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________ | | | Rich habitat Poor habitat Rewards Per Indiv. No. of competitors.

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Presentation on theme: "________ | | | Rich habitat Poor habitat Rewards Per Indiv. No. of competitors."— Presentation transcript:

1 ________ | | | Rich habitat Poor habitat Rewards Per Indiv. No. of competitors

2 ________ | | | Rich habitat Poor habitat Rewards Per Indiv. No. of competitors X

3 Problems in the study of animal contests Why are displays so common and fights so rare? How do animals “decide” what kind of combat will occur and who will win?

4 A simple model of competition Two strategies: HAWKS: Always escalate DOVES: Always display, then retreat before contest escalates into fighting Payoffs: Win: +50 Lose: 0 Injury: -100 Waste time in Display: -10

5 ½(50) + ½(-100) = ½(50-10) + ½(-10) = 15 Hawk Dove Opponent: Attacker: Hawk Dove

6 EVOLUTIONARY STABLE STRATEGY A strategy which, if adopted by all members of a population, cannot be invaded by an alternative.

7 At equilibrium: Mean payoff from hawk strategy dove strategy = (Prob. meeting hawk)* (Payoff vs. hawk) + (Prob. meeting dove)* (Payoff vs. dove) (Prob. Of meeting hawk)* (Payoff vs. hawk) + (Prob. Of meeting dove)* (Payoff vs. dove)

8 At equilibrium: Mean payoff from hawk strategy dove strategy = h = frequency of hawk (1-h) = frequency of dove -25(h) + 50(1-h) = 0(h) + 15(1-h) -25h + 50 – 50h = 15 – 15h 35 = 60h 7/12 = h

9 Assume: V = gain in fitness to winner C = reduction in fitness from injury Conclude: (1) H is an ESS if ½(V-C) > 0 or if V > C (2) if V < C, a mixed strategy is the only ESS ½(V-C)V 0V/2 H D HDHD

10 CONCLUSIONS: 1.One individual’s fighting strategy depends on what others are doing. 2. The ESS depends on the other strategies in the game. 3. The ESS depends on the values assigned to each strategy.

11 The bourgeois strategy II. Juncos Cage A A1 A2 A3 A4 A5 A6 Cage B B1 B2 B3 B4 B5 B6 Cage X A1 B1 B2 A2 A3 B3

12 The bourgeois strategy II. Juncos Cage A A1 A2 A3 A4 A5 A6 Cage B B1 B2 B3 B4 B5 B6 Cage X A1 B1 B2 A2 A3 B3 Cage A A1 A2 A3 B1 B2 B3

13 CONCLUSIONS: 1.One individual’s fighting strategy depends on what others are doing. 2. The ESS depends on the other strategies in the game. 3. The ESS depends on the values assigned to each strategy.

14 Most animal contests don’t escalate into fighting but instead involve displays. Displays occur because there is no single “best” strategy. Animals must assess their opponents and make decisions based on these assessments.

15 How do animals “decide” how to behave in contests? When should they be hawks and when doves? How do animals assess their opponent’s strength and willingness to escalate?

16 Natural selection favors individuals that give displays because: Dominant animals will win contests Subordinate animals will avoid costly fights

17 SIGNALS USED IN DISPLAYS Can convey info about: Size, strength, physical ability Intentions Can be: Reliable Unreliable

18 Problems in the study of signals of “intention” 1.If displays have evolved to signal an animal’s imminent behavior, why are they such poor predictors of attack? 2. Why are there so many threat displays?

19 ________ | | | Effec- tiveness Frequency RareCommon

20 1.Giving displays is always adaptive. 2.Individuals can display using either reliable or unreliable signals. 3. Although signalers will try to bluff, selection will favor recipients that attend only to reliable signals.

21 Most displays are honest indicators because they involve a cost. Skeptical recipients weed out unreliable signals.

22 ________ | | | Grapple Duration (sec) Difference in body length (mm)

23 Aggression in thrips Outcome: Display followed by: Withdraw. 1 fight >1 fight stabbing Opponents Sm. vs. Lg. 79% 18% 3% ---- Lg. vs. Lg. 35% 26% 26% 13% Sm. Vs. Sm. 16% 6% 44% 34%

24 Baboon male “contest” wahoos High-ranking males call most often and at the highest rates All males compete most often against rivals of similar rank Contests between similarly-ranked rivals:  longer  more calls  higher calling rates  more likely to result in fighting

25 WHY HAVE BADGES OF STATUS? From the signaler’s perspective: Subordinates avoid costly fights Dominants can too You may deceive your opponent From the recipient’s perspective: Often a quick, reliable way to assess an opponent without fighting

26 Badges of status may appear arbitrary (bizarre feathers, bright colors, smelly secretions, loud calls), but what they signal is not (size, physical condition). Badges of status are often based on characters that cannot be faked.


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