During life, animals need to fulfil some vital activities. They need to eat and drink, to reproduce (so as to pass on their genes to the next generation), to sleep and to avoid being eaten. Sleep appears to be fundamental to almost all animals. There is wide variation in sleep duration among the different animal phyla. Some animals (eg. brown bat) sleep for up to 20 hours each day. Across different animal groups, poor sleep negatively impacts development, cognitive ability and longevity.
Sleep in herbivores, carnivores and omnivores
In animals, there is an inverse correlation between body mass and sleep duration. This is primarily due to large herbivore species (such as elephants, giraffes and cattle) having low amounts of sleep compared with carnivores and omnivores. Bigger animals tend to sleep less, because they have to spend so much time eating. Under laboratory or zoo conditions, carnivores sleep more than omnivores, who in turn sleep more than herbivores. However, one needs to remember that in such environments, food is not a critical limiting factor and may affect the quantity and quality of the sleep.
Types of elephants
Elephants are the largest land living mammals in the world. There are three species of elephants, namely, the African bush or savanna elephant (Loxodonta Africana), the African forest elephant (Loxodonta cyclotis) and the Asian elephant (Elephas maximus). The African forest elephant is smaller than the savanna elephant. There are three subspecies of the Asian elephant. Several external characteristics differentiate these species, and these have been well documented. Rge African and Asian elephants' sleeping habits have evolved in their own unique way.
Brain size in elephants
Elephants have the largest brain of any land mammal. The brain of an African elephant weighs around five kilograms. It is around three to four times the weight of an adult human brain. There are around 260 billion neurons (nerve cells) in the elephant brain, again around three times the number observed in the human brain. An important difference is that most neurons in an elephant brain are found in its cerebellum (Latin for ‘little brain’) rather than its cerebral cortex.
Sleep in African elephants
They are considered to have the shortest amount of sleep among land mammals. In the wild, on average, they sleep for around two hours each day. Their sleep is polyphasic (that is occurring in four to five short bursts with each burst lasting for five – 30 minutes). When they go to sleep and wake up, depends on environmental conditions (such as the temperature, the humidity, the wind speed and solar radiation). The elephant diet consists of low calorie elements. They need to eat around 300 kilograms of low quality food each day. They need to graze for long periods, in order to eat their daily food requirements, and this contributes to their low amount of sleep.
Most of their sleep occurs between 1 a.m. and 6 a.m. They tend to sleep at different places each night. On some nights, they do not sleep at all. For instance, some African elephants have been found to travel for nearly two days without sleeping. When they get to sleep after such a journey, they do not compensate with extra sleep. Although they can sleep both lying down and standing up, most of the sleep is done standing. This makes them less vulnerable to predators. They lay down every third or fourth day and do so for around an hour. Elephants in captivity spend a longer period sleeping lying down. In the wild, sleeping whilst standing would allow them to move sooner, if the need arises. Mammals lose tone in their skeletal muscles during rapid eye movement (REM) sleep. Thus, in order to have REM sleep, they would need to lie down. REM sleep is considered important for memory consolidation and is the stage in which one has vivid dreams. In general, elephants spend only a little time in REM sleep. Wild elephants usually do not have sufficient sleep time to dream, whilst captive elephants have been found to do so. Herds tend to sleep together. Whilst one group of animals are asleep, the others remain awake. In general, the matriarch of the herd gets the least amount of sleep.
In general, the accepted way to study sleep is by recording brain waves. As elephants have large frontal sinuses, it is difficult to record such brain waves. Sleep is assessed by attaching a motion sensor to the trunk. When they are sleeping, their trunks stop moving. If the trunk is not moving for five minutes, they are likely to be sleeping. Orexin (hypocretin) neurons are found in the hypothalamus of an elephant brain. These neurons control the balance between satiety and arousal. When an elephant has had enough to eat, these neurons stop firing and they can go to sleep.
Sleep in Asian elephants
Asian elephants are awake and more active during dawn and dusk. Their sleep is spread out over several short naps, usually whilst standing. Sleeping behaviour has been investigated during 294 nights in female Asian elephants. These included seven circus elephants and five zoo elephants, and included a calf. They were continuously recorded for seven to 16 days via time lapse videography. Seasonal changes and developmental effects on sleep behaviour were assessed. Sleep occurred in standing and lying positions. Sleep onset occurred after 9 p.m., and increased progressively, reaching a maximum between 1 a.m. and 4 a.m. The total sleep time in adult zoo elephants was four to 6.5 hours per night. More sleep occurred during winter. The total sleep time in the calf reduced from 8.1 to 5.1 hours during a 15 month period. Standing sleep in the calf occurred for the first time at the age of nine months.
A study also assessed sleep behaviour in a group of Asian elephants at the Dublin (Ireland) Zoo. A total of 704 nights of elephant behaviour were recorded over a 33-month period. Closed circuit television cameras with infrared abilities were used to record behaviour. Eight elephants were studied: three related adult cows (both while they were pregnant and not pregnant), one sub-adult cow, one adult bull, two bull calves and one cow calf. Adults slept for an average of three hours and 33 minutes per night. Calves slept an average of five hours and eight minutes per night. The bull elephant showed a consistent sleep duration, while sleep in the other elephants varied through pregnancy, parturition and the immediate postpartum period. One of the elephants who became a mother during the study slept 68% less in the first nine months after giving birth. Another mother slept 13% less after parturition. A third female elephant slept 10% more, after giving birth. The reduction in sleep duration appeared to be primarily due to calf guarding behaviour. The establishment of successful suckling was also a factor.
On rare occasions, the adult cows were given the option of sand and concrete surfaces. The adult bull was given this option nightly. Both sexes always chose to sleep on sand. Elephants live in related matriarchal herds in the wild. In the wild, they are in contact with sand and soil. In a non-wild setting, mimicking such environments would permit appropriate levels of sleep and improve wellness.
Captive vs. wild elephants
Elephants in captivity tend to sleep for an average of four to six hours each day. They do not have the risk of predation or the pressures of having to find food and water.
Conclusions
Understanding the patterns of sleep in animals such as the elephant, would help us understand them better and provide important clues to aspects of conservation. It would help us develop beneficial conservation and management strategies. A comparison with sleep patterns observed in other land (including humans) and aquatic mammals, should help us better understand sleep disorders in humans and provide potential clues to its management.
(The writer is a medical doctor and a nature and wildlife enthusiast)
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The views and opinions expressed in this article are those of the author, and do not necessarily reflect those of this publication