>>what are the "signs" of a hook up that you look for?
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usually there will be a "yellowish" stain or stains and this will sometimes have a little blood in it. If you put them in a clean container and you have a yellowish/milky stain there is a pretty good chance they hooked up or he tried like hell.
The stain will be obviously different from poop or urates.
Now, they can and will hook up without leaving noticeable evidence, I've seen this as well. I saw a pair hook up and checked on them when they had separated and there were no visible signs. This is not the norm though, at least in my experience, they almost always leave a sign.

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Corn snakes and rat snakes...No one can have just one.
"Resistance is futile"
Jimmy Johnson
(Draybar)
Draybars Snakes
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