I have had extensive turtle guilt of late – that is, there just haven’t been enough turtles on Tet Zoo for a while... by which I mean, there haven’t been any. But fortune smiles on the thing that it smiles on, and a few neat photos have recently fallen into my proverbial lap, encouraging me to write the article you’re reading now. My huge friend and colleague Mathew Wedel owns a Box turtle Terrapene carolina. It’s called Eastie... don’t judge; this is because the animal is an Eastern box turtle (or is she? I wonder if Eastie is a Three-toed box turtle). Anyway, Eastie recently found part of a deceased rat’s head while on a backyard jaunt, and proceeded to deliberately snip away at the broken braincase and eat the bone fragments. This bone-eating carried on for about 20 minutes, and Matt thought it interesting enough to take the photos you see here.

The eating of bones – osteophagy – is well known for turtles, has been recorded in several species, and is observed easily enough in species kept in captivity (like Testudo tortoises). Whenever this subject is mentioned (believe me, it’s always cropping up in conversation), many people recall the photo in David Attenborough’s Life on Earth that shows an Aldabran giant tortoise Aldabrachelys gigantea* scavenging on the carcass of a conspecific (Attenborough 1984). As you can see here, it’s not entirely clear what the tortoise is doing, but it looks like it’s gnawing at dried skin and muscle, not bone. Incidentally, the photo was taken by Attenborough himself. I did used to have a very neat photo showing gnaw marks that a pet tortoise (belonging to my late friend and colleague David Cooper) left on a cow bone – to my frustration, I can no longer locate it.

* Yup. No Dipsochelys dussimieri for Tet Zoo, thank you very much. Did you follow the debate in the ICZN Bulletin?

Why are these turtles eating, and gnawing at, bone? Turtles seem to have high calcium requirements, with some studies indicating that they require unusually high quantities in order for shell and skeletal development to occur normally (Fledelius et al. 2005). In part, this is probably due to the extensive amount of extra bone they grow for their shells, but females also need extra amounts of calcium when forming eggs and growing eggshells: data from female musk turtles at least shows that skeletal bone density drops markedly during the time that eggshells are being formed (Edgren 1960).

The impression you get from what’s been published is that turtles are especially ‘calcium hungry’ relative to other reptiles (errr.. you want this quantified? Hey, I'll get back to you on that). And it seems that they recognise that bone is a valuable calcium source: they’ll not only eat small bones and nibble on large ones, but will also eat bone fragments out of dried scat belonging to big cats and other carnivorans (Bally 1946, Loveridge & Williams 1957). They can also be persistent and determined to eat the bones that capture their interest, sometimes returning to eat or gnaw at a bone just seconds after the observing humans have moved away (Esque & Peters 1994). In contrast, they tend not to immediately return to the plants they’re eating (Esque & Peters 1994), a fact which suggests that bone is an especially valuable resource that needs to be taken advantage of as quickly as possible.

Furthermore, tortoises are sometimes so keen to ingest calcium that they’ll swallow stones and even take mouthfuls of calcium-rich soils. This has been documented for wild Desert tortoises Gopherus agassizii* in the Mojave Desert (Esque & Peters 1994) and also in captive box turtles (Kramer 1973). Esque & Peters (1994) described 12 observations of soil- or stone-eating in Desert tortoises. This was out of a total of 991 observations of foraging tortoises, so it certainly doesn’t seem that these ingestion events are frequent enough to be easily observed in the wild. The geophagous animals were mostly adult females, mostly indulging in the behaviour during the spring.

* Note that the Desert tortoise has recently been split into two species (with there being suspicions that yet further taxa await recognition within the complex). The animals discussed here almost certainly still belong, however, to G. agassizii. The other one is the Morafka desert tortoise G. morafkai, found east of the Colorado River.

Despite the rarity of stone-eating observations, about 60% of 185 x-rayed wild Desert tortoises contained stones within their intestines, indicating that stone-eating is normal and common in some species. Esque & Peters (1994) suggested that this behaviour might help maintain a healthy gut pH, help nullify toxic plant compounds, control intestinal parasites, or assist in maintaining beak shape. They also made the interesting point that people should probably avoid removing bones from places inhabited by Desert tortoises – by doing so, we might be altering the pattern of nutrient cycling, and literally depriving tortoises of rare resources that they need.

The ingestion of stones and bones is pretty neat, but perhaps even more interesting is the mining behaviour also practised by Desert tortoises. Marlow & Tollestrup (1982) described how members of some Desert tortoise populations deliberately mine and eat specific layers of soil at key locations that they visit at regular intervals. Their activities mean that these locations are maintained for years as special mines (Marlow & Tollestrup 1982). Yes, mining tortoises.

Finally – in one more effort to partially relieve the turtle guilt, I leave you with these really neat and charismatic photos of a wild Common snapping turtle Chelyda serpentina that Jason Noble took at Catoctin Wildlife Preserve and Zoo, Maryland. I just love the turtle’s expression.

For previous Tet Zoo articles on turtles, see...

UPDATE: Stewart Macdonald kindly supplied the following photos, showing a site in Arizona (a dry creek bed) used as a mine by Desert tortoises. In the upper photo, cavities on the sides of the creek were being used by mining tortoises when the photo was taken (one of these cavities is visible high up on the side of the creek wall). There's some suggestion that the overhang visible in the photo at the bottom might have been caused by the sediment-eating activities of the visiting tortoises, but this isn't confirmed. Thanks to Stewart for the permission to use these images.

Refs - -

Attenborough, D. 1984. The Living Planet. Collins, London.

Bally, P. 1946. Tortoises eating bones. East African Uganda Natural History Society 18, 163.

Edgren, R. A. 1960. A seasonal change in bone density in female musk turtles, Sternothaerus odoratus (Latreille). Comparative Biochemistry and Physiology 1, 213-217.

Esque, T. C. & Peters, E. L. 1994. Ingestion of bones, stones, and soil by desert tortoises. Fish and Wildlife Research 13, 105-111.

Fledelius, B., Jørgensen, G. W., Jensen, H. E. & Brimer, L. 2005. Influence of the calcium content of the diet offered to leopard tortoises (Geochelone pardalis). The Veterinary Record 156, 831-835.

Kramer, D. C. 1973. Geophagy in Terrapene omata Agas­siz. Journal of Herpetology 7, 138-139.

Loveridge, A. & Williams, E. 1957. Revision of the African tortoises and turtles of the suborder Cryptodira. Bulletin of the Museum of Comparative Zoology 115, 163-557.

Marlow, R. W. & Tollestrup, K. 1982. Mining and exploitation of natural mineral deposits by the desert tortoise, Gopherus agassizii. Animal Behaviour 30, 475-478.