After more than 30 years of studying cotton-top tamarins in the forests of Colombia, you might think we would have a pretty good idea of what's on their menu. And we do! But one of the things I love most about studying wildlife is that no matter how long you've been observing a species, there's always something new to discover.
Recently, our field researcher German was following cotton-top tamarins through the forest at Proyecto Tití's Los Titíes de San Juan Reserve when he noticed something interesting. The tamarins were feeding on the fruits of two climbing plants that we had not previously documented in our long-term feeding study: Monstera sp. and Chiococca alba. Both were growing among the forest vegetation, using surrounding trees for support. Now, finding a monkey eating a piece of fruit might not sound particularly exciting. But for those of us who have spent decades trying to understand how cotton-top tamarins survive in Colombia's tropical dry forests, discovering new foods is like finding another piece of a very complicated puzzle.
What's on the Cotton-Top Tamarin Menu?
Cotton-top tamarins are opportunistic feeders, which means they take advantage of a wide variety of foods available in their forest habitat. While fruits are an important part of their diet, they also eat insects, spiders, larvae, nectar, tree gums and other exudates, and occasionally flowers, fungi, and even small vertebrates such as lizards and frogs.
Their ability to switch among different foods is especially important in tropical dry forests, where food availability changes throughout the year. In 2022, my colleagues and I published a study in the American Journal of Primatology examining 22 years of feeding observations from 21 groups of cotton-top tamarins at our long-term research site, Hacienda El Ceibal. With more than 10,000 hours of observations collected between 1999 and 2020, we were able to document an extraordinarily diverse diet that included plant foods from more than 80 species of trees, shrubs, and climbing vines. We recorded fruit consumption from 54 different plant species, although the particular fruits eaten varied considerably from year to year.
Some fruits were clearly favorites. Mayna grandifolia, known locally as puerco espín, was consumed every year of our study. Other important foods included hobo (Spondias mombin), mora (Maclura tinctoria), negrito (Trichilia acuminata), and muñeco (Cordia collococca). But the tamarins didn't depend on just a handful of plants. In any given year, we observed them eating fruits from an average of 18 different species, demonstrating just how varied their menu can be.
A Menu That Changes With the Seasons
One of the most interesting findings from our research was how cotton-top tamarins adjust their diet as rainfall changes. During the late rainy season, from August through November, fruits become more abundant, and the tamarins take full advantage of them. During the dry season, when fruits are less plentiful, they rely more heavily on tree gums and other exudates.
Insects and other invertebrates remain important food sources throughout the year. We also found that cotton-top tamarins were remarkably flexible in their feeding behavior during years of unusually high or low rainfall. Despite dramatic differences in annual precipitation, their overall fruit consumption remained relatively consistent. This ability to take advantage of different foods may be one of the reasons cotton-top tamarins have managed to survive in such a challenging and changing environment. But there is an important catch. Dietary flexibility only helps if there are enough different foods available in the forest. When forests are cleared, fragmented, or degraded, the tamarins lose not only their homes but also the variety of foods they need to survive.
Two New Discoveries at Los Titíes de San Juan
This brings me back to German's recent observations. While following cotton-top tamarins through Los Titíes de San Juan, he observed them eating fruits from two plants that were not included in the species list from our 22-year study.
The first was Monstera sp., a member of the same plant genus that includes the popular houseplant often called the Swiss cheese plant. In the forest, however, these plants are quite different from the decorative plants sitting in our living rooms! Many Monstera species are climbing plants that use trees for support, producing distinctive leaves and fleshy fruiting structures. While we often focus on trees as important sources of food for wildlife, plants growing up and around those trees can also provide valuable resources.
The second discovery was Chiococca alba, a climbing plant in the coffee family, Rubiaceae, that produces small, fleshy fruits. Like Monstera, it can grow among the surrounding vegetation, adding another layer of complexity to the forest and potentially another food source for the animals living there.
What makes these observations particularly interesting is that neither plant was documented in our extensive feeding study at Hacienda El Ceibal. That doesn't necessarily mean cotton-top tamarins have never eaten these fruits before. It may simply mean that these plants weren't available in the areas where we conducted our earlier research, that they were rarely consumed, or that we hadn't happened to observe the tamarins feeding on them. And that's exactly why continuing field observations at different sites is so important!
A Tiny Fruit With a Big Question
What's particularly interesting about these two discoveries is how the cotton-top tamarins eat the fruits—and what might happen to the seeds afterward.
The fruit of Chiococca alba contains a seed approximately 4 mm in size, surrounded by white, fleshy fruit that the cotton-tops seem to enjoy. But here's the question: Do they swallow the seed along with the fruit, or do they discard it after eating the pulp? Given what we know about cotton-top tamarin feeding behavior, I suspect they may swallow the seed, but we need to collect fecal samples to confirm whether these seeds actually pass through their digestive tract. The Monstera sp. presents a different and equally fascinating story. Its fruiting structure consists of a cluster of tiny fruits containing very small seeds. In the video German captured, we can actually see the cotton-top tamarins eating the entire cluster of fruits and swallowing those tiny seeds!
Why is this important? Because cotton-top tamarins aren't just consumers of fruit—they can also be important seed dispersers. In our 22-year feeding study, we found seeds from 85% of the fruit species consumed by cotton-top tamarins in their feces, providing evidence of their potential role in moving seeds throughout the forest. Interestingly, we also found that some larger seeds were discarded rather than swallowed, making the question of what happens to the 4 mm Chiococca alba seeds particularly intriguing.
These new observations raise some fascinating questions. Could cotton-top tamarins be helping to disperse Monstera and Chiococca alba throughout Los Titíes de San Juan? Do the seeds survive passage through their digestive tract, and could they eventually germinate into new plants? We don't have those answers yet, but collecting fecal samples and examining the seeds will help us begin to understand these relationships. It's fascinating to think that while a cotton-top tamarin is simply enjoying its next meal, it may also be helping to plant the next generation of forest!
Why These Discoveries Matter for Conservation
At Proyecto Tití, we spend a great deal of time thinking about how to restore forests that have been lost or degraded. We've planted more than 140,000 trees to help reconnect fragmented habitats and create more forest for cotton-top tamarins. But restoring a forest isn't simply a matter of putting trees in the ground. We need to understand which plants provide the food, shelter, and other resources that wildlife depends upon.
Our 22-year feeding study helped identify many of the trees, shrubs, and vines that are important in the cotton-top tamarin diet. These findings have helped guide our thinking about forest restoration, particularly the importance of maintaining a diversity of native plants that provide food during different seasons. German's observations remind us that we still have more to learn. Climbing plants and vines are sometimes overlooked in restoration planning, yet they can be an important part of the forest's food supply. If cotton-top tamarins are feeding on Monstera and Chiococca alba, we want to understand when these plants produce fruit, how frequently the tamarins use them, and whether they might provide food during times when other fruits are scarce.
There is also an important connection between feeding behavior and forest regeneration. If tamarins swallow seeds and deposit them elsewhere in the forest, they may contribute to the dispersal of plants that provide food for future generations of wildlife. Understanding these relationships can help us make better decisions about which plants to protect and encourage as we restore tropical dry forests.
The Forest Still Has Secrets to Share
One of the greatest rewards of working with Proyecto Tití is seeing how our understanding of cotton-top tamarins continues to grow. Our long-term research has taught us an enormous amount about their behavior, reproduction, social relationships, and feeding ecology. Yet a simple observation by German in the forest reminds us that there are still discoveries waiting to be made. I find it particularly exciting that these observations come from Los Titíes de San Juan, where our conservation efforts are protecting and restoring an increasingly connected landscape.
As we continue to study the tamarins living in this forest, we're learning not only about the animals themselves but also about the plants and ecological relationships that make their survival possible. Perhaps Monstera and Chiococca alba are occasional treats. Perhaps they are important seasonal foods that we've simply overlooked. Or perhaps different groups of cotton-top tamarins have their own favorite foods, depending on what's available in their particular corner of the forest. And perhaps, by eating these fruits and dispersing their seeds, cotton-top tamarins are helping maintain the diversity of plants that make up their forest home. For now, we have two new items to add to our growing list of foods consumed by cotton-top tamarins, and plenty of new questions to explore.
After all these years, it seems our cotton-top tamarins are still introducing us to new things on the menu. And that's one of the reasons I never get tired of studying them!
Scientific reference: Savage, A., Snowdon, C. T., Soto, L., Castro, J., Medina, F., Emeris, G., Garcia, L. C., & Guillen, R. (2022). The impacts of seasonal variation and climate on food utilization in a population of critically endangered cotton-top tamarins (Saguinus oedipus) in Colombia: A 22-year longitudinal study. American Journal of Primatology. https://doi.org/10.1002/ajp.23415