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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

Groups, competition, aggressive communication

MSc ACSB module 2006/07

SB week 10

benefits of group life
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
penalties of group life
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
mean vs variance
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)
who bears the costs in animal groups
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
how kinship affects skew in groups
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
vehrencamp s model
Vehrencamp’s model
  • So a family group may be exploitative … especially if very close kin are involved
helping vs breeding
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
profitability of breeding and helping
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
testing emlen wrege s model
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
signalling status
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?
rohwer s experiments
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
m ller house sparrows
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
why should the weaker rival avoid participation in serious contests
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.
threat is used to resolve contests
Threat “is used to resolve 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
assessment based models
Assessment-based models
  • 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
noble s simulation model 4
Noble’s simulation – 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
tough guys 2
Tough guys (2)
    • 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
Evidence for information transfer - ??
  • 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
Enquist (1985) Honest signals model
  • 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
Noble’s analysis of the Enquist model
  • (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
  • 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