Part of “Monkey 101” is learning how monkey mothers do not hold on to their offspring. The neonate must hold onto the mother; if they don’t, they fall to the ground. Mother can’t afford to waste her energy on one that is not 100%.
This is a very simplified explanation. Some of the infants that arrive could have been a victim of an innocent act from a tourist. Or a true natural act when a family saw the mother sloth push one of the twins off knowing she would not be able to tend for both of them. Or, the mother saw something we never would and abandoned it. Many species—many reasons.
Many species has what is called “social behaviors” that are passed down to their young. The example I use for “us humans” is a handshake. How many times have we seen a young child display some confusion over which hand to use? This is only because he doesn’t have that social behavior down pact in the brain, but when he does, it is as natural as breathing.
A female spider monkey will lactate for up to 24 to 30 months, depending on the sex. When she quits, the male will immediately leave her side and go to the other cohorts of the troop. The female will stay with her mother until she is sexually mature and leave the natal troop for another. Some biologists say this separation or sexual maturity doesn’t occur for as long as eight years.
The young female, being with her mother for this large amount of time will see a birth or two and also allows the social behavior of tending for their young will be passed on.