There are many fascinating facts about the species known as the Hermann tortoise. Eventually, when studying the species, you might hear of a concept called Jackson’s Ratio. This is actually quite easy to explain. It is the body mass index for Hermann tortoises. By taking the length of the carapace, in centimetres against the tortoise’s mass, in grams, you can determine how typical your tortoise is of others of its breed and age.By using the tortoise’s mass, and determining how close this is to what passes for normal with the species, you would for example be better equipped to decide whether to let your tortoise hibernate or not. When hibernating, tortoises do not eat and live off the stored up food resources within their body. When a tortoise is underweight or ill, its well-being can be seriously endangered by hibernation.
Practically this boils down to taking the measurements. To start, you could weigh your tortoise and convert the amount to grams. Next, you should get the front to back length of the flat carapace, which of course means that just holding a tape measure across the top of its dome would not do the trick. The more effective method involves putting the tortoise down on some blank paper and pencilling the front and back end onto your paper. By measuring this distance, you would have arrived at the carapace length of your Hermann tortoise.
Once you have all the measurements, you should get ready to do a few sums. As with humans on the BMI, the mass of your tortoise should be divided by its carapace length cubed. For an example of this principle in action, let us consider a hypothetical tortoise of about 6 inches (15cm). That is within the range of an adult tortoise, which can get up to 8 inches (20cm). You have just weighted the same tortoise and arrived at a mass of 22.9 ounces (650g). Jackson’s Ratio is based on metric measurements. Your amounts should be changed to grams and centimetres.
With both variables handy, it is time to hit the calculator. Take the length and multiply that twice by itself, in your instance 15 x 15 x 15. The 650g should be divided by this total, which, by the way is 3375. In the case of your hypothetical tortoise, the value you arrive at would be around 0.1925. Looking this up on the Jackson’s Ratio, you will find out that your tortoise has the correct weight for his or her size. An amount between 0.19 and 0.21 means your Hermann tortoise is a healthy and fairly typical specimen.
A tortoise with a score of 0.17 or under should be regarded as too scrawny. This result should definitely play a role in the decision of whether your tortoise hibernates or not. You may also need to look into the reason for this. While a tortoise with a very empty bladder might get a different reading than one with a full bladder, a consistent score under 0.16 should concern any conscientious tortoise owner. Likewise, if your tortoise gets a reading over 0.23, you need to find out why it is so heavy. A female could be preparing to lay eggs, but a professional should ascertain this.
Jackson’s Ratio was arrived at by the rigorous efforts of a Dr Oliphant Jackson in the mid 1970s. It can be used to assess the hermann tortoise or the Spur-thighed tortoise. If you are a regular on websites relating to tortoises, you may stumble across an application that just lets you enter the variables and will calculate for you where your tortoise stands on the graph.
Anyone who wants to ensure that his Hermann tortoise is in peak condition for hibernation should be grateful for the existence of this convenient test.