Mastering Biology Practice Test

Survivorship curves show how a cohort declines in numbers as time passes, and using relative age makes different species comparable. The x-axis often expresses age as a percentage of the species’ maximum lifespan, so you move from birth (0%) to death (100%) on a common scale. This normalization lets you compare mortality patterns across species with very different lifespans, without being tied to actual years. The y-axis then shows how many individuals remain alive or what proportion survives at each relative age. That’s why the x-axis being life spans as a percentage of the maximum life span is the best description.