Frogs are intriguing creatures that attract the curiosity of many people. There are also many misconceptions about these animals.
In general, frogs have 3 sleep-like states, namely cataleptic sleep, catatonic sleep, and cataplectic sleep. Frogs do not sleep as humans or other mammals do. Even though immobile and at rest for long periods, some frogs remain alert and react to external stimuli.
During periods of rest, the brain waves of frogs such as the American bullfrog (Lithobates catesbeianus) are of low voltage and fast electrical activity which is associated with alertness in other animals.
What Exactly Is Sleep?
Put simply, sleep is a period of reduced activity that is characterized by changes in brain wave activity, breathing, heart rate, body temperature, and other physiological functions.
French scientist Henri Pieron proposed a behavioral definition in 1913. The behavioral features of sleep include:
- The spontaneous use of a stereotypic or species-specific posture during periods of immobility
- The maintenance of behavioral immobility,
- An elevated behavioral response threshold to arousal stimuli
- A rapid behavioral state reversibility upon stimulation
After the discovery of variation in brain waves related to sleep in humans, and other mammals, electrophysiological criteria were added to the definition of sleep.
Later on, a sleep state associated with rapid eye movements (REM sleep) was identified first in humans, and then in cats. This state was referred to as ‘paradoxical sleep’.
Following this discovery, electroencephalography, electromyography (EMG), and electro-oculography (EOG) were accepted as valid methods to identify sleep in mammals.
As a result, two main states are now recognized in mammals, as well as in birds: slow-wave sleep, also called non-REM sleep, or quiet sleep, and paradoxical sleep, also known as REM sleep or active sleep.
What We Know About Sleep In Frogs
While both quiet sleep and active sleep have clearly been identified in mammals and birds, their presence in frogs remains highly debated.
In 1982, Ida Gavrilovna Karmanova conducted a study to understand the evolution of sleep in vertebrates – and concluded that amphibians and fish show three forms of rest or sleep-like states (SLS), named ‘protosleep’ or ‘primary sleep’.
- Cataleptic (plastic muscle tone) or protosleep 1
- Catatonic (rigid muscle tone) or protosleep 2
- Cataplectic sleep (atonia)or protosleep 3
Cataleptic sleep appears mainly during the day when the eyes are open, whereas Catatonic and Cataplectic sleep are observed during nighttime rest periods.
The arousal threshold increases relative to the type of protosleep, and the heart rate decreases accordingly.
Some Frogs May Remain Alert Even During Periods Of Rest
In 1966, J. Allan Hobson studied American bullfrogs (Lithobates catesbeianus) and found that the frogs did not fulfill either the threshold or the electrographic criterion of sleep.
The study found that even though immobile and at rest for long periods, American bullfrogs remain alert, and able to react to external stimuli at any time during the rest-activity cycle with the same speed and intensity they have in a non-rest state.
In addition, during periods of rest, the brain waves of American bullfrogs were found to be of low voltage and fast electrical activity which is associated with alertness in other animals.
Other Frogs Have Reduced Alertness During Periods Of Rest
Researchers studying the sleep behavior of Squirrel tree frogs (Hyla squirella) and American green tree frogs (Hyla cinerea) found that during day-time periods of rest during the day, the frogs only reacted to moderately intense and direct stimulation such as shaking the plant.
The researchers concluded that the resting tree frogs had an elevated threshold to sensory stimulation and could be said to be asleep behavioral definition.
Differences in the sleep behavior of American bullfrogs – and Squirrel tree frogs, and American green tree frogs, may be due to the fact that they are adapted to different lifestyles.
American bullfrogs are adapted to a terrestrial lifestyle, while the tree frogs are adapted to an arboreal lifestyle.
How Do Frogs Sleep?
There are over 7,000 described frog and toad species, living in a wide range of habitats from trees to caves, to fast-flowing waterbodies. For this reason, the exact sleeping posture, and behavior may vary among different frog species.
That said, there are a few common behaviors that have been observed in several species of frogs when in a sleeping state.
In the earlier mentioned study, the researchers studying the sleep behavior of Squirrel tree frogs and American green tree frogs; observed at least 20 species of each species of frog in a sleeping state.
All of the frogs were:
- Immobile
- Had their eyes closed
- Their limbs tucked under their body
- And their heads pointing downwards, towards the tips of the leaves on which they sat.
After modertly intense and direct stimulation, the frogs ‘awakened’, and asummed a crouching posture, opened their eyes, and sometimes even attempted to escape to a neighbouring plant.
When Do Frogs Seep?
The majority of frog species are primarily either nocturnal or crepuscular and are most active in the dark and twilight hours. However, frogs can sleep at any time of the day.
According to Ida Gavrilovna Karmanova (1982), frogs show Cataleptic sleep mainly during the day and show Catatonic and Cataplectic sleep during nighttime rest periods.
Where Do Frogs Sleep?
Where frogs sleep depends on the type of habitat they are adapted to.
- Terrestrial frogs spend most of their time on the ground and will sleep in cool areas sheltered from direct sunlight – such as in leaf litter, logs, rock crevices, caves, underground burrows, and other cool, hidden locations. Many toads and burrowing frogs may also sleep buried under the soil surface.
- Arboreal frogs spend most of their time in and around trees and will sleep on leaves, in tree hollows, bromeliads, the leaf sheaths of large leaves (such as banana leaves), and other cool, hidden locations in the canopy.
- Aquatic frogs spend most of their time in water, and will sleep underwater; between rocks, at the base of aquatic plants, under subgmerged rocks, logs, and in other sheltered locations.
| Type Of Frog |
Where They Sleep |
| Terrestrial frogs | In leaf litter, hollowed-out logs, rock crevices, caves, underground burrows, and other cool, hidden terrestrial locations. |
| Arboreal frogs | In tree hollows, bromeliads, the leaf heaths of large leaves (such as banana leaves), and other cool, hidden locations in the canopy. |
| Aquatic frogs | Between rocks, at the base of aquatic plants, under submerged rocks, logs, and in other sheltered locations. |
Scientific Research On The Sleep Behavior Of Frogs Is Limited
There are over 7,000 known frog and toad species on the planet, and less than 1% of these species have been studied in the context of sleep.
For this reason, what we currently know about the sleep behavior of frogs is very limited, and may change in the future, with more scientific research.
Some authors previously reported that some frogs, fish, and other animals such as crocodiles never sleep. However, these ideas have been challenged, and the current scientific consensus is that virtually all animals (including all frogs), show some form of sleep or at least sleep-like states.
Sleep enables frogs to conserve, or restore energy, and also performs many other important functions.
Conclusion
Like virtually all animals, frogs show some form of sleep.
Humans and other mammals have 2 sleep states, namely; slow-wave sleep, also called non-REM sleep, or quiet sleep, and paradoxical sleep, also known as REM sleep, or active sleep.
The presence of these sleep states in frogs is not clear and is still highly debated, although some authors suggest quiet sleep (non-REM sleep) may exist in frogs.
Not all frogs sleep in the exact same way. American bullfrogs (Lithobates catesbeianus), have been found to remain alert, and able to react to external stimuli, even though immobile and at rest for long periods.
On the other hand, Squirrel tree frogs (Hyla squirella) and American green tree frogs (Hyla cinerea) in a sleep state have been found to only react to moderately intense and direct stimulation (such as shaking the plant they are sleeping on).
Differences in the sleep behavior of different frog species may be because they are adapted to different lifestyles.
Commonly Asked Questions
Do frogs sleep with their eyes open? While sleeping, frogs generally close their translucent third eyelid, known as the nictitating membrane. Since this membrane is translucent, frogs’ eyes may appear open when they sleep – and they can still see and react to their surroundings.
How to tell if a frog is sleeping? You can generally tell a frog is sleeping by observing its behavior and posture. When a frog is sleeping, it will be immobile for long periods, have its limbs tucked under its body, its head pointing downwards, and its eyes may or may not be covered by a nictitating membrane.
Featured image credit: Daz Smith (CC BY-NC-ND 2.0)
Sources:
Hobson, J. A., Goin, C. J., & Goin, O. B. (1967). Sleep Behavior of Frogs. Quarterly Journal of the Florida Academy of Sciences, 30(3), 184–186. http://www.jstor.org/stable/24314832
Pieron, H. (1913). Le probleme physiologique du sommeil. Paris: Masson, 1-520
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