Factual error: When Moto Moto and Gloria were digging for water, they threw all the jewels and gold into a pile. The gold in the basket would weigh more than 1200 pounds. Also, real gold comes fused to other rock and rarely in pure form.
Factual error: When young Alex is in the crate, he floats from Africa to New York, dead against all prevailing currents.
Chosen answer: There's two reasons. Firstly, infants are only allowed to work for short periods of time, hence why when you see babies onscreen that are heavily featured, invariably twins are used to alternate to not fall foul of the rules. Secondly, it may be that one of the voices didn't give the filmakers as much diverse dialogue as they needed so they brought in another actor to get the rest they needed.
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