Territorial Contests. Chapter 22: Territorial Signals. Games again. Territories. Ownership asymmetry

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1 Territorial Contests Chapter 22: Territorial Signals Ownership asymmetry New twist: what significance? Previous models have only considered asymmetries in resource value and cost of conflict (fighting ability) BGYD :9 1 BGYD :9 2 Games again Hawk-Dove With bourgeois variation War of Attrition Sequential Assessment What predictions for territorial signalling? 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 BGYD :9 3 BGYD :9 4 1

2 e.g. Great Tit (Parus major) 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 BGYD :9 5 BGYD :9 6 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 floaters BGYD :9 7 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? BGYD :9 8 2

3 Bourgeois convention Conventional settlement based solely on ownership asymmetry Is this a stable solution? Bourgeois is a conventional strategy, uncorrelated with fighting ability and territory value Strategies: Hawk: Escalate Dove: Display Bourgeois: Hawk if owner Dove if intruder Assume owner 1/2 time Hawk-Dove-Bourgeois game V= territory value to winner D= cost to loser Result: -Bourgeois only invades if cost of fighting greater than benefit of winning -Value of territory must be low BGYD :9 9 BGYD :9 10 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 BGYD :9 11 Asymmetric Games revisited Hawk-Dove War of Attition Sequential Assessment Role of payoff-relevant asymmetry BGYD :9 12 3

4 Asymmetric Hawk-Dove Size and ownership Large owner small intruder Small owner large intruder Opposing asymmetries (ownership, size) Let x be probability larger wins Asymmetric Hawk-Dove assessor hawk Bourgeois/anti -bourgeois BGYD :9 13 BGYD :9 14 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? BGYD :9 15 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 BGYD :9 16 4

5 Asymmetric war of attrition V = benefit of winning k = 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 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 Contests longer (S bigger) when V/k ratio similar between two contestants BGYD :9 17 BGYD :9 18 e.g. Great Tit removal experiment Contests in great tits Sequential Assessment Prediction: value of territory increases with tenure, decreases with absence Assumes 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 BGYD :9 19 Higher V will move givingup line lower BGYD :9 20 5

6 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 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) BGYD :9 21 BGYD :9 22 e.g. Elephant Seals BGYD :9 23 BGYD :9 24 6

7 BGYD :9 25 BGYD :9 26 Long fights in elephant seals Winners either small owners or large intruders Winning owners do better (reproductive success) than winning intruders 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 BGYD :9 27 BGYD :9 28 7

8 Implications for territorial signals Assessment of neighbours versus floaters 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 BGYD :9 29 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 BGYD :9 30 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 Intruders Only a problem if they get too close: e.g. Coqui frog BGYD :9 31 Just distance, no who s who. BGYD :9 32 8

9 How do coqui frogs compete with neighbours for air-time? Brush & Narins (1989) Animal Behaviour 37:33-44 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 BGYD :9 33 Simulation Model 1) Silent (monitor) 2) Activiation (40 ms) 3) Call (400 ms) If no other calls detected during state 1, activate with probability π 5) Random Delay (0-800 ms) 4) Refractory (1140 ms) Effects of group size and π BGYD :9 34 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? 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? BGYD :9 35 BGYD :9 36 9

10 Field Results 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? Yes. More or less. BGYD :9 37 BGYD :9 38 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? BGYD :9 39 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 BGYD :

11 Banded Wren, Thyrothorus pleurostictus Complex Neighbourhood How to study this? Must identify who sings when and where. Must manipulate singing to test hypothesized mechanisms. Technically difficult. BGYD :9 41 BGYD :9 42 Microphone array Many 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. Quick selection of playback from library of song types. Interactive playback BGYD :9 43 BGYD :

12 O Yoda O Yoda 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 BGYD : BGYD :9 46 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? BGYD :9 47 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 BGYD :

13 Neighbours vs Floaters Neighbour interactions Ongoing low-level interaction always testing the boundaries Can escalate Overlapping: very rude Floaters Interlopers subject to immediate escalation Variety Fine for banded wrens How general is this? What about species with fewer song types or less flexible song learning? BGYD :9 49 BGYD :9 50 e.g. Song Sparrow 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. BGYD :9 51 BGYD :

14 playback Type match (neighbour) Type match (stranger) Intro element match 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 BGYD :9 53 BGYD :9 54 Back to Great Tits Obtain information on the status of potential rivals by eavesdropping on interactions in neighbouring territories. Clever playback experiments BGYD :9 55 BGYD :9 Peake et al. (2001) Proc R Soc B 268:

15 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 BGYD :