Hyaenas as taphonomic agents
Taphonomic agents are collectively responsible for the creation of certain fossil assemblages, see the article on taphonomy. Hyaenas are among the agents of accumulation and modification of such assemblages.
Raymond Dart initially interpreted the fossil remains at an australopithecine site at Makapan as bone fragments that were collected and used by australopithecines. This was later to become known as the "osteodontokeratic culture", due to the hypothesised use of tooth, bone and horn fragments, by australopithecines as tools and weapons. Dart supported his hypothesis by saying that:
- hyaenas are not bone collectors;
- certain skeletal elements were more abundant, indicating selection by australopithecines;
- the biased faunal representation was an indicator of hominid hunting preferences; and
- some of the breakage patterns observed, such as spiral fractures, could only be produced by australopithecines.
Which hyaenas are involved?
The family Hyaenidae is made up of four extant species; the striped hyaena (Hyaena hyaena), brown hyaena (Parahyaena brunnea), spotted hyaena (Crocuta crocuta) and the aardwolf (Proteles cristatus). The aardwolf is specialised to a diet of ants and termites, so will not be mentioned again.
The three remaining species, as well as extinct species were all originally thought to fill the same ecological niche, sharing behaviour and feeding habits. This was shown to not be the case. Spotted hyaenas are now known to be active hunters, preying mostly on medium to large-sized ungulates . Spotted hyaenas predominantly feed on fresh meat, seldom taking carcasses back to dens, unless to avoid interference competition, which juveniles have been observed doing.
Hyaenas as bone accumulators
Bone accumulations are characteristic of the breeding dens of scavenging hyaenas, as spotted hyaenas do not take carcasses back to their dens as frequently. The bone assemblages of spotted hyaenas have been examined in the past . These dens have contained the remains from a wide variety of species, mostly the long bones and skulls of medium to large ungulates, as well as higher proportion of unidentifiable bone chips than is present in den accumulations of either of the scavenging hyaenas. this greater abundance has been thought to be due to the greater bite force of the spotted hyaena, but the bite force of the brown and striped hyaena has yet to be measured accurately.
Spotted hyaenas digest large bones, seldom passing fragments in their scat. They do however sometimes regurgitate bone fragments, as well as hair and hooves in oral casts. Brown and striped hyaenas also swallow bone fragments, which are either completely digested or passed, and can be found in the scats or coprolites.
Skinner & van Aarde and Skinner et al. found that assemblages of spotted hyaenas were generally smaller than those of brown hyaenas, both in number and variety of bones present. The remains of spotted hyaena collections seldom had more than a few hundred bones in them, while those of brown hyaenas were in the thousands.
Hyaenas as modifiers of bone
In 1980, Brain categorised damage to bones by hyaenas into two types; premolar cracking of small, more manageable bones, and incisor/ canine gnawing on larger, uncrackable bones.
Maguire et al. (1980) independently compiled a list of the several types of marks and modifications to bone that hyaenas are capable of. These types of damage are however not unique to hyaenas, but all nine types have been found in accumulations of the three species of bone accumulators, but in differing proportions:
- Ragged-edge chewing: corresponding to Brain's incisor/ canine gnawing, but broadened to include smaller bones as well.
- Shallow pitting: thought that this type of damage was caused by the teeth of juveniles, but not known in 1980 if this was the exclusive cause.
- Punctate depressions or perforations: caused by the premolars of adult hyaenas. Sometimes a puncture mark is only found on a single surface of a bone.
- Lunate or crescent-shaped fracture scars: also produced by the premolars, when the bite strength is sufficient to split the bone, so that the fracture line passes through the perforation.
- Striations or gauge marks: usually short, straight and roughly perpendicular to the to the long axis of the bone.  suggested that these striations may be caused by juvenile animals.
- Continuous or close, irregular and randomly-orientated grooves: appear to be the result of more persistent gnawing, than in the case of striations, and are most commonly found on the ends of long limb bones. Theses grooves also tend to be longer and confined to a smaller region of the bone than striations.
- Scooping or hollowing out: cancellous bone is removed from the ends of long bones by the incisors.
- Acid-etching and erosion: caused by bones being swallowed and exposed to gastric acids. Found only in regurgitated and fragments that have passed right through the alimentary canal, and been passed.
- Splintering or shatter-cracking (comminuted fractures): usually encountered as jagged transverse breaks across the shafts of limb bones.
Distinguishing hyaena accumulations from those of other taxa
Hominid vs. Hyaena
- Carnivore-ungulate ratio: in hyaena accumulations, the ratio of carnivore plus ungulate remains is at least 20%, while in hominid accumulations it is usually less than 10%.
- Damage to bone surfaces: damage distinctive to hyaenas (listed and described above) is common to modern assemblages, occurring on at least 50% of the bones. These damage patterns are less commonly found in fossil assemblages, possibly due to weathering of the bone surfaces prior to fossilisation. Therefore a low percentage of carnivore-damaged bones cannot be used to exclude hyaenas as the bone accumulator.
- Bone breakage: hyaena accumulations can characterised by an abundance of "bone cylinders", long bones which have had their epiphyses eaten out or removed. In contrast hominid accumulations will tend to have long bones with the epiphyses intact, but with broken shafts.
- Cranial-postcranial element ratio: the ratio of cranial-postcranial ratio tends to decrease with ungulate size in hyaena accumulations. Postcranial elements tend to from the adults of larger ungulates, while cranial material more commonly belongs to juveniles of the larger ungulates. No such trend is apparent in hominid accumulations.
- Representation of small, hard bones: these bones are uncommon in hyaena accumulations, but are common in hominid accumulations and are superabundant in very fragmented samples.
- Age profiles: age (mortality) profiles tend to be attritional in hyaena accumulations, and may be either attritional or catastrophic in hominid accumulations, depending on how the animal was obtained.
- Abundance of horns and horn-cores:
Three of the criteria established by Cruz-Uribe and that of Stiner were discarded by Pickering as ways of distinguishing between hyaena and hominid collections. These criteria may still be useful in trying to distinguish between collections made by different hyaena species.
The criteria discarded by Pickering's with his reasons were as follows:
- Cranial-postcranial element ratio:
- Representation of small, hard bones:
- Age profiles:
- Abundance of horns and horn-cores:
Later work by Kuhn
Leopard vs. Hyaena
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