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The Jackson Ratio

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Testing your tortoise's health statistics

Introduction

In 1976 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 Testudo 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 weights are plotted on a graph, they provide a very useful guideline.

The data can also be calculated as a weight to length ratio, now known as the "Jackson Ratio":

Jackson Ratio:

Calculating the Jackson Ratio:

Measure the weight (in grams)

Measure the Straight Line length   (in cm)

 Calculate the weight divided by the length (in cm) cubed-

(weight/ length*length*length)

Results:

Jackson ratio of

• less than 0.16: An underweight tortoise needing special care.
• of 0.17: Light for hibernation, unless an active male, or an animal with unusually flared scutes.
• of 0.19: Normal, and safe to hibernate
• of 0.21: A very good weight, ideal condition.
• of above 0.23: Possibly too heavy, check for puffiness (water retention)

How to assess your tortoise:

When weighing, there are several points to bear in mind.

1. A tortoise that has Just emptied its bladder of 50 ml of water will be 50 grams lighter, and conversely it will be heavier after a drink.
2. A female carrying eggs will be deceptively heavy, even if not eating.
3. 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 in the ratio, or, at a rougher guide, 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 over-wintered until its weight is restored. Many tortoises which die in hibernation 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.

The most crucial information for the tortoise keeper are records- what ratio did your tortoise hibernate at last year, and what ratio did it come out of hibernation at. Is the ratio worsening from year to year, or improving.

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 over-wintering is given in the BCG care sheet for Mediterranean tortoises, or contact the BCG local regions for Health Check dates.

A full Jackson ratio booklet is available on request (send SAE, A5 size) to:

The British Chelonia Group,
c/o Dr R Avery,
School of Biological Sciences,
University of Bristol,
Bristol BSS I UG