Tossing cane toads into the freezer causes them no pain and is the most humane way to get rid of the amphibian pests — but just make sure they don't thaw out too soon.

That is the message from Australian scientists who used pain-sensing electrodes wired into toads' brains to find out whether freezing them — a method long-condemned as inhumane by animal ethics groups — was the best way to go.



Researchers implanted small electrodes in the brains of cane toads and then put them into a refrigerator for a few hours, before transferring them to a household freezer.

Professor Rick Shine from the University of Sydney said the toads quietly slipped into unconsciousness as they were transformed into toadsicles, and the toad's brains did not register any evidence of pain during the entire process.

"What we saw was that the toad's brains just switched off," he said.

"By the time they went into the freezer there was almost nothing happening and it slowly went down.

"The difficulty is of course, you can't ask a cane toad if something hurts.

"People have used all kinds of indirect measures ... this was really the 24-carat gold way to measure what's happening inside a toad's brain when you're doing something to it."

But Professor Shine warned the toads had to be kept in the freezer for days, otherwise they could defrost and come back to life.

"Make sure it's in the fridge for a few hours so that it's not going to feel pain when it goes into the freezer," he said.

"Once it's in the freezer, leave it in there for a couple of days ... depending on what else is in the freezer and so forth, it may not be all that far below zero, and especially if you have quite a few toads together in a bag, it takes a while for it to freeze to the point where it actually kills them.

"So if you don't want an unpleasant surprise when you pull out your bag of frozen toads, leave them in there a bit longer."