Introduction When each offspring has its own package of resources for development, the complete package of resources is lost when it dies. On the other hand, when a number of individuals share a communal pool of resources, the death of one embryo will result in the loss of only part of the resources. This is because the remaining off- spring can use the excess energy, perhaps even feed on their sibling; and in doing so, grow slightly bigger. Here the magnitude of this overlooked effect is explored and optimal strategies to make the best of a bad job are calculated. Since asexuals frequently have substantial fertility losses due to developmental fail- ure (Uyenoyama 1984), this model is applied in a comparative study of a flatworm with obligate sexual and asexual (parthenogenetic) forms. The reason offspring will in general perform better when they receive more resources is because mothers do not produce the fittest offspring possible (Godfray & Parker 1991). Rather, mothers have to trade-off the fitness and number of offspring (Smith & Fretwell 1974). For instance, let us assume that an offspring requires a minimum of I 0 resources to be viable. Further investment will lead to fitter offspring. However, this increase in offspring fitness does not continue for ever, and should start to approach a limit, say S m . This argument can be made more explicit by using an equation by Charnov, Downhower & Brown (1995) that relates energy invested in an offspring, I, to offspring fitness, S (see Fig. 1): S(I) = 0 if I < I 0 and S(I) = S m (1 – e –d (I – I 0 ) ) if I > I 0 , eqn 1 where d specifies a decay factor of the line. The opti- mal allocation can be found graphically by drawing the tangent that passes through the origin (Smith & Fretwell 1974; Lloyd 1986; Charnov et al. 1995). Notice that the optimal division of resources from the mother’s viewpoint specifies fitnesses below S m . Therefore, offspring will in general benefit from fur- ther resources; it will only mean that the mother uses her resources less efficiently. Redistribution of resources is possible when off- spring feed from a communal pool of resources or when they receive parental care over an extended time. In species that do not provide either of these two, off- spring packaging is normally an irreversible and single event that will not be open to later alterations. In such species the avoidance of risks under uncertain condi- tions can play an important role in determining the optimal strategy (Yoshimura & Shields 1992). Functional Ecology 1999 13, 786–792 © 1999 British Ecological Society ORIGINAL ARTICLE OA 000 EN Reducing losses to offspring mortality by redistributing resources J. M. GREEFF,* M. G. STORHAS and N. K. MICHIELS Arbeitsgruppe Michiels, Max-Planck-Institut für Verhaltensphysiologie, PO Box 1564, D-82305 Starnberg, Germany Summary 1. It is shown that reallocation of resources from dying offspring to their surviving siblings leads to significant reductions of fitness losses due to early developmental errors. 2. The reason resource reallocation can improve offspring fitness is because mothers do not provide offspring with the optimal amount of resources from the offspring’s point of view. Rather, mothers trade their investment per offspring off against the num- ber of offspring. Hence, surviving offspring can use reallocated resources fruitfully. 3. Animals suffering high offspring mortality can reduce this cost by producing large packages of resources shared by offspring. This allows for better reallocation of resources. Furthermore, by overstocking their resource packages with eggs they can anticipate embryo mortality and obtain offspring that will on average be more optimal in size. 4. In accordance with our prediction, parthenogenetic flatworms studied here produce larger cocoons than sexuals and they overstock smaller cocoons with eggs. However, higher embryo survival in large cocoons may also explain both these phenomena. Key-words: Embryo mortality, flatworms, offspring packaging, offspring size–number, parthenogenesis Functional Ecology (1999) 13, 786–792 786 * Present address: Department of Genetics, University of Pretoria, Pretoria 0002, South Africa