An animal behaviour study has challenged the assumption that complex, dynamic social networks are unique to animals with big brains.
Elephants, dolphins, some carnivores and primates maintain long-term social links despite frequently splitting and merging - a phenomena known as fission-fusion dynamics.
The mechanism allows groups to adjust to variable ecological demands without losing the benefits that come with grouping.
Various hypotheses have attributed the evolution of large brains in humans and other primates to the highly socio-cognitive demands that complex societies create.
But in a paper published today in the Proceedings of the Royal Society B, European researchers have discovered bats can form similar social networks.
"Our study shows that animals with a peanut-sized brain, like these bats, can also have stable long-term relationships," says study co-author Professor Gerald Kerth of the University of Greifswald in Germany.
Ten of thousands of observations
Kerth, along with Professors Nicolas Perony and Frank Schweitzer of ETH Zurich in Switerzland, say their study may shed light on the link between complex social groups and social cognition - the ability to learn about others within a group - in mammals.
Kerth and his co-authors observed two colonies of wild Bechstein's bats (Myotis bechsteinii) over a five-year period. The colonies were exclusively female, as the males are solitary. One colony fluctuated between 11 and 18 bats, while the other comprised between 33 and 42 bats.
The bats were monitored with microchips implanted in the first year of life, and a sample of wing tissue was collected for genetic identification. Over the five-year study their individual size, as well as yearly reproductive status was determined. They also collected 20,500 individual roosting observations.
By studying roosting partners and then applying a weighted network analysis, they showed individuals of different age, size, reproductive status and families maintain long-term social relationships. They also found that the social structure differed between the colonies.
In the larger colony they detected two stable subgroups, each made up of several family lineages. Links between the groups were mainly maintained by older bats.
Kerth and colleagues found the bats break-up colonies into communities of about 20 individuals for several reasons. They believe it may be due to constraints in their cognitive ability, and to maintain the number of social links within a range where cooperation still works.
"How complex social systems function is a key question in biology, economics and social sciences," they write.
"Answering it requires information about the structure and dynamics of the social network, which characterises the relationships among group members."
They add detailed long-term data sets on such dynamics in wild mammals such as this are scarce.
Link weaker than previously thought
The researchers conclude that the link between social complexity and social cognition in mammals is weaker than previously assumed.
"The crucial social factors that make our social lives cognitively demanding have to be better defined," Kerth says, "We need more studies dealing with the question of where simple mechanisms are sufficient to maintain social complexity, and where high cognitive abilities are needed."
"It would be fantastic to know more about where the cognitive limits of the bats are, and what the benefits are of having stable social links in bats," says Kerth.
Dr Terry Ord of the University of New South Wales in Sydney says the study is impressive.
"They've really been very thorough. It's very rare to have the type of data that they've been able to accumulate, and over such an extent of time," says Ord. "You really need to have this type of data if you want to demonstrate long-term partnerships between social animals."
Ord says that although the findings are interesting, they're not necessarily surprising.
"In hindsight, it's one of those things where you go 'Hmmm, well, that kind of makes sense'. But you wouldn't necessarily anticipate it off the bat. No pun intended."