How does an individual's optimal behaviour depend on its quality? An analysis based on relative ability

We analyse how an organism's behaviour is predicted to depend on its quality when there is a trade-off between survival and reproduction and fitness can be written in the form of a product of these two factors. The analysis is general, but we focus on two specific cases: the level of display of a male trying to attract a female and the level of effort of a parent caring for young. In each case, an increase in the level of behaviour increases the risk of mortality. The analysis is based on a consideration of how the performance of a high-quality individual relative to a lower-quality individual depends on behaviour. If relative performance increases, then higher-quality individuals are predicted to work harder; conversely, if relative performance decreases, then higher-quality individuals are predicted to work less hard. We also analyse the consequences of optimal behaviour. In particular, we show that the young of a high-quality parent may have a lower survival probability than the young of a lower-quality parent. We highlight other cases in which fitness has a product form, but where the product terms are not necessarily survival and reproduction. Our analysis still applies to these cases and again shows that relative, rather than absolute, perfomance is the appropriate biological variable to consider.