How do Marine Mammals Sleep in the Sea?

Whilst sleeping can be the easiest thing in the world for most people, sleeping for animals can sometimes provide unique challenges. Marine mammals need to breathe air during the night but they have to stay under the ocean's surface, so how do they do it and still get a good nights kipper?

Almost everyone will agree that they could do with several more hours sleep in a 24-hour period, it maintains our optimal brain function as well as being involved in the repairing of heart and blood vessels. Ongoing sleep deprivation has been linked to increase disease rates, poor physical health and at a basic level, increased stress. No known animal exists that does not sleep and in some environments, long periods of vulnerability whilst sleeping can be a fatal strategy.

Therefore, it is difficult to imagine how the marine mammals of our world get the quantity and quality of sleep they need to survive. Nevertheless, scientists from Aarnhus University in Demark believe they have found a behavioural breakthrough in how we understand sleeping patterns in wild harbour porpoises.

Sleeping is often debated on as the true function is not known; however, we know from behavioural studies that there is a set criterion for the behaviour: stereotypical sleeping posture; behavioural inactivity; increased resistance to arousal and 'wakefulness' when enough stimulation is applied. This behaviour has been extensively studied in captive marine mammals and has been concluded using this criterion for animals like Bottlenose Dolphins and even used for wild Sperm Whales.

Cetaceans (marine mammals such as whales, dolphins and porpoises) utilise the strange strategy of unihemispherical sleep, where only half of their brain sleeps at a time. This is because marine mammals require movement and air at the surface for breathing. The trait is shared with some bird species who use this to rest in areas of high predation or during long migratory flights. Scientists understood that the Harbour Porpoise used unihemispherical sleeping but the sleeping behaviour had not been investigated.

Recently, an international research team made use of advances in behavioural tagging technology, which can now be used against other published work for defining sleep behaviourally. Seven Harbour Porpoises, Phocoena phocoena, were tagged with behavioural data loggers in Danish waters recording 1884 to 2755 valid dives per animal.

Researchers identified a particular dive that the porpoises expressed that could be a potential period of sleep for the animal. These dives, known as parabolic dives, have a noticeable lack of vocalisations which are the echolocation clicks they use during hunting, which indicates that this silent behaviour is significantly different to others previously seen.

These dives were often, to a shallower depth to other classified dive types and found to have a stereotypically low-energy profile. Additionally, they also contained fewer rolls and a lower vertical decent rate, meaning these dives are intentionally slower than others.

If this data is significant in the application of behavioural studies, it means these marine mammals spend an important period of their lives engaged in parabolic dives and therefore, potentially sleeping.

This research has wide applications to both science and industry. Fisherman could reduce the potential for bycatch of Harbour Porpoises by managing the depth of casting nets regarding the parabolic dive depths of the animal.

It also has functions to future research in passive acoustic monitoring techniques, as these quiet periods of vocalisations during sleep means individuals may not be correctly monitored using previous data. The next step is to analyse this in contrast to other marine mammals and to develop the tagging technology further to allow more complex research to take place.

Author: Thomas Llewellyn

Reference:
Wright, A.J., Akamatsu, T., Mouritsen, K.N., Sveegaard, S., Dietz, R. and Teilmann, J., 2017. Silent porpoise: potential sleeping behaviour identified in wild harbour porpoises. Animal Behaviour, 133, pp.211-222.