Skip over navigation

The Impact of Chronic Stress on Immunity and
Reproductive Hormones in Zebu Cattle (Bos indicus)

Adviser: Andrew P. Dobson

Allison M. Tracy

Ecology and Evolutionary Biology

“The biggest surprise of my thesis was that I thought it was informative, coherent, and interesting at the end, and so did other people.”


My thesis topic emerged from research I conducted at the Woods Hole Oceanographic Institution in the summer after my sophomore year. Although my thesis ultimately involved researching the relationships between chronic stress, immunity, and reproduction, my work on lobster shell disease was my underlying inspiration because it made me think about the scientific and social problems at the intersection of the environment and disease.

Going into junior year with a broad understanding of my interests allowed me to identify a thesis adviser. I spoke to several professors about potential projects, but it took a few referrals before I met with Professor Andrew Dobson. I liked his personality, and my ideas coincided well with his research. Looking back, these were two important ingredients. He suggested that I work on a project he was developing in Tanzania to assess the epidemiology of several ungulate diseases in the Serengeti, and I was excited about this from the beginning. I changed the study site to the Mpala Research Centre in Kenya at the beginning of the summer in order to have better access to resources, such as vehicles and the Internet. At the time, I did not anticipate the value of having the support of a research community, but being able to draw on the expertise of other scientists at Mpala who studied epidemiology and endocrinology was invaluable. I had to be proactive about this and find opportune moments to ask questions. Their advice and support complemented the advice I received from my adviser.

During the fall semester, I did some important background reading and devised potential study designs. Although I spent the spring in Panama through the Department of Ecology and Evolutionary Biology (EEB) study abroad program, I was able to design an outline of my research plan using my own goals in conjunction with a project proposal that Professor Dobson had drafted with other partners on the main project. The main proposal focused on the role of cattle, their interaction with wildlife, and the resulting social context. I found the entire context compelling and useful, but I isolated a small segment for my research because the scope of a thesis is much smaller than typical ecological studies. The topic I reviewed for my spring junior paper (JP) was “The effects of stress on reproduction and immunity.” 

My JP research supplied me with the information to refine my study design. I reviewed the literature on the effects of acute and chronic stress on reproduction and immunity to identify the major questions and uncertainties in the field. I had a general plan for my research in Kenya before writing my JP, but thoroughly understanding past research allowed me to develop a specific question, account for common errors, and follow precedent. Finding the primary questions about the intersection of epidemiology (the study of disease) and endocrinology (the study of hormones) supplied me with many possible research questions. Expressing a specific research question was the most difficult and most important step for me. After comparing the existing knowledge with the ecological context in which I would study stress, immunity, and reproduction, I formulated the following question: “What is the effect of chronic stress on immunity and reproduction?” I focused on Bos indicus, a species of cattle that is common in Kenya, because that was the central study species of the main project proposal. Cattle are relatively easy to sample and very important for the economic, political, and social climate in Kenya.

As I was developing my question and my research plan, I also was figuring out the logistics of traveling to Africa. Several aspects of my plan changed during my first few days in the field, but I think that is to be expected. I spent the first week of June at the Lincoln Park Zoo in Chicago, learning the protocols for collecting hormones from fecal samples. We had designed a study that incorporated approximately three samples from each of 50 individuals over the course of six weeks. However, I realized that the need for a car and a field assistant at Mpala necessitated changes in this plan. Fortunately, there were other Princeton students there and we shared a car and helped each other with research.

My question changed only slightly over the course of my research, but I certainly went through a process of simplifying my goals. I changed the sampling regimen to fit with the logistics at Mpala, but the new design still allowed me to assess the effect of chronic stress on immunity and reproduction through a comparative study. There were several times during the course of my fieldwork when I thought I would have to change my question more drastically. It was particularly difficult to maintain the controls I needed to make sure other factors did not skew my results. However, quickly recognizing errors and figuring out a way to account for them in writing helped immensely. The fact that I wrote down everything when I did it was very important when I sought to justify errors later, whether I had known about them at the time or not. When I thought of a possible error, I would write down everything I knew about it, remembering that not everything I wrote would have to go into my thesis!

Writing a thesis is like other independent projects in some ways, except that you are more involved and the challenges you run into are more difficult. The scope of the project means that you will absolutely encounter obstacles, but you just have to accept that and then know how to adapt and whom to ask if you cannot solve the problems yourself. It is important to try to solve everything by yourself if possible, but finding the point at which you need to ask for help can be difficult. Conferring with my adviser over the more major problems was very valuable, but sometimes I just had to go with my own instincts. Another way to look at the comparison between normal work and a thesis is that things can seem rushed and questions remain unanswered upon finishing other academic projects. There are still many unanswered things after the thesis process is over, but you have the opportunity to look further into the problem. 

My thesis also allowed me to interact with professors and graduate students more closely. Discussing my ideas with my adviser was always very interesting, and I learned a lot about a branch of biology that I hope to pursue further. I was not only answering questions about writing my thesis, but also developing a general approach to biology. The fact that I am interested in Professor Dobson’s research enriched the process for me. I was continually discovering more of his intriguing projects, past and present. It is important to avoid tunnel vision in your relationship with your adviser because they have interesting things to say about many other topics. Graduate students also have great advice and encouragement. Listening to what they think about the field in general can be incredibly useful.

The biggest surprise of my thesis was that I thought it was informative, coherent, and interesting at the end, and so did other people. I had spent so much time with my research that I was not sure how it would all turn out. My adviser, my friends, and a few graduate students had helped me with pieces, but I was the one with the whole picture. In retrospect, I think that one way to gauge your progress is to jump on the opportunity when people ask you what your thesis is about. If you can get them interested in a few minutes, that is a good sign. Of course, clear writing also is essential.

There were many important things that I gained through my thesis research, but I think the most useful was the ability to engage in academic discussion of my work with my peers. This may seem surprising given that it is independent work, but working with other students was one of the most rewarding and valuable aspects of my thesis because I had people to discuss my research with, both during fieldwork and throughout senior year. Students obviously rely on their advisers and other professors for advice, but it was immensely helpful to have other students who were familiar with my project from the beginning. They provided a different perspective, and I could express my ideas, concerns, and failures in a more casual atmosphere. Our exchange of ideas kept me interested in my thesis and helped me solve problems. This realization changed my outlook on accomplishing tasks in general. 

Perhaps it is as a result of my friendly conversations about my thesis that I am still interested in it. I was able to further the questioning of traditional ideas on the relationships between hormones, reproduction, and immunity by challenging the relationship dictating the suppressive impacts of chronic stress. I think it is fascinating that animals may be able to escape the negative consequences formerly perceived as the predetermined results of chronic stress. This work is compelling in the context of the increasingly severe environmental challenges that animals face. Moreover, understanding how cattle deal with chronic stress may contribute to a better understanding of the interactions between domestic and wild ungulates in Kenya. I find it very satisfying that my thesis has both intellectual and practical applications, even though both are speculative.

To close, I would like to offer a few pieces of advice. First, some of your plans will not work out the way you wanted them to, but you can usually rework your ideas to solve the problem if you are patient and calm. Second, do not expect other people to do things for you, and you will be pleasantly surprised when they do. And finally, engage with your thesis as something you want to do instead of something you have to do. Be able to differentiate between complaining and discussing your frustrations as a way to move on. I adopted the mind-set that my obstacles were just part of the process, and I discussed them with other EEB seniors in hopes of resolving them. This is an excellent way of embracing the thesis process, and it definitely contributed to the sense of accomplishment I felt when I held a bound copy in my hands.

The Impact of Chronic Stress on Immunity and
Reproductive Hormones in Zebu Cattle (Bos indicus)

Allison M. Tracy

Andrew P. Dobson

Professor of Ecology and Evolutionary Biology

“The most rewarding aspect of advising Allison’s thesis was the opportunity to work with someone who picked new concepts up very easily and quickly realized how all the different pieces might fit together.”

The best thing about senior thesis advising at Princeton is that it gives you a variety of opportunities to interact with extraordinarily bright students. This provides new insights into my own research and opens up new areas for future exploration. The sad thing is that they walk out of the door after graduation and you often never see them again.

Thesis advising seems to use an almost entirely different part of the brain from lecturing or running a precept. Lecturing is the closest we get to being in Hollywood or on Broadway, although we have to write the script and do the entire show live with no re-takes. This simultaneously has to be combined with a visceral response on when to adjust speed and cadence of delivery to get the main points across, while desperately restraining our worst tendencies to ad-lib! Essentially, when we lecture, we need to be very serious and highly trained comedians. Thesis advising involves working one on one with someone whose strengths and insights we have to learn and nurture over the 12 to 15 months when we work with them. It’s much harder to switch off at the end of a thesis advising session and see if you’ve done a good job; new questions keep appearing and different ways to collect or analyze data. When I’m lecturing, I can play a role; when I’m advising, I have to be me!

My interactions with Allison Tracy over the last academic year were really fantastic. I think we both learned new things about the project each time we met to discuss, plan, and analyze the results. 

There were frustrating moments early on when we were trying to work out getting research clearance for the project. This is always a huge hassle as we have very little time to get through an administrative process that usually takes six months. So the adviser has to have sufficient confidence in the student that they can pull the strings to speed up the process. Lots of students think they just have to get the funding, and Africa will welcome them with open arms. It’s much more complicated than that, and the adviser is usually putting their future research clearance on the line every time they take a student into the field. I never had any worries about Allison in this respect as her personality and incredible work ethic convinced me that whoever she worked with would benefit from interacting with her. It’s very rarely that easy.

For me, the most rewarding aspect of advising Allison’s thesis was the opportunity to work with someone who picked new concepts up very easily and quickly realized how all the different pieces might fit together. The huge strength of this thesis is that it combines an arduous but well-balanced collection of field data in a tropical country with detailed laboratory analysis, and the development of mathematical models and statistical analysis, pulling all the different pieces together. Although I often get to advise a thesis that focuses on one or two of these approaches, it is very rare to work with a student who has strong skills in all of these areas.

I learned a lot from this thesis. Initially, I thought we would collect a very useful set of data that could be used as supporting material for a larger research grant application, but Allison had such a diversity of skills that we were able to explore the interaction between stress and disease both in her data and in the mathematical models that we worked together to develop. As with any interesting model, we began to see counterintuitive patterns in the model’s behavior that provided insights into why the data Allison had collected were not straightforward to interpret. This is when any sort of science gets really exciting; you begin to develop a deeper understanding of the system you’re studying and how the parts that operate at different spatial and temporal scales are coupled together.

In terms of advice for future thesis writers, I would point out that advisers are much more interested in concepts than location. I have lots of people turn up at my office and say they really want to work in Africa and then have no explanation of what they’d like to work on or why. Much more rarely, someone appears and says I was really intrigued by this problem that came up in class, I think it might provide insight to more general classes of problems. I might be able to test these ideas in this system, but I was wondering if we could discuss the best way to do this in my thesis. Guess which one gets to go to work in Africa?