Estimating Island Fox Age Using Cementum Rings In Teeth

Slides of tooth specimens - S. Baker

In October 2019, Friends of the Island Fox funded a research study by Stacy Baker and Juliann Schamel, via a donation from Safari West, to investigate the use of cementum analysis as a method to determine island fox age at death. The results are in and the answer is, well mostly yes, but...

To conduct the analysis 31 teeth from island foxes, whose age was known when they died, were sent off for testing. Of the 31 samples, 28 were aged accurately to within 1 year of their known age, two teeth were underestimated by 3 years and one was underestimated by 4 years. This underestimation primarily occurred with older foxes.

plot of accuracy in aging island foxes using tooth cementum, S. Baker 2021

The data suggests that for younger and middle age foxes–up to approximately age 7–cementum analysis can provide an accurate assessment of age at the time of death. (A more accurate assessment than estimating age by tooth wear.) For more senior island foxes, however, the accuracy of the method drops significantly. 

What are the factors that led to a misread of the older fox teeth? 

According to Baker, the study's principal scientist: "Tooth condition may perhaps play a role, as two of those teeth out of four were determined to be of lower quality for aging according to the Matson's Lab. This also may be unavoidable, since broken and decayed teeth become more prevalent as individuals age." (Matson's Laboratory in Manhattan, Montana, specializes in aging mammals using teeth.) With the limited sample size from the older fox population, more analysis is warranted to tease out the factors that may play a role in the reading error. In the wild, foxes are known to live 10–12 years on some islands. (K Dearborn personal communication)

Friends of the Island Fox is committed to research that will expand understanding of island fox biology and behavior. The more we know about island foxes, the more we can protect their future. It's Science for Fox Sake! - Mike Watling, FIF President

Full Report: Using Cementum Annuli to Estimate Age in Island Foxes by S. Baker, 2021 

This research project was made possible through Friends of the Island Fox, a donation from Safari West, and in partnership with the National Park Service, Santa Barbara Museum of Natural History, and UC Davis.

Gathering Specimens For Island Fox Tooth Research

In 2019 Friends of the Island Fox supported a research project investigating whether cementum ring structures in teeth could be used to accurately determine island fox age at death. Research Project by Stacy Baker and Juliann Schamel.

Baker and Schamel updated us on their progress mid-March 2020.

"We went up to the Santa Barbara Museum of Natural History last week to collect teeth from the first seven skulls that they are processing for their collection," Schamel wrote in mid-March. An additional five skulls have since been processed.

The researchers explained that the island fox skulls came from Dr. Leslie Woods and the CA Animal Health and Food Safety Lab at UC Davis, which is responsible for conducting necropsies on island foxes when they die on the Channel Islands. These specimens are vital for biologists to understand cause of death, as well as underlying health problems. When an island fox dies, a necropsy (similar to an autopsy for a human) examines organs and tissues in detail to understand the cause of death. Necropsies are vital to identifying disease or any other threat, which might endanger other island foxes. The bodies of these individual animals are stored in special freezers at UC Davis to preserve tissue samples for future scientific investigations, like the tooth project.

During the final stages of the preservation process, cleaned skulls are soaked in cold water for several days. The researchers explain that "At that point in the process, the teeth are very loose and easy to pull out..." A single lower canine tooth was extracted from each skull. Once all of the skulls have been processed, Schamel added, "I think we will be ready to send all the teeth to Matson's Laboratory in Montana!"

See Mike Watling's story for more on how each tooth will be processed.

The plot below shows how fox age estimated by tooth wear does not always match-up with actual island fox age.

We know the actual age of island foxes that are micro-chipped as pups. Biologists performing health checks in the field, however, do not have access to this age information when they are examining individual foxes. They estimate the fox's age by the amount of wear on its teeth. This estimate puts the fox in one of five categories: 

• Age Class 0 - pup to 1 year
• Age Class 1 - young adult
• Age Class 2 - adult
• Age Class 3 - mature adult
• Age Class 4 - senior

The plot above shows data from 1300 exams of island foxes compared with their known ages. The larger the dot the more individuals fell into each category. The more you examine this plot the more you will see. For example: a 4-year-old island fox was almost as likely to be estimated as Age Class 3 as Age Class 2. A few 4-year-olds were estimated as Age Class 1 or Age Class 4. What do you see in the plot for island foxes that are 6 years old? Is tooth wear a reliable way to estimate island fox age? 

This data shows why the cementum research is so important. Island foxes live longer and eat a different diet from the southeastern gray foxes for which the tooth-wear age classes were originally designed.

This project also highlights the broad scientific community involved in researching island foxes. Biologists from Channel Islands National Park are working with the Santa Barbara Natural History Museum, UC Davis, and Matson's Laboratory in Montana, which specializes in cementum age analysis of mammal teeth. Science brings people together.

As this research project moves forward FIF will keep you updated on the findings. Scientific data is vital to making informed conservation decisions that maintain sustainable populations of island foxes.

Friends of the Island Fox invites biologists, ecologists, and other researchers to contribute to our understanding of island foxes. Applications for the FIF Research Grant 2020 are now available.

The island fox cementum analysis research project was made possible through a donation from Safari West.

Your Donations Help Make Research Possible 

How Structures in Teeth Could Provide Accurate Age Information for Island Foxes?

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

Cementum is a thin mineralized tissue covering the root surface of teeth and functions as a tooth supporting device which anchors the tooth in the socket. Unlike bone, cementum is produced throughout one's life and forms annual layers (annuli)–a fact that makes it useful in aging techniques.

Cementum Analysis
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

Once the tooth is received at the lab it is cleaned to remove any dirt and soft tissue, then undergoes processing to decalcify the tooth in preparation for analysis.

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 sections are stained with a special histological dye that is taken up by annuli at varying degrees (light blue or darker blue) depending on the amount of cementum laid down in a given year. This provides both a highlight and contrast for the purposes of counting the number of growth rings (aging) under a microscope.

The picture of cementum annuli in the photo below indicates the growth rings with the black arrows.

Image 4 - see sources below

Significance
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?

Friends of the Island Fox is committed to research that will expand understanding of island fox biology and behavior. The more we know about island foxes, the more we can protect their future.

- Mike Watling, FIF Advisory Committee 

Image sources

• 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