Groups, competition, aggressive communication

1. Groups, competition, aggressive communication MSc ACSB module 2006/07 SB week 10

2. 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

4. 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)

5. 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

6. 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

7. Vehrencamp’s model • So a family group may be exploitative … especially if very close kin are involved

8. 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

9. 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 & 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

11. 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?

12. 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

13. 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

14. 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

17. Noble’s simulation – MODEL 4 • (1998) Tough guys don’t dance • http://www.comp.leeds.ac.uk/jasonn/Research/Aggression/index.html • 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

18. 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

19. 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

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 > 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

21. 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

22. 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 • Noble’s papers and additional info at http://www.comp.leeds.ac.uk/jasonn/Research/Aggression/index.html