Bear rolls down hill.
Not at all. Look what happens when a person or animal sits around doing nothing. They get out of shape and are less physically adapted to handle those survival related tasks when the need arises. So even when the activity is not necessarily related to training for a survival skill, it helps animals and ancient people stay fit and healthy.
I’m not sure that’s the right way of looking at it.
There’s no inherent reason why an organism should need exercise to build muscle or cut fat. Human metabolism happens to often favor fat storage and muscle wasting as an prehistorical adaptation to a poor diet and/or frequent famines. But there’s no law that a species’ metabolism do that.
So I think it’s more to do with building specific kinds of agility and skills, than needing to get fit.
We’re talking about animals. Are there any animals where this isn’t true?
Yes. But the calories an animal burns having fun could have been burned doing some other activity that directly benefitted it, like hunting for food. The exercise gained would have been the same either way.
Sure. But that’s different than sitting around doing nothing.
It’s an extremely interesting question. It brings up the basic question of “what is consciousness?”
We put off gratification in order to work and build up resources, then we put aside time to have fun. We have to try to focus on fun. Whereas animals just exist- they look for food, they avoid danger, they have fun. I sometimes wonder, how much self-consciousness does a creature need to really appreciate enjoying something? If I’m kayaking on a beautiful river, does the great blue heron I see ever stop to abstractly think, “nice river I fish on, and kickass day today!”
I would not overthink it. Simply put, it feels good.
Thanks for this. I’ve been making an effort to play more with Wee Weasel and of course the fear that I’m not properly encouraging his development is ongoing. This helps me see the purpose of play more clearly and know whether or not I am hitting the mark.
I would add to this there is a difference between passive entertainment and play. I can spend all my day browsing my phone and feel terrible. But if I instead focus on creative pursuits I feel vitality. There is also a need for bordeom to compel play. We are increasingly no longer bored, and thus no longer creative, and it’s affecting our mental health.
I’m not really sure I understand the question as all animals’ metabolisms are different.
The point is that there is no law of metabolism that excess calories must be stored as fat. Animals have evolved metabolisms that provide the right balance between storing calories and getting so big as to struggle to hunt or evade predators.
If you’re a marine mammal, the balance is way over at the “store fat” end, since as well as storing energy, blubber provides necessary insulation and buoyancy.
For land mammals the balance is way over at the other side. And it broadly works as it’s been fine-tuned by natural selection.
Beyond humans, domesticated animals, and animals that are otherwise overfed by humans, obesity is basically not a thing.
Also, another problem with your point is that for many species, it’s primarily infants that indulge in play. But infants generally have less need of exercise since they need a lot of energy to continue to grow.
He looks like he is having fun.
My initial thought was that “fun” is some sort of evolutionary/instinctual/training function. I still think that is mostly the case. But i’m not sure what why the gorilla is having fun here. Maybe there is something about this activity that releases endorphins/dopamine. Maybe knowing when to cool off on a hot day is instinctual? why not splash around in a pool?
I figure, reptiles don’t.
Lizards, snakes, and turtles are born already knowing what they need to do to live their lives.
If you have a pet reptile, any activities you consider “play” is probably attempts to escape an uncomfortable situation.
Aside: we have different definitions of “fun”. I would say sex is, or can be, enjoyable. It’s in the same category as eating, listening to music, reading a book. I don’t know whose definition is more mainstream, just that it is jarring for me to hear that sex is “fun”.
The question was part of USAF upper class hazing:
Mister, is sex fun or work
Must be fun sir, if it was work they’d have me doing it
Interesting, but is eating “fun” or “work”, they have him doing it. I guess eating USAF food is work, 
Ok. I really don’t want to get bogged down in semantics and I don’t think it affects my point at all. So just say sex is enjoyable, not fun.
The questions, and tentative answers, remain the same.
I don’t know how many species have sex just for the “fun” of it. But humans aren’t the only ones (i think). Many other species just copulate when they are fertile for the purpose of procreation.
But i think this too is evolution, humans are fertile year round. It would make sense that we find it “fun”
Just my opinion, i am not a biologist.