"I want to try and breed horned frogs, but had a few questions before purchasing some. First, how long do they take to reach sexual maturity? Second, when in hibernation do I only need to lower the humidity in the tank and the leave temps the same? Third, will they drown in a pool consisting of 2 1/2 inches of water, they can easily climb out. And last, will they be ok if there is gravel under there substrate? I have a aquarium set up for grays trees with a waterfall and small pool, I plan to put 3in of dirt on top of the gravel and then some moss on top of that."
Ok well I'll answer in parts for you.
They take at least a year before you can breed them.
It's not hibrnation it's brumation, and you need to lower the temperature into the low to mid 60's and let the humidity drop but keep the soil from completely drying out.
And yes they can drown in 2 1/2" of water, because my female did when the male was amplexing her.
And do not use gravel.
And they will need more than 3" of substrate to brumate before breeding, more like about 6" of substrate.
I hope this helps. I plan on breeding my Cranwells' Horned Frogs here at the end of winter.

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