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Chapter 22: Territorial Signals

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Territorial Contests

• Ownership asymmetry– New twist: what significance?

• Previous models have only considered asymmetries in resource value and cost of conflict (fighting ability)

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Games again

• Hawk-Dove– With bourgeois variation

• War of Attrition• Sequential Assessment

• What predictions for territorial signalling?

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Territories• Fixed area from which intruders are excluded by

an owner• Different types, with different values and costs

– Breeding: small, only mating or nesting– Feeding: larger, enough to support owner– All-purpose: largest, has it all

• Neighbourhoods– Networks of contiguous territories

• Territorial signal: a long-distance threat signal associated with territory defense

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e.g. Great Tit (Parus major)

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Territory ownership and auditory signals

Great tit song

• Experimental removal of owners

•Broadcast of songs from speakers

•After 12 hours, new owners only in control areas

•Slower establishment with multiple songs

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Territorial Conflicts• Owners• Intruders

– Neighbours: other territory owners in neighbourhood• Encounters initially aggressive• Settle to stable associations with repeated low-level

encounters• Escalation only if trespass• Recognise neighbours and judge location

– Floaters: non-owners searching for a territory• Immediate aggression from owner• Potential asymmetries in fighting ability and resource

valuation• Owners should distinguish neighbours and

floatersBGYD45 2003:9 8

Distinguish neighbours & floaters

• Why?– Owners usually win contests– Neighbourhoods often have stable boundaries and

membership• Asymmetries

– Payoff-relevant• Fighting ability• Resource value

– Payoff-irrelevant• Owner/intruder (this is a new twist)

• How do these affect outcome?

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Bourgeois convention

• Conventional settlement based solely on ownership asymmetry– Is this a stable solution?

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Hawk-Dove-Bourgeois game

V= territory value to winnerD= cost to loser

Bourgeois is a conventional strategy, uncorrelated with fighting ability and territory value

Strategies:Hawk: EscalateDove: Display Bourgeois: Hawk if owner

Dove if intruderAssume owner 1/2

time

Result: -Bourgeois only invades if cost of fighting greater than benefit of winning-Value of territory must be low

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But:• Unlikely that truly bourgeois strategy is stable

(requires V < D)• Territory games are really scrambles rather than

contests– Ownership will increase territory value– Value of winning higher if ownership is respected– Cost of losing lower if non-owners have little chance

of getting territory– Desperado strategy

• Assessment of fighting ability and resource valuation likely to be important– Therefore need other models

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Asymmetric Games revisited

• Hawk-Dove• War of Attition• Sequential Assessment

• Role of payoff-relevant asymmetry

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Asymmetric Hawk-Dove

• Size and ownership– Large owner – small intruder– Small owner – large intruder

• Opposing asymmetries (ownership, size)• Let x be probability larger wins

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Asymmetric Hawk-Dove

assessor

hawk

Bourgeois/anti-bourgeois

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ESS

• Three outcomes, depending on parameter values– Hawk– Assessor– Bourgeois or Anti-bougeois (common sense

or paradoxical strategies)• This suggests that ownership asymmetries can be

stable basis for conventional settlement under some conditions. But how realistic is this?

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Limitations of Asymmetric H-D• No role for differences in territory value

(between owner and intruder)• Predicts paradoxical conventions (not

observed)• Predicts bourgeois strategy with V < D

(unlikely to be stable for reasons given above)

• Doesn’t allow varied investment of cost• Assumes perfect knowledge of x

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Asymmetric war of attrition

V = benefit of winningk = rate of cost accrual

V/k ratio determines how long an individual is willing to fight

Contestants assess relative V/k ratio

Select high or low persistence time separated by cutoff value ‘S’

Contests longer (S bigger) when V/k ratio similar between two contestants

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Asymmetric war of attrition

• Hard to separate V & k• Hard to examine role of payoff-irrelevant

asymmetry• Can explain owner advantage by

ownership increasing territory value

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e.g. Great Tit removal experimentContests in great tits

Prediction: value of territory increases with tenure, decreases with absenceAssumes costs (k) similar

Results: -Returning owners won after short absences, lost after longer absences-Longest contests for intermediate absences-No evidence of ownership convention-No evidence of asymmetry in fighting ability

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Sequential Assessment

Higher V will move giving-up line lower

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Sequential Assessment• Original version

– V is fixed– Giving-up line: quit if your estimate of your relative

fighting ability crosses threshold• Territorial variation

– V different for owner & intruder– Therefore different giving-up lines– Can hold either V or fighting ability to model effects

separately• V has stronger effect

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Predictions• When fighting ability is similar, owners win

because they have a higher V• Contests longer if intruder has higher fighting

ability (this lowers his giving-up line)• Contests longer if both think they’re owners• Contests won by intruders are longer than

contests won by owners (this is not predicted by WoA, first three are)

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e.g. Elephant Seals

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Long fights in elephant seals

• Winners either small owners or large intruders

• Winning owners do better (reproductive success) than winning intruders

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General conclusions from territorial games

• Simple conventions like ‘owner always wins’ are unlikely

• Territory owners value territories more than intruders– Investment in reproduction– Increased probability of additional mates– Increased knowledge of food and hiding places– Stable boundaries with neighbors

• Territory valuation more important than fighting ability

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Implications for territorial signals• Territory signals should be designed to:

– Primarily display value placed on territory ownership (motivation to fight)

– Secondarily display fighting ability– Be individually distinctive

• Example: Territorial defense calls in cricket frogs– Frequency of initial call by territory owners correlated with body size– Subsequent responses to intruders lower in frequency– Drop in frequency correlated with likelihood of attack

– Calls contain information on both fighting ability and of motivation to fight

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Assessment of neighbours versus floaters

• Neighbours and floaters differ in degree of threat to owners– Frequency & repetition of encounters– Value of territory– Prior knowledge of strengths

• Owners need to assess these differences

• Neighbours need to be recognized as such

• Production of individually variable signals

• Discrimination of these signals

• Memorization of proper location of a neighbour

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How?

• If you have a territory, then by definition an intruder is a rival inside your territory.– Therefore must be able to judge location

(distance) of rival signal– If you want to tell neighbours from floaters,

you need something more sophisticated

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IntrudersOnly a problem if they get too close: e.g. Coqui frog

Just distance, no who’s who.

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How do coqui frogs compete with neighbours for air-time?

• Brush & Narins (1989) Animal Behaviour37:33-44

• Based on computer network models– Competition for bandwidth– Avoid calls overlapping (jamming)– Stereotyped signals – Fairly regular timing– Prefer quiety but will compete for air-time

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Simulation Model

1) Silent (monitor)

2) Activiation(40 ms)

3) Call (400 ms)

5) Random Delay (0-800

ms)

4) Refractory (1140 ms)

If no other calls detected during state 1, activate

with probability π Effects of group size and π

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Compare with real frogs

• Record individual members of calling choruses– Measure degree of call overlap (jamming)– Do they avoid overlapping with each other?– Is there a limit to how many neighbouring

singers they respond to?

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Model results

• To minimize call overlaps– Small group size– Optimal π value

• don’t call every time there is silence, hold back a bit (how much depends on group size)

• Do the frogs do this?

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Field Results

Yes. More or less.BGYD45 2003:9 38

Field Results• Frogs only interact with two or three nearest

neighbours, ignore the rest (irrelevant)• Avoid jamming

– But occasionally deliberately jam. So it’s a bit more complex.

– Nevertheless, not all rivals are treated equally• This only looks at strategies for timing of a fixed

(stereotyped signal). What about more complex signalling strategies?

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e.g. Songbirds• Complex neighbourhoods• Complex acoustic signals

– Varying complexity among species• Dear enemy phenomenon

– Owners less aggressive toward invasion by neighbours than toward floaters

• Know each other, can measure one another’s V/k ratios more accurately, short contests

• Highly complex territorial signalling– What’s it all for?

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Song Repertoires• Males of many songbird species produce

a great variety of songs (“song types”)• Neighbours often share similar song types

(a fraction of their repertoires)• Evidence that females prefer males with

large repertoires (but won’t deal with that now).

• Repetoires also used in territorial singalling

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Banded Wren, Thyrothoruspleurostictus

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Complex Neighbourhood

How to study this?

Must identify who sings when and where.

Must manipulate singing to test hypothesized mechanisms.

Technically difficult.

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Microphone arrayMany microphones (eleven in this study).

Multichannel recording.

Triangulate to calculate location of each call recorded.

Must also obtain complete record of each male’s repertoire.

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Interactive playback

Quick selection of playback from library of song types.

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“O” “Yoda” “O” “Yoda”

http://birds.cornell.edu/BRP/BaWrSongMatch.htmlBGYD45 2003:9 46

Simple case: counter-singing

• Can indicate response to particular receiver by alternating calls with him– Rate, duration etc. can form basis of contest

• Many creatures do this– Insects, birds, mammals– Birds with larger song repertoires may do

more

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More sophisticated: song sharing

• Interacting with neighbours individually– Many neighbours, everybody singing, how to

know who’s challenging who?• Signal varying levels of aggression

(escalation)• Test intruder

– Who is he?– Does he know my songs?

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Graded Escalation• Song-type matching

– Reply with the same song– Indicates who receiver is and that sender recognises

him individually (maybe)• Repertoire matching

– Reply with a different song but still one that is in his repertoire

– Lower level of aggression• Fail to match

– Either not engaging or revealing floater status

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Neighbours vs Floaters

• Neighbour interactions– Ongoing low-level interaction always testing

the boundaries– Can escalate

• Overlapping: very rude

• Floaters– Interlopers subject to immediate escalation

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Variety

• Fine for banded wrens– How general is this?– What about species with fewer song types or

less flexible song learning?

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e.g. Song Sparrow

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Alternative song matching

• Song sparrows have fewer song types– Less sharing with neighbours

• Still use song matching (in a broader sense)– Matching by singing a song type that has

some recognizable similarity.

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playback

Type match (neighbour)

Type match (stranger)

Intro element match

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Eavesdropping

• Cardinals that hear a neighbour put the run to an intruding floater respond more quickly and aggressively if he flees into their territory– May not be aware otherwise– Floaters in this situation usually avoid fleeing

into the adjacent territories

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Back to Great Tits

• Obtain information on the status of potential rivals by eavesdropping on interactions in neighbouring territories.– Clever playback experiments

BGYD45 2003:9 56Peake et al. (2001) Proc R Soc B 268:1183-1187

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Results

• Males respond more aggressively to intruders they have overheard to be losers than to those overheard winning.– Songs used in playbacks were identical, only

timing varied (ie alternate, overlap, etc.)– Therefore no absolute quality information from

song itself, effect must be due to perceived outcome of previous interaction