Tie one on: ‘nest tying’ by wild chimpanzees at Bulindi—a variant of a universal great ape behavior?

Abstract

With data accumulating from a growing pool of chimpanzee field studies, new behaviors as well as novel variants on common behaviors continue to be described. Nest construction is a universal behavior in wild great apes. Among chimpanzee populations, reported variation in nest building behavior mostly reflects environmental constraints. Despite the ubiquity of nest making by chimpanzees, only ground nesting has been recognized as a behavioral variant, potentially determined by both environmental and social factors. In a study of nests made by chimpanzees (Pan troglodytes schweinfurthii) in Bulindi, Uganda, I identified a hitherto undescribed nest construction technique, termed ‘nest tying’. Five observed nests lacked strong weight-bearing structures beneath them, such as large branches or a supporting trunk. Instead, the nests appeared ‘tied’ (or ‘tethered’) to an adjacent trunk by looping leafy stems or palm fronds around it and interweaving these into the nest mattress, securing the nest against the trunk; thus, nest tying presumably functions to provide added stability and support. This preliminary report presents a description of the observed nests. Irrespective of whether nest tying constitutes true knot making—commonly considered absent in wild great apes—this nest construction technique would seem to require advanced dexterity and a sophisticated understanding of the mechanical properties of the plants used. Forest fragments in Bulindi are highly degraded. Thus, nest tying—and construction of integrated nests (i.e., utilizing multiple plants, often small trees and shrubs) generally—may be promoted by a relative paucity of suitable nesting trees at this site. Still, insofar as nest building is learned in chimpanzees, different construction techniques including nest tying are potentially acquired through social learning. Further investigation is required to ascertain the prevalence and acquisition of this nest construction technique at Bulindi, and to verify its presence or absence in other habitats.