New York, May 22 (IANS): Female chimpanzees who rank low in terms of reproductive success seek out other girl pals with similar status, finds a new study on social relationships among apes.

Unlike most primates, female chimps are loners compared to males.

"They spend about half their time alone or with dependent kids," said study co-author Steffen Foerster from the Duke University in the US.

Despite their reputation for being aloof, recent studies suggest that some pairs of female chimps hang out together more than others.

To find out if some females forge stronger bonds than can be explained by chance encounters or range overlap alone, the researchers analysed 38 years' worth of daily records for 53 adult females in the Gombe National Park in western Tanzania.

Over the decades, the females were spotted in more than 600 female-female pairs.

The team found that some female-female relationships are tighter than others.

Not surprisingly, mothers, daughters and sisters formed the strongest bonds.

But among unrelated females -- which made up more than 95 percent of the twosomes they studied -- low ranking females were more likely to seek each other out than females from other social ranks.

The researchers are still working out whether the low ranking pairs are true buddies, friends of convenience or merely acquaintances.

"It does not necessarily mean that they like each other," Foerster said.

"The lowest ranking females are the newest to arrive. When a female migrates into a new group, she starts at the bottom of the social ladder. It may be that they are not really that into each other, but that they need to tolerate being in the same space," he noted.

The study appeared online in the journal Animal Behaviour.