Answer:
Longitudinal study
Step-by-step explanation:
A longitudinal study (or longitudinal survey, or panel analysis) is a research design requiring repeated measurements of the same variables (e.g., humans) across short or long periods of time (e.g. using longitudinal data).
Studying the elephants across long periods of time to determine their intelligence.
It is often a form of observational study, although it could also be described as randomized longitudinal experiments.