"He's a happy monkey," said Musk, the chief executive officer of electric-vehicle maker Tesla Inc. and backer of numerous other futuristic projects, including Neuralink Corp., a startup focused on developing a brain-computer interface. He said videos of the plugged-in simians would be released soon, perhaps in around a month.

Musk was speaking on Clubhouse, a private social app where users host off-the-cuff, informal conversations. Although Clubhouse seems like nothing more than an app to hop onto various conference calls, it has become a popular platform for an estimated 5 million users, a jump from 3 million 10 days earlier.