“A tiger enrichment program was used to document actual bone damage unequivocally caused by claws.” As soon as I read that line in the abstract of this new study, I was hooked. An image hovering over the abstract pulled me in further. Embedded inside a piece of wood is a piece of bone. Reads the caption, “Figure 1. Bovid femur bolted in log, accessible to paws, but not jaws.” All this in the name of paleontology.
Fossil bones are rarely found pristine and intact. In the early days of their afterlives, bones can be burrowed into by insects, tumbled along stream beds, trampled, and otherwise come to bear the marks of postmortem history under the banner of what researchers call “bone modifications.” Sifting through these clues is part of the science of taphonomy – reconstructing how an organism died and what happened afterwards – and among the common clues that paleontologists regularly puzzle over are gouges, scrapes, and pits that seem to record bite marks. But when is such damage a clue of a prehistoric bite and when is it an indicator of scraping claws? This question is what led paleopathologist Bruce Rothschild and coauthors to devise bony toys for tigers.
After carefully removing flesh from cow femora – thigh bones – so as not to damage the thin, surrounding membrane called the periosteum, researchers bolted one bone at a time inside a piece of wood. This allowed the tiger at Wichita, Kansas’ Sedgwick County Zoo to reach inside and paw the bone, but not mouth the scientific plaything. True to feline nature, the big cat was quite curious about the object. “The tiger expressed major interest in the object, pawing at the bone in the log,” Rothschild and colleagues write, “but was unable remove it from its bolted location.”
The tiger’s claws scratched the bone in at least four places. Each shallow rut created by the tiger “appeared marred and lacked periosteal covering compared to unscratched (control) regions” of the bone, the researchers report. Microscopic examination confirmed that the tiger’s claws punched through the outer periosteum to leave shallow scratches on the actual bone beneath.
Despite being composed of a material softer than bone, the tiger’s claws were able to slightly damage the cow femur. That’s a simple finding, but one that suggests that paleontologists should be on the lookout for claw scratches as they pore over fossil bones.
Frustratingly, however, the PLoS One study doesn’t go beyond a simple experiment to demonstrate that claws can damage bone tissue. The authors provide no details about the tiger’s behavior during the time the scratches were created, nor do they attempt to categorize what might separate a prehistoric claw mark from a bite wound or other damage. Likewise, I’m quite curious to know how claw scrapes created by tigers differ from those left by other cats, birds of prey, and other clawed vertebrates, as well as how scratches created by pawing differ from claw damage on bones that big cats grasp and gnaw. Documenting that claws can scratch skeletons is only a bare bones beginning to a thread of interrogation that may help us better understand the postmortem fates of creatures that perished so very long ago.
Rothschild, B. Bryant, B., Hubbard, C., Tuxhorn, K., Kilgore, G., et al. 2013. The power of the claw. PLoS ONE 8, 9: e73811. doi:10.1371/journal.pone.0073811