Mob culture unravelled

Abstract

One of the most intriguing issues in animal behaviour is that of altruism, particularly between unrelated individuals. For if such altruism is not reciprocated, biologists believe that it would not be able to persist as cheats would win out. So when does it pay to help others? One puzzling case is that of prey mobbing potential predators. Members of many species of small birds will mob a potential predator when spotted. They will utter characteristic sounds that attract other prey individuals of different species to the vicinity. Mobbing usually causes the predator to leave the area, which gives an opportunity for prey individuals to continue their normal activity. But mobbing behaviour also carries risks of injury or death. The individual that first raises the alarm to the presence of a predator may be at increased risk of predation by attracting the predator's attention, especially if not quickly joined by other prey individuals in the mob. Two researchers, Indrikis Krams and Tatjana Krama, at Daugavpils University in Latvia, have looked at whether there is any evidence of reciprocal altruism in mobbing behaviour in the presence of different prey species (Proc. Roy. Soc. B 2002, published online). They tested whether a male chaffinch (Fringilla coelebs), a migrant bird in Latvia, would instigate a mobbing attack on a predator when in a community of returning, migrant birds unknown to each other at the start of the breeding season compared with chaffinches that joined a sedentary group of birds that are more likely to know each other. Migrant birds, forming anonymous communities at the beginning of the breeding season, may also develop closer social ties during the course of the summer, the researchers considered, that may allow the development of reciprocal help. The birds inhabit small patches of woodland in a mosaic landscape of open land comprising bogs, lakes and rivers. Such woodland patches present relatively isolated habitats with little movement of birds between patches during breeding. Fellow migrant species include tree pipits, starlings, wrens, dunnocks icterine warblers, garden warblers, blackcaps, whitethroats, willow warblers, flycatchers, robins and song thrushes. All these birds show mobbing behaviour of potential predators. The researchers expected that the male chaffinches would be less likely to initiate a mob at the beginning of the breeding season when surrounded by strangers. However, they suggested, mobbing activity might increase as the breeding season advances and they knew their neighbours. Using a stuffed owl deployed around the patch of woodland containing the birds, the results support these predictions. In contrast, they looked at chaffinches that had joined a woodland patch inhabited by sedentary species where there was a greater likelihood that the birds already knew each other. Here they found the chaffinches were much more confident in raising the alarm as soon as they arrived, compared with those in association with fellow migrants who did not know each other. The results suggest the first clue that reciprocal altruism may exist between members of different species – when they know each other – in tackling a common enemy.