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 ) 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.