Helping effort and future fitness in cooperative animal societies

Little attention has been paid to a conspicuous and universal feature of an imal societies: the variation between individuals in helping effort. Here, we develop a multiplayer kin-selection model that assumes that subordinates face a trade-off because current investment in help reduces their own futu re reproductive success. The model makes two predictions: (i) subordinates will work less hard the closer they are to inheriting breeding status; and (ii) for a given dominance rank, subordinates will work less hard in larger groups. The second prediction reflects the larger pay-off from inheriting a larger group, Both predictions were tested through a field experiment on the paper wasp Polistes dominulus. First,,ve measured an index of helping e ffort among subordinates, then we removed successive dominants to reveal th e inheritance ranks of the subordinates: their positions in the queue to in herit dominance. We found that both inheritance rank and group size had sig nificant effects on helping effort, in the manner predicted by our model. T he close match between our theoretical and empirical results suggests that individuals adjust their helping effort according to their expected future reproductive success. This relationship has probably remained hidden in pre vious studies that have focused on variation in genetic relatedness.