HYPOTHESES FOR THE PRODUCTION OF EXCESS ZYGOTES: MODELS OF BET-HEDGING AND SELECTIVE ABORTION.

Two hypotheses can explain the overproduction of zygotes. Bet-hedging assumes that optimal brood size varies unpredictably among breeding attempts. Excess zygotes are produced so that the number of independent offspring can be flexibly adjusted downward to the optimum number for that attempt. Selective abortion suggests that parents overproduce zygotes, identify those with the highest fitness expectations, then kill or abandon those with lower fitness in order to concentrate investment in those with the best prospects. Both hypotheses for the overproduction of zygotes work in principle, alone or together, and can lead to impressive levels of zygote overproduction. For both hypotheses, high levels of zygote overproduction are only attained when the unit cost of an aborted embryo is low relative to the cost of an independent offspring. Under bet-hedging, it is also important that the variability of environmental conditions important for breeding success be high. The two hypotheses together make clear when a parent could increase its fitness by killing or abandoning its offspring.