Complex sentence: Captive females breed as young as four years of age, but in the wild, they usually start breeding only when six years old, which likely indicates they need to be large enough to avoid being killed by aggressive males.
Split into simpler sentences: Captive females breed as young as four years of age. But in the wild they usually start breeding only when six years old, which likely indicates that they need to be large enough to avoid being killed by aggressive males.