When we pick up a pen to write, we tend to use either our right or our left hand. This lateralisation helps us react rapidly and cuts down on how much nerve tissue we need. But if you are a predator, lateralisation may be costly: if prey can anticipate your angle of attack, it may help them escape. This predictability might select against lateralisation in predatory species. However, Ralf Kurvers from the Center for Adaptive Rationality in Berlin, and an international team of collaborators suspected that group-hunting sailfish, Istiophorus platypterus, might have the best of both worlds. As sailfish attack schools of sardines by slashing and tapping their prey with elongated bills, the team wondered whether the communal fish might dodge the costs of lateralisation while exploiting its benefits by hunting in a pack, where, despite individual-level lateralisation, attacks still come from all angles.
Snorkelling off the coast of Cancun in Mexico, the researchers filmed wild sailfish hunting and analysed the movies back in the lab. The team then identified individual sailfish using each fish's unique dorsal fin morphology and found that sailfish consistently attack from either the right or the left, suggesting that the fish demonstrate lateralisation. Knowing that the fish appear to favour one side over the other, the team suspected that the teeth lining the dominant side of the fish's bills would be more abraded, in much the same way that humans are more likely to injure their dominant hand. To test this idea, they compared the condition of micro-teeth in sailfish bills and found that tooth abrasion was always more pronounced on one side or the other. Together, this behavioural and morphological evidence points to lateralisation in individual sailfish.
So, if sailfish are lateralised, does this reduce their hunting success by helping sardines to pre-empt their angle of attack? The team answered this question by seeing how the strength of lateralisation in each sailfish correlated with its sardine capture success. The result was clear: increased lateralisation promotes hunting success and sailfish are more likely to successfully snatch a sardine if attacking from their preferred side.
Given that lateralisation often incurs costs for predators, how does this trait help sailfish on the hunt? The team suspected that group hunting could reduce the costs of lateralisation if hunting groups contain a mix of right- and left-billed sailfish. To test this idea, the team randomly selected individuals that they observed hunting at least three times, and put them together into random groups that varied in size from 1 to 15 fish. They calculated how lateralised these groups were by averaging the mean laterality of each of the group members. As the hunting groups increased in size, the overall degree of lateralisation across the entire group declined. That is, although individuals may be lateralised, the net lateralisation of the group is reduced as the number of left-lateralised fish cancels out the number of right-lateralised animals. So, hunting in a group provides the unexpected benefit of allowing individual predators to exploit the benefits of lateralisation, while their prey still have no idea which direction the next attack is coming from.
