The team, led by Karline Janmaat from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, recorded the behaviour of five chimpanzee females for continuous periods of four to eight weeks, totalling 275 complete days, throughout multiple fruiting seasons in the Tai National Park, Cote d'Ivoire.
They found that chimpanzees fed on significantly larger trees than other reproductively mature trees of the same species, especially if their fruits emitted an obvious smell.
Interestingly, trees that were merely checked for edible fruit, but where monitoring could not have been triggered by smell, or the sound of fallen fruit, because the trees did not carry fruit, were also larger.
The researchers found that chimpanzees checked most trees along the way during travel, but 13 percent were approached in a goal-directed manner.
These targeted approaches were unlikely initiated by visual cues and occurred more often when females foraged alone and when trees were large as opposed to small.
The results suggested that their monitoring was guided by a long-term "what-where" memory of the location of large potential food trees. For their results, researchers analysed which of nearly 16000 potential food trees with different crown sizes were actually approached by the chimpanzees.
Long-term phenological data on individual trees indicated that the interval between successive fruiting seasons, and hence the minimal "memory window" of chimpanzees required for effective monitoring activities, could vary from two months to three years.