It needed at least -- at least -- an orangutan in a rubber suit.
The Humane Society film-set check-ins don't cover animals' dignity.
Yes, in the rough of the climactic scene, the aliens were portrayed by trained orangutans in alien masks. And roller skates. Haha, what? Why roller skates? Well, Spielberg thought that the apes' physiology, unpredictable behavior, and skating movements would seem suitably alien -- and, failing that, it would be hilarious to try.
But it turns out that wrangling orangutans isn't an easy job, particularly in a roller rink. The vision was for a party of alien-costumed orangutans to skate down a ramp toward the camera, but the orangutans had other ideas. Every time they tried to film the scene, the orangutans would refuse to participate and latch onto their handlers. On one of the few occasions that they managed to convince the apes to participate, the lead immediately ripped off its alien mask and started skating backwards. Which, if you ask us, would have been one hell of a twist to end a movie on.
"So we just saved the monkeys for Indy 4."