The Costs of Group Living

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The Costs of Group Living

• Eoin Kilkenny
• Charlotte Larsen
• Dave McCabe
• Paul McAteer
• Sophie van Lonkhuyzen

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Introduction

• We have just seen the benefits

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Introduction

• We have just seen the benefits
• Why dont all animals live in groups?
• Must be costs

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Introduction

• We have just seen the benefits
• Why dont all animals live in groups?
• Must be costs (exponential)
• If costs outweigh benefits

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Some examples

• Competition for resources
• Aggression
• Increased risk of infection/disease/parasites
• Inbreeding
• Mis-directed parental care
• Predator attraction
• Prey disturbance
• Defaulters/cheaters

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Intraspecific Competiton

• The more individuals there are the more
  competition interactions can occur
• g no. of individuals
• g(g-1) no. of interactions
• The larger the group the closer g(g-1) gets to g2

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Intraspecific Competition

• Simple competition (consumptive)
• Getting in each others way
• Kleptoparasitism

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Simple Competition

• The more animals there are in a group that share
  a resource the less each gets
• Consumptive competiton
• e.g. Carabid Cave Beetles
• The more individuals present in the group the
  less they eat.

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Just getting in each others way

• Searching in a group is better than searching
  alone.
• But sites will overlap!!! - affected by search
  patterns of individuals by relative positions
  to other group members.
• Trade off
• As far from other group members as possible to
  minimise risk of researching areas.
• As close as possible to take maximum advantage
  form sharing the finds of others.

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Kleptoparastism

• This is food being stolen
• Cost to both parties involved. Time and energy
  spent
• Only used when it is advantageous
• e.g. Oyster Catchers

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References

• Free et al 1977, On the inadequacy of simple
  models of mutual interference for parasitism and
  predation
• Griffith Poulson 1993 Mechanisms and
  consequences of intraspecific competition in a
  carabid cave beetle, Ecology, 74(5) 1373-1383
• Stillman et al 1996 Modeling interference from
  basic foraging behaviour, journal Animal ecology,
  66, 692-703

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Aggression

• Aggression within group not just due to foraging.
• Main reason is dominance hierarchies.

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Dominance Hierarchies

• Due to fact that higher social ranking means more
  and healthier mates.
• When new groups are formed, an animal can learn
  whether it is dominant or subordinate to each
  other group member by engaging in fights with
  each other individual in the group.

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Costs of aggression

• Process is time-consuming. Less time allocated to
  foraging/feeding or breeding.
• Can cause injury and so reduce an individuals
  fitness and also that of the group as a whole.
• Energetically costly

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Group size

• Larger group generally means more fights.
• Feeding time for group overall is reduced

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Increased risk of disease, infection and parasites

• Transmission rates increased due to closer
  proximity of individuals.
• e.g Hoi et al (1998) found parasite
  infestation in bee eaters decreased with
  increasing nest distance.
• 2. Positive relation between group size and
  load

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Inbreeding

• Breeding between close relations
• Can lead to reduction in genetic diversity
• May result in inbred individuals exhibiting
  reduced health and fitness and lower levels of
  fertility.

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Misdirected parental care

• Cuckoldry, brood parasitism and adoption.
• Helping or feeding of young birds by non-parental
  conspecifics. Been reported in more than 150sp
  (Brown, 1978).

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References

• Brown, J.L. 1978. Avian communal breeding
  systems, Ann. Rev, Ecol. Systems 123-155.
• Hoi. H.,Darolova. A., Konig. C., and Kristofik.
  J. (1998) The relation between colony size and
  breeding density and ectoparasite load of adult
  European bee-eaters. 156-163.

• Krause.J, Ruxton.G.D. 2002. Living in Groups
  41-52.

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Detection by predators

• Living in groups can be beneficial for confusing
  predators post-detection
• Generally, detection of a group is easier than
  detection of an individual.

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Detection by predators

• Animals that avoid predation post-detection. Fast
  movingaggregate.
• Animals that avoid predation pre-detection.
  Crypsis. Solitary.

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Disturbance of prey

• More likely to detect more than one predator
• Sit and wait tactics
• Animals that hunt by stealth

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Interference

• Interaction between two predators that reduces
  searching efficiency.
• Waders that hunt by touch tend to aggregate where
  as those that hunt by sight tend to disperse.

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References

• Goss custard ,J. D. (1970) Feeding dispersal in
  some overwintering wading birds.Social behaviour
  in Birds and mammals pp3-34. Academic press,
  London.

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Defaulters

• Those who dont carry out the jobs associated
  with group living

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Defaulters

• Those who dont carry out the jobs associated
  with group living
• Natural selection acts on the individual, not
  group
• Benefits become less for group as a whole

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Mechanisms

• Cheaters lower group productivity by not
  cooperating
• Freeloaders with grouping tendencies both
  lower group productivity and increase group size
  past optimal size
• Ref Solving the freeloaders paradox, Leticia
  Aviles, July 2002.

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Defaulters

• Those who dont carry out the jobs associated
  with group living
• Natural selection acts on the individual, not
  group
• Benefits become less for group as a whole
• Examples in our society

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Defaulters

• Those who dont carry out jobs related to group
  living
• Natural selection acts on the individual, not
  group
• Benefits become less for group as a whole
• Examples in our society
• Examples in groups in the wild

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Reference

• Solving the freeloaders paradox Genetic
  associations and frequency-dependent selection in
  the evolution of cooperation among nonrelatives
• Leticia Avilés 
• Department of Ecology and Evolutionary Biology,
  University of Arizona, Tucson, AZ 85721
• Edited by Edward O. Wilson, Harvard University,
  Cambridge, MA, and approved August 26, 2002
  (received for review July 8, 2002)
• http//www.pnas.org/cgi/content/abstract/99/22/142
  68

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Overall conclusion

• It is for these reasons among others that groups
  do not always form or form large numbers.
• But for the true impact of these costs on the
  group size, much more research needs to be done.
• Anyone for tea?