He is having fun. There can’t be any other reason. He is using his inherited skills, which are awesome, just for the hell of it and I for one am mightily impressed. Or perhaps they knew they were being filmed and decided to show off? A remote possibility I believe.
On the internet, there is a lot of artwork of monkeys doing handstands but I see few real examples of it. The BBC has a photograph of a young monkey doing a handstand. Monkeys are more athletic than humans. Or that seems to be the case but one person says that they are not more athletic but simply stronger. I think he is wrong. This person on the Quora.com website says that the only sport a chimpanzee can beat a human at would be powerlifting. I disagree. Gymnastics is something that chimpanzees (a great ape) and monkeys would excel at far above the competency of humans in my view.
Perhaps the question as to whether apes are more athletic than humans is wrong because humans are apes! But apes that aren’t humans are stronger and more agile than humans. Although humans have better fine motor skills than apes.
Chimps can’t pick and choose a few muscle fibres at a time as humans can which gives humans the ability to delicately manipulate objects with fine motor skills. It is said that this is one reason why chimps are stronger than humans but looking at the anatomy of a chimp, and how muscular they are, I don’t think this is as a good argument.
Of course, monkeys have to lift their weight because they live in trees and have to carry their weight on their arms. They’ve evolved to do this. Humans walk and therefore we don’t need as much muscle. And humans use their intelligence to make gadgets to help them carry weight. I wonder whether this has gradually undermined the muscularity of humans. The lazy human looking for ways to minimise effort.
Monkeys have a higher strength to body mass but apparently, they have less stamina and tire faster than humans. Humans tire slowly and are able to work much longer but at a lower power level compared to monkeys. And monkeys and chimpanzees have to work the muscles far more than humans do.
Returning to the video on this page, you can’t help but be taken by the fantastic balance that this monkey has and the alacrity with which he goes from walking on his legs to walking on his hands. It’s a seamless transition due to great strength, balance and athleticism.