Marmosets, small New World monkeys, are increasingly utilized as animal models in various biomedical research fields, including neuroscience, infectious disease, behavioral research, obesity, and reproductive biology. Their small size, short lifespan, and high fecundity make them advantageous for research purposes. However, a comprehensive understanding of their nutritional requirements and standardized dietary practices is lacking, hindering their effectiveness as models for human health studies. This article explores the dietary needs of marmosets in their natural habitat and compares them to the diets provided in captivity, highlighting the challenges and potential solutions for optimizing their health and well-being.
Marmoset Diets in the Wild
Gum eating has played an important role in marmoset evolutionary biology; it is central to the natural diet of marmosets, has nutritional value, and provides a fermentable substrate. In their natural environment, marmosets exhibit a diverse diet that includes:
- Tree Gum: Marmosets are well-known for their habit of gnawing on trees to extract gum, which forms a significant part of their diet.
- Insects: Insects provide a crucial source of protein and other essential nutrients.
- Fruits: Fruits offer vitamins, minerals, and carbohydrates, contributing to a balanced diet.
The role of gum in the diet also appears to vary from species to species, playing a more central role in the diets of C. jacchus and C. penicillata. Another participant noted that gum eating has played an important role in marmoset evolutionary biology; it is central to the natural diet of marmosets, has nutritional value, and provides a fermentable substrate.
Dietary Practices in Captivity
Approaches to the care and management of captive marmosets for research vary widely between institutions. The literature regarding nutritional requirements in marmosets is sparse, with most published reports providing information on a limited number of specific diet components in relatively small numbers of animals. In captive settings, marmoset diets often consist of specially formulated খাবার, with the goal of meeting all their nutritional needs in a controlled manner. However, dietary regimes are more influenced by anecdotal experience and animal food preferences than by solid evidence, which has led to a wide variety of dietary regimes among institutions housing marmosets ranging from cafeteria style kitchen made diets, to commercial purified irradiated diet with no fresh foods. These diets may include:
- Commercial Marmoset Diets: These are typically single-item diets designed to provide all the necessary nutrients. Such a diet does not require gluten.
- Supplemental Foods: Facilities may offer additional foods like fruits, vegetables, insects, and treats to enrich the diet and provide variety. A list of supplemental foods that were offered before the study commenced is given in Supplementary Table S1.
Researchers have experimented with different types of treats for training and rewards. Some use mini marshmallows, others use drops of Ensure or sugar water, and others use gum. Several attendees reiterated that it is ideal for treats to be aligned with an overall healthy diet and, crucially, that any treats be counted as part of the animal's diet.
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Challenges in Captive Diets
Despite the intention to provide optimal nutrition, several challenges exist in replicating the natural diet of marmosets in captivity.
- Nutrient Requirements: Marmosets' exact nutrient requirements are not well-established, leading to variation in diets between colonies and dietary husbandry informed by practice or anecdotes.
- Digestibility: Digestibility of vitamin D tends to be unusual in marmosets. Experimental testing has exposed some unexpected results. For example, when examining a single-item diet at three separate colony locations, each sample demonstrated a large amount of variability in diet digestibility. Although one would expect healthy animals to process a single-item diet similarly, some animals presented with high levels of vitamin D while others were deficient.
- Individual Differences: Individual differences complicate the task of defining nutritional needs of marmosets and suggest that underlying problems, disease, or other dysfunction may contribute to biological unpredictability.
- Gum Replacement: Marmosets naturally consume gum, which provides energy, minerals, and a fermentable substrate. However, gum is an incomplete food; if a marmoset eats nothing but gum, it will die. It's role in intestinal health and gut microbiome requires further study. Moreover, if research indicates positive effects from the inclusion of gum in the marmoset diet, the question follows of whether the benefit is specific to gum or if a saccharide, agar, or other fermentable carbohydrate could serve the same purpose.
- Obesity: Obesity prevalence in marmoset colonies has been increasing and may indicate an overestimation of energy requirements for this species in captivity or possibly diets that are highly palatable and lead to overeating.
- Metabolic Bone Disease: Over time, vitamin D deficiency can affect healthy bone density. Susceptibility to inflammatory bowel disease (IBD), in some form, has been implicated in poor weight gain in young marmosets and weight loss in older animals, with associated higher morbidity and mortality. Digestive efficiency in captive marmosets was correlated with vitamin D status; animals with poor digestive efficiency were at high risk of vitamin D deficiency leading to metabolic bone disease.
- Variation in Practices: This variability in dietary husbandry may result in high variation in nutrient intakes between individuals, both within and between colonies, which can have metabolic and physiological consequences that can contribute to unexplained variation in experimental outcomes.
The Role of Vitamin D and Iron
During the discussion, participants considered the wide variation in vitamin D supplementation practices between laboratories. One attendee noted that the picture is further complicated by the fact that some laboratories allow marmosets access to outdoor enclosures, where exposure to sunlight can increase endogenous vitamin D. Several research groups are exploring ways to measure vitamin D precursors to untangle the beneficial or detrimental impacts on health of dietary vitamin D.
Indoor lighting can also influence vitamin D and other factors, and researchers are working to determine which lighting approaches are best. The challenge with marmosets is that they can be anywhere in the enclosure and the level of vitamin D an animal gets is highly dependent on how close the animal is to the light source. Compensating for this by using lights that are too strong can risk eye damage both for animals and their caretakers.
Labs have reported different downstream effects of high vitamin D supplementation, including soft tissue calcification, though it is unclear whether this is actually related to vitamin D. It has previously been hypothesized that vitamin D metabolism differences may be related to…
Addressing Nutritional Challenges
To improve the health and well-being of captive marmosets, several strategies can be implemented.
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- Standardization: Harmonization of diet and nutritional guidelines is equally important. Although recommendations for nutrient requirements for healthy marmosets exist, experimental testing has exposed some unexpected results.
- Detailed Reporting: To enhance the evidence base for dietary guidelines, it will be important for researchers to share information openly and in detail, reporting not only what foods were offered but a full nutrient profile breakdown of each food, detailing what was offered versus what was consumed, and indicating whether the choice of food varies depending on the time of day or other factors. It also would be helpful to report and compare levels of nutritional components such as vitamin D in the blood, as these data could affect research results and comparisons across laboratories.
- Individualized Assessment: Examining Differences Between ColoniesAlthough recommendations for nutrient requirements for healthy marmosets exist, experimental testing has exposed some unexpected results. For example, digestibility of vitamin D tends to be unusual in marmosets. Individual differences like these complicate the task of defining nutritional needs of marmosets and suggest that underlying problems, disease, or other dysfunction may contribute to biological unpredictability.
- Enrichment and Training: Importantly, food used as enrichment, training, or rewards must be accounted for as part of the animal's diet and should not compromise nutrition, Power said. In the same context, food is not a good enrichment item unless it is used in creative ways (e.g., puzzle boxes, to reinforce marmosets' natural behavior).
Impact of Diet on Research Outcomes
These findings emphasize the potential impact of diet on research outcomes and the need for communication about feeding practices among facilities. Furthermore, factors such as age, bedding, cleaning procedures, gut microbiome, catching, fixation, and sedation need to be taken into consideration, particularly when studying the immune system or related diseases. For example, a study comparing the effects of chlorine-based disinfectants on captive marmosets found that their use resulted in elevated levels of chromosomal disorders (Delimitreva et al. 2013).
Animals with vitamin D deficiency likely exhibit different immunological functions than seen in those without this deficiency. These differences could contribute to significant unintended variation in research results.
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