Researchers from the Max Planck Institute for Psycholinguistics in Nijmegen, The Netherlands, found that orphaned chimpanzees are less socially competent than chimpanzees who were reared by their mother.
They observed that orphaned chimpanzees frequently engaged in social play, but their play bouts were much shorter and resulted in aggression more often.
This suggests that chimpanzee mothers endow their offspring with important social skills, researchers said.
"Orphaned chimpanzees had more difficulties to successfully coordinate their social play interactions," said Edwin van Leeuwen from the Comparative Cognitive Anthropology Research Group at the Max Planck Institute for Psycholinguistics.
"Since social play comprises a complex context in which signals about intentions need to be communicated, it seems that orphaned chimpanzees have missed out on valuable lessons from their mothers," van Leeuwen said.
Leeuwen and his co-authors Innocent Mulenga and Diana Lisensky compared the play behaviour of 8 orphaned and 9 mother-reared juvenile chimpanzees at the Chimfunshi Wildlife Orphanage Trust in Zambia.
In this institution the orphan chimpanzees are initially cared for by humans. As soon as they are strong enough - usually with one or two years of age - they grow up in an orphan chimp group.
"The chimps in the study were between four and nine years old, so they have kind of been raising each other," said van Leeuwen. The orphaned and mother-reared chimpanzees matched in age and sex.
The scientists found the orphaned chimpanzees engaged in social play more frequently than the mother-reared juveniles, although for shorter amounts of time.
However, social play of the orphaned juveniles resulted more often in aggression than social play of the young chimps that were reared by their mother.
"Although the orphaned chimps were motivated to play it seems that they were less able to coordinate their play bouts and prevent them from resulting in aggression," van Leeuwen said.
Just like in humans, chimpanzee mothers seem to be important for the development of adequate social skills in their offspring, the researchers concluded.


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