Living beside familiar neighbours boosts a squirrel's chances of survival and successful breeding, new research shows.
The study measured year-to-year survival of North American red squirrels - and found keeping the same neighbours was so beneficial that it outweighed the negative effects of growing a year older.
However, living near genetic relatives did not improve survival rates.
The research - part of the Kluane Red Squirrel Project - used 22 years of data on squirrels in Yukon, Canada, within the traditional territory of the Champagne and Aishihik First Nations.
"These squirrels are solitary - each defending a territory with a 'midden' (food stash) at the centre - so we might assume they don't cooperate," said lead author Dr Erin Siracusa, of the Centre for Research in Animal Behaviour at the University of Exeter.
"However, our findings suggest that - far from breeding contempt - familiarity with neighbours is mutually beneficial.
"Defending a territory is costly - it uses both energy and time that could be spent gathering food or raising pups.
"It may be that, after a certain time living next to one another, squirrels reach a sort of 'agreement' on boundaries, reducing the need for aggression.
"Competition is the rule in nature, but the benefits identified here might explain the evolution of cooperation even among adversarial neighbours."
The study looked at the "neighbourhood" within 130 metres of a central territory, examining both "kinship" (how closely related the squirrels were) and "familiarity" (how long individual squirrels occupied adjacent territories).
Researchers also studied survival rates and breeding success - for males this was measured by number of pups sired, and for females it meant pups surviving their first winter.
The team were surprised to find the benefits of familiar neighbours outweighed the effects of ageing.
Ageing alone reduced annual survival rates from 68% (age four) to 59% (age five).
However, squirrels that maintained all their neighbours had a 74% chance of surviving a year from age five.
"Although we don't have evidence of direct cooperation among familiar neighbours - such as working together to fight off an intruder - it's clear that neither benefits if their neighbours die," Dr Siracusa said.
"Whatever the nature of their interactions, our study shows that even solitary species have important social relationships."
Dr Siracusa said the lack of evidence in the study for kinship being beneficial doesn't necessarily mean related individuals do not cooperate.
"Genetic relatedness in the neighbourhoods we studied was relatively low, and it's possible that kin might be important at a smaller scale than the 130m radius we used," she said.
"Other studies have found that related squirrels are less likely to rattle (an aggressive sound) at each other, and kin will sometimes share a nest to survive the winter."
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The Kluane Red Squirrel Project was started at the University of Alberta and has been led by researchers at the University of Alberta, University of Guelph, University of Michigan and University of Saskatchewan.
Funders included the Natural Sciences and Engineering Research Council of Canada, the National Science Foundation, the American Society of Mammalogists and the Arctic Institute of North America.
The study, published in the journal Current Biology, is entitled: "Familiar neighbors, but not relatives, enhance fitness in a territorial mammal."
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