A guide for differentiating mammalian carnivore taxa responsible for gnaw damage to herbivore limb bones

1. What they did

During the late Pleistocene (the last Ice Age), North America had more kinds of large carnivores than today, including an enormous bear with no modern counterpart (Arctodus), a lion larger than modern African lions (Panthera leo atrox) and a wolf with massive, hyena-like teeth (Canis dirus). Their bones are often missing from fossil collections, but the marks their teeth left on the bones of their prey may still be there. Gary Haynes set out to describe the gnaw damage that each group of large carnivores leaves, so that it can be recognised in fossil bone collections.

• Feeding experiments. Captive bears, wolves, large cats and hyenas in three zoological parks were fed fresh limbs or limb bones of domestic cattle, and the author recorded the sequence of damage, how long it took, how long the animals stayed interested in the bones, and what happened when several animals gnawed.
• Field studies. He studied wild wolves, bears and their prey in northern North America, lions and their prey in southern Africa, and hyenas and the carcasses they killed or scavenged in Africa, following fresh carcasses and skeletons over time.
• The sample. He looked only at the thigh bones (femora) and shin bones (tibiae) of herbivores weighing 300 kg or more, such as cattle, bison, moose, elephant, African buffalo and giraffe. Table 1 lists the bones examined for each carnivore: 205 for wolves, 65 for spotted hyenas, 23 for lions, 13 for brown bears, and between 2 and 7 for black, polar, sun and spectacled bears, tigers and jaguars.
• Fossils. He then compared this with fewer than 100 late Pleistocene bison and moose limb bones from Alaska and the Yukon.

2. What they found

General patterns

• Large carnivores use carcasses in surprisingly patterned, predictable sequences. The author grouped the damage into light, medium and heavy stages. Differences between carnivores are easiest to see at the medium and heavy stages.
• Light gnawing by any carnivore can look the same: bones lightly gnawed by lions can never be told apart from bones lightly gnawed by bears or any other carnivore.
• Teeth leave pits when they bite down and furrows, scratches or score marks when they move. Unlike a true tool cut, a tooth groove is seldom smooth and flat-walled; it has uneven edges and breaks unevenly through the layers of bone.

Spotted hyenas

• Femur: early on, scrapes and dents about 1 x 2 cm, the size of hyena cheek teeth. With more use, the greater trochanter (the knob at the top of the thigh bone) is half removed, and the shaft is ground, crushed and pulled apart piece by piece. In the end less than a third of the shaft may remain, heavily tooth-marked with well-rounded edges.
• Impressions in the spongy bone inside are cone-shaped and round-bottomed, about 3 to 5 mm wide and deep for adults.
• Tibia: the top end is gouged out, then removed; shaft fragments may be up to 5 cm long or longer, with heavy scoring across the shaft.
• A femur taken to a den or fed to a captive hyena may reach the final stage within an hour.

Wolves

• Femur: the cheek teeth expose the spongy bone at the ends. The stump of the greater trochanter may keep a rim of compact bone about 5 mm high around the spongy bone.
• In the final stages the shaft survives as a hollow cylinder, with scratches up to 3 cm long or longer, 1 mm deep and 2 to 3 mm wide, mostly across or diagonal to the shaft and most common near the ends.
• Wolves mark compact bone less heavily than hyenas, except at dens and homesites, where they spend much more time chewing bones.
• Tibia: the front crest at the top is removed, with clear furrows in the spongy bone.

Bears

• Most bears do not gnaw heavily on bones once the soft tissue has dried or gone, although individuals and species differ widely.
• Their broad cheek teeth grind down and crush bone, leaving a flattened or faceted stump and a “mashed” look, rather than biting through it. A compact rim higher than the spongy bone is rare.
• Fewer than 5 pits, about 6 mm deep and 10 to 20 mm long, each about the size of one large bear cheek tooth.
• There may be no tooth marks on the shaft at all; the few that occur are short sets of parallel scrapes (seldom wider than 1.5 mm or longer than 9 mm) that look like rodent gnawing.
• Black and brown bears seldom badly damage tibiae of large prey such as bison unless wolves have first removed the ends.

Large cats (lions, tigers, jaguars)

• Large cats seldom keep gnawing on large bones.
• They bite off the greater trochanter, bite under the femoral head, and scrape off the trochlear rims with the carnassials, leaving a few deep grooves from single tooth cusps, usually larger and fewer than those of hyenas or wolves.
• The key sign is rough, irregular marking from biting into or through the spongy bone of the ends: wide, deep marks that can be counted. Lion cubs may leave a rim like that left by adult wolves.

Comparison table (Table 2)

Each group is scored from 1 to 5 on three features: tooth marks on compact bone (hyenas most, then canids, bears and cats least); grinding off versus biting through bony knobs (bears grind, cats bite through, canids and hyenas in between); and the shape of tooth impressions in spongy bone (bears square or rectangular, canids and hyenas cone-shaped, cats an “axe-edge” or elongated V shape).

Fossil bones

• Much of the gnaw damage on Pleistocene bison bones from Alaska and the Yukon closely matches the damage modern northern wolves do to modern bison. A few bison and moose bones show brown bear damage. The author saw no damage that clearly came from large cats, but notes his samples from any one place were small.
• Bones from the Bluefish Caves archaeological site in the Yukon, about 13,000 to 14,000 years old, show a “kennel” gnawing pattern, which the author links to domestic dogs gnawing at homesites.

3. What they concluded

• When these types of damage appear on bones, they can be confidently assigned to particular groups of carnivores. However, they need not always occur, because how much a carcass is used depends on local conditions of hunting and feeding.
• Wolf-type damage on the fossil bison bones suggests that local prey provided enough food for predators, that scavengers were fairly uncommon, and that predator and prey relationships, at least between bison and wolves, were probably much like today’s.
• The author suggests (and calls it only his opinion) that lion gnaw damage should still be recognisable in Pleistocene collections. Examining large samples of Pleistocene bison limb bones could support or contradict another author’s idea that Panthera leo atrox and Arctodus were the main predators of bison in Beringia.
• How many prey bones show gnaw damage depends on prey vulnerability, feeding group size and other factors. Because several carnivores often feed from the same carcasses (lions, leopards, cheetahs and hyenas in Africa; bears and wolves in North America), bone collections can only be interpreted fully with very detailed taphonomic observations.

4. How tracking was or was not used

The study is about reading one kind of sign: the marks carnivore teeth leave on bones. The author identified gnaw damage from known carnivores in captivity and on carcasses in the wild, and then used those signs to infer which carnivores had fed on fossil bones. No trailing or footprints were used.

5. Tracker or observer certification

Not reported. All observations were made by the author. In the zoo feedings the carnivore was known; for wild carcasses the paper does not say how the carnivore responsible was confirmed.

6. Other technologies, skills or innovations

• Controlled feeding experiments with captive carnivores, recording the sequence of damage over time.
• Monitoring known carcasses in the wild as they were eaten and weathered.
• A simple scoring table (Table 2) comparing canids, hyenas, bears and large cats on three damage features.