In October 2019, Friends of the Island Fox funded a research study, via a donation from Safari West, to investigate the use of cementum analysis as a method to determine island fox age at death. The current standard method for estimating age in island foxes is based on the wear of the upper first molar. This method is imprecise as tooth wear is heavily influenced by diet and varies significantly between islands and even among habitats on the same island.
Knowing the age at death is very valuable as it allows the National Park and other island managers to track the island fox lifespan. This is important as a fox who dies at age 9 or 10, after reproducing for many years, has contributed to the survival of the species. A fox that dies at 3 or 4 may not have had a chance to replace itself in the population or pass on its genes.
|Image 1 - see sources below|
To determine the age of an island fox after it has died, the lower canine tooth is removed and sent to the lab for aging.
|Photo provided by S. Baker 2019|
This makes the tooth very soft and pliable. The tooth is then preserved by fixing it in formaldehyde, also known as formalin, to preserve proteins and vital structures within the tooth. Next, it is embedded in a paraffin wax block which provides a support medium to make it easier to cut thin slices for examination.
The paraffin embedded tooth is cut into sections, approximately 15 microns thick (about the size of a droplet of mist or fog) on a device called a microtome, which is very similar to a deli slicer. The thin sections are mounted on glass microscope slides as the final step prior to analysis.
|Paraffin embedded sample on a microtome being sectioned, courtesy M. Watling 2019|
The picture of cementum annuli in the photo below indicates the growth rings with the black arrows.
|Image 4 - see sources below|
If it is possible to accurately determine a fox's age at time of death, it can be used to monitor average longevity and to investigate many questions across all six of the Channel Island. Questions such as:
- What age group of foxes are most likely to be hit by cars?
- How old were foxes collected during annual mortality monitoring?
- Do males or females have a greater lifespan?
- Mike Watling, FIF Advisory Committee
- Image 1 - The chemical and microbial degradation of bones and teeth. Advance in human palaeopathology. John Wiley & Sons - Scientific Figure on ResearchGate. Available from https://www.researchgate.net/figure/Simplified-cross-section-of-a-tooth-incisor-and-jaw_fig4_292711354 (accessed 17 Dec., 2019)
- Image 4 - Tochigi, Kahoko & Aoki, Yukino & Maruyama, Tetsuya & Yamazaki, Koji & Kozakai, Chinatsu & Naganuma, Tomoko & Inagaki, Akino & Masaki, Takashi & Koike, Sinsuke. (2019). Does hard mast production affect patterns of cementum annuli formation in premolar teeth of Asian black bears (Ursus thebetanus)? PLOS ONE. 14.e0211561. 101371/journal.pone.0211561