Efficiency as a foraging currency in animals attaining a gain below the energetic ceiling

Abstract

Previous research has found that efficiency, or, more precisely, the foraging gain ratio (FGR), is a valid currency in foraging theory when (1) there is a limit to the energy that can be assimilated by the forager and (2) a forager is trying to meet an energy requirement. The FGR is b/(c - cr), where b is the rate of metabolizable energy intake, and c and cr are the rates of energy expenditure while foraging and resting, respectively. Here I show that, when energy expenditure has a cost besides energy, animals should also choose the option with the highest FGR when they are aiming at a given positive daily gain. The next question is which gain they should aim for? Researchers have shown that observed intake levels of growing ruminants are close to the levels predicted by maximization of the efficiency of oxygen utilization. This currency can be approximated by (B - C + Cr)/C, where B is the daily metabolizable energy intake, and C and Cr are the total and basal daily energy expenditures, respectively. By simulating growth at different intake levels, I found that mass-specific oxygen consumption rate is indeed minimal at the observed intake levels. This is the first study in which these efficiency measures (FGR and the efficiency of oxygen utilization) are combined.