If you've had an annual checkup recently, your doctor probably ran a blood test to look at things such as your cholesterol levels. Researchers are also interested in the levels of cholesterol and other lipids in the blood of chimpanzees. This is largely to help zoos and other institutions take care of human-managed chimpanzee populations - but it may also (someday) help us understand more about cardiovascular health in humans.
To learn more about the study of chimpanzee lipid levels, and how much we still have to learn, we spoke with Kimberly Ange-van Heugten and Ethan Renfro. Ange-van Heugten is a co-author of a paper on the work and Alumni Distinguished Undergraduate Professor of Animal Science at NC State. Renfro is first author of the paper and a Ph.D. student at NC State. The paper, "Comparative Analysis of Serum Lipid Profiles in Sanctuary-Housed Chimpanzees (Pan troglodytes verus) at Tacugama Chimpanzee Sanctuary," is published open access in the journal Veterinary Sciences.
The Abstract: What was the research question you were setting out to address with this work?
Kimberly Ange-van Heugten: During the fall of 2024, Anneke Moresco - an adjunct professor in NC State's College of Veterinary Medicine - and I were fortunate to represent NC State on a team of chimpanzee medical care experts at the Tacugama Sanctuary in Sierra Leone. The trip was organized to conduct full medical health checks on as many of their chimpanzees as possible. Despite chimpanzees being a very popular species, and although they have been held in zoological facilities for over 100 years, research into their true nutritional needs is sparse. Therefore, one of the health-check goals was to assess the nutritional status of the chimpanzee population and their diet.
We came up with the idea of comparing serum lipids specifically for this paper as part of Ethan Renfro's Ph.D. program because we realized that some of the lipid concentration levels were higher than we expected from prior literature. Our research questions became:
1) How do the chimpanzee serum lipids (cholesterol, HLD, LDL, triglycerides, etc.) at Tacugama compare to human values and chimpanzee values measured at other locations (both in native countries and in U.S. managed facilities)? and
2) If the serum lipids are in fact different in different places, do lipid levels also differ by sex, age and chimpanzee subspecies? And if so, why might this be important for future medical care of this endangered species?
TA: What is serum in this context? And what are serum lipids?
Ethan Renfro: Serum is a portion of whole blood that does not have red blood cells remaining in suspension - it's the clear liquid that is left behind once blood cells and platelets have been removed.
Typically you can get serum samples by drawing blood and then having it clot, which can be done naturally once it touches a surface outside the body. But you can also draw blood into tubes that encourage clotting such as a serum separator tube. Additionally, serum can be separated from whole blood via centrifuge, or by a combination of these techniques.
When analyzing serum, you can find values for minerals such as electrolytes, like sodium and potassium. You can also see differences in feeding responses before and after a meal by measuring circulating blood glucose for short-term energy and lipids for long-term energy status.
Serum lipids are fats that are found in blood serum. These include several forms of cholesterol and triglycerides. For this study, we examined triglycerides, total cholesterol, high-density lipoproteins, low-density lipoproteins, and very low-density lipoproteins; commonly abbreviated as HDL, LDL, and VLDL, respectively.
Triglycerides are the main form of lipid and fat storage in the body and can be found in circulation commonly travelling from the gastrointestinal tract to the adipose tissue (or body fat), where excess energy is stored. Cholesterol is a key component of our cell membranes and is a precursor to Vitamin D. HDL, LDL and VLDL are lipoproteins that circulate in the blood to transport lipids and fats around the body for energy utilization and storage. In humans, HDL tends to get a reputation as the "good" cholesterol with LDL being the "bad" cholesterol due to LDL's influence in the development of cardiovascular diseases in humans. Though, of course, it's not quite that simple.
TA: Why would you be interested in studying serum lipids in chimpanzees?
Ange-van Heugten: The short answer is that we wanted to know the "normal" serum lipid values for a large and healthy chimpanzee population in a native range country consuming a semi-known diet to help us make better dietary choices to support the health of managed chimpanzee populations. Now for the long answer.
Chimpanzees are a critically endangered species. In order to assist the dramatic conservation needs of this species, scientists must better understand how to best house, feed and care for these great apes. As a comparative animal nutritionist, my primary goal is to study what healthy native animals eat in their natural habitats. This way I can learn more about natural diet nutrients to better implement similarities in human-managed situations. The chimps at Tacugama are not "wild" anymore - they are rescued individuals - but they do live in their native land and much of their diet is native vegetation. Being that wild animals are much more difficult to get blood samples from, our goal was to study nutritional health parameters in chimps as close to their wild compatriots as possible.
Serum lipids are a common blood metabolite analyzed in human lab settings and the local human hospital was able to run these samples for us in a timely manner. Additionally, we were fortunate to be able to analyze the general chimpanzee diet at Tacugama. As a result, we could compare the serum lipids with the lipids fed to the animals. We studied all this information because serum lipids are important for a chimpanzee's essential biological functions, such as energy storage, hormone regulation and vitamin absorption.
Serum lipids also serve as a crucial comparative model for understanding all great ape health parameters and the evolution of cardiovascular disease in different species. Too little and too much of each serum lipid can be unhealthy.
TA: So, what exactly did you look at? And what did you learn?
Ange-van Heugten: This study involved measuring cholesterol, triglycerides, HDLs, LDLs and VLDLs from 77 healthy chimpanzees at Tacugama. These five serum lipid values were also compared by age, body condition score, housing group and sex within the Tacugama population.
These results were compared to other known chimpanzee "normal" values and to "normal" human values.
We found that total cholesterol was higher in Tacugama males than females. LDLs also tended to be higher in males. Average Tacugama VLDL and triglyceride values exceeded previously reported "normal" values for human-managed chimpanzees. HDLs varied by Tacugama housing group, possibly due to environment, physical activity and age. Differences we found from data published on managed chimpanzees and human reference intervals suggest that chimpanzees at Tacugama are unique with respect to their lipid values. Future studies are needed to assess whether these differences are healthy variations in the lipid metabolite processing for the subspecies of chimpanzee found in Sierra Leone or if they represent a higher disease risk.
TA: Does this raise any new research questions for future work?
Renfro: While chimpanzee populations are declining due to anthropogenic factors such as development, mining, energy production, climate change, and introduced diseases, there are also large human-managed populations found in sanctuaries and zoological institutions worldwide. Being our closest relative, chimpanzees have been used extensively in U.S. biomedical research prior to the NIH decision to phase them out beginning in 2015. Therefore we know a lot about ourselves due to research done on chimpanzees. It stands to reason that we should now return the favor by improving our ability to care for these animals.
Just like we would go to the doctor for routine wellness exams, most sanctuaries and zoos conduct regular wellness exams as an integral part of their veterinary care programs and overall management. If we have the opportunity and resources to provide a higher standard of care for chimpanzees, why would we not? I am fortunate to be in a position to do a small part that can help us provide better care for managed chimpanzee populations - with the hopeful end goal of a robust chimpanzee population eventually returning to their native range, free of human interference.
TA: Many people suffer from heart disease related to serum lipids. Could investigating serum lipids in chimpanzees help us understand some of the drivers related to heart health in humans?
Renfro: While cardiovascular disease (CVD) is a leading cause of mortality for both humans and chimpanzees, studies have shown that what causes CVD varies between species.
In comparing the serum lipid values for these chimpanzees to human reference values, only five of the 77 individuals assessed were within human reference values for all five lipids. If these animals were subject to the same biochemical pathways that lead to CVD and atherosclerosis in humans, we would not see the lifespan in captivity that we do and would see many human-managed chimpanzees dying of CVD - which simply isn't happening. These animals were marked as generally healthy by the human cardiologists who assessed them during physical exams, so they clearly have a different method to deal with these elevated serum lipid concentrations.
Exactly how this evolutionary mechanism arose, and if it can be replicated, remains to be seen.
