Most of us start to tire after about half a day without any sleep. Staying awake for five in a row would be extremely difficult and even if you could manage it, you’d be a physical and mental wreck by the end. But not all animals suffer from the same problem. A dolphin can stay awake and alert for at least 5 days straight, chaining together all-nighters without any noticeable health problems or loss of mental agility.
The two halves of a dolphin’s brain can sleep in shifts, “shutting down” one at a time so that the animal is always half-awake. They can truly sleep with one eye open, an essential skill for an animal that has to be constantly watching for predators and timing its breaths.
Previously, Sam Ridgway from the US Navy Marine Mammal Program found that bottlenose dolphins could respond to a distinct noise for 5 straight days without any dip in accuracy. He trained a female called Say to recognise occasional 1.5-second beeps amidst a background of shorter 0.5-second ones. When she heard the longer tone, she pressed a lever for food. Say was slower to respond at night, but overall, her reaction times didn’t slow over the five day run.
It was an impressive performance, but one that didn’t require much in the way of thought. This time, Ridgway wanted to see if the dolphin’s mental skills would take a hit after five days of continual vigilance. To do that, he trained Say, and another male called Nay, to make different noises for two different visuals – a whistle for a single, vertical, green bar of light, or a burst of sonar pulses for three, horizontal, red bars. Incidentally, dolphins are colour-blind; the colours were for the scientists’ benefit.
In Shark Bay, off the Western coast of Australia, a unique population of bottlenose dolphins have a unusual trick up their flippers. Some of the females have learned to use sponges in their search for food, holding them on the ends of their snouts as they rummage through the ocean floor.
To Janet Mann at Georgetown University, the sponging dolphins provided an excellent opportunity to study how wild animals use tools. Sponging is a very special case of tool use – it is unique to Shark Bay’s dolphins and even there, only about one in nine individuals do it. The vast majority of them are female. A genetic analysis revealed that the technique passes down almost exclusively from mother to daughter, and was invented relatively recently by a single female dolphin, playfully named “Sponging Eve”.
Dolphins tend to sponge only in deep water, which is why little has been done to study this behaviour since its discovery a decade ago. Now, Mann has published the first detailed analysis of dolphin sponging. She watched every dolphin who knows the technique and analysed how much time they spent on it and what it meant for their success at raising calves. This incredibly thorough analysis revealed that sponging dolphins are the most intense tool-users of any animal, except for humans.
Mann confirmed that the dolphins were using the sponges to root out potential meals. On days when the sea was exceptionally clear, she could see the animals swimming slowly along the sandy bottom while wearing their spongy muzzles and disturbing the sand. When they spotted something, they dropped the sponge and probed about with their beaks, often surfacing with small fish that they quickly swallowed. Meal in throat, they retrieved their unusual hunting aid and started again.
The technique worked for humans too. Mann’s team tried it themselves for four hours with sponges over their hands, and consistently ferreted out the same species of fish – the spothead grubfish. Before the sponges were used, the fish were completely invisible to the divers but once revealed, they were easily spotted, tracked and found again when they reburied themselves. A single photo of a sponging dolphin with a fish in her mouth, while blurry, suggests that they too could be after grubfish.
You might not be that impressed to receive a clump of grass or branches on a first date, but a boto dolphin might think differently. A new study suggests that these Amazonian dolphins wave bits of flotsam to attract mates.
The boto is a freshwater river dolphin that swims through the currents of the Amazon and the Orinoco. They are elusive creatures that are difficult to study, so very little is known about their social lives.
Tony Martin from the University of St Andrews spent three years in the Amazonian Mamiraua reserve studying the behaviour of botos. During this time, he spotted over 200 groups of dolphins playing with objects, a behaviour that other scientists have noted throughout the animals’ range and was recently filmed in the “Fresh Water” episode of Planet Earth.
The botos pick up branches, sticks, vegetations and lumps of clay in their mouths, often thrashing them against the water surface or throwing them with jerks of the head. None of the objects are edible and the carriers often swim in a ritualised way, spinning slowly with their head above the water.