Mongoose teamwork hides darker side
22 February 2010, by Tom Marshall
Banded mongoose society looks harmonious and cooperative, but research over a decade and a half shows this is just a thin veneer hiding fierce competition.
Mongoose groups have an unusual social structure that is helping scientists understand why some animals are willing to make sacrifices to help others, and what the limits are to this kind of cooperation.
The findings are emerging from a long-term study programme monitoring ten mongoose packs living in Uganda. 'Banded mongooses are a great model system for studying the evolution of cooperation in animals - their social structure has some unique and very interesting features,' says Dr Michael Cant of the University of Exeter, who initiated the project in 1995. The research inspired Banded Brothers: The Mongoose Mob, a new BBC 2 documentary that's being aired from 21 February 2010.
One area of particular interest is the way mongoose mothers synchronise their childbearing. In meerkats and many other social animals, only one female at a time gets to breed, and those further down the pecking order have to content themselves with looking after the breeding female's young in the hope of spreading their genes by helping young cousins and other relatives survive.
In contrast, in banded mongoose groups, several females breed at the same time, and they give birth on exactly the same day. This helps protect their pups. New research suggests that if young mongooses are born too early, older and higher-status females are likely to kill them in an effort to increase the chances their own young will survive. Once these high-ranking females have young of their own they won't kill pups - female mongooses don't seem to be able to tell one pup from another, and so they don't risk killing their own offspring.
But there are also costs to giving birth too late: if pups are born later than their peers, there's a good chance they'll lose out to older pups in the competition for food and adult attention. The result is a remarkable degree of birth synchrony, whereby up to 10 females all give birth at the same time.
By temporarily dosing female mongooses with contraceptives to limit the number of births, the researchers have learned that the optimum number of females that can give birth at the same time is around three. A mongoose born with no peers of the same age has almost no chance of living to adulthood.
'The adults just don't seem to take things seriously if there aren't enough pups,' Cant explains. 'They don't spend much time looking after them, or feeding them, and they sometimes fail to leave babysitters behind to protect them when the group goes off to find food.' But if there are too many pups, on the other hand, there is not enough food to go round and several will starve.
Because it lets more pups avoid death by infanticide, synchronised childbirth improves low-ranking females' chances of breeding successfully. But it is also likely to push the number of pups in the group well over the optimum total, so it harms the group as a whole. If the food supply runs out, middle-aged females are driven out in often-violent ejections; these females have a better chance than their younger rivals of surviving on their own and founding another group.
So while synchronised birth looks on the surface like a cooperative way to let more individual mongooses bear children, it seems to have evolved in response to routine infanticide and is effectively a way for young females to seize advantage at the expense of their more senior rivals.
'The more we look into this question, the more complex we find it is,' Cant says. 'Birth synchronisation seems to be a cooperative behaviour, but as you look deeper you see lots of conflict beneath the surface. Mongoose society is balanced on the edge between cooperation and conflict.'
He says that compared to the closely-related but much better-known meerkats, ruled by a dictatorial queen, mongoose dominant females have lost authority. They need to allow some females to breed to reach an optimum number of youngsters, but this exposes them to the risk that even more females will seize the opportunity and that the group will end up with too many offspring.
Cant also highlights his team's investigation of 'escorts' - males without the high status needed to breed, who each form a close relationship with a single youngster and devotes considerable time to looking after it and keeping it out of trouble. 'This is unique among mammals - there are no other examples that we know of adults having a one-on-one caring relationship with young animals that aren't their own offspring,' he explains.
The group is now focusing on new areas, such as ageing in mongooses. Females live for a maximum of nine years or so, whereas males can reach 13. Cant thinks this may be because females can start breeding at just a year old, whereas males must often wait years to get their chance, as only high-ranking males get to breed.
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