Four closely related civets, a small nocturnal animal found in Africa and Asia, have made the same geographical area in the rainforests of Borneo home. Typically, closely related animal species have difficulty coexisting because they are competing for the same or similar resources. Despite eating the same figs, binturong, small-toothed palm, masked palm, and common palm civets do coexist together. To understand how they coexist, researchers used a compound-specific nitrogen isotope analysis to understand the degree of faunivory (eating animals). The results were published in Progress in Earth and Planetary Science on October 2, 2024.
Strictly using observational methods to analyze the behaviors of civets have suggested insufficient. This is because their behavioral characteristics -- nocturnal and semi-arboreal (climbing trees 30-60 m in height) -- make them difficult to observe. Fecal sampling is also not sufficient to gather data about the civets' diet.
"The cryptic ecology as nocturnal, solitary forest dwellers makes it difficult to discern the mechanism by which these four closely related species coexist. Observational studies indicate that fruits dominate their diets, while faunivory, or eating both other animals and insects, is rare. We suspected that faunivory was a larger part of their diets and investigated by applying stable carbon and nitrogen isotopic analyses," said Miyabi Nakabayashi, an associate professor at Hiroshima University's Graduate School of Integrated Sciences for Life in Hiroshima, Japan.
Having different food sources is the easiest way for species having similar behavioral ecology to coexist in the same geographical area. Due to the generally low and unstable productivity of fruits in Borneo's tropical forests compared to Sumatra Island, fruits alone may not be the sole source of food for all four civet species.
Researchers used stable isotope analyses on each species to determine the nitrogen isotope ratios in the hair of the civets, which would show how much animal protein the civets were eating.
Each civet species was captured in box traps. Hairs were pulled out and used for the bulk stable isotope analysis and compound-specific nitrogen isotope analysis of amino acids. Local insects and fruits were also collected, based on their proximity to the civets' habitats and observations of the civets' diet. Compound-specific nitrogen isotope analysis of amino acids was completed on the hair of two individuals for each civet species, insect, and plant samples. The civets' trophic positions (TP), or their positions in the food web, were determined based on the stable nitrogen isotope ratios of the two amino acids, glutamic acid and phenylalanine.
The results showed that faunivory was much more common in three of the civet species than previously thought.
"Among the four species, the bulk stable isotope analysis revealed distinctly low nitrogen isotope ratios in binturongs, suggesting that binturongs exhibit the lowest degree of faunivory among them. Binturongs had the lowest trophic position, almost similar to exclusive plant-eating animals, estimated from the nitrogen isotope ratios of amino acids, followed by small-toothed palm civets, masked palm civets, and common palm civets. The trophic levels of the latter three species are in the range of omnivorous animals. These results suggest that the varying degree of consumption of animal sources, such as insects, is the key mechanism of niche partitioning in these four Paradoxurinae civet species in Borneo," said Takumi Tsutaya, an assistant professor at the Graduate University for Advanced Studies (SOKENDAI) in Kanagawa, Japan.
Looking ahead, researchers will continue to look at tropical rainforests and how ecologically similar closely related species coexist in close geographical areas.
"Tropical rainforests have, by far, the largest species number compared to forests in other regions. This study revealed one of the coexistence mechanisms of animals inhabiting there. We would like to find other factors that enable multiple closely related species to coexist, not only for civets but other animals as well," said Nakabayashi.
Other contributors include Yoko Sasaki, Nanako O. Ogawa, Naoto F. Ishikawa, and Naohiko Ohkouchi of the Japan Agency for Marine-Earth Science and Technology in Kanagawa, Japan; and Abdul Hamid Ahmad at the Universiti Malaysia Sabah in Sabah, Malaysia.
The Inui Memorial Trust for Research on Animal Science, the Shikata Memorial Trust for Nature Conservation, the Fujiwara Natural History Foundation, JSPS Core-to-Core Program, A. Advanced Research Networks (Wildlife Research Center of Kyoto University), "Evolutionary Studies of Complex Adaptive Systems" Research Grant, and Grants-in-Aid for Scientific Research from JSPS supported this research.
Since its foundation in 1949, Hiroshima University has striven to become one of the most prominent and comprehensive universities in Japan for the promotion and development of scholarship and education. Consisting of 12 schools for undergraduate level and 4 graduate schools, ranging from natural sciences to humanities and social sciences, the university has grown into one of the most distinguished comprehensive research universities in Japan. English website: https://www.hiroshima-u.ac.jp/en