Cooperative Breeding

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Cooperative Breeding

• Lecture 12

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Cooperative Breeding

• Individuals other than genetic parents help raise
  young
• gt200 species of birds
• gt 100 species of mammals

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Cooperative Breeding

• Individuals other than genetic parents are called
• Helpers
• Auxiliaries
• Skutch 1935

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Cooperative Breeding

• How can this evolve?
• Evolutionary perspective
• Seems maladaptive
• Two questions
• Why do helpers not breed?
• If they cant/dont breed, why do they help?

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Hamilton 1964

• Proposed the first theoretical framework to
  address these questions
• Theory of inclusive fitness
• Can help explain evolution of phenotypically
  costly behaviours

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Inclusive fitness

• Fitness (Inclusive fitness) has two components
• Direct fitness offspring
• Indirect fitness also includes other related
  individuals

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How could cooperative breeding evolve?

• If young males or females cannot acquire mates
• Cannot find enough resources to produce offspring
• Helping their sisters may be making the best of a
  bad situation

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Two questions

• Why dont helpers breed?
• Why do they help?
• If the fitness benefits that accrue to helpers
  exceed the costs
• Helping should evolve

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Examples Florida Scrub Jay

• Helpers are young that fail to disperse
• Help defend natal territories
• Help feed young
• Defend nest when adults are away
• Feed and protect fledglings

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Silver-backed Jackal

• Young remain with parents for the following
  breeding season
• Groups of 3-5 adults raise newborn offspring
• Non-breeders are helpers

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Silver-backed Jackal Helpers

• Provision nursing females
• Feed young
• Guard pups when parents are away feeding
• Play with young
• Teach hunting skills

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Both Scrub Jays Jackals

• Small groups
• Helpers are offspring from the previous
  litter/clutch

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Other cooperative breeders

• Sometimes form more complex larger groups
• Complex social organizations
• Extended rather than nuclear families
• Overlapping generations
• More than one breeding pairplural breeders

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Grey-breasted Jays

• Extended family groups
• 8-18 individuals
• Permanent all-purpose territories
• Several breeding pairs within each family

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Helpers

• Help
• Provisioning young
• Nest defence
• When young are fledged they are fed
  indiscriminately by group members

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Banded mongooses

• Packs of up to 40
• Plural breeders
• Helpers at den
• Several females breed synchronously

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Banded mongooses

• Young are the same age
• Suckle indiscriminately
• Group members bring insects to feed the young
• Males also guard the den

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Other cooperative breeders

• Are highly gregarious White-fronted bee-eaters
• Colonies of 50-300 birds
• Colony is composed of smaller extended family
  groups or clans
• Helping occurs within clans

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Bee-eaters

• Unlike grey jays, bee-eater helpers attach
  themselves to a single nest
• Participate in all aspects of breeding
• Excavating the nest
• Defence
• Feeding females
• Incubation
• Caring for fledglings

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Bee-eaters

• When nests fail
• Adults will help at other nests
• Birds switch back and forth between helper and
  breeder roles

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Cooperative breeding and mating systems

• The previous examples involve species with
  monogamous breeding pairs
• Cooperative breeding is also found with other
  breeding systems
• e.g., acorn woodpeckers

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Acorn woodpeckers

• Coastal California
• Groups with
• 1-4 breeding males
• 1-2 breeding females
• Up to 8 non-breeding helpers

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Acorn woodpeckers

• Helpers are usually grown-up offspring
• Only one nest is tended
• Most group members help
• incubate the eggs
• Defend nest
• Feed young

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Acorn woodpeckers

• No pair bonds formed
• More than one male will mate with a breeding
  female
• In about 25 of cases two females contributed
  eggs to the communal nest
• Females mate with more than one male
• Some males mate with both females
• polygynandrous

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Primates

• Saddle-backed tamarins
• Groups consist of a female and two or more adult
  males
• Males
• Grown up young
• Or unrelated male that shares access to the
  female (polyandrous)

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Saddle-backed tamarins

• Females always have twins
• extra male helps carry young as the group moves
  through the forest

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Naked Mole-rats

• Live in subterranean colonies
• 40-300 animals
• 1 breeding female
• 1 3 breeding males
• Females breed polyandrously with the breeding
  males

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Naked Mole-rats

• Breeders perform direct offspring care
• Non breeders (both sexes) maintain the tunnels
• Division of labour
• Small mole-rats build tunnels bring food
• Larger moles rats defend it against predators

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Do helpers help breeders?

• Increase success of breeding attempt
• Increase the frequency of breeding
• Enhance breeder survival

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Evidence that helpers help

• Correlations between group size and offspring
  survival
• Silver-backed Jackels
• Red Cockaded Woodpeckers
• Moorhens

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Problem with correlational evidence

• Numbers of helpers could be correlated with
  territory quality
• Territory quality could be the factor responsible
  for the differences in offspring survival

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Multivariate approaches

• Statistically remove effects of other factors
• white fronted bee-eaters trend remained
• Red cockaded woodpeckers no effect of group size
  when age of breeders was included as a covariate

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Experimental removals of helpers

• Grey-crowned babblers (Brown et. al 1982)
• matched territories for quality and helper number
• Removed helpers on half the territories
• Territories with helpers had 3X nesting success

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Other ways helpers help

• Increasing the number of breeding seasons
• Grey crowned babblers
• Breeding pairs re-nest sooner if they have
  helpers

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Other ways helpers help

• Increasing survival of parents
• Pied Kingfishers
• Acorn woodpeckers
• Breeding pair is more likely to survive if they
  have helpers

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So we have evidence that

• In many species, helpers do help
• Increased breeding success
• More frequent breeding attempts
• Increase survival of breeding pair

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But what is the benefit to helpers?

• Raising their own offspring which are more
  closely related should be preferable to helping
  raise more distantly related relatives

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Two routes to the evolution of social groups

• Individuals aggregate to form groups because of
  the advantages of living in groups
• Remain in groups as a result of some kind of
  constraint on dispersing

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In many cases

• Helpers are young that delay dispersal from their
  natal territory
• What ecological features tip the balance in the
  direction of delaying dispersal?

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Habitat saturation hypothesis

• Helpers dont breed on their own because they
  cant
• Need a territory to breed
• Cant displace older more experienced males
• Best to wait on a familiar natal territory of
  proven quality with familiar relatives

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Demography of typical cooperative breeders

• High juvenile and adult survival
• Permanent residency on all purpose territories
• Surplus of mature individuals relative to
  territorial vacancies

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Shortage of breeding openings

• Has been suggested as a reason for cooperative
  breeding in many species
• Acorn woodpeckers
• Red cockaded woodpeckers
• Silver-backed jackal

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Benefits of philopatry hypothesis

• The habitat saturation hypothesis emphasises the
  costs of dispersing early
• Benefits of philopatry hypothesis emphasises the
  gains of staying at home

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Evidence for the natal philopatry hypothesis

• Many cooperative breeders will breed
  cooperatively even when there appears to be
  suitable vacant territories available
• If variance in territory quality is high it may
  be better to wait for a high quality territory

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A shortage of breeding territories

• Is not sufficient to explain cooperative breeding
• Many species live as floaters when there are
  territories available

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Koenig and Pitelka (1983)

• Suggested that offspring should remain on natal
  territories
• If optimal breeding habitat is limiting
• And
• Marginal habitat is rare

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Fitness vs habitat quality
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Examples of steep habitat quality Fitness curves

• Acorn woodpeckers
• granaries
• Green woodhoopoes
• Cavities made by other species
• Banner-tailed kangaroo rats
• Mounds used for security

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Other factors that will influence early dispersal

• Risk of mortality associated with the act of
  dispersal
• Probability of finding a mate after dispersal
• Probability of reproducing successfully

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Shortage of potential mates

• Skewed sex ratio will affect probability of
  finding a mate
• Bee-eaters, red cockaded woodpeckers have male
  biased sex ratios

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Prohibitive cost of reproduction

• If environmental variation is high
• It may be better to delay dispersal during harsh
  years when the cost of reproduction is high

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Experimental tests

• Shortages of territories
• Scrub Jays
• Long term studies
• Number of breeding territories are stable
• Numbers of helpers varies from year to year

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Experimental tests

• Competition over vacant territories
• If suitable territories are scarce competition
  for vacancies should be intense
• In several species intense battles occur after a
  territory holder dies or is experimentally
  removed
• Losers return as helpers on their natal territory

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Experimental tests

• Superb blue fairy wrens
• Experiments where males were removed
• Helpers moved into vacant territories
• If females were also removed they remained as
  helpers
• Males respond to availability of territories and
  females

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Experimental tests

• Red cockaded woodpeckers
• Excavate cavities in live pine trees
• Experimental additions of cavities
• Helpers moved into experimental cavities

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Acorn woodpeckers
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Why do helpers help?

• Helping could increase the fitness of the helper
• Non-adaptive consequence of parental care
  instincts

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How might helping help the helper

• Increase survival
• Enhance probability of future breeding
• Increase fecundity when it does breed
• Increase production of non-descendent kin

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How might helping help the helper

• Increase survival
• If large groups are advantageous, helping
  increases the group size and therefore will be
  beneficial

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How might helping help the helper

• Enhance probability of future breeding
• Territory inheritance
• Experience in a territory may lead to a
  competitive edge

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How might helping help the helper

• Increase fecundity when it does breed
• Helping may allow inexperienced breeders to gain
  experience

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How might helping help the helper

• Increase production of non-descendent kin
• In bee-eaters probability of helping is similar
  to the degree of relatedness