By Nadene Stapleton
(Doctor of Veterinary Medicine, Australia)
Chelonia inhabit many different habitats. Part of the reason for their success is their ability to fill a very narrow environmental niche. This ability has evolved over eons - leaving each species perfectly adapted to a particular temperature range, humidity level, light intensity, day length, and diet. Though we try, when keeping such species in captivity, we cannot hope to perfectly match their natural environmental conditions. Even the most knowledgeable tortoise keeper must continually monitor and adjust the habitat to keep it optimal.
When you consider the vast majority of illness in captive tortoises and turtles could be prevented with better husbandry, it is evident that many keepers get even the basics wrong. This being the case, it should be painfully obvious that keeping two or more different species in the same environment is completely inappropriate. You cannot possibly maintain correct environmental conditions to adequately provide for more than one species. Slight variations in dietary and temperature requirements will mean one (or both) species will be kept sub-optimally. What may seem like a small compromise on your part can mean the difference between survival or debilitation for the animal.
It is also important to understand that certain species of chelonian and individuals within species have different resistance and susceptibility to disease. Many agents such as mycoplasma and herpes viruses can infect "carrier" tortoises. These tortoises show no clinical signs of illness, but are able to spread the disease causing debilitation and sometimes death in other tortoises.
Similarly, what may be considered "normal" bacteria in some tortoises can prove rapidly fatal in others. Overcrowding leads to increased competition for basking sites, food and water and waste material build up. Different aquatic species may also require conflicting water to land ratios. These factors will all contribute to injury, stress, decreased immunity and increased susceptibility to disease. When we choose to keep non-domesticated species, it is our responsibility to emulate their natural environment as closely as possible. It is our duty to minimize stress and do everything we can to keep these animals healthy. Mixing species in captivity is irresponsible as it always means one species is compromised in favor of the other, or even worse, that both are compromised.
(c) Nadene Stapleton, DVM/Tortoise Trust 2008