Dental metrics in white-tailed and mule deer – Journal of Zoology Blog

Dagher, J. and Greiner, E. (2024). Variation in white-tailed and mule deer dental metrics is associated with precipitation gradients in the Americas. Journal of Zoology, vol. 324, pp. 325-338. https://doi.org/10.1111/jzo.13220

The relationship between the physical environment and adaptation is crucial in understanding the selective pressures driving evolution. Furthermore, evidence from the fossil record is necessary for reconstructing past environments. But how can we use fossils to comprehend past environments? One way of answering this question is by thoroughly investigating how tooth morphology and wear appear in the modern analogs and then applying this knowledge to the fossil record. For example, herbivorous ungulates (hooved mammals) are prolific in the fossil record and are often studied to understand what vegetation was available at the time. If we produce a more holistic and comprehensive reconstruction of those environments, we can better understand what selective pressures were acting on fauna in that particular spatiotemporal range.

When Dr. Elliot Greiner first approached me with the idea of investigating tooth wear in modern white-tailed deer and mule deer, I was a second-year undergraduate student. After months of literature review and discussions with Dr. Greiner, we developed a project examining tooth wear and adaptations to tooth wear in the genus Odocoileus, in white-tailed and mule deer. Dental metrics such as the hypsodonty index (relative tooth crown height) and mesowear score (average lifetime tooth wear) are vital for understanding the dietary makeup and environmental contexts of large herbivores. In looking at these species of deer, we were better able to understand how these metrics operated on a taxon that occupies a variety of environments and consumes variable amounts of grass.

From the University of Michigan Museum of Zoology, University of Florida Museum of Natural History, the Los Angeles County Museum of Natural History, and the Field Museum, Dr. Greiner and I collected craniodental measurements and observations from over 300 deer specimens. As an undergraduate, this was my first time working in museums; getting behind the scenes of the collections was an invaluable experience!

After coming home, one of the largest obstacles to this project was correlating location data from museum specimens with environmental data from which we can meaningfully understand the dental metrics across environment types and precipitation gradients. To do this, I acquired data on “ecoregions” from the Environmental Protection Agency and correlated those ecoregions with the county that the specimens were collected from. These data also provided mean annual precipitations for the specified ecosystems, which was essential for the analyses. As the ecoregions were extremely specific, I classified them into broader environmental categories. Thus, the craniodental data could be linked to environmental data.

We found that the hypsodonty indices of the lower second and third molars correlate with mean annual precipitation and environment type, with the tallest molars occurring in environments that are traditionally more open and arid, while the mesowear scores did not present as strong of a relationship. We postulated that, as tooth morphology is highly dependent on fallback foods, this pattern may be attributed to the foods that the deer are consuming in times of food scarcity. Therefore, we posit that hypsodonty may help deer broaden their dietary niche so that they can subsist during periods of low food availability.

We aim to strengthen the body of information that uses tooth morphology to understand vegetation availability. This has implications for understanding deer dietary ecology and also for the broader picture of using selenodont molars (as seen in deer) as a proxy for paleoenvironmental conditions. By understanding how tooth morphology may vary within taxa based on the environmental conditions, specifically precipitation, we can better apply these methods to the fossil record.

All data and R code for the analyses are located here: https://github.com/jennadagher/Odocoileus_HI_MS

Jenna Dagher