Perhaps the most frequent question I am asked concerning turtles and tortoises is how much and how often to feed them. Turtles that are overfed can become obese, just as other animals (and people) can. Although no definitive studies have been done on the health consequences of obesity in chelonians, I feel that it is best to err on the side of caution. Turtles are trapped in their shells - too much excess flesh could compress and injure internal organs. A decreased lifespan or chronic health problems may also be consequences of obesity.
With aquatic turtles, obesity can be detected by simply looking at the turtle. If you startle the turtle, causing it to withdraw its head, do the back legs shoot out? When the turtle sits on its basking area, do billows of flesh spill out around the turtle's legs? If so, then the turtle is probably too fat. Hatchlings and juveniles can also be fed too much. Rather than becoming obese, juvenile turtles divert the excess food into increased growth, which might lead to problems such as shell deformities or a decreased life span.
My wife and I err on the side of caution in feeding. Within a few months of hatching, we put our turtles on an every-other-day feeding schedule. We feed our aquatic turtles in separate containers, in order to monitor how much food each animal is eating. Each turtle receives as much food as it can eat in a few minutes. One rule of thumb is to feed the turtle as much food as would fit in its head and neck. The only time we provide more food or feed more often is when a turtle is sick or noticeably underweight. At that point, a more frequent feeding schedule is desired, to help the turtle in its recovery.
An alternate-day feeding schedule, especially on hatchlings, might seem to be rather limiting. However, keep in mind the normal environments of most turtles. They are not fed top-quality food every day (or even more often than that!) in the wild. They grow slowly but steadily, except those species that hibernate or estivate. I have heard cases of turtles that have grown quickly enough to breed after only a couple of years, when the same species in the wild takes over a decade to reach breeding size. I feel that such rapid growth is often one of the primary reasons for shell deformities ("pyramiding"), in addition to the potential for long-term health consequences.
Exactly what to feed a particular species of turtle or tortoise is beyond the scope of this article. Investigate the natural history and diet of the chelonian in question, and try and duplicate it if possible. For many tortoises, this means a high-fiber, low protein diet of mainly grasses with occasional vegetables and fruits. For semi-aquatic turtles, a combination of vegetables and insects would be appropriate, while for land turtles, an omnivorous diet of vegetables, fruits, insects, earthworms and other invertebrates should be provided. Having said that, we deviate from that naturalistic diet for some of our animals - our semi-aquatic turtles are fed mainly Tetra's ReptoMin and Wardley's Reptile Ten. When fed in moderation, we have raised hatchlings of many species to adulthood with no health problems on "foodsticks" supplemented with the occasional treat of insects, earthworms, or fruit.
Turtles that are noticeably obese should be placed on a reduced-food diet. Often, this diet is harder on the owner than the turtle. Turtles, just like other animals, can be trained. Many of them, for example, have been trained to associate "begging" behaviour with the arrival of food, and will beg whenever they get the chance. While this behaviour is endearing, giving in to it may be exactly the wrong thing to do. In addition to reducing the amount of food and the number of feeding times, the total captive environment should be evaluated. One common contributing factor is the size of the cage. Turtles that are kept in too small an area do not get the exercise that they need to burn off the food that they consume. All types of turtles and tortoises should be provided with areas that are big enough - we try and keep indoor turtles in areas that are three to five times wider than the turtle is long, and twice that ratio in length. When in doubt, remember that bigger is always better for turtle enclosures but not for turtle feedings!
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