Turtles may dig around in the rocks at the bottom of their tank for a few reasons. Your turtle may be eating the rocks, or she may be hungry or bored. However, rock-digging behavior may indicate that your turtle is looking for a place to deposit her eggs.
Eating the Rocks
Lithophagy, the act of eating rocks, is a well-documented phenomenon in turtles and countless other animals, including crocodilians, birds and lizards. The reasons for this behavior are poorly understood and vary from one species to the next. The practice may aid digestion, provide nutrients, or serve some other purpose -- most likely, the process meets several needs.
While small stones typically pass through the digestive tract of a turtle, large rocks may obstruct their intestines, leading to a medical emergency. Accordingly, some turtle enthusiasts keep their turtles in bare-bottomed tanks, which lack a substrate entirely. Others are not troubled by the behavior and continue to utilize gravel as a tank substrate. You also can use gravel that is too large for the turtle to consume.
Looking for Food
Hungry turtles may forage for food amid the rocks at the bottom of their tank. This is a natural behavior, which is also seen in wild turtles looking for aquatic insects or crustaceans, hiding amid the rocks. While hunger-initiated rock digging is not necessarily a problem, it suggests that you should consider increasing the amount of food offered to your turtle.
Fighting off Boredom
Turtles kept in small cages or those kept in cages that lack any complexity may become bored. Of course, it is impossible to know if turtles experience feelings like boredom, but their behavior is consistent with the emotion. Usually, these behaviors will resolve themselves if you increase the habitat complexity.
To make the habitat more complex, add more basking sites, rearrange the furniture or incorporate live plants into the tank. It may be necessary to increase the size of the habitat before adding more cage furniture, to avoid overcrowding the enclosure.
Gravid turtles may begin pacing their tank or digging in their gravel as the time for egg deposition approaches. This is especially true when the turtle has no access to a proper egg-deposition site. This is a serious issue requiring immediate attention, as the lack of suitable egg laying sites can cause turtles to retain their eggs, which can lead to serious health complications or death.
Avoid this problem by providing all mature female turtles with an egg-laying site at all times. This is even important if your female does not live with a male; females may retain sperm from earlier matings, or they may produce infertile eggs.