Goat Paper: An Actualistic Taphonomy Experiment
Taphonomy is the study of decomposition and fossilization, and its processes play a large role, from shaping fossils to preservation quality of organisms and final placement of fossils within the rock strata. The feeding behaviors of predators often result in skeletal dislocation, one reason for the lack of full skeletons within the fossil record (Oliver and Graham 1994). Bone dislocation via scavenging is caused by both scavenger birds such as vultures as well as mammals such as foxes, dogs and wolves. Larger mammals tend to target the hind legs first, presumably because the back of an animal possesses more meat, and possibly is easier to drag away (Oliver and Graham 1994).
We aim to see this dislocation in action as well as other natural processes, such as decay, that occur when carcasses are left in the open. We will leave three goats (genus Capra) uncovered on the ground for nine weeks (January 27th to April 1st). Two goats will remain inside a wire cage to prevent them from being dragged off, while the third will remain out in the open with no protection. In this way we will see the difference between decay untouched by predation versus that left vulnerable to predation. We expect to see the goats in the cage to decay with the bones left in situ, and disturbances to the goat caracass left in the open.
The Kohl Family Farm donated three goats, two adults and one kid, for this experiment. Prior to our experiment setup a larger predator had scavenged one of the goats, dragging away its legs and rump and leaving its stomach, head, and forelegs. The kid and the fully intact goat were placed in a wire cage behind the compost heap at Washington & Lee University. The spaces of the cage wiring were too small for most scavengers to enter and stopped the goats from being carried off, enabling the bones to stay as intact as possible while still being exposed to other taphonomic processes, such as insect predation and the weather. Boards covered two sides of the cage to hide it from public view. The mutilated goat was placed out in the open for maximum exposure to predators. In order to catch the predators in action, a trap camera was set up. Once a week observations were made on the state of the goats’ decomposition. The observations became more frequent as the weather warmed and decomposition became faster, becoming once a day during the final week.
The motion camera captured evidence of predation on the goat that had been left exposed: significant decay on its open entrails was also observed (figures 2 and 3). A fox and two vultures were seen actively feeding on the carcass, while an opossum, a skunk, a dog, and a rat were seen to inspect it. The opossum appeared twice. The first time (February 2nd) it had not eaten any of the corpse, which was frozen due to it being 26 degrees Fahrenheit. The second time the opossum appeared in March, and whether or not it fed on the corpse is unclear from observations and the trap camera.
On March 15th the fox dragged the goat carcass about five yards from its original starting point, leaving behind the stomach and other internal organs. The goat’s ribs and spine were clearly visible, and the cheekbone was exposed partially. The goat’s head up to the eye-socket and jaw remained untouched. Black vultures also scavenged the carcass. By March 24th the exposed goat had disappeared from the near vicinity and was no longer in sight, so it is possible that the fox dragged it further away. Aside from the activity of insects such as flies and their maggots, the goat carcasses within the cage decayed slowly. The observations demonstrated little change in outward appearance from February to April, and no movement from their original positions.
Our experiment demonstrates effectively the importance of scavenger activity in taphonomy and its role within the creation of the fossil record. The goats protected from predation remained in the position that they were placed in. The freezing conditions in February and early March may have slowed down decay, but it had no effect on the position of the goats. Because of their relative protection from the elements and from scavengers, the corpses could decompose in situ. The fact that the goat chosen for exposure had been preyed upon before our experiment began shows the swiftness of scavengers, and thus how quickly a carcass must be buried before dislocation happens. This original scavenger was large and strong enough to tear off the hind legs of the goat and drag it off. However, a scavenger does not have to be a wolf or large dog to drag corpses; smaller scavengers such as foxes can drag the front legs and head of an adult goat a distance of five yards or more. The fox further separated the forelegs and head of the goat from its entrails, probably because the stomach still contained undigested feed. This however would not usually show up in the fossil record due to the extremely rare preservation of soft body parts. The vultures ate on the site of the corpse and thus did not contribute further to skeleton dislocation except for some possible bone breakage or slight disarticulation of the skeleton. The vultures did not appear until the unknown scavenger and the fox had fed, probably because the corpse was not sufficiently opened for them to feed. Thus the animals that most dislocate animal bones are the larger mammals, whereas birds mostly scavenge on site and may cause small dislocations. However this study suggests that vultures are not the dominant scavengers, and therefore their effects on the fossil record are smaller than that of larger predators. Larger scavengers create the most dislocation.
This experiment demonstrated how the taphonomic process of scavenging effects the preservation of the fossil record, primarily that some gaps in fossil record are based on processes from animals. For example the rump area of the goat had already been scavenged before our experiment began, and the rest of the goat was dislocated only seven weeks after exposure. It is unlikely in nature that carcasses would remain where scavengers could not reach, thus, unless a corpse is buried quickly, it will disappear from its death area.
In order to learn more about surface taphonomic processes, it would be conducive to explore the effects of climate and flowing bodies of water. Observing animal carcasses placed in or near a creek or river would help us quantify the effects of rain runoff and predation on the movement of corpses or their bones. Using carcasses of different sizes, such as a squirrel vs. a goat or cow, would also better allow us to see whether size of the corpse effects whether the predator carries away the full carcass or only a portion of it. Furthering the current research in these manners may enable paleontologists to predict which taphonomic processes dislocated the bones of a given species, and what most affects this dislocation.
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Oliver, J. S., & Graham, R. W. (1994). A catastrophic kill of ice-trapped coots: time-averaged versus scavenger-specific disarticulation patterns. Paleobiology, 229-244.