The vast majority of european tortoises kept in human care grow pyramidal bone plates in the centres of the horn shields covering the dorsal shell.
The causes for the development of these humps have as yet not been scientifically identified.
I will here briefly illustrate my own observations on this subject.
Firstly, you need to distance yourself from the idea that all tortoises grow to the same shape. This is mere wishful thinking we tortoise keepers tend to engage in. It also does not happen in nature. In the wild, every population of tortoises also contains individuals with humpy and irregular shells. This does not necessarily need to be pathological. Entirely smooth and regular tortoise shells are today found only in entirely original habitats that have escaped alterations by man.
Dalmatian tortoise with a smooth shell in a river valley in Croatia.
A Testudo hermanni boettgeri with regular growth but a humpy shell in Greece.
A Testudo hermanni boettgeri with a smooth but irregular shell in Greece.
A Testudo hermanni hermanni with smooth growth but an extremely domed carapace in Tuscany.
The formation of minor humps as such is initially not necessarily pathological and does not mean any impairment for the affected animal.
But, how do these humps develop?
It is generally presumed that carapace humps may be a result of the animal living on a substrate that is too dry. However, my years of field studies appear to indicate that the formation of humps cannot be exclusively related to the degree of ambient moisture and humidity. Rather, the reason for a formation of humps must be sought in the actual water household of the tortoises. As a matter of fact, these tortoises live only in habitats that are marked by a high level of relative humidity.
I have been finding smooth specimens mainly in populations living in habitats where water is freely available. Hump-shelled tortoises, on the other hand, are chiefly encountered in areas where there are no streams or other bodies of water; these habitats likewise sport a high level of humidity, though.
I have encountered animals with very prominently humped shells in a very arid, water-deficient coastal habitat on the island of Sardinia. As a result of failing rains, the water supply in this region has decreased to such an extent over the past years that hundreds of tortoises virtually dry out every year now. It only takes one extremely dry summer to cause juveniles to grow a humpy bone shell.
Testudo hermanni hermanni that died of thirst on Sardinia.
I know keepers who used to keep their animals on a dry substrate and at a low level of humidity and nevertheless raised tortoises with smooth shells. However, these keepers routinely bathed their hatchlings and subadults, which provided the animals with a regular opportunity to balance their water households.
It follows that in principle it should be enough for a smooth shell growth if the tortoises are able to take in sufficient amounts of water on a regular basis. My experiences and observations suggest that ambient moisture and humidity levels just cannot be the sole reasons for a development of a humpy shell.
On the other hand, a substrate with a certain degree of moisture and relatively high humidity levels are nevertheless vital for young tortoises (and obviously older ones as well). These animals are known to absorb moisture via their skin. High levels of relative humidity are a notable characteristic of all tortoise habitats in the wild and play a major role in keeping the respiratory system healthy.
Both in nature and in captivity, a formation of humps caused by insufficient moisture alone would not be detrimental to the health of animals thus afflicted. However, there is more to it, as you can see from these two shells that have been sawn open along the longitudinal axis.
The upper shell belonged to a "power-raised" animal that had been on an entirely inadequate diet of animal protein and died much too early. The formation of extreme humps has also affected the spinal column. I found the lower shell on Sardinia and have dissected it for illustrative purposes. It originates from the decidedly humid habitat near the sea referred to above. Even though this shell is relatively heavily humped, the spinal column has grown normally and related health problems cannot be supposed.
The actual problem therefore does not lie with the humps themselves, but is rather related to a dietary regime in human care that is often inappropriate and much too rich in protein. It is aggravated by an insufficient water supply from which very many captive specimens suffer. Like with ourselves, some 70% of the body substance of a tortoise is made up of water, and all bodily processes require a watery medium to run properly. Even a loss of body fluids of as little as 10% can have a negative effect on the metabolism.
The presence of a water bowl alone does not necessarily mean that the animals take in enough water. This a problem most often not realized, simply because it is not commonly known! But because our feeder plants cannot be compared to the plants growing in the natural habitats of the tortoises, our animals need to drink much more water than their cousins in the wild.
The pathological development of humpy shells thus only occurs if European tortoises are not kept and fed in accordance with their inherent biological needs. You can read up what "keeping and feeding according to natural needs" means on my homepage and in much more detail in my book.
Here, I want to point out just two points:
1. European tortoises are strict vegetarians and should therefore not be given foods of animal origin. This applies in particular to young tortoises at any stage of growth. Even though it is repeated over and again - tortoises in the wild do not consciously consume meat or carrion, but only feed on plant matter.
2. To keep European tortoises healthy in captivity demands that distinct differences be created between day and night temperatures. This fluctuation amounts up to 20 degrees Celsius in nature on a regular basis! Yes, you got it right, twenty degrees Celsius or 68 degrees Fahrenheit.
Why this is so you can read up in my latest book, "NATURALISTIC KEEPING AND BREEDING OF HERMANN'S TORTOISES".
A Dalmatian tortoise with a smooth shell in Croatia