Beyond leopards: tooth marks and the contribution of multiple carnivore taxa to the accumulation of the Swartkrans Member 3 fossil assemblage

1. What they did

Leopards have long been seen as major bone collectors in the South African caves where early human fossils are found. Bob Brain proposed that leopards brought in most of the small and medium-sized animals (Size Classes 1 and 2, 4.5 to 104 kg) at Swartkrans, while larger cats such as sabretooths may have brought in the larger animals. Later, de Ruiter and Berger (2000), after finding an eland in one modern leopard den and a zebra in another, suggested that leopards alone could explain most of the bones in these caves, whatever the size of the animal.

The authors tested this idea with direct evidence from the fossils themselves: the tooth marks carnivores left on the bones.

• The sample. From the Swartkrans Member 3 deposit (about 1.0 million years old), they re-analysed 1466 bone specimens: all ungulate limb bone shaft pieces at least 5 cm long, plus smaller ones with surface marks. Of these, 532 carried tooth marks.
• Measuring tooth pits. They chose 25 tooth-marked specimens at random and made silicone molds of 70 tooth pits (small round dents left by teeth). They measured the greatest length and breadth of each pit on the molds with a precision caliper. Molds protect the fossils, and the pits often show more clearly as bumps on a mold than as shallow dents on the bone.
• Comparing. They compared the fossil pits, split by the size of the animal (small and large), with tooth pits made on the shafts of limb bones by modern carnivores of known identity in two earlier studies.

2. What they found

• Across all 70 pits, the average length was 2.59 mm and the average breadth 1.98 mm. These are larger than the pits made by small canids (members of the dog family), cheetahs and leopards, and fall within the range of Selvaggio’s modern spotted hyena sample.
• Bones of small animals (46 pits): average length 2.15 mm and breadth 1.70 mm. These match carnivores with less robust teeth, and compare most closely with the leopard sample.
• Bones of large animals (24 pits): average length 3.43 mm and breadth 2.53 mm. These match the large dogs, spotted hyenas and lions of the modern samples, not leopards.
• The fossil pits were well below the averages for spotted hyenas and lions in the Domínguez-Rodrigo and Piqueras sample, so another carnivore may also have been involved. The authors suggest brown hyenas, which have slightly less robust teeth than spotted hyenas and of which a few are in Member 3. Some large animal bones may also have been chewed by extinct carnivores, for which there are no good modern data.
• Identifying the carnivore from pits on the dense bone of limb shafts is less certain than from pits on the spongier bone of the limb ends.
• Hominids left cutmarks and percussion marks on bones of all size classes (105 specimens), so they used a wide range of carcasses, unlike the carnivores, which appear to have divided carcasses by size.
• Modern leopard feeding remains show that leopards do leave tooth marks on limb bone shafts while removing meat, mostly near the bone ends, though at low frequencies. So if leopards had been as active with large carcasses as with small ones, the authors would expect to see a leopard signal on the large animal bones too.

3. What they concluded

• Leopards probably did contribute significantly to the small animal remains in Swartkrans Member 3, but they were not major contributors of the large animal remains.
• The large animal bones were probably collected by carnivores able to modify bone more intensely than leopards, including large canids, spotted hyenas and lions, and possibly extinct carnivores.
• The results falsify the idea that leopards alone collected most of the bones, and support Brain’s original view: leopards specialised on smaller prey and other predators on larger prey.
• Evidence of what modern leopards can kill and carry is not enough on its own to explain fossil assemblages. Direct fossil evidence, such as tooth marks, is a better guide, and each cave deposit needs to be judged on its own evidence rather than by one model for all.

4. How tracking was or was not used

No tracking in the field. The study is a form of sign interpretation: it reads feeding signs (tooth pits) on fossil bones to identify which kinds of carnivores chewed them, using the size of the pits measured on molds and compared with tooth pits of known origin.

6. Other technologies, skills or innovations

• Silicone (polyvinylsiloxane) molds of tooth pits, measured with a high precision caliper instead of touching the fossils.