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The evolution of cooperative breeding, in which nonbreeding members of a
group help to raise the breeding members' young, can result from one or
more of three possible advantages for helpers: (1) immediate, direct
advantages, such as immediate improvement in survival; (2) indirect
advantages from kin selection; and (3) delayed, direct advantages for
helpers that join a queue for eventual succession to an advantageous
reproductive position.
The conditions under which either of the first two kinds of selection can
provide a sufficient explanation for the evolution of cooperative breeding
are relatively well understood. We have now developed quantitative
conditions under which the third sort of selection can provide a
sufficient explanation.
More than one of these sources of selection are likely to influence the
evolution of cooperative breeding in any particular case. Nevertheless,
we need quantitative conditions under which each provides a sufficient
explanation in order to understand how the three sorts of selection could
complement each other.
Delayed advantages provide a sufficient explanation for helping if joining
a queue for a favorable reproductive position and helping to raise the
breeding individuals' subsequent young increase the spread of an
individual's genes. A complete explanation thus requires conditions (1)
for the evolution of delayed breeding, (2) for the curtailment of cheating
among helpers, and (3) for the evolutionary stability of queuing behavior.
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Delayed breeding can evolve when the eventual reproductive advantages
compensate quantitatively for the delay in the onset of reproduction.
For a population of stripe-backed wrens in Venezuela, this condition for
the evolution of helping is satisfied for males of all ages and for
females to age 4.
When helpers contribute to the increased fecundity of breeding members of
groups, queuing for succession to breeding status in a group amounts to a
special case of delayed reciprocity. The young that an individual helps
to raise become the individual's helpers when it succeeds to breeding
status in the group. In this special case of reciprocity, cheating by
failing to help cannot spread, provided (1) demographic conditions
favoring delayed reproduction and queuing for breeding remain the same for
successive cohorts and (2) individuals queue for breeding positions.
The evolution of this form of delayed reciprocity thus requires that
queuing behavior satisfy the conditions for an evolutionarily stable
strategy. Of several possibilities, the one most likely to explain
evolutionarily stable queuing in stripe-backed wrens appears to be
mutualistic cooperation among acquainted individuals.
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