Proto-Cooperation: group hunting sailfish improve hunting success by alternating attacks on grouping prey

TL;DR

This study reveals a novel form of proto-cooperation in sailfish, where alternating attacks on prey increase hunting efficiency without complex coordination, supported by empirical data and a mathematical model.

Contribution

It introduces the concept of proto-cooperation in group hunting, demonstrating how alternating attacks enhance success without requiring spatial coordination.

Findings

Group hunting increases prey injury and capture rates.

Alternating attacks improve hunting efficiency up to 70 group members.

Free riding strategies are only advantageous under high attack costs.

Abstract

We present evidence of a novel form of group hunting. Individual sailfish (Istiophorus platypterus) alternate attacks with other group members on their schooling prey (Sardinella aurita). While only 24% of attacks result in prey capture, multiple prey are injured in 95% of attacks, resulting in an increase of injured fish in the school with the number of attacks. How quickly prey are captured is positively correlated with the level of injury of the school, suggesting that hunters can benefit from other conspecifics' attacks on the prey. To explore this, we built a mathematical model capturing the dynamics of the hunt. We show that group hunting provides major efficiency gains (prey caught per unit time) for individuals in groups of up to 70 members. We also demonstrate that a free riding strategy, where some individuals wait until the prey are sufficiently injured before attacking, is only beneficial if the cost of attacking is high, and only then when waiting times are short. Our findings provide evidence that cooperative benefits can be realised through the facilitative effects of individuals' hunting actions without spatial coordination of attacks. Such "proto-cooperation" may be the pre-cursor to more complex group-hunting strategies.