Sources of Variation in Carnivore Tooth-Mark Frequencies in a Modern Spotted Hyena (Crocuta crocuta) Den Assemblage, Amboseli Park, Kenya

1. What they did

Archaeologists use carnivore tooth marks on broken limb bones to work out whether early humans or carnivores got to a carcass first. If carnivores ate first, the middle shafts of the limb bones tend to be heavily tooth-marked; if humans stripped the meat and marrow first, carnivores leave fewer marks there. But the percentages of tooth-marked bones vary widely even among experiments where the order of feeding is known, which has led some researchers to question the method. The author set out to find the sources of that variation.

The author studied the bones collected in 1975 from the surface of the Amboseli Airstrip Den, a natal and communal spotted hyena den used for many generations. At the time, a group of 15 spotted hyenas lived there, so digging was not allowed. The collection is dominated by wildebeest and zebra, followed by Grant’s and Thomson’s gazelles.

• The sample. All mammal limb bone pieces with little weathering (732 specimens), so that tooth marks were not hidden by cracking or flaking.
• What was recorded. For each piece: species, body size class, which bone, which part of the bone (end, shaft near the end, or middle shaft), maximum length, and whether and where tooth marks were present. Each piece was checked under strong light with a hand lens, following published guidelines for identifying tooth marks.
• What was compared. Tooth-mark frequencies against fragment length, prey body size, the different bones and their parts, and published bone density values.

2. What they found

Fragment size

• Longer pieces were more likely to be tooth-marked. The share of marked pieces rose from 14.3% for pieces up to 20 mm long to about 95% for pieces 80 to 100 mm long. Overall, 71.0% of pieces were marked.

Prey body size

• Middle shafts of larger (size 3) mammals were marked more often than those of smaller (size 2) mammals: 75.3% versus 63.4%, a significant difference.
• But the larger animals’ bone pieces were also longer (64.1 mm on average, against 45.1 mm). When only pieces of similar length (20 to 60 mm) were compared, the difference almost disappeared (59.0% versus 57.7%, not significant).

Different bones and parts of bones

• Tooth marks were not spread evenly across the different bones. There were more marked tibia shafts near the knee, humerus shafts and ends near the elbow, and radius middle shafts than expected, and fewer marked humerus middle shafts.
• Bone ends varied hugely, from 15.4% to 100% marked depending on the bone. Shafts near the ends varied from 45.8% to 75.0%.
• Middle shafts were much more consistent, from 71.6% to 82.4% across all bones.

Bone density

• For bone ends, the less dense the bone, the more often the surviving pieces were tooth-marked (a significant negative relationship).
• Shafts near the ends showed a weaker negative relationship that was not significant.
• Middle shafts showed no significant relationship with bone density.

3. What they concluded

• Tooth-mark frequencies depend on fragment size, on the mix of bones and bone parts in an assemblage, and, for ends and near-end shafts, on bone density. These come from carnivore behaviour and the physical properties of bone, so they are likely to apply to bone collections made by other bone-crunching carnivores, and to archaeological sites.
• Anything that breaks bones into smaller pieces (trampling, sediment pressure, weathering), not only carnivores, can lower the share of tooth-marked pieces. This makes it risky to compare simple experiments with archaeological sites that have been through many destructive processes.
• Because carnivores eat the least dense parts first, the pieces that survive from those parts carry more tooth marks. Processes that destroy low-density bone at a site can remove exactly the most heavily marked pieces.
• The author suggests three corrections: compare pieces of similar length; analyse each bone and bone part separately; and, given density effects, rely mainly on dense middle shafts, which show no link with density and are consistent across bones.
• These variables are not the full story, and more research is needed to find other sources of variation.