Freezing rodents is not an acceptably humane way of killing them - their death is prolonged and they are in distress - going through the same process of freezing that other mammals do.
Pinkies and fuzzies can be killed by snapping them quickly and hard on the back of the head with your finger (hold mouse by hips and snap finger off of thumb onto the back of the head directing the blow forward and down). They are immediately unconscious and die from cardiac arrest quickly after- though they do kick a lot as a result of the massive nerve stimulation that results from the severe trauma to the brain. Cervical dislocation in very small mice often results in the head separating from the body because the tissues are so fragile.
Hopper and adult mice can be killed by cervical dislocation as Zoe describes or with some variation on that. I suggest that you practice with a dead mouse or two before trying it on a live one because it is important to make sure that the neck is completely broken the first time time, so the animal does not suffer. I find that if you hold the head and pull the tail you are as likely to break the back as the neck and it is preferable to make the break as close to the head as possible so that the massive trauma produces unconsciousness instead of just the paralysis you get with a broken back. I place them on a brick so they have something to grab on to and they tend to pull themselves forward when you hold their tail. Then I bring a solid metal rod (the metal part of a screwdriver) down quickly across the top of their neck right behind their skull. By pushing it down quickly and firmly the neck is broken completely but the skin is not damaged, so there is no bleeding (there will be swelling under the skin). This results in immediate unconsciousness and rapid death.
CO2 is a great method for people who can't do the hands on, but direct blunt trauma to the head or cervical dislocation are acceptably humane and produce rapid unconsciousness and death. As has been mentioned in a couple recent responses, decapitation is preferred for rats - it is hard to do cervical disolation on adults because of the strength of the neck muscle, so blunt trauma to the head (whacking) is what is most often used.
mary v.
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Mary VanderKop
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