The energetic cost of competition: Gerbils as moneychangers

Using behavioural bioassays, we measure the cost of both inter- and intraspecific competition to a foraging gerbil, Gerbillus allenbyi. The bioassay is the amount of foraging activity in field enclosures of 2 ha, and the difference between foraging activity in an experimentally manipulated 1-ha subplot compared to the matched, unmanipulated twin hectare next to it. The bioassay depends on assuming that natural selection has fitted the gerbils to assess the comparative values of feeding opportunities and competitive pressures, although these occur in different currencies. The gerbils perform quantitatively and consistently, smoothly adjusting their foraging effort to the combination of disparate opportunities and costs. Measuring their responses as behaviour allowed us to estimate the costs of competition in the currency of energy flow, that is, the rate at which we experimentally add seeds to subplots.