I need help with MGS3 (frogs) :(

I'm trying to collect all the frogs in Metal Gear Solid 3, but I'm stuck. I'm cowardly reading a guide that explains the locations of the frogs and it says that there's one in the Dolivodno area (the bridge) the second time you get there (during operation Snake Eater, nightime), on top of a post. But I can't find it!

So I thought that maybe I already found that frog the first time I played the game. My question is: do frogs you find in a previous game disappear in games you play later?

Help me! I can't go on :(

No, they don't disappear, you have to get all of them in one playthrough.Jack7Of7Spades7

I read I can get them in different plsytroughs, as long as I load the file it gives you when the game is cleared.