Researchers of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, have now found that cooperative relationships are facilitated by an endocrinological mechanism involving the hormone oxytocin, even when these are between non-kin.
They collected urine samples of 33 chimpanzees from Budongo Forest, Uganda, and measured their urinary oxytocin levels after single episodes of a specific cooperative behaviour, mutual grooming.
They found that oxytocin levels were higher after grooming with cooperation partners compared with non-cooperation partners or after no grooming, regardless of genetic relatedness or sexual interest.
This suggests that in chimpanzees oxytocin, which acts directly on neural reward and social memory systems, plays a key role maintaining social relations beyond genetic ties and in keeping track of social interactions with multiple individuals over time.
In non-human primates and other social animals strong and enduring social bonds are typically seen between genetically related individuals but also, occasionally, between non-kin, same-sex individuals.
In humans and other social mammals the neuropeptide hormone oxytocin plays a central role in facilitating bonding between kin and mating partners.
"Our results demonstrate that a rise in oxytocin was dependent upon the combined effects of social grooming with a bond partner," said researcher Catherine Crockford.
"Crucially, oxytocin levels were similarly high after grooming with non-kin and kin bond partners. This suggests that, in chimpanzees, oxytocin plays a key role in maintaining social relations beyond immediate genetic ties," she said in a statement.
"This is the first study that measures the levels of the hormone oxytocin on wild animals in a non-invasive way," said researcher Roman Wittig.
"We have developed a tool with which cross-species comparisons that link underlying physiology and behaviour can eventually be made of social mammals in their natural environment," Wittig said.
