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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