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Yes, this variable appears to change depending on environmental pressures. The more complex the environment (new niche, variety of food sources, social changes), the more time is required for the young to reach maturity and adapt to it. In times of scarcity (simplicity), the young matures quickly so they can give birth before they die. I'm sure I'm mangling the idea, but it's something along those lines.

Interesting. I'll have to read more about that. Thanks