  A further source of ambiguity in the definition of biological altruism concerns the time-scale over which fitness is measured. Conceivably, an animal might engage in a social behaviour which benefits another and reduces its own (absolute) fitness in the short-term; however, in the long-term, the behaviour might be to the animal's advantage. So if we focus on short-term fitness effects, the behaviour will seem altruistic; but if we focus on lifetime fitness, the behaviour will seem selfish—the animal's lifetime fitness would be reduced if it did not perform the behaviour.