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Groups, competition, aggressive communication. MSc ACSB module 2006/07 SB week 10. Benefits of group life. Food Food-finding, e.g. colony information exchange Changes the variance of feeding success Co-operative hunting, - lions, hunting dogs Safety

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Groups competition aggressive communication l.jpg

Groups, competition, aggressive communication

MSc ACSB module 2006/07

SB week 10

Benefits of group life l.jpg
Benefits of group life

  • Food

    • Food-finding, e.g. colony information exchange

    • Changes the variance of feeding success

    • Co-operative hunting, - lions, hunting dogs

  • Safety

    • Dilution of risk from predators - pelagic fish

    • Increased vigilance - pigeon flocks

    • Co-operative defence- buffalo

  • Breeding

    • Co-operative territorial defence

    • Helpers at nest/den - scrub jay, dwarf mongoose

    • Eusocial societies - naked mole rat

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Penalties of group life

  • Cooperation → larger prey can be killed…

    • BUT low rank + big group limits food intake by subordinate members of group

  • Larger groups reduce predation risk…

    • BUT increase competition at food sources

  • So, benefits must be traded off against costs

    • What currency underpins for this trade-off?

    • Mean intake per hour, or per day

    • Variance in intake rates across time

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Mean vs. variance

  • Safety (low variance) may be more valuable long-term than a high rate of intake

    • High variance → a greater risk of ‘going bust’, even if mean return is quite good

  • Grouping can alter variance in food intake

    • Thompson, Vertinsky & Krebs (1974) JAE 43, 785-820Realistically-simulated flocking in titmice - flocking had much greater impact on variance than on mean return (MODEL 1)

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Who bears the costs in animal groups?

  • Dominants get more of the benefits from life in the group, subordinates bear more of the costs.

    • Skew within the group

  • Sibly’s ‘thought-experiment’ model:

    • groups expected to bring benefits and costs

    • costs greatest for lowest-ranked ##

    • Assuming free entry/exit, group will be stable at the size at which: a subordinate does as poorly in the group as it does when living independently

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How kinship affects ‘skew’ in groups

  • Vehrencamp, 1983, Anim. Behav. 31, 667-682(MODEL 2)

  • In kin-groups, lowest-ranking members incur direct costs to benefit dominants; but if dominants are related, subordinates get indirect benefits

    • Subordinates’ inclusive fitness is increased if the breeding success of these dominants is enhanced

    • The closer the relationship Dom →Sub, the greater the skew that can be imposed on Sub

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Vehrencamp’s model

  • So a family group may be exploitative … especially if very close kin are involved

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Helping vs. breeding

  • Harassment in W. Fronted Bee-Eaters

    • Harassment halts many breeding attempts

  • Emlen & Wrege, 1994, Nature 367, 129-132 (MODEL 3)

  • Decision model to predict choices when faced with reproductive decisions

    • Calculated which options are most profitable

    • Takes age, relatedness, etc into account

    • Takes alternatives of breeding, helping into account

    • Payoff matrices for different conditions

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Profitability of breeding and helping

  • (a) is payoff to males of breeding

  • (b) payoff to M or F of helping

  • (c) payoff to females of breeding

    • If young female tries to breed and fails, no second-string benefit from helping, so low payoff in top left hand cells

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Testing Emlen & Wrege’s model

  • Calculated expected payoff from staying at home vs pairing with a particular male, for each female that paired

    • 7 of 74 (9%) of females seem to ‘get it wrong’

  • Compared ‘errors’ with frequency in simulations in which

    • Females paired randomly

    • Females assess benefits of staying home to help, then choose male mate randomly

    • These simulations gave fewer ‘correct’ decisions

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Signalling status

  • Access to resources in a group often depends on status – so animals may signal their status

  • Larger, darker bib below bill on dominant Harris sparrows (“studlies”) - Rohwer

  • Birds take note of the size of the bib in their interactions

    • so this badge of status allows a dispute about rank to be resolved without fighting

  • Why is the dark bib an honest signal of status?

    • If wearing a large badge gives victory at minimum cost, why don’t subordinates pretend to be dominant by growing larger badges?

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Rohwer’s experiments

  • Rohwer (1974) Behaviour 61, 106; (1978) Anim. Behav., 26, 1012

  • Experiments in which badge size was increased or reduced experimentally

    • Dominants with bleached-out badges had to fight hard for access to food etc - badge works

    • DO win eventually - so really are better fighters

    • Subordinates with enlarged badge don’t win…

    • UNLESS also injected with testosterone at the time that the badge is enlarged

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Møller – House sparrows

  • Møller (1987) Anim. Behav., 35, 1637-1644

  • High-ranked birds had larger badges

  • Attack most common between males with equal badge sizes (in field, not lab flocks)

    • Attacks on birds with a similar-size badge more frequent among high-ranking birds

    • So subordinates with a deceptively large badge will face more attacks than if honest

    • Larger badges impose a greater cost from other bird’s attacks – limits cheating

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Why should the weaker rival avoid participation in serious contests?

  • DeCarvalho et al, 2004, Anim. Behav., 68, 473-482, measured energy of the 3-phase contests of the Sierra Dome spider

    • Phases have expenditure of 3.5x, 7.4x, 11.5x BMR

  • Costs are substantial, and later phases are more costly, so it pays a potentially-losing rival to quit early rather than to persist through phase 1 to phases 2 or 3.

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Threat “is used to resolve contests” contests?

  • In many contests, differences between the individuals [in size, weight, fighting prowess (RHP), value of the resource] are probably used to decide the outcome

  • Combat involves energy costs, and risk of injury

    • Threat displays allow animals to interact to resolve disputes without physical combat

  • But if A gains victory more cheaply by giving an intimidating display, why does B not give an equally- or more-intimidating display and win instead?

    • The problem of honest signalling in animal contests

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Assessment-based models contests?

  • Many think that differences in size/weight/etc. are assessed during contest

    • Larger/heavier rival* wins in fish, spiders, etc.

    • Later phases of contest likely to provide more information about weight-difference than earlier phases

    • *BUT see Taylor & Elwood 2003

  • Leimar, Austad & Enquist, 1991, Evolution 45, 862-874

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Noble’s simulation – contests?MODEL 4

  • (1998) Tough guys don’t dance


  • Animats in contest change their position on the attack-flee continuum (θ ) over time

  • Being aggressive carries cost; being attacked carries cost. Animats differ in fighting ability (f)

  • Neural network implementation

  • Animats know own fighting ability, their place on the attack-flee dimension θ, and what they are doing; and can see what opponent is doing

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Tough guys (2) contests?

  • Blind: get zero info about opponent’s θ

  • Exptal: can evolve to make use of input that gives info about opponent’sθ

  • Unfakable: full info - given opponent’s fighting ability, f, so don’t really need θ

  • Did not evolve to use this info about θ, although over time fewer contests were settled by fights, and 41% of fights were between opponents with well-matched f-values

  • Evidence for information transfer l.jpg
    Evidence for information transfer - ?? contests?

    • Contests are longest when opponents are equally matched => pick up info about size difference from interaction

    • Taylor & Elwood (2003) show that same pattern can occur if each rival persists for a time related to its size, with no assessment – need to tease apart effects of larger’s size, smaller’s size, and difference in size, to test assessment models

    Enquist 1985 honest signals model l.jpg
    Enquist (1985) Honest signals model contests?

    • Contestants strong/weak. Know own strength but not that of opponents MODEL 5

    • Stage 1 – produce signal A/B ESS strategy do this to show whether self strong (A) or weak (B)

    • Stage 2 – decide to

      • Attack unconditionally - ESS Strong do this if signal A

      • Attack if opponent did not withdraw – ESS Strong if signal B

      • Withdraw

    • ESS if ½ V-C > V-D where C is cost of fight between equals, D is cost to weak of just being attacked by strong one, V is value of victory

    Noble s analysis of the enquist model l.jpg
    Noble’s analysis of the Enquist model contests?

    • (2000) Talk is cheap: production system implementation of Enquist’s strong/weak signal honest-signalling model (1985)

      • Strategies that evolved from a random starting point rarely employ information about opponent’s signal

      • Enquist’s ‘honest’ strategy was vulnerable – displaced by mutants that didn’t use information about signal

      • If Noble increased the cost of ‘just being attacked’ to unrealistic levels, information about opponent’s signal WAS now employed in some simulations

    References l.jpg
    References contests?

    • Pulliam & Caraco (1984) Chapter 5 in Krebs & Davies (Eds) Behavioural Ecology (2nd Ed – only this edition). Blackwell, Oxford.

    • Krebs & Davies (1993) An introduction to behavioural ecology (3rd Edn) Chapter 6, 7

    • Bradbury, JW and Vehrencamp, SL (1998) Principles of animal communication. Chapter 21 (and parts at least of Chapter 20, esp. pp. 649-658, 662-665, 668-676)

    • Enquist et al. (1985) Anim. Behav., 33, 1007-1020

    • Noble’s papers and additional info at