You found a frog in your toilet? Relax and be grateful that it wasn't a snake because more than one person has opened the toilet lid to find a serpent curled up in the drain opening, and in at least one instance, the snake responded to being disturbed by biting. Compared to a situation like that, your frog problem is pretty benign, but even so, you probably want to know how it got in there so you can keep frogs out of your toilet.
They Come Through the Sewers
The toilets in your house as well as all the other drains empty into a common sewer. If a frog happens to find its way into the sewer, the toilet is the most likely place for it to emerge because toilet drains have a large diameter, and they aren't protected by drain covers.
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How would a frog get into the sewer? Apart from the unsavory but remote possibilities that it entered your septic system through the drain field and crawled through the septic tank or it swam through the municipal sewer system from a point far away, it's possible that the frog got in through a break in the sewer pipe. If so, it's performing a valuable service by alerting you to that fact.
Take a walk around the yard and look for squishy ground or standing water and note any strange odors. If you find anything, call a plumber right away because a broken sewer pipe is not something to leave until the weekend. All the sewage from your house is going into the groundwater, and it's a health hazard for you and your neighbors.
They Come Through the Vents
It's far more likely that the frog in your toilet came from above and not below. The plumbing vents that serve all the drains in your house, including the toilets, all converge to one or two main stacks that rise through the roof. Vent stacks are constructed with 2- or 3-inch pipe, both of which offer plenty of room for a frog to fall into, and the hapless amphibian is most likely to wind up in the toilet closest to the main stack.
Vent stacks are notorious for providing havens for birds and small animals, as you may have just discovered, and if you want to keep them out, you should cover the vent opening with a screen. Not only will this keep frogs out of your toilet but it will prevent leaves and debris from falling into the vent and clogging it, which can cause all sorts of plumbing problems in your house.
They Come Through the Bathroom
An even more likely possibility is that the frog came into your house through an open door, hopped into the bathroom and jumped into the toilet because the lid was open. If you don't want that to happen again, keep the toilet lid closed, and if the thought of a frog anywhere in your house disturbs you, make an extra effort to keep all the doors closed.
It's a safe bet that the frog doesn't want to be in your toilet any more than you want it there. This might be a good time to install a pond in the yard so frogs can find a place to go that is more appealing than your toilet. Now, all you have to worry about is all that croaking.