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Cooperative breeding is quite widely distributed among animals. ... of birds are cooperative breeders (individuals other than parents help raise offspring), ... of mammals and over ... species of fish (so pretty rare in fish - mainly ....)

4-9%, 3%, 20, cichlids (great rift valley lakes in east africa)


The ... ... hypothesis is composed of two steps, the first being habitat .../ecological constraints, and the second being the fitness benefits of helping ... those of not helping (directly and indirectly).

What does this indirect/direct part mean?

ecological constraints (e.g. shortage of habitat, risky reproduction or dispersal), saturation, exceeding

Direct - increase or enhance future survival and reproduction of helper
Indirect - by helping, helper might increase survival or productivity of the helped breeders
- fitness benefits often better than not helping at all

- Independent breeding is constrained --> grown offspring delay dispersal and "stay at home" --> Grown offspring help to rear later broods


Estimated that, in species with helpers, if constraints on independent reproduction are relaxed, then helpers will...

disperse from their native territories and take up the opportunity to breed independently (i.e. reduction in helping)


Various studies support this,
e.g. in sociable weavers, when additional ... was provided, helpers were more likely to breed independently rather than staying at home.

In red-cockaded woodpeckers, the provision of additional ... ... lead to helpers leaving their natal territory and breeding independently.

In superb fairy wrens, if territories were vacated through the ... of ..., helper males dispersed to take up these vacancies

These intraspecific studies suggest that indeed constraints are in operation and relaxing them allows more independent breeding.

food (relaxing food constraint)

nest holes

removal, males


If we compare cooperative with non-cooperative species, according to the ECH, we'd predict that constraints on reproduction are ... in cooperative species than they are in non-cooperative species. This is very obviously not the case. In non-cooperative UK species such as blue tits, magpies and blackbirds, a substantial proportion of these populations are ...-... (e.g. ... males in blackbirds unable to attract females due to biased sex ratio). Unpaired males never become helpers at the nest. Magpies often take ... years before they are able to defend a territory. Those that are waiting to become breeders never become .... Why do non breeders help in some species, but not in others? hmmm

stronger, non-breeders, 20-25%, 2-3, helpers

The question is: can we explain the inter-specific distribution of cooperative breeding?


The first thing to notice is that there is a very strong ... component to cooperative breeding. For example, there are ... species of fairy wren and they're all cooperative. 25% of Australian corvidae are cooperative (well above 4-9% average). In many group, e.g. seabirds, there are no cooperators.



Looking across the avian tree of life, we can see >... independent transitions to or from cooperative breeding in birds. This is a useful consideration for comparative analyses.



Hypothesis 1 for explaining the distribution of cooperative breeding across the avian phylogeny: does ecology ... independent breeding in some groups, who therefore become cooperative, but not in others, who therefore remain non-cooperative.



There are 45 species of African .... Rubenstein and Lovette (2007) categorised these as either cooperative or non-cooperative breeders. They then looked to see whether cooperation was associated with any particular habitat (savannah vs non-savannah). They found that cooperation was strongly associated with ... living.

What is it about savannahs that make them conducive to the evolution of cooperative breeding? Hypothesised that the association was attributed to the ... of the environment. Resources vary more through time and space in unpredictable environments so independent reproduction may be difficult - may be better to live in a group rather than disperse independently. Found that predictability and ... of savannah habitat significantly lower than in desert or forest habitats. Supported their hypothesis. (and hypothesis 1?)

starling, savannah (all savannah species cooperative, most non-savannah species non-cooperative)

predictability, constancy


Is environmental unpredictability associated with cooperative breeding on a global scale?

Proportional richness of cooperative breeders in 9310 species of non-marine birds (as marine birds are all non-cooperative) analysed and found that cooperative breeding is concentrated in certain environments such as Subsaharan Africa and Australia, parts of the middle east etc.

Predictability of environments globally was then measured according to two variables: temperature and precipitation. Measured within years (seasonality) and among years. Matched this up with the prevalence of cooperative breeders

Found that the hypothesis was supported: prevalence cooperative breeding is associated with greater variation in rainfall, both between and among years. However, the association is pretty weak. There is a huge amount of overlap in the types of environment that cooperative and non-cooperative breeders live in.


The predictability hypothesis has also been tested in mammals. What was foynd?

Distribution of cooperative breeders (hotspots in east and southern africa and north america) associated with decreased predictability of rainfall AND TEMPERATURE among years, when compared to monogamous breeders.

Again supports hypothesis, but rather weakly.


So evidence from analyses of birds and mammals suggests that cooperative breeding is indeed associated with unpredictable environments. But the evidence is not in anyway conclusive. One can also not infer that...

the environment has selected for cooperative breeding. It could be that cooperative breeding allows for colonisation of harsher environments


Cornwallis (2017) tested this hypothesis that cooperative breeding allows colonisation of harsher environments and, by looking at the transitions from benign to harsh environments of cooperative and non-cooperative bird species, found...

that the transition from benign environments to harsh environments was much more pronounced for cooperative lineages than it was for non-cooperative lineages.

However there are caveats with this study - predicting environments from ancestral traits is difficult, attributing transitions to particular nodes in the radiation of birds.

(reconstructed and assessed ancestral traits in avian evolution)


Overall, evidence from hypothesis 1 that ecology constrains breeding more in some species than others is...

far from conclusive


Hypothesis 2 is the idea that it is not external constraints which cause offspring to stay at home, but that there are benefits of ... (staying at home) and these benefits select for delayed ....

philopatry, dispersal


This hypothesis 2 has been tested in a few species, one of them being the ... ... in coastal California. In this species, sons usually spend the winter on their natal territory, and may help in the following year. ... is a key food resource for this bird during the winter. Access to this is increased by staying at home. Manipulating the availability of this resource by removing clumps of this ....
They found that...

western bluebird, mistletoe, mistletoe

the resource wealth at home and the potential benefits derived from this was a key player in determining whether sons (not daughters or parents) were philopatric or not. (see table on photo on phone).


A study into green ..., a species in which both sexes may stay at home for several years before dispersing to breed, found that females (but not males) who stay for a while have higher ... (goes against ecological constraints hypothesis). This study suggests that philopatry may be ... rather than forced by external constraints.

woodhoopoes, LRS, beneficial (e.g. better parents, acquiring helpers or territory)


Some authors have argued that there isn't much difference between the idea of ecological constraints n dispersal and benefits of staying. In both cases, offspring must weigh up the relative benefits of staying against those of dispersal. The ecological constraints hypothesis (hyp 1) and the philopatric benefit hypothesis (hyp 2) really differ only on the emphasis placed on..

the pros and cons of staying and/or leaving

- nevertheless, looking across taxa, the preponderance of evidence seems to be in support of the idea of constraints rather than benefits of staying.


Hypothesis 3, one of the main competitors of the ECH is the idea that ... ... traits predispose some lineages to cooperate.

life history


This idea fits very nicely with the distribution of cooperative breeding through the avian phylogeny. It is very patchily distributed in certain families. The argument goes that those taxonomic lineages with slow life histories appear to...

have a low rate at which breeding vacancies arise - in those lineages we see the evolution of cooperative breeding


What do we mean by slow life histories?

- increased longevity
- decreased clutch size
- migration

these are all associated with a slow rate at which breeding vacancies open


This idea was tested by Arnold. In a comparative analysis she looked across the global avafauna and identified those families with slow vs fast life histories. She found that..

in those families that were long-lived with small clutches, cooperative breeding was clustered. Argued that cooperation was more likely to occur in these low fecundity taxonomic lineages. In those families with a fast life history (short lived, high turnover rate, large clutches), cooperation was never found, no matter the ecological conditions. This supports the life history hypothesis.


It has since been pointed out that these particular sets of life history traits are not only associated with cooperation or non-cooperation, but were also associated with...

tropical and temperate regions of the globe. So it could be that it is not the life history traits themselves that drive the evolution of cooperation, but rather the environments that the birds live in.

Slow life history --> more likely in tropical regions like subsaharan africa, australia

Fast life history --> more likely in temperate regions

Problem - very hard to distinguish life history from phylogeny and biogeography etc. they all appear to be wrapped up together.


Hypothesis 4 is that cooperative breeding may be associated with ... ....

brood parasitism


There are two potential relationships going on here with hypothesis 4.

The first is that brood parasites (such as bronze cuckoos) may want to target cooperative breeders because...

The second is that, from the host's point of view...

there are more carers at the nest who may provide more food for the offspring than would a simple pair (may be beneficial for survival of offspring, who are often much larger than host)

having helpers may improve their defence against the cuckoos/brood parasites (more pairs of eyes etc.), so having more helpers selected for when brood parasitism is common.


This idea was first tested in the specific context of the Horsfield's bronze cuckoo vs the superb fairy wren. It was found that the mass of parasite chicks and likelihood of fledging...

Secondly, the idea that parasitism selects for cooperative breeding was tested by looking at the amount of time spent mobbing cuckoos when they approached the nest in small vs large groups. It was found that...

were higher in nests where there were helpers than in nests where there were no helpers - so parasites prefer cooperative breeders

time spent mobbing was significantly greater in larger groups than in small groups and the proportion of nests that were parasitised in large groups was significantly smaller compared to small groups - supports idea.


The global distribution of cooperative breeding is also associated with...

the distribution of obligate brood parasites


In African and Australia, hosts of brood parasites are...

more likely to be cooperative breeders, though the direction of association is unclear.


So none of the 4 hypothesis are conclusive as predictors of sociality. Why is predicting sociality so difficult?

- cooperative breeding includes many diverse social systems which differ in complexity (catch all phrase)
- constraints (which may be selecting for cooperation) are diverse - food, nest sites, territories, mates etc.
- phylogeny, biogeography and life history traits covary

The debate continues.... DUN DUN DUNNN


Kin selection is a selection that operates on...

traits due to their beneficial effects on the fitness of relatives