Mixing species is something to avoid. Take it from me. I once owned five juvenile sulcatas that all lived together in a single species group. In 1998, I got my first leopard tortoise, a three pound wild caught female. I had her quarantined for all of the spring, summer and fall, just to be sure. I then introduced her to her new winter quarters, which she shared with the sulcatas. Though I'd read that mixing species was a bad idea, I figured these animals inhabited a similar environment and had similar dietary needs. Besides that, she was always very shy, so I figured the competition might loosen her up a bit.
I was wrong. Within one week, all five of my juveniles were showing symptoms of respiratory distress. The leopard was fine, and by the second week, she was the only one eating. I spent over eight hundred dollars in vet bills only to lose every one of my sulcatas. The vet took blood, stool, and saliva samples from all of my animals, and discovered that they were all carrying a gram-negative bacteria to which the leopard was apparently immune. Sure enough, leopards were not afflicted by this disease, as I introduced two more animals, a male and a female, the year following with no problems.
I've also had a friend who had lost his two Indian Star tortoises when he decided to keep them in the same winter quarters as his leopard tortoises (and his were captive bred leopard tortoises!).
The Mediterranean Testudo complex seems to be more resistant to disease, and I have never kept Egyptians, so i couldn't say for them. I own marginateds and Turkish black greeks. I personally keep them separate as well, though, as the males do have a tendency to harrass the females regardless of species, and nothing good can come of such stress. The males are also quite vicious toward eachother, to the extent that I have to keep them separated from everyone (I've caught my oldest Greek male chasing hatchlings of his own lineage when I introduced them; he will even tackle my wife's ceramic turtles if I put them in his pen!). I also used to keep russians, and I've found that once acclimated and disease free, they can usually be temporarily mixed with other Testudo with no problems. Permanent mixing is not best, though, given the aggression factor. Another curious topic is corprophagy, or the tendency by tortoises to eat feces. All tortoises engage in hindgut digestion (whereby microbes break down the ingested food, as opposed to stomach acids and bile in humans). These microbes are different for every tortoise, and field herpetologists have hypothesized that many tortoises enrich their gut flora by consuming such things. However, I think it safe to say that most tortoises in the wild would not be consuming dog, cat, oppossum, rat, or other such animal feces, and as dog feces tend to be relatively parasitized, this may not be the best thing in the world for the tortoise to be eating. How is one to control this if a dog shares the yard?
Certainly, it is no surprise that people have successfully kept different species from different environments together, but this is best avoided, in my opinion. Certain animals demonstrate apparent immunity, and I can't explain this. However, in general, tortoises are solitary animals that need a spacious, stress-free environment. Certainly, wild animals will come into contact with tortoises kept outdoors. The ramifications of this cannot be readily measured, so I could not say that this does no harm. No one is a stranger to the problems posed by ferral goats, cats, pigs, rats, and insects that were introduced to the Galapagos by ignorant nineteenth century sailors. Arizona, California, and Nevada all have laws against releasing captive reptiles into the wild for this very same reason. Though I'm far from expert on the subject, there is a clinical basis upon which the tortoise trust and other organizations have grounded their conclusions. So, play it safe. Quarantine, don't overcrowd, and don't mix species.
T.G.
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