Thursday, March 15, 2018

Rodents gnaw bones

Rats eat dead bodies; be it human or another organism. These animals are mostly plant eaters, but mice and rats have evolved to ingest human waste, meat, fat, and including bones. This behavior is peculiar and important to us, those that study bones from the fossil or archaeological record.

Figure 1. Extensive gnawing by gray squirrels Sciurus carolinensis. Note the parallel, fan-shaped grooves.

Many organisms are instantly attracted to human waste and trash. Also to animal and human burials. It seems that rodents are attracted to the bones the most, which they gnaw and chew to acquire calcium salts and other nutrients. Gnawing also helps them file their ever-growing teeth.

This physical activity leaves physical evidence: characteristic, almost unmistakable, double grooved, fan-shaped marks on the objects they gnaw. For paleontologists, archaeologist, and forensic anthropologists, these marks are important and diagnostic of burial conditions, timing, and environment.

Figure 2. Gnaw-marks made by the Brown rat Rattus norvegicus. Note and compare to figure 1.
The grooves made by mice and rats are much straighter, and closely packed, with a smaller width.

Research during the past decades has shown that these marks are tell-tell signs of exposure and scavenger activity. A cadaver that is exposed, or unburied, attracts dogs, raccoons, and rodents, which eat, chew, and gnaw the parts that are exposed, available, or that are most attractive to them, leaving their markings behind. Several of them, like the African porcupine, take bones back to their burrow, where over the years, a collection or cache of bones builds up. These animals are modifiers, and their modifications can help determine how long those remains were unburied and who had access to scatter them.

In the case of rats, it had been assumed that they were most attracted to old, dry bones. But evidence from human and animal cadavers had been contradictory. A recent article, “Rodents as Taphonomic Agents” by Walter E. Klippel and Jennifer A. Syntelien, discuss this very issue. They found that in fact, rats are most attracted to fresh bones, especially those that still preserve yellow marrow, and fat. Remains with these characteristics are usually less than 30 months old, even under direct exposure to the elements and other modifiers. Their experiment showed that rats and canids (dog family), preferred fresh remains; usually, those that were less than a year old.

On the other hand, experiments on body farms, show that other rodents like squirrels do prefer older, dry bones, that have been exposed over several years. This is important because it can provide a confirmable timetable to estimate time of deposition and exposure for remains that bear these markings. For forensic anthropologists, the identification of these marks (along with other indicators such as insects and vegetation) can provide a time-since death, in the case of human cadavers resulting from accidents or homicides. For paleontologists and archaeologists like me, they can provide evidence of a nearby scavenger fauna, or proximity of several scavengers to human dwellings, and the approximate time of burial or exposure for those remains. Overall, providing much more information that can be gleaned from the bones alone.

Figure 3. Rodent marks made by a Cuban hutia (likely the large Capromys sp.), on a extinct hutia's femur shaft.
Note that there are characteristic, smaller, perpendicular striations.

We have found similar markings in Cuban archaeological deposits, but these are not referable to rats or squirrels since these rodents are not native to the island. However, they are referable, based on size, to the Cuban native hutias; rodents of the family Capromyidae, with several endemic species on the island. These markings suggest that hutias also were attracted to indocuban refuse, where they could gnaw on bones. The variation in the size of the marks also suggests that more than one species gnawed on refuse bone-remains that Cuban Indians discarded. Our evidence indicates that capromyid rodents were the most important bone dispersers and modifiers on the archaeological and paleontological deposits of the island much before Columbus rediscovered the New World (see Orihuela, Jimenez and Garcell, 2016).

Figure 4. Rodent gnaw marks on the distal end of an extinct hutia's tibia.
These smaller marks were likely made by a medium sized hutia, spiny rat
from Cuba's extinct fauna.

But how quickly did they seek out the bones? Or for how long were the remains available for these rodents? What role did the introduction of domesticated dogs affect these natural processes? We do not have concrete answers for these questions yet, but we have research on the way that can help clarify some of these issues and their importance in the study of the past and for historical sciences.

Stay tuned to find out!

Here is a brief bibliography for those that would like to read these interesting articles:

Fisher, J. W. (1995). Bone surface modifications in zooarchaeology. Journal of Archaeological Methods and Theory 2(1): 7-68.

Haglund, W. D. (1992). Contributions of rodents to postmortem artifacts of bone and soft tissue. Journal of Forensic Sciences 37:1459-1465.

Haglund, W. D., D. T. Ready, y D. R. Swindler (1988). Tooth mark artifacts and survival of bones in animal scavenged human skeletons. Journal of Forensic Sciences 33: 985-997.

Klippel, Walter E., y Jennifer A. Synstelien (2007). Rodents as taphonomic agents: Bone gnawing by brown rats and grey squirrels. Journal of Forensic Sciences, 52(4):765-773.

Orihuela, J., O. Jimenez Vazquez, and Jorge F. Garcell (2016). Modificaciones tafonomicas bioticas en restos oseos de depositos arqueologicos y paleontologicos en las provincias de Mayabeque y Matanzas, Cuba. Cuba Arqueologica.

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