Food, fear and physics drive collective behaviour in the ocean's pelagic zone.

Understanding how complex, coordinated patterns arise in animal groups has been an important theme in behavioural ecology, revealing much about the mechanisms of the evolution of behaviour. While fish schools are a well-known example, they represent just one case among numerous and diverse collective behaviours exhibited by pelagic animals, including schooling and swarming, vertical migration and cooperative foraging. We argue that key features of the pelagic ocean, including the limitation and patchy distribution of food resources, the lack of cover and the motion of the medium itself, have repeatedly led to the evolution of collective behaviour in ocean animals. Examples from animals as small as plankton and as large as whales, the surface and the deep and invertebrate and vertebrate taxa illustrate how fear, physics and food lead to the evolution of group behaviours. Collective behaviour itself also drives the development of group behaviour in other animals in the habitat. These feedbacks result in group behaviours that simultaneously serve multiple purposes, nested collective behaviours and multi-species collectives. Observations of life in the largest living space on Earth are providing new insights into the evolutionary drivers and adaptive significance of collective behaviour and group living. This article is part of the theme issue 'Multi-predator feeding aggregations in pelagic waters'.