Observation. The ones in captivity, it’s obviously easy. In the wild, you need to spend a protracted period of time following a group of animals from birth to grave. For some animals, it’s fairly easy because they don’t naturally live very long. For something like a tortoise, you might have several generations of naturalists observing over the course of multiple lifetimes.
That said, we can also make some very accurate predictions from models based on birth rate and changes in population sizes.
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