Chap.14 Aggression鄭先祐 (Ayo) 教授

國立台南大學 環境與生態學院生態科學與技術學系

環境生態研究所 + 生態旅遊研究所

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Aggression Fight or flight?Game theory models of aggression

The Hawk-Dove Game The war of attrition model The sequential assessment model

Winner, loser, bystander, and audience effects

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Aggression

Conflict → fight or flight?Dominance hierarchies (rank

orderings of the individuals) in a group.Intruder aggression (Fig. 14.1)

When a wasp (left) approaches a nest, guards at the nest determine whether it’s a hive mate or an intruder. Intruders are aggressively repelled.

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Fight or flight? If fighting is costly, then once it is clear that an

animal is losing a fight, it will often be beneficial for it to signal subordination, and hence reduce future costs.

It may inhibit its aggressive behaviors. Color change may be particularly good

communication vehicle in aggressive contests, where color may be linked to “badges of status”, and hence color change can quickly indicate an individual’s relative rank in a hierarchy and whether it will engage in aggressive behaviors. (Fig. 14.2) (Atlantic salmon)

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Serotonin and aggression In crustaceans, increased serotonergic

function leads to enhanced aggression and high social status.

案例： when lobsters are paired up in fights, they

generally escalate their aggressive behaviors through a series of ritualized combats.

Once an individual loses a fight, however, it avoids aggressive interactions for days.

But losers can be made more aggressive if they are given injections of serotonin (Fig. 14.3)

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Fig. 14.3 serotonin and aggression

.A small individual was made subordinate by being matched against an individual that was 30% larger than it was. When serotonin was continuously infused into

subordinates (red bar), their aggressive levels surged. Subordinates returned to their pre-infusion levels within 30 minutes after serotonin infusions were turned off.

(A) the intensity of the aggression over time, and (B) the duration of the aggression.

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Game theory models of aggressionAll game theory models must have

some sort of variable that represents the value of the resource being contested.

Deciding to fight (Fig.14.4) (A) one of the many resources animals will

fight over is food, as shown here by these vultures that are fighting over a carcass.

(B) males also fight over females, here, male elephant seals are fighting over access to reproductively active females.

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Value of a resource Two individuals contesting a resource may not

assign the same value to that resource. For example (Fig. 14.5), imaging that two

animals – one of whom is starving, and the other of whom is hungry, but not starving – are contesting a ten-pounds of meat.

To a starving animal, ten-pounds of food might make the difference between life and death, while to a less hungry animal the value of the ten pounds of meat might be much lower.

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Territory Consider the value of territory to a potential

intruder and to a territory holder The territory holder will value a contested area

(its territory) more because it has already invested time and energy in learning where the resources in such a territory are located

Table 14.1 Resource value and fighting. The value that an individual animal assigns to

some resource affects how long contests last to what extent they escalate.

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The hawk-dove game

Imagine that individuals can adopt one of two behavioral strategies when contesting some resource: (1) hawk- wherein a player will escalate

( 增加占有 ) and continue to escalate until either it is injured or its opponent cedes( 讓出 ) the resource,

(2) dove – wherein a player displays as if it will escalate, but retreats and cedes the resource if its opponent escalate.

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V = the value of the contested resource C = the cost of fighting

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Evolution of fighting behavior: game theory

Why don’t animals always fight with maximum effort? Natural selection favors the individual that

passes on more of its genes Game theory can help to understand

the evolution of conflictGame theory: predicts an animal’s

optimal behavior While taking into account the behavior of

other animals

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The game-theory model Players: the combatants Strategies: Different decisions available to

players Assumed to be heritable Successful strategies increase in the population

Payoff: measures the costs and benefits for each strategy

Currency: used to measure the payoff Relates to fitness (number of offspring produced or

number of calories acquired) A payoff matrix: organizes the values of the

payoffs of each strategy against the other strategies

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One game theory model: hawk-dove The simplest game-theory model of

aggression Two players fight over a resource Each opponent can play one of two strategies:

hawk and dove Hawk strategy: immediately attack its

opponent Dove strategy: flee immediately if

confronted by a hawk Display if confronted by another dove

If a hawk meets a hawk or a dove meets a dove Each opponent has a 50% chance of winning

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The payoff matrix for the hawk-dove game

Three variables measure a currency that relates to fitness: V = the value of the resource being

contested W = the cost of being wounded in a fight D = the cost of displaying to an opponent

Add some numbers: V = 30 W = 60 D = 5

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Payoff for hawk – hawk interaction If an animal playing the hawk strategy meets

another hawk Both attack immediately One hawk wins the resource: its payoff is V The other hawk will be wounded: its payoff is –W

The average payoff for a hawk vs. hawk interaction Payoff for the winning hawk + the payoff for the losing

hawk Divide by 2 to get the average V – W

2

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Payoffs for other interactionsHawk against dove

The hawk immediately attacks The dove flees Hawk wins the resource, so its payoff is V

Dove against hawk The dove immediately flees The dove does not get injured Nor does it win anything - its payoff is 0

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The payoff for dove vs. dove

One wins the resource The other walks away

Both pay the cost of display The payoff for the winning dove is V-D The payoff for the losing dove it is just –D

Sum these and divide by 2 V ─ D ─ D V ─ 2D V

= = ─ D 2 2 2

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Box18.1 How many hawks and doves? Neither hawk nor dove is an evolutionarily

stable strategy. Rather, the stable equilibrium composition of

the population is some combination of hawks and doves in a mixed ESS.

The stable proportion of hawks and doves occurs when the average payoff for the hawk strategy equals the average payoff for the dove strategy.

Assume: p = the proportion of hawks in a population 1- p = the proportion of doves

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Box18.1 How many hawks and doves?

Payoff for dove strategy = p (V-W)/2 + (1-p) V

Payoff for hawk strategy = p (0) + (1 – p) (V/2 – D)

At equilibrium…. -15p + (1-p)30 = 0 + (1-p) 10 -15p + 30 -30p = 10 – 10p 30 – 45p = 10 – 10p 20 = 35 p p = 0.57

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Understanding the gameThe currency = units of fitness

These strategies are heritableSuccessful doves have offspring

That also play the dove strategyHawks give rise to hawksGame-theory models predict whether

strategies in a population Increase in frequency Remain stable Or disappear

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An evolutionarily stable strategy (ESS) A strategy that, when played by all

members of the population Cannot be invaded by another strategy

If the dove strategy is an ESS All members of the population play the dove

strategy If an animal playing hawk entered

All of its opponents would be doves The hawk strategy will do well

The hawk’s genes increase The hawk strategy increases in frequency

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Is the hawk strategy an ESS? Will the population eventually become all

hawks? If the population is comprised of all hawks The average payoff drastically decreases

If a dove enters the population It won’t win But it won’t be wounded during half its battles The frequency of the dove strategy would increase

Neither a “pure hawk” strategy nor a “pure dove” strategy is an ESS

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A mixed ESS is stable A mixed ESS: some combination of hawk

and dove strategies that is stable The stable proportion of hawks and

doves occurs When the average payoff for the hawk strategy

equals the average payoff for the dove strategy A certain proportion of animals always

plays hawk And another proportion always plays dove

Or all animals play both hawk and dove

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Can a hawk or dove strategy be stable?If the value of a resource (V) is

greater than the cost of being wounded (W) A pure hawk strategy is an ESS

If V < W, a mixed ESS will result

A pure dove strategy is never an ESS

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Bourgeois butterflies In the Speckled wood butterfly, territories are

not set in space. That is, rather than having territory with a set

place in three dimensions, a male has a territory that is an open patch defined by well-lit areas that emerge when the sun breaks through the clouds.

When a male comes upon an empty well-lit patch, he immediately occupies it and secures a mating advantage, compared to males not in sunlit territories.

Resident wins rule (Fig. 14.6)

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Antibourgeois Mexican spiders

These spiders establish their territories under rocks, and when an intruder approaches a territory, the territory holder flees rather than fighting.

The former territory holder then searches for a new territory.

Fig. 14.5 reverse bourgeois strategy

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The war of attrition ( 磨損 ) model

Three underlying assumptions (1) individuals can choose to display

aggressively for any duration of time (2) display behavior is costly – the longer

the display, the more energy expended (3) there are no clear cues such as size,

territory possession, and so forth, that contestants can use to settle a contest.

Fig 14.8 the probability that a contest will last a certain length of time is a function of the value of the resource.

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The sequential assessment modelA third game theoretical model of

aggression is called the sequential assessment model.

The sequential assessment game examines contests in which the level of aggression varies from relatively mild to very dangerous.

範例： In Nannacara anomala (fighting fish)

(Fig. 14.10) Mouth wrestling (Fig. 14.11)

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Mouth wrestle in fighting fish

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Winner, loser, bystander, and audience effects

Winner and loser effects In blue-footed boobies In Rivulus marmoratus (fish) In copperhead snakes Mathematical models

Bystander effectsAudience effects

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Blue-footed boobies Fig. 14.13 winning and losing booby chicks

(A) when subordinate chicks (green curve) who have experienced many losses are paired against neutral singleton chicks (red curve), they display very little aggressive behavior.

(B) when dominant chicks (orange curve) who have recently won many aggressive interactions are paired against neutral chicks (red curve), they are very aggressive. The pattern begins to dissipate after four hours.

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In Rivulus marmoratus (hermaphroditic fish)More controlled experimental

studies of winner and loser effects have been undertaken in fish than in any other group.

Why? (1) aggression is common in fish (2) aggression in fish is easily quantified

in controlled laboratory settings (3) the endocrinology of aggression in fish

has been well documented in numerous species.

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In Rivulus marmoratus

Wins (W), losses (L) or neutral (N) The wins and losses an experimental

fish received before it was tested were controlled by the researchers.

By comparing fish in the WW versus LW and LL versus WL treatment (Table 14.3), the penultimate aggressive interaction a fish experiences also affects its current probability of winning or losing.

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In Rivulus marmoratus

While penultimate interactions were important, their impact on winning or losing a current interaction was not as powerful as the outcome of the interaction immediately preceding the interaction underway.

asymmetry in winner and loser effects- that is, the loser effect was not stronger than the winner effect, nor vice versa.

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In copperhead snakes Copperhead snakes (Agkistrodon contortrix)

Males had had no aggressive interactions for 6 to 12 months prior the study.

“size” contests Two males that differed in size by

approximately 10 %. In all 32-trials, the larger male emerged as

dominant and gained reproductive access to the female.

Winner and loser effects Ten winners and ten losers from the “size”

contests, and each was matched against a same-sized male copperhead that had no prior experience.

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In copperhead snakesNo winner effects

The prior winners were not more likely to win again.

But have a loser effects, that is, losers were more likely to lose

again. losers lost all contests even with smaller

opponents. The plasma corticosterone was

significantly greater in losers than in winners or controls (Fig. 14.15)

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Mathematical models Computer simulation that examined winner

and loser effects when individuals assessed each other’s fighting ability and could choose to fight or flee.

Results: (Table 14.14) Winner effects alone produce hierarchies in

which the rank of individuals all the way from top rank (α) to bottom rank can be unambiguously assigned.

Loser effects alone produce hierarchies in which a clear αindividual exists, but the relationship among other group members remains murky.

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Table 14.4 winning, losing, and hierarchy formation

When winner effects alone are at play (green box), a clear linear hierarchy exists, with the position of each individual clearly delineated.

When loser effects alone are at play (orange box), only the alpha individuals is clear.

Loser effects tend to outweigh winner effects (red box)

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Bystander effects

When the observer of an aggressive interaction between two other individuals changes its assessment of the fighting abilities of those it has observed, the bystander ( 旁觀者 ) effect - sometimes called the “eavesdropper ( 偷聽者 ) effect” – is in operation.

案例： green swordtail fish

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Bystander effects of swordtails Eavesdroppers could either observe

aggressive interaction between a pair of other or such interactions were blocked from view by an opaque partition.

When eavesdroppers were then paired against winners of the previous contests, those who had observed the previous contests were less likely to (A) initiate interactions, and (B) escalate aggressive interactions. Those who were paired with the losers were

more likely to initiate aggressive contests. The difference between orange and green bars

for losers was not statistically significant.

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Cichlid fish

When eavesdropping males watch a fight between a pair of other males, their androgen levels rise (Fig. 14.17). Testosterone levels increased

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Audience effects Individuals involved in appressive interactions

change their behavior if they are watched. Fig. 14.18 audience effects in chimpanzees.

Chimpanzees that were victims in severe aggressive interactions emitted distinctive screams.

Significantly longer screams were emitted when fights were watched by an audience that to the aggressor (orange bars) than when the audience did not contain such an individual (green bars).

This screaming strategy was successful, as victims who emitted longer and more intense screams were able to entice support from observers, who would proceed to intervene and break up fights.

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Ayo NUTN website:http://myweb.nutn.edu.tw/~hycheng/

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