How Do Whales and Dolphins Sleep?

A question I get asked a lot as a marine biologist is, “How on earth do whales sleep?” And this is, in fact, a very good question. Dolphins and whales belong to a group called ‘cetaceans’ by scientists, and they are, like us, mammals. This means that they breathe air, suckle milk from their mothers and have hair (the ‘hair’ bit has been lost in cetaceans to make them more streamlined). As we all know, most mammal babies sleep a lot of the time – just think of human babies and puppies. In fact, a new born human baby must sleep 17 hours a day. All in all, you will spend about a third of your lifetime asleep! Babies spend so much of their time in “la la land” because sleep is a very important part of development. Sleep is needed for physical growth and for recovery, the immune system, brain development, learning, memory, and information processing as well as many other systems of the brain and the human body.

Sleeping Sperms Whales (image from National Geographic)

It has always been assumed that marine mammals like dolphins and whales babies need as much sleep as infant mammals on land. Therefore, it came as a major surprise when a study by Lyamin and his colleagues (2005) showed that baby killer whales and bottle-nosed dolphins and their mothers hardly sleep after the birth , the days a baby was assumed to be in critical need of sleep. In fact, the babies and their mothers remained mobile and active for 24 hours of the day for the first month, swimming and avoiding obstacles and rising to the surface to breathe.

A Killer Whale mother with her calf (image from hdwallpaperc.blogspot.com)

Cetaceans are different to land mammals in that they need to remember to breath. You and I can simply take a breath of air, and generally not notice that we are in fact breathing – our brains physically regulate our breathing in response to our body’s needs. This is not the case in cetaceans - they have to swim to the surface in order to breath. Just like us, whales and dolphins can drown if they don’t come up for air.  When they do “sleep”, the adults don’t fall asleep completely like we do because they need to be aware of when they need to breathe. Generally, whales and dolphins rest for long periods whilst holding their breath, floating at the surface or resting just as the bottom of the shallow water.

Humpback whales sing and sleep in this strange position! (image from williaminram.com)

The study showed that as the baby got older, both it and its mother gradually increase their time asleep until they reached the sleeping time of normal adults, but never more than that. Cetacean babies simply do not sleep as much as mammals on land. The researchers think that this may be because normal mammal baby sleep behaviour and the importance of this sleep in development and survival simply did not evolve in cetaceans. Whale and dolphin babies may simply not need to sleep for such long periods after birth.

A bottle-nosed dolphin mother and her baby come to the surface to breathe (image from zooborns.com)

 Bottle-nosed dolphin babies need to breathe every 3 to 30 seconds, and when they do the mother keeps swimming which forces the baby to keep swimming. Mom also has to help and support the baby .This doesn’t allow any time for either mom or baby to get much sleep. Previous work has shown that when adult cetaceans sleep, one hemisphere (half) of their brain activity slows and they close either one or both eyes. Mother cetaceans only closed their eyes (either one or both) for less than 1% of their 24 hour day.  Calves less than one month old showed little more eye closure time (1.5% of 24 hours), although these periods were no more than 30 seconds long because the calf needed to breath. This is surprising because such disruptive sleep patterns in other mammals (like humans) is non-restorative, meaning the body gets so little rest you may as well not have slept at all.

A mother and her calf (image from National Geographic)

The study measured the stress hormone cortisol in three killer whale mothers before and after they gave birth. There was no significant increase in the measures after the calf was born, which means that stress did not cause the reduction in sleep behaviour for the mothers - it was their baby's fault!

Four captive bottle-nose dolphins and their calves were observed during their first month after birth. Both showed the same pattern of sleepless behaviour, with neither the mothers nor their caves resting at the surface until they were older than one month of age, after which resting time increased until it was similar to that of normal adults. Injecting cortisol into other male and female dolphins did not change their sleep behaviour. This means that stress was not the cause for the lack of sleep in mothers and their calves – the reason for their lack of sleep is that the calf has to breathe every few seconds and the mother has to help and support it. In the wild, the mother would also have to remain alert to any dangers that may threaten her calf.

Humpack whale mothers and calves stick close together, with the mother helping the calf breathe, stay afloat and safe from predators (image from motelmoka.com)

The ability to remain active and alert helps newborn cetaceans. Constant swimming helps maintain body temperature until they have gained enough weight to develop insulating blubber that will do the job for them in adulthood.

Threats to baby cetaceans include predators like this Great White Shark (image from National Geographic)

This research has changed the way we think about sleep. Other animals, from fruit flies to mammals, suffer severe and sometimes fatal consequences if they do not sleep. Cetacean babies grow and develop just like other mammal babies, despite hardly sleeping. How do they manage? What makes their brains different? The mystery remains to be solved.

Bubbles and Whale Strandings

The end of March saw the stranding of 19 adult pilot whales on Noordhoek beach on the western side of the Cape Peninsula, South Africa (see the News24 article). The strandings are the second on this particular beach in recent years, with 55 false killer whales beaching themselves in 2009. There are deep public divisions about what to do in such circumstances. Some want the animals to be “left alone”, and for “nature to take its course”. Others desperately want the animals saved, and bemoan the euthanisation of the whales. Despite this intense public interest (the authorities had to close off the area to keep the fascinated public at bay), the debate as to the causes of cetacean strandings still rages within the global scientific literature.

A stranded pilot whale on Noordhoek Beach, South Africa (see News24)

 The truth is that we simply do not know exactly what causes cetaceans (the scientific term given to whales and dolphins) to strand. Some papers suggest navigational disorientation due to anomalies in the magnetic field of a certain area is the cause of strandings. Because cetaceans utilise the magnetic field of the earth to navigate, a disruption in this system may cause the animals to “get lost” and head towards shore. Another theory is that the tight social groups of whales and dolphins is a cause – if one individual, especially if it is the ‘leader’ of a social group, becomes ill or injured the others will follow it when it beaches. An emerging explanation is that the increased ‘noise pollution’ in the ocean leads to a breakdown in the communication between whales, causing confusion and strandings. A more sinister follow on from that theme is that intense noises created by humans underwater (such as military sonar or underwater mining explosions) results in internal damage to the animals.

A mass stranding of pilot whales (from http://www.smh.com.au/news)

There is no doubt that anthropogenic (man-made) sounds have increased dramatically over the past century, due to the industrialisation of the world’s oceans. Shipping traffic, commercial activity such as oil drilling and exploration, research and military sources have all added to the din below the waves. I have had the privilege to listen to hydrophone recordings taken recently in False Bay of South Africa, and it has stuck me just how noisy it is down there – not just due to natural sounds of the waves and the dolphin dawn chorus, but because of boat traffic. We are particularly sensitive to the effects of pollution we can actually see – a seal entangled and drowned in an old net, an albatross hooked on a long line hook. But we also need to take into account those aspects of pollution we cannot see and how that affects the world around us.

Noise pollution frequencies in the ocean. Note how the noise generated by shipping falls right over the frequency band used by baleen whales to communicate; and how intense the frequencies of sonar are in comparison (from http://www.wired.com)

Sound travels a lot further and faster in water than in air, and marine mammals in particular utilize this very effectively for communication, hunting and orientation. It should be no surprise then that anthropogenic disturbances in the underwater realm have a marked impact on the physiology and behaviour of animals sensitive to sound. The issue needs to become a major conservation concern because it involves human behaviour influencing the behaviour of other organisms and in some cases, harming them. For example, baleen whales communicate by low frequency calls which happen to be on the same frequency band as the increasing ambient noise from shipping traffic (see Tyack, 2008). Does this decrease the range over which the whales communicate? Does it mask their calls or even confuse them? The effects of anthropogenic sounds on cetaceans in particular range from call silencing, displacement and temporary hearing loss to physiological injury of the hearing canal, internal bleeding and stranding. The effect is dependent on the level of noise (the intensity, amplitude and frequency). The most intense anthropogenic sound source comes from seismic exploration and sonar utilised by the military. 

 

An advert by the Whale and Dolphin Conservation Society (see www.wdcs.org/stop/pollution/)

Jepson and his colleagues published a brief communication in Nature in 2003 which discusses the impacts of military sonar on marine mammals. Even though it is not a recent publication, the intense public interest created in light of the recent strandings calls for the examination of the paper. In 2002, fourteen beaked whales were stranded in the Canary Islands (off the coast of Spain) close to where a mid-frequency military sonar exercise had been conducted.  The animals were dissected (as most stranded whales are, including those stranded at Noordhoek), and these autopsies revealed that the animals had not died of illness. However, there was widespread vascular congestion (the overfilling and swelling of the veins with blood as a result of an obstruction in the vessel) as well as prominent, widespread capillary haemorrhages due to the blocking of the blood vessels by blood clots and air bubbles. These bubbles in the blood vessels were also present in several vital organs. These bubbles and the associated damage of organs are usually associated with severe decompression sickness (the bends). If a human diver rises too quickly to the surface, dissolved gases come out of the blood and form bubbles inside the body as the pressure increases again. Cetaceans (deep diving beaked whales in particular) were thought to have physiological adaptations to cope with pressure changes as they dive, such as exhaling before diving or collapsing of lungs. Jepson et al (2003) argue that the formation of these bubbles may be due to changes in the diving behaviour these animals because of the sound disturbance, such as ascending from depth too quickly. It has also been suggested that sonar pulses force dissolved gas in the blood out of solution through changing pressures. These Canary Island strandings are not unique in the presence of bubble-associated tissue injury. Strandings around the world have revealed gas bubbles in blood vessels and gas-filled cavities in the functional parts of the vital organs. The liver was most affected, with gas-filled cavities taking up between 5 and 90% of the volume of the organ. These cavities are not filled with the bacteria associated with the decomposition of a body, and are therefore caused by something before death. 

So, what did cause the strandings of those pilot whales on the beautiful beach in Noordhoek? Necropsies are still taking place, and I look forward to the publication of the results. Until then, we will continue to ponder the deaths of these magnificent creatures. Was it natural? Or was it, once again, the fault of humans?