Cooperative Breeding

A. Cooperative breeding is a social systems in which some group members defer thier own reproduction, even as adults, and help care for the young of a few breeding individuals

B. Helpers are typically related to breeders and are often individuals that do not disperse instead aid in the rearing of their siblings

C. In a very few cases, helpers are not related to breeders or to the young they help rear (e.g. dwarf mongooses, wild dogs)

D. Cooperative breeding is relatively rare -- found in only about 3% of birds and mammals (roughly 200-300 bird species and about 120 mammal species)

II. Two important steps in the evolution and expression of cooperative breeding

A. STEP 1 -- Potential helpers must decide whether to disperse and attempt to breed on their own or to remain in their natal group and accept a nonreproductive position

Why not disperse? Two hypotheses

[1] Habitat or mate saturation hypothesis -- the probability of successful dispersal and breeding is low because of lack of availabile habitat or mates, so you may be better off staying where you are until your chances of successful dispersal improve

[2] Group-living advantages hypothesis -- the benefits of group-living (such as we've talked about before) may be great enough to outweigh the costs of foregoing reproduction

Most evidence supports the first hypothesis

Ex. In acorn woodpeckers, as the shortage of territories becomes more severe, a greater proportion of yearlings delay dispersal

Ex. When cooperatively-breeding Seychelles warblers introduced onto a vacant island, all young dispersed to begin reproducing on their own. But as available territories became filled up, family breeding units began to form again.

B. STEP 2 -- If potential helpers decide to remain in their natal groups, they must decide whether or not to provide help

Why provide help? Several possible reasons

[1] To increase your own inclusive fitness

Requires that helpers be relatives of breeders and predicts that the degree of helping will covary with the degree of relatedness of the helper to the recipient

Ex. In scrub jays (top) and Seychelles warblers (bottom) the percentage of nonbreeders in a group who help is greater when potential helpers are more closely related to nestlings

So how can helpers increase their own inclusive fitness?

A. By improving the survival of breeders

Ex. In grey babblers, parents with helpers provide much less of total food to young, and therefore, presumably, have greater survivorship themselves

B. By improving reproductive output of breeders

C. By improving helper's own chance of survival (e.g. through benefits of grouping)

D. Note: must control for other factors such as territory quality

[2] To enhance own breeding opportunities

A. Nonreproductive group members may be more likely to inherit a territory (though this is not well-documented)

B. Helpers may be able to inherit a mate when a former breeder dies

C. Helpers may recruit siblings they have helped raise as coalition partners to acquire mates or territories

Ex. In lions, brothers may emigrate together to form a new group or take over a pride and then share mating opportunites

[3] To gain parenting experience

A. In several species of callitrichid primates, individuals that have helped rear their siblings have a better chance of having their own offspring survive to weaning

B. Reproductive output (clutch size) in some species of birds increases with experience both as a breeder and as a helper

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