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Tracing their lineage back to the Triassic Period (230 million years ago), box turtles are fascinating and popular pets. A well cared for box turtle can live a relatively long time compared to other pets of a similar size.
Box turtles have very specific care needs that should be carefully researched before adopting one. As part of ongoing attention to your box turtle’s well-being, you will want to pay attention to their growth to ensure they’re staying healthy and living their best possible life. The average size of an adult box turtle is 5–7 inches and their growth rate will be affected by various factors including their diet and living conditions. Read on to find out more
Box Turtle Overview
In this article we will be referring to North American box turtles from the genus Terrapene, which are the species most commonly kept as pets. Box turtles are medium sized terrestrial turtles and got their name from their ability to pull their head and all four legs completely into their shell and seal up like a box!
They are fascinating pets but they do have a long life span (commonly over 20 years) and very specific needs that need careful research before deciding if they are the right pet for you. It’s also important to check the legalities of owning a box turtle where you live before adopting one, and you should never capture or adopt a wild caught turtle.
Box Turtle Size and Growth Chart
Hatchling box turtles are 1–2 inches and typically weigh 0.1 pounds. They can grow up to 4 inches in their first year and then typically grow 0.5–1 inch per year until they are fully grown. Once they reach 4–6 years of age, they have usually done most of their growing and growth either ceases or becomes slower. Adult Box turtles can weigh up to 2 pounds, but can be as light as half a pound (8 ounces).
This growth chart is a general guideline on what to expect. Size and growth will vary depending on the species of box turtle, their diet, health, and reproductive status and will be affected by environmental factors.
| Age (years) | Average Shell Length |
| 0–1 | 1–2 inches |
| 1 | 2–3 inches |
| 2 | 3–3.5 inches |
| 3 | 3.5–4 inches |
| 4–6 | 5–7 inches |
Factors Affecting Box Turtle Growth
Like many animals, male box turtles are usually slightly larger than females, but in some species of box turtles this can actually be the other way round. Let’s look at some of the things that affect a box turtle’s growth rate and size.
Dietary Factors
Box turtles are omnivores, but tend to have a more carnivorous diet for the first 5–6 years of their life when they need more protein for growth and development. As adults their diet contains more plants.
Adequate calcium is also essential for a turtle’s growth and development and as with most reptiles a proper calcium to phosphorus ratio is vital. Box turtles require more dietary calcium than phosphorus and generally the calcium to phosphorus ratio across the diet should be 1.5:1 to 2:1. Low calcium, high protein foods may need to be supplemented with calcium. Consult with your exotic veterinarian to get the right diet plan for your turtle. Nutrition is complicated and will vary depending on their age, reproductive, and health status as well as the temperature and season.
Environment and UVB light
The best environment for box turtles is a safe, enclosed outdoor location with regular sunlight for basking. Turtles require UVB light exposure to help them synthesize vitamin D3 that is vital for them to be able to utilize calcium.
Adult box turtles living indoors need a very large terrarium with a suitable basking heating source and UVA and B light source.
Genetics
Box turtle genetics play a significant role in the animal’s size, reflecting the size of their parents. If the parents were small, it’s likely that their offspring will inherit this trait. There are several subspecies of box turtles commonly kept as pets. The Gulf Coast box turtle is usually larger than both the Eastern box turtle and three-toed box turtle.
Health Conditions and Illness
Good overall health is important for growth. If your turtle seems to be growing slowly or is underweight then it may be unwell. Book a check over with your exotic veterinarian for assessment.
Can I Tell My Box Turtle’s Age Just from Their Shell?
It’s possible to get a very general (and somewhat inaccurate) idea of a box turtle’s age by counting the rings on the scutes. Scutes are the ‘scales’ that cover the turtle’s shell.
- Look for the rings on the scutes. You don’t want to count the scutes themselves; they don’t reflect the turtle’s age.
- The rings normally alternate between wider rings of one color and narrower rings of another. In theory, the wider ring represents a period of good eating, typically a warm season, while the narrower ring represents a time of scarcity, often in the winter.
- If you count the rings and divide by 2, you can get an easy but very rough idea of your box turtle’s age.
When Do Box Turtles Stop Growing?
Many things can affect a box turtle’s growth rate. Wild and pet turtles are the same genetically but their growth rate and size is often different due to their diet and surroundings. In general wild box turtles take a bit longer to reach their full size due to the lack of food at some points, and the fact that turtles will not grow while hibernating (technically called brumation in reptiles).
Adult size is usually reached by 4–6 years of age and 5–7 inches. Growth either ceases or slows right down around this age.
Pet box turtles also normally live longer than their wild counterparts, in part due to the absence of predators. The increased stability of their living conditions also contributes. As long as you take care of their diet and the environment’s temperature, humidity, and light, your box turtle can easily live for 25–35 years, depending on the species. That said, some live much longer, up to 100 years in some cases!
Measuring Your Box Turtle
To measure shell length in a turtle, you will typically measure the length of the carapace (the top shell), from the front to the back. This is often referred to as the straight carapace length (SCL). Don’t measure over the top of the curved shell as this will affect your measurement and add extra length.
You can keep your turtle still on a surface that you can mark. Mark the front and back edges of the carapace and then move the turtle and measure between the marks using a measuring tape.
Remember that all turtles may carry Salmonella spp and so careful hygiene is essential after handling.
Final Thoughts
Box turtles can be great pets for the right person. They may be quiet but they are not low-maintenance and have very specific needs. The best way to see them reach optimal size is to provide the best possible environment, diet and care, so they have everything they need to grow and thrive.
Featured Image Credit: Steve Byland, Shutterstock
