On the Evolution of Cooperation
Newsletter, 03.02.2015
The evolution of cooperation is still one of the great mysteries of evolutionary biology, because it is contrary to the general principle that evolution promotes only selfish genes. This week, a paper is published in the Proceedings of the National Academy of Science (PNAS), where we present the V formation flight as a rare example of true cooperation in animals.
One year ago, in a highly regarded NATURE publication, we were able to demonstrate convincingly that birds in V formation flight can save energy. Individuals use the aerodynamic upwind generated by the bird flying in front. This results in an asymmetry of costs depending on the position in the formation. Such a system could be exploited by cheats, which only fly in convenient positions. The system would thus become unstable.
In the current publication, we show that young Northern Bald Ibises get to a stable flight formation despite cost asymmetry, through mutual cooperation. During a human-led migration flight all birds were equipped with miniaturized GPS loggers, which store their positions with high accuracy. The data show that the birds frequently change their positions in the formation while flying partly on energy-saving following positions and partly on energetically unfavorable leading positions. The individuals alternate their positions in a way that on average all individuals in the group equal their energy balance. The adjustment is done on a pair wise level and is known as direct reciprocity. Due to this mutual cooperation the formation flight can become an evolutionary stable strategy.
Since Robert Trivers postulated direct reciprocity as a potential mechanism for cooperation in the 1970’s, only a handful of studies could provide tentative evidence that animals might be capable of using such a strategy. One of the few well-documented examples is the sharing of blood by vampire bats observed by Gerald Wilkinson. The general scarcity of convincing cases of reciprocation in animals even made some biologists doubt, whether this strategy can be a salient mechanism on its own. The clearness of the evidence found in this study was, therefore, far beyond our expectations. We suggest that several characteristics of migratory flights might have favored the evolution of a cooperation system based on direct reciprocation. Among these some are a substantial benefit to the individual when flying in formation and as a result, a high selection pressure for formation flying, a favorable payoff structure and ample opportunities for repeated interactions on the long distant journeys.
As a study species we have chosen the Northern Bald Ibis as a critically endangered migratory species, because an ongoing reintroduction scheme funded by the European Union LIFE+ initiative, run by the pioneering conservation organization Waldrappteam, allowed us to closely monitor the birds during migration. The current study does not only promote our understanding of the evolution of cooperation but also provides insight into the universally known but still puzzling phenomenon of formation flight. It is rather outstanding for an animal conservation project to have such a high basic scientific output like the reintroduction of this charismatic ibis species.
Citation: Voelkl B., Portugal SJ., Unsöld M., Usherwood JR., Wilson A. & Fritz, J. 2015. Matching times of leading and following suggest cooperation through direct reciprocity during V-formation flight in ibis. Proceedings of the National Academy of Science of the USA (PNAS), Vol. xxx, Pp xxxx-xxxx.
PNAS Link: http://www.pnas.org/cgi/doi/10.1073/pnas.1413589112
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