In 1976 the late Dr Oliphant Jackson, MRCVS, recorded the weights and measurements of a large number of healthy and sick Mediterranean tortoises, both Testudo graeca (Spur-thighed Tortoise) and T. hermanni (Hermann's Tortoise). He observed that in healthy tortoises there is an optimum body weight, which can be used as one of the criteria to assess the state of health of these species and their suitability for hibernation.
If the figures for the average weights are plotted on a graph they provide a very useful guideline. The data can also be calculated as a weight/length ratio, now known as the "Jackson Ratio"; but for practical purposes, Dr Jackson recommended using the graph, which also shows the lower weight limits of healthy tortoises.
To make use of the graph, you must take your measurements in metric units (millimetres and grammes). The carapace is measured in a straight line, which can be achieved by pressing the front of the shell against a vertical surface to push the head right in, and pressing another vertical surface against the tail end. Measure the distance between these two points in millimetres. The weight (in grammes) can be found using an accurate pair of kitchen scales, or for smaller specimens you may need a balance or electronic scales.
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Now use your data to find a point on the chart: draw a vertical line on the graph corresponding to your tortoise's length, and a horizontal line across from its weight. Mark the spot where the lines meet, and you can then see if it falls within the range for healthy tortoises.
When weighing, there are several points to bear in mind. Firstly, a tortoise that has just emptied its bladder of 50 ml of water will be 50 grammes lighter, and conversely it will be heavier after a drink. Secondly, a female carrying eggs will be deceptively heavy, even if not eating.
Thirdly, an overweight tortoise may be full of body fat or fluid, and is not necessarily healthy. If there are no apparent problems with your tortoise, and its weight falls within the healthy range on the graph, you can safely assume that it can be hibernated. However, if it falls into the "dangerously low" category, veterinary attention should be sought, and the animal should be overwintered until its weight is restored. Many tortoises which die in hibermation simply have insufficient body reserves to see them through the winter. Similarly a grossly overweight tortoise should have a veterinary check-up to investigate possible abnormal causes.
A tortoise that is still growing may increase its length by up to 2 mm in a good summer, with a corresponding increase in weight. It is recommended that tortoises are weighed at least once a month as a more reliable guide. Regular weighing will give you an early indication of any adverse trend and alert you to the possible need for veterinary treatment.
A recent publication with further information on Jackson Ratio for the Spur-thighed Tortoise and Red-eared Terrapin is: Blakey, C. S. G. and Kirkwood, J. K.( 1995). Body mass to length relationships in chelonia. The Veterinary Record 3 June 1995, pp. 566568. Further information on pre-hibernation checks and overwintering is given in the BCG care sheet for Mediterranean tortoises.