Though mongooses be but little, they are fierce. These gutsy mammals are famous for facing the worst dangers of their African habitats head-on, from venomous snakes to big cats. But they also get into nasty fights with each other. Surprisingly, when conflict breaks out, it seems these animals are most likely to target their own family.

Banded mongooses spend their lives in close-knit groups, and much of the time they're models of good social behaviour. The whole crew works together to find food, raise and feed newborns, and even care for the old or sick. When danger approaches, the mongooses will often congregate around the youngest members to protect them.

But member mongooses are allowed to stick around only until one of the dominant females decides otherwise. If there are too many breeding adults around, it can take away resources from the offspring of the group's leaders. When that happens, the ladies in charge (usually the oldest in the pack) start kicking out other members – and these confrontations can be shockingly violent.

Scientists at the University of Exeter spent 18 years studying banded mongooses in the wilds in Uganda. In that time, they saw lots of forceful evictions, and they noticed a pattern: the dominant females weren't kicking out random group members – they were specifically going after their own close relatives!

"Targeting close relatives for eviction like this is the opposite of what we would expect social animals to do," said Faye Thompson, lead author of the new study. When it comes to social animals, nature typically favours those who show preferences for their siblings and cousins – this is called 'kin selection'."

Why the unusual behaviour? The researchers wondered if it had to do with reducing inbreeding caused by mating within the family, or if cousins were more intensely competing with each other, but neither of these hypotheses held up under scrutiny. Instead, it seems that the dominant mongooses are taking advantage of family ties: when under threat of eviction, close relatives are less likely to fight back.  

"Our research shows that related females submit more easily because they are more sensitive to the costs they inflict on their relatives by fighting to stay in the group," Thompson explained. "As dominant banded mongooses need to evict rival females to reduce competition for their own offspring, the best strategy is to target close relatives."

"But we're family!" These two mongooses are prepared for a fight. Image: Dave Seager

So while the unfortunate low-ranking members are sticking true to familial kindness, the dominant females are relying on strategic betrayal. And being evicted from a group isn't a polite affair. The mongooses that are kicked out – usually females, though males are sometimes targeted as well – often end up suffering injuries or even dying in the process of being violently removed.

"We've long wondered why some individuals are marked out for violent attack and eviction, whereas others are permitted to stay," said study co-author Michael Cant. "Our new study shows that a crucial determinant is whether victims can put up a fight."

In the time the researchers observed the mongooses, this familial rivalry came into play only when a dominant female decided to evict group members old enough to defend themselves. If they aimed to remove a particularly small or young mongoose (including the occasional infanticide), it didn't seem to matter if the helpless targets were relatives or not.