Kathryn McGrath was awarded a Leakey Foundation Research Grant in our spring 2016 cycle for her project entitled “Understanding stress-related enamel defects in wild mountain gorillas.” Click here to read a short summary of her work. Here she updates us on her progress.
A female chimpanzee canine thin section.
I received a Leakey Foundation Research Grant one year ago, and since then, I’ve traveled around Europe to collect data for my dissertation, analyzed much of it, presented at five (!) conferences (and was accepted to present at two more), and got one award. I actually had to bring my own microscope to image great ape canine thin sections, which was more complicated than I had initially hoped, though not quite as tricky as that time I brought an X-ray gun as a carry-on.
Teeth grow in incremental layers, and just like trees, they can be counted and measured to learn about the rate and duration of their growth. I chose canines for my dissertation because they take the longest to form in great apes (think male gorilla fangs), and thus provide a nice, long window into the early life of the animals to which they belonged.
The first chapter of my dissertation focuses on what information we can glean from the outside of great ape canines, because most teeth can’t realistically be cut in half to learn about how they grew. Luckily, unlike trees, most of the growth rings in teeth are also visible from the outer surface, and they appear as horizontal waves around the tooth. When normal growth is disrupted by events like injury or illness, more pronounced grooves are formed, like the one pictured here.
A male mountain gorilla’s lower canine (left), and a digital elevation model of a defect (right).
These grooves or defects are fairly well-understood in humans because researchers have easier access to health records and other information about the way we live, but opportunities to study their causes in great apes are rare. We know that great ape canines usually have defects, but only recently have researchers begun to measure their shape using sophisticated imaging techniques, like the one used here. I scanned defects in a number of great ape species, created digital elevation models (pictured), and extracted coordinates to compare their depths. Some researchers have proposed that depth is related to the severity of the insult that caused the defect, like a life-threatening illness as opposed to a minor cold, while others have suggested that defect depth has more to do with the way each species grows their teeth. Different rates of enamel growth are hypothesized to correspond to different geometries in the underlying tissue, which might lead to discernible variation in defect depth between species.
In our study, we found that mountain gorillas, the fast-growing, leaf-eating gorillas, have shallower defects than other great apes, including closely-related western lowland gorillas. We think that this mostly has to do with the way that mountain gorillas grow their teeth, although future studies incorporating health and behavioral records from when individual gorillas were alive will assess what kinds of life events might correspond to these defects. We want to know, for example, whether social stress corresponds to defects in the developing teeth of a gorilla, or if they are more commonly associated with physical injuries and illness.
A surprising result of this study is that the deepest defect in the sample belonged to a western lowland gorilla named Haloko who was born in the wild and captured to live in a zoo when she was around 2-3 years old. Based on previous studies of western lowland gorilla canine development, we think that this major defect, which is about ten times deeper than the typical defect in her species, might correspond to her capture from the wild. We can’t be sure of the timing because we aren’t able to create thin sections from her teeth, which is the only way to reliably count the growth increments, but I hope to follow up on this story in my postdoctoral research by studying other great apes that were captured from the wild as infants to see if they have similarly marked defects in their teeth.
Haloko, courtesy of the Smithsonian’s National Zoological Park (left), and a replica of her lower canine with a major defect that might correspond to her capture from the wild (right).
What I’ve learned so far is that the processes behind dental defect formation are super complicated. On the one hand, there are clear species differences in defect depth, and preliminary data from associated thin sections suggest that this has a lot to do with the way these different animals grow their teeth. On the other hand, examples like Haloko with very traumatic histories and corresponding deep defects imply that the severity of the growth disruption might also have something to do with depth. Even though there is still a lot to learn from our closest living relatives in terms of what these defects mean, this study provides an important caveat for researchers interested in studying dental defects in our fossil ancestors. We should be careful when interpreting defect “severity” without measuring a number of teeth from the same species first. A defect that might appear shallow in one specimen might actually represent the species-typical pattern, as in mountain gorillas.
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The most meaningful part of my work as a Leakey grantee is sharing my passion for teeth with kids. At the most recent American Association of Physical Anthropologists meeting in New Orleans, I participated in Education Committee-organized outreach sessions with AP Biology students at a local high school. We talked about chimpanzee behavior, the scientific method, the latest fossil finds from South Africa, and of course, gorilla teeth.
AAPA morning outreach team: John Mitani, Caitlin Schrein, the author, and John Mead
There was a special moment when I walked around the room holding a replica of a chimpanzee canine that came from the Tulane University collections, right in their backyard. This animal has remarkably deep defects all along its canines, representing some kind of repeated growth disruptions throughout early life. In my day-to-day routine, when I’m struggling to find the motivation to power through excel spreadsheets and scientific papers, I think back to those moments when at least a few kids in the classroom seemed to really appreciate what I’m doing, and I feel recharged.
Male chimpanzee canine with repeated defects.
This story is far from over, and I still have about one year left before I become Dr. McGrath, but I would like to express my deepest gratitude to The Leakey Foundation for their support of not only my research, but also my outreach and science communication. Every bit of evidence suggests that we need to prioritize sharing our science, and the Leakey Foundation is doing just that.
I’d also like to thank Sireen El Zaatari for hosting me at Universität Tübingen; Marcia Ponce de León, Christopher Dean, Wendy Dirks, Donald J. Reid, Kristofer Helgen and Darrin Lunde for access to specimens; Arnaud Martin for access to equipment; the Mountain Gorilla Skeletal Project and partners including Dian Fossey Gorilla Fund International, the Rwanda Development Board, and the Gorilla Doctors; Briana Pobiner from the AAPA Education Committee; my interns, dissertation committee, and coauthors; the National Science Foundation; George Washington University and my advisor, Shannon McFarlin, for everything.
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