The advantages of keeping herps outdoors are many. They include lower maintenance, excellent ventilation, and access to unfiltered sunlight. Not only is the last believed important for vitamin D synthesis in reptiles (especially chelonians), but it has been well documented to have positive behavioral effects. In many situations, outdoor space is much less limited than indoor space. Thus, outside vivaria tend to be larger, which helps to reduce stress on the animals and allows for more natural behavioral interactions.

Only the most adverse of climates prohibit the keeping of herp species outdoors. The exact list of species which can be successfully maintained outside does, of course, depend on your precise geographical location. Sometimes two locations only a mile apart will have such dissimilar climates that the list of species suitable will be different for them. This is especially true as one moves inland away from the coast or up in altitude. Some areas may be suitable for outdoor housing of certain species only during a particular season; the remainder of the year they must be kept indoors. The list of species suited to a particular area can be expanded by such micro-habitat alterations as providing more extensive shade, a misting system, heated shelters, and chilled pools, etc.

An indoor cage is within a room, which is within a building, and an animal living in it has three levels of confinement. Outside, there is usually only one level of confinement, the vivarium itself. Thus, it is especially important when maintaining animals outdoors that the enclosure be made escape-proof.

If your locality has a lot of beasties that could potentially prey upon your captives, or if the captives are extremely athletic, it may be necessary to completely enclose the vivarium, aviary style. However, in many instances, an open pen will be sufficient. It is the building of such pens with wire fencing that the remainder of this article will concentrate on.

After having decided upon the dimensions of the future pen, the next consideration is the type of wire fencing. In order of strength, one's choices range from screen to hardware cloth to poultry mesh to sheep wire to chain link. Screen is universally unsuited as primary fencing wire; it damages much too easily. Chain link is excessive except for the largest of species; a giant tortoise is capable of going through anything less.

Normally, the thicker (stronger) the wire, the larger the mesh opening. Thus, a fencing of sufficient strength to retain an adult may have mesh large enough to allow the passage of hatchlings. Anticipate the appearance of hatchlings and design their containment into the pen. If there is no one wire that will contain both adults and neonates, then you will have to lay a secondary small mesh over the primary fencing, or as recommended by many turtle keepers, run a board along the base inside the pen. A board will serve the further purpose of providing a visual barrier which in some instances is beneficial in preventing rubbing against the pen side, or the inhabitants getting head or limbs tangled or injured by the wire.

In the U.S., wire mesh comes in widths of whole feet. You can save yourself a lot of time cutting up wire and generating sharp edges by having the heights of your fences correspond to the pre-sized widths of the fencing.

No turtle or tortoise requires a tall fence; three feet is usually more than adequate. A low fence has the advantage of allowing the keeper access to his charges from all sides of the enclosure. One need only lean over to pick up an animal or put down a food dish. However, low fences should not be used when certain predators (including humans and dogs) are a potential problem.

Even to turtles, excessive height poses only minimal deterrent to climbing up and over a fence. Stinkpots (Kinosternon odoratus), wood turtles (Clemmys insculpta), Pacific pond turtles (Clemmys marmorata), and a few other species have all been known to escape by climbing straight up and over six foot fences! Even fairly clumsy species can climb up a corner. Installation of a climb barrier is an effective deterrent. Indeed, climb barriers should be used to surround all pens containing climbing species and across the corners of all other pens. Remember to plan for the unexpected neonates who are often much more adept at climbing than their cumbersome parents.

Rather than using post supports, a stronger and more permanent fence is obtained by attaching the wire to rectangular frames which form individual fence sections. These sections are then fastened (bolted) together. They can be easily disassembled to change the dimensions or location of the pen. A frame made of "2 x 2" wooden rails is usually sufficient.