At least five outside areas are required for 2.3 animals, or three outside areas for 1.2 animals. These areas should be adjacent to and interconnected with each other allowing flexibility to combine enclosures to create fewer larger areas. The area used for mixing rhinos should offer multiple escape routes for the animals and good observation points for the keepers so that the animals can be separated quickly and safely if required.
A variety of methods have been used as primary barriers, including horizontal metal rails, vertical posts of either metal or wood and solid walls of concrete, masonry or stone. Black rhinos may climb and a primary barrier should be a minimum of 1.5m (5ft) high and non-climbable. There have been a couple of reports of particularly adventurous individuals getting just their front legs over horizontal rail fences of this height. Care should also be taken that animals cannot move items of cage furniture (such as logs) up to the fence line and use these as steps to climb over.
Posts connected by horizontal heavy gauge wire (hawser wire) have also been used however care should be taken with these as some rhinos will use them to rub their horns leading to the development of deep groove or even amputation of the horn. This behaviour may be reduced if plenty of alternative horn rubbing opportunities are available.
Both dry and water filled moats have also been used. Dry moats have a preference above water filled moats because of danger of drowning, especially if young calves are kept in the enclosure. Where dry moats are used, steep drops should be avoided as there have been cases of animals falling or being pushed into them. Ditches with vertical walls are considered dangerous and are not recommended, especially not in areas where animals may be introduced to other rhinos. The recommendation is that where existing vertical walls have ditches then these ditches be modified to a gradual slope with exits on either side of the ditch being provided.
Any moat used should have sloped areas to allow animals to exit easily if they enter them. It may be necessary to drain or otherwise prevent access to water moats or other areas of deep water in winter as there is a danger of animal attempting to walk on ice and slip or fall through.
It may be useful to have an area of bars (preferably vertical ones) where the keepers can examine and train the animals outside as well as inside as this will facilitate care of the animals without the need to bring them inside. Vertical pipes or posts should be spaced 25 cm to 30 cm apart. Open bars and gates between the rhino paddocks are also important to allow socialisation of the animals prior to mixing. It is recommended that all introductions of animals be undertaken outside. Note, if bars are used, care should be taken to ensure that they are far enough apart to reduce the chance of rhinos trapping their horns and damaging them. If using poles, each should be about 30 cm thick and set into the ground in concrete. Bars or poles should be connected vertically to prevent the fence from being uprooted by the animals. If calves are kept outdoors, adequate measures are to be taken to prevent escaping. It is important to consider fence spacing and keeper access respectively in order to provide an exit in cases of emergency. When poles are used they must be treated with non-toxic compounds only.
Secondary barriers used to protect planting or provide escape areas for other species sharing the enclosure include fallen logs, large rocks and lengths of primary barrier fencing. Electric fencing has been used with mixed results. Some animals respect the fence after their first shock; others seem to be irritated by it and will just rip it out. It certainly would not be a sufficient deterrent to stop a frightened or excited animal. Aprons of small sharp rocks have been tried to discourage rhinos from walking on particular areas however these do not seem to have worked and are not recommended. Triangular profile metal strips set over a shallow pit (cattle grid) have been used in one institution to restrict access to part of a paddock. This would almost certainly not deter an excited or frightened rhino and are also not recommended. To prevent individuals from being hurt, barriers should have no sharp edges.
A variety of gate systems have been used. As with the gates used in the indoor enclosures, in general manual slides are preferred outside to swing gates or hydraulic gates both for their speed of closure and to reduce the chance of rhinos getting trapped or injured. It is important that keepers have good visibility of as much of the paddock as possible either side of the gates in order to operate them safely. When sliding gates are used, the track must be kept clean in order to reduce the chance of them seizing. All gates must be firmly secured and designed such that the animals cannot dislodge them with their horns (e.g. cap hinges so that rhinos cannot lift gates off their hinges with their horns).
It is important that animals are adequately protected from excessive disturbance from visitors. Carlstead (1999) showed that mortality since 1973 correlated positively with percentage of public access. It is difficult to make recommendations as to a suitable maximum percentage of the perimeter to which visitors be allowed access as this will depend on topography and size of the enclosure, availability of visual and auditory barriers, number and behaviour of the visitors and the temperament of the individual animals. However as a general rule it would be inadvisable to allow public to access the whole perimeter and it is important to monitor the animal’s behaviour closely and to be able to make adjustment as necessary. Public should be prevented from touching or feeding rhinos, suitable standoff fencing is probably the most effective way of achieving this.