Lecture 11 - cooperative breeding Flashcards
what is cooperative breeding?
‘Helpers’ assist breeders to raise offspring that are (usually) not their own
why is cooperative breeding an evolutionary puzzle?
- think of natural selection favouring individuals that increase their own reproductive success no that of others
describe the diversity of cooperative systems
- 4-9%of bird species
- 3% of mammal species
- describe in >10 fish species
- primitive social insects
80% of cooperative systems involved ‘helpers at the nest’ give an example of this in birds
e. g. Florida scrub jay
- pair + 1.8 helpers
- helpers feed and protect young from predators
- helpers usually related to breeding pair
an example of ‘helpers at the nest’ in mammals
e. g. silver-backed jackal
- pair + 1-3 helpers
- helpers regurgitate food to pups and lactating female
give an example of a mammal with an extreme cooperative system similar to that of eusocial insects
naket mole rats - high defree of sophistication in social system
- reproductive division of labour: queen,soldiers,foragers
- can live in colonies of 80-100 individuals
what are plural breeders and an example?
Several males & females share a nest and raise a communal brood
e. g. Banded Mongoose
- 4-40 in a group
- several females reproduce
what is the difference between plural breeders and ‘helpers at the nest’
plural breeders often have a higher degree of conflict
what overarching theory explains the diversity of social systems and phenomenon in generality
ecological contraints hypothesis (Elmen 1982)
describe the two step process of the evolution of cooperative breeding
step 1) habitat saturation/ecological constraints
step 2) fitness benefits of helping exceed those of not helping
where do the two steps fit in on an evolutionary timeline?
independent breeding is constrained
(1)
grown offspring delay dispersal and ‘stay at home’
(2)
grown offspring help to rear later broods
what is a key assumption of the ecological constraints hypothesis?
there is a better fitness return from breeding than helping (otherwise you have no need for constraints) but breeding is constrained
give an example which proves the assumption of the ECH
e. g. long tailed tits
- as a breeder an individual gets 0.5 genetic equivalents whereas the pay off from helping is just 0.14
- proves that breeding is better than helping which pushes towards the conclusion that helping must be constrained in some way
describe correlations evidence that supports the hypothesis that constraints cause offspring to delay dispersal
instead of breeding independently
- look over a number of years and see what the ecological conditions were like to see if helping varies over ‘bad’ years
e. g. acorn woodpecker stores acorns - the larger the acorn crop the less constraint on independent breeding - in years with good acorn crop there is evidence of fewer individuals delaying dispersal
what is the issue with correlational evidence
- its not causal evidence - there could be other variables involved
how can you get experimental evidence of cooperative breeding?
researchers manipulate constraints on breeding and see if there was an effect on incidence of cooperation
example of experimental evidence in the superb fairy-wren
- 60% of pairs have helpers
- always male
- strong male bias sex ratio 1.8 male:1 female
- they removed males away from territories leaving the female without a partner and open vacancies for helpers
- the vacancies were rapidly filled by helpers showing that helpers are capable of reproduction but they dont often have the opportunity
- exp 2 - male and female removed - none of the territories were filled suggesting the helpers must be gaining from staying where they are and helping
conclusion from superb fairy-wren experiment
- helpers are capable of reproduction
- habitat is limiting (ecological constraints hypothesis)
- mates are limiting
what is inclusive fitness?
direct fitness and indirect fitness
what are the direct benefits helpers get from group living?
1) increased survival -group augmentation hypothesis (Producing extra offspring benefits helpers by increasing group size)
2) increased probability of future breeding
3) direct reproduction
what is the group augmentation hypothesis?
better to live in a large group - therefore have to help with rearing the offspring
circumstantial evidence of increased survival being a benefit a group living?
Kidnapping in banded mongoose and white-winged chough - want to expand their group size at any expense even by raising non-kin
experimental evidence of increased survival being a benefit a group living?
Cichlid groups more likely to accept immigrants when predation risk high
what are 3 ways group living will increase the probability of future breeding?
1) territory inheritance
2) mate acquisition
3) increased experience (skills hypothesis)
give an example of territory inheritance
Florida scrub jay
48% of helpers eventually acquire all or part of parental territory
give an example of increased mate acquisition from group breeding
- pied kingfishers have 2 kinds of helpers 1) close kin of breeding pair 2) distant kin/non-kin
- when secondary helpers try to settle the breeding male is initially very resistant but they eventually settle
- in 41% of cases the secondary helper ‘inherited’ female when breeding male died
- in 18% they ousted breeding male
give an example that proves the skills hypothesis
Seychelles warbler - Comparison of success of 4yr old birds
- results showed that previous experience made you a successful parent in terms of hatching success and number of fledglings
how does group breeding help direct reproduction?
e.g. seychelles warbler - Helpers are usually female
44% lay eggs in helped nest
15% of all young produced by ‘helpers’
- females typically seek copulation on other territories to avoid inbreeding
what are indirect fitness benefits from group breeding
1) Increased reproductive success of relatives
2) Increased survival of related breeders= ‘load-lightening’
give an example of increased reproductive success of relatives
long tailed tit - Reproductive success increases with no. of helpers i.e. the offspring had a better chance at survival with more food etc
what is an issue with the long tailed tit study?
just correlational evidence not causation - have to manipulate an experiment to prove it
give an example of an experiment proving the indirect fitness benefit of increased reproductive success of relatives
experiment using moorhens, grey-crowned babbler and florida scrub jays
- did a control and removal of helpers - results showed significant reduction in reproductive success
what is ‘load lightening’?
if the presence of helpers means breeders have to do less it will result in the increased survival of breeders
what data supports the hypothesis of ‘load lightening’
difficult to test - only observational data supports the idea
