In the waters near Laguna, Brazil, local fishermen often stand in waist-deep water in a line or wait in a canoe, while bottlenose dolphins chase groups of carmine fish not far away and drive them to Rush to the shore. Fishermen can’t see fish in the muddy water, so they wait for the dolphin to send out a signal—such as suddenly diving into the water or flapping its tail on the surface—before they start casting their nets.
This kind of cooperation between dolphins and humans is very rare. In most places, fishermen usually see dolphins as competitors and sometimes even as prey.
Laguna’s bottlenose dolphins have been cooperating with local fishermen since at least the end of the 19th century, presumably because such cooperation is beneficial to both parties. For fishermen, by cooperating with dolphins, they can catch more big fish; and dolphins may also get similar benefits. For example, fish are more likely to be caught by them after they get confused by fishing nets.
However, not all Laguna dolphins are happy to cooperate with humans. Therefore, the researchers divided them into two categories: dolphins that are willing to cooperate and dolphins that are not willing to cooperate. “Cooperation” here refers specifically to cooperation with humans.
Interestingly, these two types of dolphins seem to “lack a common language”—their screams seem to be different when they hunt (whether foraging alone or in cooperation with humans). The dolphins who are unwilling to cooperate more often use long, slowly rising screams; the dolphins who are willing to cooperate prefer to use short, high-pitched screams, with more tortuous changes in tones.
