One of the biggest changes [with 1986 UK legal reform] was the cage size. And most of the mice at [the breeding company] were in NKP standard plastic box type cages. And one of the problems we had, some of the breeding animals preferred to breed in trios, one male and two females. And under the Home Office regulations, there wasn’t room for three adults plus a litter within those cages. When we put them back to one to one, the breeding significantly went down. So we tried all sorts of things and in the end, we just got a slightly larger cage, but being honest, it wasn’t as good, it wasn’t, you know, the breeding wasn’t as good for certain strains, the trio breeders in the bigger cages, they didn’t seem to like it.

Plastic_Back_1

I've certainly done it and I know my colleagues do as well, we're trying to move from this sort of stainless steel industrial feel to something that’s a lot softer with plastics and things like that, because it greatly influences not only the work that we do but the people…you know, create an environment that people want to be in. Because they're in there all day and if it's awful, they become depressed about it, you know, it really is grim. I mean, bearing in mind, most animal facilities won't have windows and all of that, so you have to try and make the environment they work in as pleasant as possible.

Plastic_Bottom_2

With manufacturing improvements in in the 1960s, plastic cages could finally withstand the autoclave ­(sterilisation at high heat) and were better at preventing mice from escaping. They were also lighter than metal cages, and more convenient to handle. Finally, clear plastic cages also allowed technicians and vets to easily observe the animals and check on their health without disturbing them unnecessarily. At the same time, more light exposure conflicted with mouse preference to hide and burrow.

Plastic_Front_1

"Clear plastic cages make it easier for staff to observe the animals. The animals may be able to detect staff movement and so would become more quickly accustomed to disturbance in the room. [On the other hand] transparent cages are more prone to over-illumination and it is difficult for the animals to get away from the light. [They] reduce the ability of the mouse to hide (a behaviour which seems very important).
Recommendations:
<ul>
<li>Plastic cages appear to have most advantages and are recommended for this reason.</li>
<li>Use environmental enrichment to overcome the disadvantages of different types of caging, e.g. provide plenty of bedding in metal cages and nesting material and hiding places in transparent cages.</li>
<li>Cages with solid lids should not be used without giving the animals facility for climbing and without regularly checking airflow rates."</li>
</ul>
RSPCA (1998).

Plastic_Left_1

"Attempts to make productive use of plastic within cage design had been made since at least the 1940s. However, plastic was initially prohibitively expensive and unable to compete with metal which, unlike early plastics, could withstand the high heat of the autoclave... Though the 1960s advances in plastic production produced first polycarbonate and later polysulfone, making possible affordable, lighter, warmer, and importantly transparent caging that allowed the observation of animals without their physical disturbance."
Kirk 2016

Plastic_Right_1

A mouse or rat cage must contain the animal safely, that is, be escape-proof. Mice and rats, especially when young, can wriggle through very small gaps, and their teeth make short work of all but the hardest materials, particularly where design faults result in projecting ridges inside the cage. [...] Plastic materials may be used for the lower or holding part of the cage, but a metal top (stainless-steel wire mesh) avoids destruction by gnawing, and is essential for maintaining the optimal environment by allowing adequate ventilation, and carrying the food hopper and the water supply...
LASA Working Party on Caging and Penning (1970), "Caging Standards for Rats and Mice" Laboratory Animals 4: 61–66.

Plastic Housing

How important is it to see the mice in order to care for them?