A female captures a pup belonging to another tribe and flies out of the cave
Greater spear-nosed bats form maternal tribes that go to war with each other.
Each tribe comprises up to 25 unrelated females who stick together for years.
Not only do these females cooperate to roost and find food, they fly in to rescue each other's infants from danger.
And given the chance, female members of one tribe will try to capture and kill the pups of neighbouring tribes, researchers report in Behavioral Ecology and Sociobiology.
"We were surprised by the aggression between adults and towards pups from other social groups," says Kirsten Bohn, now at Texas A&M University in College Station, US.
"Our discovery has highlighted a remarkably complex society that is organised into cooperating groups that 'war' with other groups."
Bohn and colleagues Gerald Wilkinson and Cynthia Moss of the University of Maryland in College Park, US made hundreds of visits to observe wild greater spear-nosed bats living in Guanapo Cave, a small limestone cave in northern Trinidad, West Indies.
We know that in many cases the females left the cave with the pup, because we could hear it screaming outside
Mammologist Kirsten Bohn
A single entrance to the cave leads to a small, nearly circular chamber about 10m wide and 3m tall.
Within this chamber roosts approximately 400 female bats, which form around 20 social groups.
"Given their social structure, we thought the odds were high of some sort of cooperative behaviour," says Bohn.
But the researchers were stunned by what they discovered.
Genetic tests revealed that each social group comprises females that are not related to one another. Previous research has shown that these females call to attract other tribe members to rich sources of food. Now Bohn's team has found that they also guard each other's young.
Each female gives birth once a year, and the pups are unable to fly for the first six weeks of their life. During this period pups frequently fall from roost sites in the cave ceiling to the cave floor, where they will die if not retrieved by an adult.
A tribe gathers.
When this happens, pups cry out for help, emitting high pitched 'isolation calls'. Upon hearing these cries for help, other females in the tribe fly into action, flying down to the stricken pup to guard it, often grooming the infant and trying to get it to attach to their body in a nursing position. If it does it is then flown back to the cave wall to safety.
"One would expect each pup to be visited and then retrieved by only a single female, the mother. Instead, we observed on average 17 visits per pup with over 300 visits to a single pup," says Bohn.
This guarding behaviour is particularly important for greater spear-nosed bats because of the tribes' war-like tendencies.
Female spear-nosed bats regularly fight those belonging to other tribes, the researchers have found. Females will bare their teeth, bite and chase those who are not part of their own tribe.
They also attack their pups.
Sometimes females will bite and attempt to capture stricken pups.
"This is when a female grabbed a pup by the teeth and flew off. They happened extremely quickly and we know that in many cases the females left the cave with the pup, because we could hear it screaming outside."
To prevent this happening, females would often guard the pups belonging to their tribe, regardless of whether they had given birth to the infant.
Females fly in to retrieve a lost pup belonging to their tribe
"Many females would land and just 'hang out' for long periods of time seemingly doing nothing," says Bohn. "These were the guarders. They would respond to any bat that was aggressive towards pups."
Why greater spear-nosed bats form such complex, tribal groups is difficult to understand.
"Cooperation among unrelated individuals is of great interest because, although common in humans, it is exceedingly rare in nature," explains Bohn.
"In cooperative breeders such as meerkats and other species, not only are helpers usually related to the breeding pair but they do not reproduce themselves. I can not think of any cases of adults leaving their own offspring to care for others."
While the adults are away, the pups hold on.
Bohn's team has ruled out a few reasons.
"We tested and rejected the most common models of cooperation: reciprocity and mutualism," says Bohn.
"Females are not simply reciprocally guarding each other's pups nor do they gain immediate benefits by guarding others."
Her team believes that such cooperation must benefit the bats, but in more complex ways.
Such benefits may emerge over time, as the greater spear-nosed bat is a long-lived species that can live for 20 years of more.
Bohn's studies have already shown that certain tribes stick together for up to 16 years and will roost in the same depression in a cave year after year. If disturbed, the whole tribe will move home together.
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