You sound interested enough that you might enjoy bits of the actual paper [1]. They didn’t exactly say the bees stopped playing under stress. Instead they said

You sound interested enough that you might enjoy bits of the actual paper [1]. They didn’t exactly say the bees stopped playing under stress. Instead they said the behavior isn’t a stress response (they know this because they eliminated stress from the environment). Also, the evidence that it’s inherently rewarding is bolstered by the pattern of interaction with balls. It increases over time and then drops (typical of play), and it is unrelated to mate/food finding. It’s very difficult to know what animals are thinking. The scientists say multiple times in different ways that none of this is conclusive (the longest section regarding knowing the behavior isn’t function-related). That being said, it’s a well designed experiment in my opinion. I think they did a good job of setting it up so that we can conclude: “why else would they roll the balls?” 1. https://www.researchgate.net/profile/Samadi-Galpayage/publication/364433874_Do_bumble_bees_play/links/635abaf16e0d367d91d13cca/Do-bumble-bees-play.pdf?origin=publication_detail https://www.researchgate.net/profile/Samadi-Galpayage/public...