Simply speaking the bigger the brain the more complicated things it can learn but it also takes more time for it to become good at doing the given task.
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I am not sure if what you are observing is really the lack of strength or just that the creatures didn't learn to use the muscles properly yet. Check this creature of mine. It is perfectly possible for it to lift it's own weight, it just took some time for it to learn to use muscles properly:
I would try to redownload. The file you specify is there when I download the game. Also your antivirus software may be messing things up and removing the file or preventing its creation when extracting.
Big thanks for your detailed answers, they explain a lot. Also thanks for the link to the code. I will try to have a look into it though I am not sure if I will be able to do anything with it without frying my brain.
Now, I would like to further elaborate on the reason for the bugs. Though I guess you might have already thought about what I am going to say.
A lot of my creatures suffered from this bug but I also have a creature that seems to be immune to it. That immune creature is the one on wheels, let's call it "wheeler". What is the difference between wheeler and other creatures? When the wheeler "walks", its physical state eventually returns to the arrangement that it was initially in and it keeps periodically returning to that arrangement as it "walks". On the other hand the creatures that suffer from the bug have, at the start, a very unique physical arrangement that they never ever return to as they walk. That leads me to the conclusion that the bug must be hiding somewhere in the creation/initialization of creatures.
EDIT: So I downloaded the source, installed Unity, opened the project, pressed play, got notified of 58 errors, got headache :D
EDIT2: Managed to make it work! Hmm, when loading in my old creature it doesn't work as it was working before, is the source updated with some new code that makes old brains incompatible or something?
EDIT3: Ok, I noticed things are even weirder. I launch the game inside unity, load my evolved bipedal and it doesn't want to walk at all. I build an exe, load my bipedal with that exe and the creature walks as it should. I can't figure out what's going on.
EDIT4: As far as I understand the code "thinking" happens once per frame. Frames though are generated at different intervals as FPS is something that is constantly fluctuating. Maybe this causes different behaviours at different times and thus generates those bugs?
EDIT5: After changing some "Update"s to "FixedUpdate"s it seems like simulation got a bit more consistent but I am not 100% sure just yet.
Feel free to make gif of it or anything you want, I really don't mind. I thank you for your detailed explanation but there are still some things that I do not understand.
Isn't the program supposed to preserve the best creature and so the drop in fitness shouldn't happen because of that?
Isn't the simulation deterministic so the same creature with the same brain should always behave exactly the same way?
Maybe the best creature is picked but for some reason some other creature is being displayed?
Also shoudn't the worst creatures be erased from the population and so there shouldn't be such a big gap between the best and worst creature?
You said that the bug doesn't happen often but for me it happens all the time. As you can see in the video the "best of" screen never shows anything other than a creature falling on it's face.
Are you the one who wrote the code behind the neural network or are you using some libraries for that?
And one last thing. Is there any chance of you releasing the source code to public?
BTW If you so like funny creatures here is one on wheels:
Hmm, may it be that the reason for the bug is that, you are reseting physical structure of creature but not it's brain at the start of "the best" simulation? Same reason could be the cause for that fitness drop.
I think I am experiencing the same thing. I have made a little video about it here:
Also if you wish to download the creature from that video and see for yourself how the game behaves you can get it here: https://drive.google.com/open?id=18IafZLqzAKWo3M3TC0MKXQBzBdkblKnM