non-humans examples of tool use in the context of foraging

• Egyptian vulture using stones to open ostrich eggs
• Sea otters using rocks as anvils to open sea urchins
• Archer fish - use water to shoot at prey
• Dolphin using sponges to probe bottom for flouders and protect from stony fish

• Woodpecker finch - shortening cactus spines
• Chimpanzee - modify sticks
• Winnow ants 0 use leaf pieces to sop up hemolymph of inverts a bring to colony

• 1. Pounder: destroy nest (I think)
• 2. Enlarger: (makes stringless bee nests opening bigger)
• 3. Dip stick: to get honey
• 4. Probe: termite/ant hills
• 5. Poker/spear: hunt bush babies

• Kacelnik's crow: New Caledonian crows: companion curled wire to get access
• Cheke's Eurasian jays: Selectively chooses heaviest rubber to raise water level to access food
• Green heron fishing baits: Uses bread to attract fish
• Elephants use leaf fonds to swat bugs
• e.g. Chimp knocking a drone out of the air
• Gorilla uses walking stick and land probe

• 1. Species entering foraging niche with extensive competition
• 2. Lack specialized equipment for dealing with prey
• e.g. Sea otters - only marine otters. Recently invaded marine environment
• e.g. Human and ancestors - only primates to exploit mammals as prey (other than chimps)

• Methods employed in locating and consuming food
• Have dealt with foraging strategies

Foraging as a process

• 1. Prey detection (locating food)
• 2. Which prey to attack? (What to eat)
• 3. Where to forage? (where to eat)
• 4. Anti-predator behaviour (avoiding being eaten)

• Frogs and toads: visual cues
• Bats: auditory cues

• Olfaction:
• - Detect blood from up to 4km away
• - Detect minute concentrations (1 drop/100L)

• Hearing:
• - Respond to sounds from up to 1 km away ( to low freq. sound)

• Vision
• - Detect prey at close range (15 m)
• - More rods than cones (low light sensitivity)
• - Tapetum lucidum: reflects light, focuses light onto retina

• Lateral line system
• Neuromasts respond to mechanical disturbances

• All animals generate electric fields: action potentials, muscle contractions, potential difference across membranes
• Injury - cut/scratch (2x signal released)
• Detect via Ampullae of Lorenzini

• Mosquitoes detect heat and CO2
• Pit vipers and boas use thermal signatures

• Variety of potential food items?
• Some prey/food are:
• Easier to find
• Easier to handle/process
• Easier to digest
• Have higher nutritive value

Greatest payoff from maximizing benefit and minimizing costs

• Predicts animals will select forage items that maximizes the net difference between costs and benefits
• Animals that forage optimally enjoy selective advantage
• --> Foraging has been a favorite topic of those studying optimality

• 1. Foraging as a process involves a series of decisions
• - Which prey item to choose
• - How long to stay in one area

• 2. Consequences of alternative decisions can easily be quantified
• Costs and benefits can be translated into energy units (calories):
• - Time to find and handle food
• - Caloric value of item
• - Cost of digestion

Unprofitable food requires more energy to process than it provides

Do animals make such distinctions?

YES

• Crows forage on whelks
• Fly with whelk in beak
• Drop whelk to ground
• - If it breaks, whelk eaten
• - If not, repeats drop or chooses another whelk

• Examined size choice
• Size of shell fragments
• Predominantly large shell fragments
• But smaller fragments swept away?

Conducted experiment

• Crows preferred large and rejected small whelks
• But why are small whelks rejected?

• Offered crows de-shelled whelks - presented equal number of whelks in all three size classes
• Crows ate equal numbers of different size whelks - Large not selectively taken due to calories
• --> On per morsel basis, absolute number of calories trades off with handling time (small whelks eaten faster than large whelks)
• Small not toxic nor unpalatable

Why selectively forage based on size?

• Whelk size influences the ease with which whelks break open when they're dropped from the air
• --> Easier opening decreases handling time and overall energetic cost

Tested hypothesis by dropping whelks of various sizes from different heights

• Large whelks more likely to break from any height with fewer drops (optimum ca 5.23 m)
• For all 3 sizes, there's a maximized return height

• Based on:
• Energy cost (drop height & # drops)
• Energy gain (calories/whelk)

• Net energy profit:
• - Large = 1.49 kcal/whelk
• - Medium = -0.30 kcal/whelk
• - Small = -1.23 kcal/whelk

Crows in field dropped large whelks from an average of 5.23 m above ground

Crows forage optimally

Blue whales optimize foraging efficiently by balancing oxygen use and energy gain as a function of prey density

= net energy gain/handling time

• Caloric content of item
• Energetic cost of obtaining and digesting item
• Handling and processing time, but, other factors are critical as well

• Animals are confronted with multiple potential forage items

• It depends!
• Abundance of most profitable item
• Other nutrient requirements

• When most profitable item detected, it should be consumed
• But
• - If most profitable item relatively rare, it takes a long time to find
• Therefore animal increases overall energy intake by accepting any item that provides an energy gain

• "Food conveyor" study - control abundance of different types of prey
• Used mealworm chunks of 2 different sizes
• When few mealworms of either size:Birds are without preference
• When abundance of large increased: birds took large only, even when # of small increased
• *Preference for large items not based on proportion in the population

• Choice of item cannot be based solely on energetic payoff
• Physiological processes require a variety of nutrients including essential vitamins and minerals
• --> Animals must trade-off energetic gain to obtain a balanced diet

May involve compromise of energy budget to obtain balance of nutrients

• e.g. Belovsky(1978) - Moose foraging
• - Browse on leaves and buds on deciduous trees (High energy): deficient in sodium
• - 80% foraging in time on aquatic plants: rich in sodium but fewer calories
• *Being in water likely details (avoids) pests

• Caloric content of item
• Energetic cost of obtaining and digesting item
• Handling and processing time
• Abundance of most profitable item
• Nutrient content of item

• 1. Prey detection - Locating food
• 2. Which prey to attack - what to eat
• 3. Where to forage?
• 4. Anti-predator behaviour

• Animals forage in discrete patches
• Food distribution usually clumped: animals will search in the most profitable patches
• Abundance of food items and the number of individuals competing for them influence profitability

• Proposed model to explain distribution of birds throughout their habitat
• --> Ideal free distribution

• Animals distribute themselves among habitat patches in proportion to resource availability in those patches
• --> Resource matching rule

• Habitat patch 1 (food enters 10 units/min) and Habitat patch 2 (20 units/min)
• Predict twice as many individuals to occupy patch 2 as opposed to patch 1

• Findings support the resource matching rule
• When the sides of the tanks were switched, the fish adjusted their density accordingly

• Established birds in aviary
• Allowed birds to forage among food patches with different densities of mealworms
• Birds came to prefer patches with the greatest density
• Birds continue to sample less profitable patches
• Though birds are still testing habitat (private info)

Local enhancement hypothesis (Thorpe 1956; Loman and Tamm 1980): individuals increase their foraging efficiency by observing foraging activities of conspecifics at a foraging patch, and recruiting to such patches

Information centre hypothesis (Ward & Zahavi 1973): information regarding food resources acquired at a colony or roost may enhance the foraging success of individual group members

• Animals choose patches with the highest profitability
• - Affected by average abundance of food items within patch (& number of individuals)
• - Also affected by degree of variability in food supply within patch
• --> animal may be faced with choice between a patch that reliably supplies a modest amount of food and one that cycles between large and little amounts
• - Variability = risk

• Animals that "consider" variability in food abundance in deciding where to forage
• Risk sensitivity depends on state of satiation and need of forager
• - Satiated animal at no risk of starvation likely to prefer predictable (stable) food source
• - Hungry animals selected to endure risk associated with variability in source, since moderate amount may not suffice anyway

• e.g. Caraco, Martindale, and Whittam 1980 with yellow eyed juncos
• Presented birds with choice of two seed stray
• - fixed amount (5 seeds)
• - variable amount (0 or 10 seeds)

• Examined  value (utility) of each item of food taken for:
• - Hungry birds
• - Satiated

• Satiated birds should be risk averse
• Hungry birds should be risk prone (greater utility with more seeds)

• They found:
• Satiated juncos preferred the fixed amount trays
• - had positive energy budget
• - were risk averse

• Hungry juncos preferred trays with variable payoff
• - were risk prone

• Shrews (Barnard and Brown 1985)
• Bumble bees (Carter and Dill 1990)

Willingness to endure risk reduces probability of starvation

Underlying energy budget exerts a profound influence on foraging in a social situation (Giraldeau and Caraco book on social foraging theory)

A game theoretical Model incorporating two potential foraging strategies

Producers: locate food for themselves, producing information about food that can be used by others

Scroungers: rely on information from others to make foraging decisions

Two strategies experience differential variance in return on foraging (Individuals can switch between strategies)

• Producers: high variance (potentially high payoff) 
• Predict adoption by risk-tolerant inds.

• Scroungers: low variance (but predictable payoff)
• Predict adoption by risk-averse inds.

Theoretical predictions borne out by empirical data for multiple species

e.g. Scaly breasted Munia (bird) Wu & Giraldeau

• As an animal forages within a patch, food becomes more difficult to obtain
• Food becomes depleted
• Prey take evasive action
• e.g. Plants produce chemicals against herbivory

• Ultimately favours moving to new patch
• Bell curve graph of food intake over patch time (Drickamer and Vessey 1992)

Two factors are critical in deciding when to leave patch

• 1. Capture rate at present location relative to other available patches in environment
• 2. Cost of journey to another patch
• - Predation risk
• - Energy used in travel

• Optimality model
• Maximizing long term rate of return for resources that are patchily distributed
• Predicts when animal should give up present patch and move to another
• - Two domains of prediction
• 1. Patch quality
• 2. Travel time between patches

• Giving up times inversely related to average patch quality in environment 
• e.g. Krebs et al. with Chickadees 
• Move to a new patch when return in current patch drops to average for environment (marginal value)

Krebs et al. 1974 with chickadees

• Established birds in aviary where mealworms placed in wooden dowels
• Could manipulate both average patch quality and travel time between patches

Had wonky df and used one-tailed test (sneaky)

• Patch quality results
• Giving up times
• - Time between last capture and leaving patch
• - Inversely related to average capture rate for the environment
• --> Stay longer without finding food if average patch value is low
• --> Stay less time if average patch value is high

Individual should remain in current patch longer as distance between patches increases

• With little distance between patches:
• --> little predation risk in moving
• --> little energetic cost in moving

• With greater distance between patches:
• --> Risks and costs increase
• --> Offset potential gain of moving

Gather food in cheek pouches and return it to their nest (chipmunks)

• Central Place Foragers - chipmunks
• Provided chipmunks with trays containing sunflower seeds - trays with seeds serve as patches of food
• Manipulated travel time by varying distance of trays from nest

• As distance between patches and nest increases
• --> Time spent foraging in each patch increases

• Travel time substitutable for:
• --> risk of predation
• --> effort required to forage

Established birds in aviary like Krebs, but mealworms in sawdust filled cups

Manipulated effort by placing lids on cups

Increased effort increased foraging time within a given patch

To do so places organism at risk of committing the Concorde Fallacy

Continued investment based upon past outlay with no possibility of offsetting future payoff

• not necessarily
• Greater effort required to forage in patches with hard to open lids decreases average profitability of patches in environment
• --> decreases future payoff associated with giving up
• In other cases, past investment may be correlated with future payoff, and behaviour based upon that

Travel time effect upheld Cowie 1977

• Any time a resource depletes as a function of use
• And costs are associated with traveling between patches of that resource
• Can use marginal value theorem to solve for optimal patch time
• Useful in studies of:
• - Mating behaviour
• - Social behaviour
• - Parasitoid host use

e.g. Walmart placement takes advantage of human "foraging"

• Venturia canescens oviposit on flour moth larvae (Ephestia cautella)
• Presented wasps with patches that varied in host density
• Monitored oviposition over 1 week period
• Encounter rate (with unparasitized host) at end of week did not differ among patches of different densities
• Patches depleted to same marginal value

• Uncovered mechanism of patch departure in Venturia that provides optimal exploitation
• - Host larvae produce cuticular chemical that causes Venturia habituates to the chemical and leaves the patch
• --> But with successful oviposition, effects of habituation are temporarily reversed
• --> As oviposition rate drops, habituation prevails and Venturia leaves patch

• Overall expression of optimality is subject to constraints
• 1. Competition: 
• Allospecifics (ones w/ same resource demands) and conspecifics
• 2. Predators:
• Eat without being eaten
• Foraging represents trade-off between energy payoff, nutritional balance, and safety