When reading up about the Herman Tortoises, you will sooner or later encounter a principle known as Jackson’s Ratio. Simply put, you should think of Jackson’s Ratio as the BMI or body mass index for Herman Tortoises. It uses a comparison of the carapace length of your tortoise to its weight to assess whether your tortoise is overweight, underweight or just right.The tortoise’s body weight can serve as an indicator of various important considerations, the most significant of these being the decision of whether to allow your tortoise to hibernate. Since tortoises do not eat during hibernation or for the preceding weeks, they will need enough body reserves to be able to weather this period without their health being compromised.
The process of applying this is simple. You must first weigh your tortoise in grams. Then you should measure the flat length of the carapace. Bear in mind that you will not get an accurate reading by running a length of measuring tape over the dome of the shell. The best way to achieve accurate results is by placing your Hermann’s tortoise on a sheet of paper. Make a pencil mark at the head and tail end of the carapace. That should give you the flat length of the tortoise’s carapace. The length should be converted to centimetres. One inch equals 2.54 centimetres.
The next step may involve some calculating on your side. You must divide the weight in grams by the length in centimetres cubed. An average length of a Herman tortoise is 6 inches (15cm) to 8 inches (20cm). Let us imagine your tortoise weighs 650 grams and its carapace is 15cm long. You would need to divide the 650 grams by 15 times 15 times 15, which works out to approximately 0.1925. According to the Jackson Ratio, this is ideal. In fact, a figure anywhere between 0.19 and 0.21 puts your Herman Tortoise at his or her optimum weight.
If your tortoise is 0.17 or under, you may need to reconsider about allowing it to hibernate. Below 0.16 means that your Herman is seriously underweight and you should investigate possible causes for this. Similarly, a value over 0.23 indicates that your tortoise might be carrying too much bulk. If it is female, she could be carrying eggs, but a visit to the vet to double-check would be advisable.
This scale of comparison was devised by Dr Oliphant Jackson in 1976 and applies to the Herman Tortoise and also the Spur-thighed Tortoise. There are even websites that include a Jackson’s ratio calculator, which could save you the effort of making your own sums.
If you are an owner of a Herman Tortoise, you would no doubt appreciate this guide to assessing your pet’s health.