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

Previous research has found that efficiency, or, more precisely,the foraging gain ratio (FGR), is a valid currency in foragingtheory when (1) there is a limit to the energy that can beassimilated by the forager and (2) a forager is trying to meetan energy requirement. The FGR is b/ (c — c_r), whereb is the rate of metabolizable energy intake, and c and c_rare the rates of energy expenditure while foraging and resting,respectively. Here I show that, when energy expenditure hasa cost besides energy, animals should also choose the optionwith the highest FGR when they are aiming at a given positivedaily gain. The next question is which gain they should aimfor? Researchers have shown that observed intake levels ofgrowing ruminants are close to the levels predicted by maximizationof the efficiency of oxygen utilization. This currency can beapproximated by (B — C + C_r) / C, where B is the dailymetabolizable energy intake, and C and C_r are the total andbasal daily energy expenditures, respectively. By simulatinggrowth at different intake levels, I found that mass-specificoxygen consumption rate is indeed minimal at the observed intakelevels. This is the first study in which these efficiency measures(FGR and the efficiency of oxygen utilization) are combined.