Every evening, chimpanzees build sleeping “nests” in trees. In some studied communities, individuals appear to be selective about the tree species used, which has led researchers to hypothesize whether chimpanzees prefer trees that repel troublesome insects or/and that provide comfortable and stable structures. We investigate these hypotheses, or a trade-off between both, though study of tree species preference based on their biomechanical and/or biochemical properties in the Sebitoli chimpanzee community in Kibale National Park, Uganda. The ten tree species most frequently used for nesting were compared with ten abundant in their environment but not preferred for nesting. For these 20 tree species, we determined their biomechanical and morphological characteristics such as foliar density, foliar units form (shape and size) and branch rigidity. Their spatial repellent activity, previously tested against Anopheles gambiae was incorporated into the analysis. Chimpanzees chose tree species with medium-sized and elongated foliar units, high foliar density and branch with stiffer wood. In addition, most tree species with such mechanical and morphological properties also have mosquito repellent activity. These tree properties may provide a comfortable sleeping environment enhancing sleep quality. Finally, a comparison across chimpanzee communities would be relevant to understand whether these choices are not only ecological but also cultural.
Alternatively, another explanation inspired by “the sleep quality hypothesis”, postulates that individuals choose nesting materials offering the most physically comfortable structure. Indeed, birds and orangutans build a compliant central structure, with thicker, stronger and stiffer outer elements compared to the weaker and more flexible materials used for the inside cup. In the chimpanzee case, a study in the Toro-Semliki community has shown that the most selected nesting tree species, which has potential repellent activity, is also considered as more comfortable by providing a firmer and more stable nest with rigid branches and thick foliage with small leaves.
In this study, we jointly examined the two main frameworks known to explain tree species preference in nesting behavior: mechanical comfort and/or chemical repulsion of insects. We analyzed a large data set obtained in the habituated Sebitoli chimpanzee community. This data set included previously published data on the spatial repulsive activity of the ten tree species most used for nesting and ten tree species that are abundant in their environment but not among the most chosen species for nesting. In addition, we characterized these 20 tree species based on their biomechanical and morphological characteristics (distance between nodes, foliar unit shape and size, branch flexural rigidity and flexural elastic modulus). We expect chimpanzees to select nesting trees with a potential trade-off between repellent and comfortable species but also taking into account their availability in their habitat. Repellent abundant tree species could be physically uncomfortable and conversely, so chimpanzee may need to choose or find a compromise between chemical and physical comfort.
