The metabolic costs of breeding strategy and its influence on competition
Energetically financing reproductive efforts can be achieved with a variety of strategies that fall along a continuum from those that rely on stored energy acquired prior to the reproductive period (i.e., capital breeders) and those that rely on energy acquired during the reproductive period (i.e., income breeders). Energy storage is expected to incur a metabolic cost, and the utility of a capital breeding strategy is thus expected to depend on the magnitude of the cost of energy storage, the magnitude of nonreproductive basal metabolic costs, and on environmental quality. Here I use an individual-based simulation model to explore the role of energy storage costs, differences in nonreproductive energy costs, and environmental quality on the ability of two competing species to persist, one a pure capital breeder and the other a pure income breeder. I then examine a specific case study using the European green crab Carcinus maneas to explore the cost of energy storage. I measured metabolic rates of crabs that had stored various amounts of energy as a result of differences in experimental diets and use these metabolic rates together with energy storage patterns of wild crabs to estimate the total metabolic cost of energy storage.
Results of the simulation model demonstrated that the success of capital breeders, relative to the competing income breeders, declined with the cost of energy storage, as expected. However, this pattern did not depend on the nonreproductive metabolic costs. The influence of habitat quality was complex and interacted with the costs of energy storage to determine the relative success of these two breeding strategies. Metabolic rate measurements on green crabs indicated that the metabolic costs of energy storage were ~55× greater than the costs of increased body size alone. Overall, the difference in basal metabolic rates for crabs storing a maximal amount of energy prior to the start of reproduction is 56% higher than for crabs not engaged in reproductive energy storage. This increased metabolic expenditure is equivalent to the energy content of more than 900 eggs per day. This high cost of energy storage may be a contributing factor to the documented decline of European green crabs in southern portions of their invaded range on the east coast of North America with the introduction of a second invader, the income-breeding Asian shore crabs.