Ralph Martin/BIA/Getty
By Sam Wong
When animals that live in groups take it in turns to keep watch for predators while others forage, it appears to be altruistic. But now it seems they might mainly be out for themselves.
Birds called Arabian babblers usually live in territorial and hierarchical groups of up to 20 individuals, but sometimes one low in the pecking order will go it alone. These individuals are called floaters, and they are usually attacked or chased away if seen by the territory-owning group.
Roni Ostreiher and Aviad Heifetz at The Open University of Israel have been observing Arabian babblers (Turdoides squamiceps) at the Shezaf Nature Reserve in Israel for 28 years. All birds in the study area are fitted with coloured rings so they can be identified individually.
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The babbler groups’ sentinel activity has been studied extensively but when the researchers started watching floaters, they were surprised to see them engaging in similar behaviour, scanning their surroundings for several minutes and even uttering alarm calls when they saw approaching predators, especially birds of prey.
This suggests that sentinel behaviour is at least partly down to selfish motives. “It doesn’t mean that others can’t benefit, but an individual acts as a sentinel first of all for itself,” says Ostreiher.
Vigilant spells
When Ostreiher and Heifetz compared the sentinel activity of individual floaters with their behaviour when they were group members, they found that floaters spent less time being watchful. That might be because finding food is more difficult for individuals, so they have to spend more time foraging.
But during their vigilant spells, floaters made alarm calls just as often as they did when they were group members.
Amotz Zahavi, another Israeli biologist who has spent a lot of time watching Arabian babblers, has proposed that alarm calls are a way to communicate with the predator rather than for alerting other group members. If the predator learns it has been spotted, it knows that an attack is unlikely to be successful, and so may look for prey elsewhere.
After an initial alarm call from one bird, often the entire flock will join it on the tree and issue the alarm call together, giving credence to this hypothesis. The fact that solitary Arabian babblers sound alarm calls too adds further support to the idea.
“The predator’s chance to succeed at predating is when nobody knows about its presence,” says Ostreiher. “It’s in the interest of the predator to listen to the prey, to not waste time, and to go to other areas where nobody has discovered him.”
Mutual benefit
Evidence from other species, such as meerkats, also suggests that acting as a sentinel helps the individual, because they detect potential threats with enough time to get away.
But labelling the behaviour as selfish misses two important points, says Peter Bednekoff of Eastern Michigan University. “It is the mutual benefit of shared detection that can lead to taking turns as sentinels,” he says.
Secondly, sentinels may act to protect others. For example, a study found that meerkats act as sentinels more when they have pups to protect.
It’s reasonable to conclude that the Arabian babblers’ alarm calls are a signal to predators, but some other alarm calls are clearly directed towards vulnerable young or other companions, says Bednekoff. “I think this question needs to be investigated case by case,” he says.
Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.160738
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