Kin competition within groups: the offspring depreciation hypothesis

Where relatives compete for the same resources (kin competition) and each obtains an equal share, this can favour the evolution of elevated dispersal rates, such that most resource competition is among non–relatives. We show that this effect evaporates as among–sibling dominance increases to the point where the allocation of resources is maximally unequal. We restore a kin–competition effect on emigration rates from dominance–ranked family groups by showing that where siblings form queues to inherit the breeding positions, the length of the queue affects the fitness of all individuals by depreciating the rank of subsequent offspring. Incorporating this ‘offspring depreciation’ effect decreases optimal queue lengths, increases dispersal rates and explains the otherwise paradoxical use of sinks by cooperatively breeding birds in stable environments. The offspring depreciation effect also favours the evolution of small, but consistent, clutch sizes and high reproductive skew, but constrains the evolution of alloparenting.