Dr Edwin van Leeuwen, of St Andrews University, lead author of the study, which was published this week in the journal Scientific Reports, said: "Noel approached Thomas' body, sat down close to his head, turned her upper body sideways to select a hard piece of grass, put the grass in her mouth, and opened Thomas' mouth with both of her hands.
"Then she wrapped her fingers around Thomas' chin and jaw, and used her thumbs to explore his teeth. After three seconds, she took the grass out of her mouth with her right hand, while maintaining focused grip on Thomas' mouth with her left hand, and started to meticulously poke the grass in the same dental area as where her thumbs had been.
"To date, this behaviour has never been reported in chimpanzees or any other non-human animal species.
"Chimpanzees may form long-lasting social bonds and, like humans, may handle corpses in a socially meaningful way."
Nina, her adolescent daughter, stayed at her side and observed the cleaning efforts of her mother.
The researchers say Noel may also have been trying to understand how Thomas had died. She was seen tasting the debris she had picked from his teeth. A post mortem examination revealed that Thomas was likely to have died from a combination of a viral and bacterial lung infection.
However, Klaus Zuberbuehler, also of St Andrews, said scientists should be cautious in interpreting the behaviour.
"Perhaps the chimpanzees are just challenged by the fact that a group member has suddenly become completely motionless," he said.
Thibaud Gruber, of the University of Geneva, told New Scientist that the chimps might have a limited understanding of death.
"We simply do not know if and how much chimps understand about death," he said.
"In other words, it is unclear whether this is 'corpse cleaning', or simply 'social cleaning'. But certainly, it adds on behavioural descriptions of unusual behaviour displayed by chimps when they face the death of one of their species."
