Treasure of the month

July 2017

A deer skull - gnawed by a squirrel

The treasure of the month July is a deer skull. A Hamburg citizen found it in October 2016 in a forest strip near Bergedorf and passed it on to the mammal department of the LIB. The find is unusual. So are the gnaw marks.

Who has ever found a deer skull in the forest? It's rare, because the remains of dead animals are the natural food source for numerous other animals, fungi and bacteria. While the energy-rich soft tissues are mostly quickly consumed, bones can remain lying on the surface for up to several years. In most cases, the organisms known as destructives, which include countless species of fungi and bacteria, insects and vertebrates, are in fierce competition with each other and colonize an animal carcass in a fixed sequence, the cadaveric succession. A dead vertebrate can thus be understood as an ecosystem in its own right. The speed and duration of this succession, including the number of generations that develop, is highly dependent on local conditions, especially temperature and humidity.

When all soft tissues are finally digested, bones are attractive to only a few species, but bone is not all the same. Bone marrow serves as a fat store for mammals, filling cavities in the bone. These fats are often still attractive food years later, provided the bones have not been defatted by sunlight and heat. Rats are known to readily exploit this food source, using their nail teeth to penetrate through the compact outer layer of bone into the interior.

However, the brain skull of a deer is not very attractive as a source of fat, because it is mainly composed of compact bone tissue. Therefore, there must be another motivation for a rodent to work the hard outer bone tissue with great force and energy with its teeth. The pure gnawing instinct and the need to wear down the permanently growing incisors is one possibility. But it could also be the mineral content, especially the phosphate content of the bone, or simply the nutritional value of collagen, a protein that dissipates tensile forces.

Collagen is very stable and can still be detected for a very long time, even in weathered bone. Such nail behavior on fat-free bone is known, for example, from squirrels. Even if we don't know which rodent has finally "damaged" our deer skull in such a way, it is a rarely found document for a scientifically still hardly investigated phenomenon, the consumption of bones (osteophagy) by rodents.

We would like to know more about this phenomenon, because there are interesting applications of this knowledge. In forensics, rodents can contribute to the delimitation of lying times of a corpse by their behavior. Paleobiology also uses corpse succession as an indicator for reconstructing fossil habitats and their ecological variables.