Living in Groups

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Living in Groups

• Lecture 8

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Last lecture

• Territoriality (defence of exclusive areas)
• Not all animals defend territories
• Some live in large groups

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Group size varies widely among species

• Mexican free-tailed bats
• Summer breeding colonies of gt 20 million bats
• Tree swallows
• Territories
• Male-female pair with offspring

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With species there is also seasonal variation

• Common crows
• Summer small groups (territorial pairs)
• Winter large flocks

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When should natural selection favour group
living?

• There are costs and benefits to living in large
  groups
• If the benefits of group living are greater than
  the costs associated with group living then group
  living should be selected for
• Whether these conditions are met depends on the
  biology of the species and on environmental
  conditions

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Potential costs of living in groups

• Increased competition for resources
• Food
• Nesting sites
• Increased rate of disease transmission
• Increased rates of parasitism

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Potential benefits of living in groups

• Increased ability to detect predators
• More eyes watching
• Increased probability of not being caught
• Dilution effect
• Confusion effect
• Cooperative hunting
• Carnivores that kill large prey
• Increased ease of sharing information about the
  location of ephemeral resources e.g. insect
  prey
• Information centre hypothesis

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Colonial Behaviour - Cliff Swallows

• Cliff swallows are ideal models for studies of
  the costs and benefits of living in groups
• Colony size varies greatly
• 10s to 10s of thousands of birds

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Cliff swallows

• Brown et al. have conducted a series of
  experiments and field studies
• Tested the information centre hypothesis
• As well as other costs and benefits of colony
  size

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Cliff swallows

• They compared feeding behaviour and nestling
  feeding rates over a range of colony sizes

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Unsuccessful foragers

• Follow successful foragers to feeding sites
• Identified by watching for birds returning with
  food for nestlings
• At large colonies
• Successful foragers leave more regularly
• Unsuccessful foragers have to spend less time
  waiting for a successful bird to follow

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Translated into

• Higher provisioning rates at large colonies
• Parents bring back food more frequently
• Bring back more food per trip

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Costs of livings in large colonies

• Increase in nest parasites

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Trade-off between risk of nest parasites and
efficient information transfer

• Birds from large and small colonies have approx
  equal chances of successfully fledging young
• There is a trade-off between benefits (learning
  where food is) and costs (parasitism risk)
• In fumigation experiments
• Removing risk of parasites
• Large colonies have greater success

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Protection from predators

• Animals must simultaneously forage and also
  remain vigilant for predators
• In large groups each individual should be able to
  spend more time foraging if other group members
  share the task of detecting predators

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Ring-necked Pheasants
Group Size
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Ring-necked Pheasants
Group Size
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Vigilance in Elk (Childress and Lung 2003, Anim.
Behav. 66389-398)

• Risk of predation influenced by extrinsic and
  intrinsic factors

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Intrinsic

• Sex
• Age
• Nutritional status

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Extrinsic

• Predator density
• Habitat
• Group size

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Rocky Mountain Elk

• Yellowstone National Park
• 1995 1996
• Reintroduction of Wolves
• Gradient of predator encounter rates

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Observed herds from roadsides

• Recorded
• Herd size and composition
• Activities of focal animals
• Divided activities into categories based on
  posture
• Feeding head below shoulders
• Scanning head above shoulders
• Aggression stamping
• Resting lying down

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Individual vigilance

• Measured by observing focal individuals
• Calculated time spent scanning

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Group vigilance

• Fixed interval timed scans
• Calculated
• time at least one herd member was scanning
• Proportion of individuals scanning during a
  sample scan

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Scan duration (sec) under different encounter
risks
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Vigilance varies

• According age-sex class

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Vigilance varies

• Males and females respond differently to
  variation on predation risk
• Females with young
• Detecting predators may be more important
• Males with depleted energy reserves
• Foraging may be more important

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Group size

• They also found a group size effect
• But the type of effect varied with age and gender