Groups, competition, aggressive communication

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Groups, competition, aggressive communication

• MSc ACSB module 2006/07
• SB week 10

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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

3
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,
  785-820 Realistically-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
• Siblys 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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Vehrencamps 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

10
Testing Emlen Wreges 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 dont subordinates pretend to be
  dominant by growing larger badges?

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Rohwers 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 dont 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
  birds 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.

15
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

16
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

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Nobles simulation MODEL 4

• (1998) Tough guys dont dance
• http//www.comp.leeds.ac.uk/jasonn/Research/Aggres
  sion/index.html
• Animats in contest change their position on the
  attack-flee continuum (? ) 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 ?, and what they are
  doing and can see what opponent is doing

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Tough guys (2)

• Blind get zero info about opponents ?
• Exptal can evolve to make use of input that
  gives info about opponents ?
• Unfakable full info - given opponents fighting
  ability, f, so dont really need ?
• Did not evolve to use this info about ?, although
  over time fewer contests were settled by fights,
  and 41 of fights were between opponents with
  well-matched f-values

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Evidence for information transfer - ??

• Contests are longest when opponents are equally
  matched gt 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 largers size, smallers size,
  and difference in size, to test assessment models

20
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 gt 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

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Nobles analysis of the Enquist model

• (2000) Talk is cheap production system
  implementation of Enquists strong/weak signal
  honest-signalling model (1985)
• Strategies that evolved from a random starting
  point rarely employ information about opponents
  signal
• Enquists honest strategy was vulnerable
  displaced by mutants that didnt use information
  about signal
• If Noble increased the cost of just being
  attacked to unrealistic levels, information
  about opponents signal WAS now employed in some
  simulations

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References

• 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
• Nobles papers and additional info at
  http//www.comp.leeds.ac.uk/jasonn/Research/Aggres
  sion/index.html