Bottlenose dolphin (Tursiops truncatus) sonar slacks off before touching a non-alimentary target

Odontocetes modulate the rhythm of their echolocation clicks to draw information about their environment. When they approach preys to capture, they speed up their emissions to increase the sampling rate of "distant touch" and improve information update. This global acceleration turns into a "terminal buzz" also described in bats, which is a click train with drastic increase in rate, just as reaching the prey. This study documents and analyses under human care bottlenose dolphins' echolocation activity, when approaching non-alimentary targets. Four dolphins' locomotor and clicking behaviours were recorded during training sessions, when sent to immersed objects pointed by their trainers. Results illustrate that these dolphins profusely use echolocation towards immersed non-alimentary objects. They accelerate click emission when approaching the target, thus displaying a classical terminal buzz. However, their terminal buzz slackens off within a quarter of second before the end of click train. Typically, they decelerate to stop clicking just before they touch the object using their rostrum lower tip. They do not emit clicks as the contact lasts. In conclusion, when exploring inert objects, bottlenose dolphins under human accelerate clicking like other odontocetes or bats approaching preys. Bottlenose dolphins' particular slackening-off profile at the end of the buzz shows that they anticipate the moment of direct contact, and they stop just as real touch relays distant touch of the object.