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News Birds strengthen social bonds when they sense trouble MAORI warriors use the haka to bond before battle. Now it seems that birds also demonstrate bonding behaviour when they think they might have trouble with the neighbours. Scientists know that social birds become closer immediately after conflict with other groups, but until now little was known about how the risk of future conflict influenced animal behaviour. Dr Andy Radford of the University of Bristol studied green woodhoopoes to see if they acted differently when faced with possible territorial conflict. These birds live in small groups in permanent territories; conflict between groups is frequent close to territorial boundaries, and allopreening – when one bird preens another – is an important part of group behaviour. The groups typically consisted of a dominant breeding pair and up to six subordinate ‘helpers’. Radford watched the birds in the river valleys of the Eastern Cape Province, South Africa. He noted the length of periods of selfpreening and allopreening, which individuals in the group were involved, and where in the territory the birds were when the preening took place. His results, published in Biology Letters, show that both the frequency of allopreening within the group, and the amount of time the birds spent doing it, increased when the group was at the edge of its territory, where conflict with neighbouring groups is likelier. Radford found the biggest increase was in the amount of preening given by the dominant birds to the helpers in the group. This ‘affiliative’ behaviour is likely to reassure subordinates
Chris van Rooyen
and increase closeness within the group, ensuring the birds all stick together if battle ensues. Surprisingly, when this behaviour was observed there had been no visual or vocal evidence of other woodhoopoe groups for at least an hour. This suggests that, rather than bonding in response to an immediate threat, the birds’ behaviour was in anticipation of a possible future threat. ‘It would be wrong to say this behaviour is firm evidence for forward planning in birds,’ says Radford, ‘but it is very exciting to have seen this link between potential intergroup conflict and current intragroup behaviour in the wild.’
Unique social structures could explain the menopause HUMAN females aren’t the only ones to go through menopause – some whale species also go through a similar ‘change’, and the unique structure of human and whale societies might be responsible, say scientists. Short-finned pilot whales stop breeding when they get to around 36 years, but can live until they’re 65. Killer whales stop having young when they reach about 48 years of age, but often live up to 90 years. This is in line with the socalled grandmother hypothesis, which suggests that by stopping having children early and then helping their existing offspring survive and reproduce, women still benefit by helping to pass on their genes. Among our ancestors, a woman would move to wherever her mate lived. Initially she’d be completely unrelated to members of her new ‘group’, and so would have no incentive to help them reproduce. But by having children, as she aged, she became more related to them. Then it made evolutionary sense to stop having children and help her younger relatives bring up their children. Among mammals, however, it’s unusual for the female to move away from the family she was born into – it’s usually the male that leaves his family group. Mammals with this type of social structure don’t go through a menopause, but continue breeding until they die. Elephants, for example, breed well into their sixties.
‘We were puzzled by this and wanted to understand why you don’t get grandmothers in other long-lived cooperative species,’ says Dr Rufus Johnstone from the University of Cambridge, lead author of the research, which is published in the Proceedings of the Royal Society B. Johnstone and his colleague Dr Michael Cant from the University of Exeter describe how they applied a model of relatedness – or kinship dynamics – to the two species of whale which go through menopause. They found a similar pattern of increased relatedness with age to the one seen in humans. In killer and pilot whale societies both males and females stay with their family groups, but males leave temporarily to mate with females from other family groups, called pods. This means that females are born into a pod which doesn’t contain their father. But as they get older and have young of their own they become more related to other pod members. So it makes sense for older female pilot and killer whales to stop breeding and instead help the younger members of their families raise their offspring. ‘This helps explain why of all the long-lived mammals, menopause has only evolved in humans and toothed whales,’ says Johnstone. ‘It would be good to look into the social structures of whale species we don’t know much about to see how well our theory stacks up,’ he adds.
Planet Earth Autumn 2010
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