To Sleep or not To Sleep?

Giraffes live in the African savanna, which is a mixed woodland- grassland landscape characterized by bumpy ground, many low plants and bushes, as well as several types of trees. The giraffe’s favorite food, acacia trees, grow primarily in these areas. Giraffes roam the savanna in search of food, water, and other giraffes—sometimes up to several kilometers per day. Giraffes do not live and travel in unchanging groups. Rather, loose groups called cliques are formed from time to time [1]. These cliques spend some time together traveling, foraging, and sleeping before individual giraffes, or mothers (called cows) with young giraffes, split off from the clique again.

Young giraffes (called calves) stay with their mothers for many months. They only leave the group when they are big enough and no longer drinking their mothers’ milk. Young males (called bulls) often roam together in what are called bachelor groups. They compete in playful fights until they are finally big and strong enough to go off on their own. Adult males often roam alone and only meet other females for breeding. Giraffes also spend the night together in cliques. Interestingly, it is very unusual for all the giraffes of a clique to sleep at the same time. While some giraffes are sleeping, other giraffes are always nearby, paying extra attention to protect their sleeping buddies. Let us take a closer look at the reasons for this interesting behavior.

Glowing in the Dark

Analyzing the nightly behavior of wild animals takes a lot of resources, including equipment, people, money, and time. From what you already know about a giraffe’s natural habitat, you can imagine that tracking and observing giraffes at night is not easy. Giraffes are animals that flee at the slightest danger, so observers must be very quiet and calm when approaching a group of giraffes in the wild. Therefore, we did our nightly observations by car, as cars do not signal danger for giraffes. To see the animals at night, we used thermal imaging cameras (Figure 1). In these images, the warm body of the giraffe stands out clearly against the cooler surroundings, and the animals can be easily recognized. The thermal images can tell us when giraffes are feeding, standing, walking, lying down, and, most important for this study, sleeping. With the help of the recorded videos, the behavior of giraffes can then be analyzed down to the second.

Cozy but Still Uncomfortable

In mammals, which include giraffes, humans, and many other animals, sleep is divided into two stages. One stage is called rapid eye movement (REM) sleep. In humans, REM sleep is the stage in which we dream. The REM sleep stage is fascinating, as the body appears calm while the brain is working intensely. Often, the eyes are moving quickly behind closed eyelids—that is where the name of this stage comes from.

For giraffes, an important characteristic of REM sleep is the position their bodies must be in. During REM sleep, the body experiences muscle atonia, in which the skeletal muscles become loose and relaxed and cannot respond to the brain’s signals [2]. So, an animal must rest all parts of its body during REM sleep. This means giraffes can only sleep in a very specific position (Figure 2) or their heads would fall toward the ground or they would fall over. Looking at the specific position a giraffe must be in during REM sleep, you can imagine how long it takes the animal to get up if it needs to. This means that a sleeping giraffe would be helpless against a lion attack! This could be why giraffes sleep so little! But giraffes are also protected by the behavior of the whole group. They often rest and sleep in a group of several animals, and most of the time only one giraffe is in the attackable REM sleep position. If one animal wakes up from this REM sleep position, another animal moves into this position. Basically, giraffes take turns sleeping, so the danger of being attacked becomes smaller. The non-sleeping animals carefully monitor the environment, providing additional security.

Big Animals, but Little Sleep

During the night, a giraffe spends an average of only 20–30 min in REM sleep, divided into several short sessions that last just 2–4 min. It is quite amazing that giraffes—the largest land mammals on Earth—sleep so little! The short sessions of REM sleep do not occur all at once, but are interrupted by an activity phase in which the animals feed and roam. After their midnight snack, another resting phase with REM sleep sessions follows. The rest and REM sleep phases occur more frequently after nightfall and then again in the early morning hours, shortly before sunrise. This cycle of rest and sleep phases during the night is part of the circadian rhythm, which is our body’s natural clock that helps us know when to be active and when to rest (Figure 3).

As in humans and many other mammals, young giraffes sleep more than their mothers. The older a giraffe gets, the less it sleeps in REM position. Scientists have two common hypotheses as to why giraffes sleep for such a short time. One has to do with the giraffes’ energy requirements and digestive system. Giraffes are ruminants, which are animals that have a special stomach with four parts to help them eat lots of leaves, and they chew their food again and again to get all the important nutrients from it. Giraffes have to eat a lot and often, to have enough energy—so they have to get up at night to eat, which means they cannot sleep through the night like humans and some other animals do. The other hypothesis relates to the risk of being killed by a predator during REM sleep, as we already mentioned [4]. The body position of the giraffe during REM sleep makes it easy for a lion to attack the giraffe. Therefore, scientists assume that the giraffe stays in this dangerous position during REM sleep for only a few minutes, to avoid being surprised by a lion during sleep. The behavior of the other giraffes within a group supports this hypothesis. If this position were not so dangerous, the other animals would not have to stay nearby to guard the sleeping animals.

Conclusion

Giraffes have lived on the African continent for thousands of years. But, due to habitat destruction and other human impacts, the population of wild giraffes has shrunk dramatically in recent decades. Something urgently needs to be done to protect these giants. One approach to conservation is to increase our basic knowledge of a species. The more we know about a species, such as how it behaves in its habitat and responds to changes in its environment, the better we can protect it. We want to expand this basic knowledge with our research and show what the nighttime behavior of giraffes looks like, how they sleep, and what their natural needs are. Our research shows that wild giraffes have only short sleep phases during the night. Their group behavior, in which awake and alert animals monitor the environment while others are in the vulnerable REM sleep position, is an important adaptation and safety strategy. If human impacts disturb these short sleep phases, that could have negative effects on these animals. Hopefully, our research findings will help to ensure the survival of giraffes for many years to come.

Glossary

Cliques: ↑ Groups of giraffes that can change over time and might have different members depending on the circumstances or the activity they are engaged in.

Thermal Imaging Camera: ↑ A device that produces an image using infrared radiation (heat) and functions similarly to a normal camera that produces an image using visible light.

REM Sleep: ↑ A sleep phase characterized, among other things, by rapid eye movements, often with closed eyelids. This phase is accompanied by muscle atonia.

Muscle Atonia: ↑ A phenomenon that occurs during REM sleep, in which the skeletal muscles are blocked from moving. This serves to protect the body—it prevents dreamed movements from actually happening.

Skeletal Muscles: ↑ Muscles that are mainly responsible for active body movements, for example the movement of arms and legs.

Circadian Rhythm: ↑ The internal rhythms, like sleeping and waking, that cycle over a period of ~24 h.

Ruminants: ↑ Herbivores that have a multi-part stomach. Pre-digested food is regurgitated and chewed again before being re-swallowed and going into actual digestion.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This study was partly supported by the Opel-Zoo foundation professorship in Zoo Biology from the von Opel Hessische Zoostiftung.