Jacqui's Captive Bred Tortoises

Checking if your tortoise is fit to hibernate tends to be a scary responsibility. if you are unsure or concerned ALWAYS check it out with your REPTILE vet.

The Jackson Ratio.

Please note there is a separate chart for baby tortoises!

The tortoise trust instructions are as follows:

The 'Jackson Ratio' graph may only be used with tortoises of 'average' body morphology, e.g. Testudo graeca and Testudo hermanni. This graph MUST NOT be used with Testudo horsfieldii or Testudo marginata, as it will invariably produce incorrect results. The body shape of these species is such that T. horsfieldii, being 'short and square' will tend to read 'overweight' even if it is actually underweight. T. marginata, being 'long and narrow' will, on the other hand, read 'underweight' even if it is of a perfectly average and safe weight for its size.

We also take this opportunity to point out that the 'Jackson Ratio' graph is of no use whatsoever for assessing the health of Hinge-back, Leopard, Sulcata or any other tropical, non-hibernating species. Neither is it of value in determining the suitability of Box turtles to hibernate.

This graph must ONLY be applied to Testudo graeca and Testudo hermanni and to no other species.

When measuring for the Jackson Ratio graph, use the straight line carapace method, and do not measure 'over the curve'.

There is only one way to take meaningful measurements for the purpose of computing the Jackson Ratio, and that is in a straight line.

Any measurements taken 'over the curve' of the carapace will invariably produce a false result which will indicate that the tortoise's weight/length ratio is lower than it actually should be.

The following link will enable you to understand it better and also link you to a Jackson Ratio graph for your use.

http://www.tortoisetrust.org/articles/measuring.htm

It is essential that measurements (and weights) are taken accurately, otherwise the result of the exercise will be meaningless, and may well endanger the tortoise.

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

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.

A female carrying eggs will be deceptively heavy, even if not eating.

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.

The maths:

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 divided by length x length x 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)
• Please use the following link to the Tortoise Protection Group which has a calculator on its site to take the worry out of the calculations:

• http://www.tortoise-protection-group.org.uk/site/171.asp

   

  With thanks to the Tortoise Trust & Tortoise Protection Group.