Whales, dolphins and porpoises sleep with half a brain

Silent porpoise: Whales, dolphins and porpoises sleep with half a brain

Ever wondered how and where whales, dolphins and porpoises sleep? New work by University of Canterbury researcher Andrew Wright at the released this week reveals for the first time that harbour porpoises sleep during diving.

As part of Dr Wright’s PhD research in Denmark before coming to the University of Canterbury’s Gateway Antarctica, he attached behavioural loggers to porpoises and discovered a new type of dive in the subsequent data. The dives are slow, low energy and low in echolocation clicks – the biosonar that porpoises use to find food.

Cetaceans – whales, dolphins and porpoises – sleep with only half the brain at a time because they spend their lives underwater and must return to the surface to breathe. This unusual behaviour is also seen in many migrating birds that sleep on the wing.

Life underwater means that we know little about sleeping in wild cetaceans, Dr Wright says. Applying behavioural criteria for sleep that was developed in terrestrial mammals to behavioural data from the porpoise tags, Dr Wright identified a roughly semi-circular dive form that measured up.

“Stereotypical in not only dive shape, but also the swimming movements throughout the dive, the dives are typically quiet,” he says.

This discovery raises the possibility that sea animals sleeping at depth might be more susceptible to becoming entangled in fishing nets because they are not echolocating.

Dr Wright says the work raises some interesting possibilities for resolving the conflict between fishermen and cetaceans around the world, including New Zealand’s own Maui dolphin. For example, it may be possible to reduce entanglement rates if fishermen can avoid setting nets at the depths that the porpoises and dolphins sleep at, he says.

“Although the dives make up less than 10 per cent of all the activities for each animal, even small reductions in fisheries bycatch can make a big difference to the long-term survival of many endangered cetacean species,” Dr Wright says.

However, the finding also has implications for scientists, he says. Passive acoustic monitoring technology is becoming more common. Detecting marine mammal sounds as whales and dolphins swim past, such devices were thought to detect all porpoises as they were believed to produce clicks at all times.

“However, the existence of quiet dives means that not all animals will necessarily be detected. This means the finding also has implications for industries that rely on passive acoustic monitoring to protect marine mammals from harmful effects, such as the oil and gas industry,” Dr Wright says.

Source: Andrew J. Wright, Tomonari Akamatsu, Kim N. Mouritsen, Signe Sveegaard, Rune Dietz, Jonas Teilmann. 2017. “Silent porpoise: potential sleeping behaviour identified in wild harbour porpoises.” Animal Behaviour Vol 133, November 2017, Pages 211-222. https://doi.org/10.1016/j.anbehav.2017.09.015

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