The following is how we make a restrainer for testing empathic helping. There are clearly other ways to do it. The two important features to keep in mind are:

The restrainer should be large enough that the trapped animal is able to turn around. This greatly decreases the stress of being trapped and prevents overwhelming distress.
The door should be weighted in such a way that the free animals of the size used can open it. In other words, opening the door that we have designed is difficult and requires a moderate amount of strength but it is do-able for rats weighing about 300-450 g. If the door is too difficult to open, then the rats give up and develop learned helplessness. If the door is too easy, it will open even if the free animal just bumps into it or even worse, it may open due to activity from the trapped animal.

Preliminary work:

1) Start with a “Harvard Apparatus Unheated Rodent Restrainer” for a rat of 350-700 grams. Each restrainer comes with two doors. Remove the cardboard adhesives on these doors.

2) Place the “rear” door, the one with incised holes, in its appropriate slot.

Principal work:

1) The front door should be replaced with a replica that does not have an inlet.

2) Drill two holes into the upper left corner of this new “front” door.

Both holes should be parallel to the left side, one centimeter away from the left side.
1. The first hole is to be ½ centimeter down from the top side.
2. The second hole is to be 2 centimeters down from the top side (or 1½ centimeters down from the first hole).
These holes are to be big enough to fit two screws (D: 4mm, L: 10mm; slotted pan-head) that can keep in place a 4½ centimeter long solid plastic rod. Fasten such a plastic rod into the door with two screws. (NOTE: the rod and door should overlap for 2½ centimeters and the rod should extend above the Plexiglas for another 2 cm).

3) Create a half-door that has the same width as the front door.

The bottom of this half-door should be 3 centimeters up from the bottom of the front door.
It should have the same outline as the front door until it is 2 centimeters away from the top side of the front door.
At that point, the half-door should be cut to make an isosceles triangle where the top-most point is equidistant from the right and left-hand corners on the top side of the front door. (The side-lengths of this isosceles triangle should be around 5 centimeters).

4) Drill three new holes into the front door and three corresponding holes into the half-door.

These holes should echo the isosceles triangle. The two holes on the base line should be ½ centimeter away from the right and left hand sides. The third hole should be 1 centimeter down from the uppermost tip of the isosceles triangle. (NOTE: in step 4, you will be drilling a total of six holes).

5) Cut three 1-centimeter long solid plastic rods.

Screw each rod into the holes you drilled in step four, so that the half-door and the front-door are connected with only 1 centimeter of space between them.

6) Take one 50-gram or two 25-gram weights and secure them onto the 4½ centimeter long plastic rod that you built in step 2b.

7) Place this completed door into any two slots in the restrainer, and you are done!

How to Make the Mason Lab Rat Restrainers: Photos

Preliminary work

Principal Work