The allocation of resources to growth and reproduction are essential components of life history theory, particularly for organisms with indeterminate growth where there is the possibility of allocating resources to growth throughout their lifetime. Since the amount of energy an individual can obtain is finite, tradeoffs between somatic and reproductive investment can have a large influence on fitness. Current theories about resource allocation state that under both constant and fluctuating food, evolution will favor strategies with increasing allocation to reproduction. This work has mainly focused on a narrow range of high food abundance. However, these levels are unrealistic ranges in natural systems. Here we expand this knowledge by studying a larger range of natural food abundances, ranging from abundant to near-starvation. We use the common freshwater zooplankton Daphnia as a model organism because resource allocation can be observed non-destructively and they exhibit indeterminate growth. We raised four genotypes of Daphnia pulicaria under seven densities of the algae Chlamydomonas reinhardtii, and included both the presence and absence of maternal effects. We measured age-specific growth, reproduction, survivorship and respiration, which enabled us to study the allocation of resources to growth and reproduction across treatments using dynamic energy budgets.
Our results show that individual daphniids increase the proportion of energy allocated to growth as food decreases, which is independent of whether maternal effects are present. These results are contrary to previous work at higher food densities, which suggest food density has no impact on energy allocation to growth. To better understand this, we used dynamic energy budgets, and estimated both the weight-specific proportion of energy allocated to growth, and net production (energy available after maintenance). When net production is large, individuals allocate more energy to reproduction than growth as they grow. However, as individuals grow and maintenance rates increase, net production decreases towards zero. At low net production, individuals switch allocation entirely to growth at the cost of reproduction for all genotypes. The pattern of increasing lifetime energy allocation to growth as food decreases emerges because individuals encounter low net production earlier in their life at lower food. Our results reveal an opposite trend to what is predicted by most bodies of theory, and will contribute to a better understanding of resource allocation its implications for life-history evolution.