Tortoises all seem to grow at highly variable rates, dependent on all sorts of environmental and genetic factors, so comparing tortoises, even those of the same age, is often useless. If you measure them monthly, and your sub-adult tortoises are increasing in weight and length over time, then you're doing well; if you want a bit more than that, then continue reading below.
Susan Donoghue, DVM wrote an article about tortoise weights, containing the Donoghue Ratio, that gives us a target weight for most tortoises. The data is based on 76 tortoises and box turtles representing 11 species and works fairly well even for young tortoises, but may not be as applicable to hatchlings.
I use the formulae below to get an idea of whether my tortoises are over or underweight.
The Donoghue Ratio
The basic formula gives us the 'ideal' target weight: SCLcm3 x 0.191 = tWTgr
(Straight-line Carapace Length, in centimeters, cubed, times 0.191 equals the target weight in grams)
Tortoise BMI (tBMI) Formula
This formula gives us a number that helps us better understand generally whether a tortoise is under- or over-weight: cWT/tWT = tBMI
(current weight divided by target weight equals the tortoise BMI)
Target Weight: imagine a tortoise with an SCL of 12.3 centimeters and a current weight of 349 grams... the target weight should be (12.3)3 X 0.191 = 355 grams (the target weight, rounded off)
tBMI: this imagined tortoise has a cWT of 349g and a tWT of 355g: 349/355 = 0.983 (tBMI - a little underweight)
You can determine if your tortoise is underweight, normal, or overweight based on these results.
(Note- information offered in the table below should be used as general suggestions only)
0.66 or lower: Very underweight. Needs immediate care. Start with lengthy and repeated soaks in warm water and/or carrot baby food. Consider seeking veterinary help.
0.66 to 0.83: Dehydrated and/or underfed. Needs appropriate care in the form of more soaking and more frequent access to nutritious/appropriate food for the type of tortoise.
0.83 to 0.99: A little underweight. Review and correct care, soaking, and diet as appropriate for the type of tortoise.
1.00: 'Ideal' target weight. Note that few tortoises will hit this exactly, even with the best of care.
1.01 to 1.16: A little overweight. Review and correct care, soaking, and diet as appropriate for the type of tortoise.
1.16 to 1.33: Overweight. Adjust care, diet, access to adequate space for exercise.
1.33 or over: Significantly overweight. Adjust care, diet, access to adequate space for exercise. Consider seeking veterinary help.
Appendix I. Getting a good SCL reading.
The easiest way to get an accurate SCL without tools like calipers is to:
- Place a metric ruler on the floor or table, '0' against the wall or some sort of block.
- Set the tortoise on the ruler, head towards the wall, positioned so the shell touches the wall.
- Place a block, box, etc. behind the tortoise, on the ruler, and move it so it touches the back of the tortoise's shell firmly.
- Move the tortoise and read the ruler at the block.
Appendix II. Adjusting the formulas to predict target length.
Using weight to predict length. You can switch the Donoghue formula a bit to determine the target length of a given weight of tortoise: SCLcm= cube root of (WTgm/0.191).
- Adkins, Mark. Tortoise Library, 2012, tortoiselibrary.com/health-and-medical-2/healthy-weight-and-size/.
- Barthel, Tom. “The Hydration Equation.” Reptiles, July 2007.
- Donoghue, Susan, 1997. "Nutritional status of tortoises using morphometrics to assess body condition". Vivarium Magazine, Volume 8 Number 2.
- Mader, Douglas R., DVM, ed. Reptile Medicine and Surgery. Saunders Elsevier, 2006.
- Pingleton, Mike. The Redfoot Manual: A Beginner's Guide To The Redfoot Tortoise. Art Gecko Press, 2009.
- Tortoise Forum, www.tortoiseforum.org/. Multiple care guides for various types of tortoises, as well as articles on tortoise care and medicine.