Tuesday, September 4, 2012

Guest Blog: Don’t look a gift horse in the mouth? How about looking at 170 digital radiographs of deer mouths!

This week we are featuring a guest blog post by Zoe Morris, a PhD candidate at Western University, and our Ancient Images Lab technician. You may remember Zoe from some of our past blog entries, featuring artifact scanning and proof of concept testing on the MicroCT. In this post, she introduces one aspect of the research she has completed for her PhD thesis - determining the order of tooth formation (dental mineralization) in white tailed deer. Her research has been informed by examining radiographs (x-rays) of deer jaw bones completed at Sustainable Archaeology using the Faxitron Digital X-Ray. Why deer? Read on to find out!

Don’t look a gift horse in the mouth? How about looking at 170 digital radiographs of deer mouths! 
By Zoe Morris, PhD Candidate Department of Anthropology, Western University

White-tailed deer are one of the most adaptable North American mammalian species, found as far south as the jungles of Central America and as far north as the boreal forests south of Hudson Bay. They are the most important Canadian and American hunted species, providing millions of dollars in revenue annually. White-tailed deer are also significant crop pests, causing more damage to crops than any other large-bodied animal, they are involved in thousands of deer-car collisions a year causing property damage and human loss of life and they can be vectors for a variety of diseases that affect livestock.
A browsing white-tailed deer in the Medway Valley Heritage Forest, neighbouring the Sustainable Archaeology facility in London. Photo by Zoe Morris.
White-tailed deer’s adaptability to human altered landscapes has brought them in close proximity with humans for thousands of years and they are the most ubiquitous animal find at many North American archaeological sites, constituting a major source of protein and an essential source of skins for clothing and shelter for past human groups.  Because of the modern and archaeological significance of white-tailed deer, understanding their habitat-use throughout the year has implications for ecological research, hunting policies, and archaeological understanding of past life-ways.

In order to better track the movement and diet of Ontario white-tailed deer over the first year and a half of life, I determined the dental mineralization sequence of white-tailed deer teeth.  There is extensive literature on the order of eruption of white-tailed deer teeth and this information is used by hunters, ecologists and zooarchaeologist to estimate the age of deer under two years old (dental wear is used after two years of age).  However, there is no previous research on the mineralization sequence for white-tailed deer.

In order to examine dental mineralization I examined 170 radiographs (x-ray images) of juvenile (less than two years), Ontario white-tailed deer mandibles and maxilla.  The majority of radiographs were completed at the Sustainable Archaeology Ancient Images Laboratory located in London Ontario. Additional radiographs were completed at the Royal Ontario Museum in Toronto, Ontario and Canadian Museum of Nature in Gatineau, Quebec.
All of the radiographed deer were aged based on their dental eruption. I also estimated their season of death. For example, most deer in SW Ontario are born mid-May or early-June. Therefore if a white-tailed deer died at six months old, it probably died approximately November. We could then begin to work out when each tooth was forming by examining the radiographic images and estimating which teeth were forming during which season. (Left image: sixteen white-tailed deer mandibles at various dental mineralization stages. Digital Radiograph courtesy of Sustainable Archaeology Ancient Images Laboratory).

Once the dental formation sequence was completed, it was confirmed using oxygen isotope analysis. Oxygen isotopes were obtained from the serial sections of deer teeth. Oxygen isotopes are variations of oxygen atoms whose ratios vary depending on factors such as temperature, precipitation, and geography.  This ratio is preserved in tissues such as dental enamel.  Because precipitation and temperature varies greatly in SW Ontario with season, we were able to track seasonal variations in the oxygen of both modern and archaeological deer teeth. Happily, the variation corresponded to the estimated season of dental formation!

Why is this significant? We can now use those same teeth to track other information about white-tailed deer. For example, I was able to examine whether deer were eating corn or other food sources during the first two years of life using carbon isotopes from the dentition. Carbon isotopes reflect the foods we eat and can vary based on whether a plant is warm weather adapted (like corn which was domesticated in Mexico) or cool weather adapted (like most indigenous SW Ontario plants).  For more information on the final results of this study, please see my dissertation, to be defended in 2013 (fingers crossed).

A white-tailed deer skull with mandibles (~six month old) from Griffith Island, Ontario. Note the second molar is forming but has not yet erupted.  Digital Radiograph courtesy of Sustainable Archaeology Ancient Images Laboratory

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