Dolphins plan their dives
Underwater recordings reveal the cetaceans delay reaching the surface in order to gather intelligence useful for future hunts. Tanya Loos reports.
Dolphins have been found to ignore the pressing need to breathe in order to plan for their next hunting dive, a behaviour scientists say comprises the first evidence of planning in sea mammals.
Dolphins and other mammal and bird marine predators, such as seals and penguins, forage for food bound by two strict parameters – the need to find enough food at depth, and the need to return to the surface to breathe. Penguins and seals have been shown to increase their oxygen store before they dive if they anticipate a particularly deep excursion, but direct evidence of future planning has been lacking in cetaceans.
The brainy superstars of the avian kingdom, the ravens, have been shown to plan, with scientists devising complicated puzzles to confirm that the birds can and will delay gratification if it means greater reward in the future. It is somewhat trickier to find evidence of planning in wild populations of animals such as dolphins, even though they are known to be highly intelligent.
Risso’s dolphins (Grampus griseus) dive down several hundred metres to hunt shoals of squid off the coast of the US. A multi-agency group of scientists led by Patricia Arranz and Peter Tyack from St Andrews University in the UK used data loggers and echo-sounders to track the mammals and their prey to determine what strategies the mammals use. The results are published in the Journal of Experimental Biology.
The team attached data-loggers to 33 dolphins to record depth and movement, as well as the sounds they emitted. The locations of the squid shoals were recorded by echo-sounders. Then, back in the lab, information from 37 dives was analysed.
The study revealed that at the start of a dive, the animals selected an echolocation range that targeted the best prey-populated depth encountered on the previous one – “which can be interpreted as dolphins recalling information to plan the next foraging dive,” says Arranz.
The team also found that the dolphins were echolocating throughout their ascent to the surface after feeding at depth, even though at that point they had lower motivation to eat and higher motivation to breathe.
The researchers regard this as evidence that the dolphins are planning their next dive – sampling prey at different depths with their buzzes and clicks to build up a mental map in anticipation of their next hunting sortie.