How Can This Seabird Get Enough Sleep When It Spends Weeks in the Air?
Great frigatebirds can’t land on the water, so they’ve evolved to take the ultimate power naps
Sleep is nearly universal in animals, being found in species as different as jellyfish, fruit flies and whales. Though much about why we sleep remains mysterious, its ubiquity suggests sleep is doing something important, a conclusion further supported by the observation that bad things happen when animals (or humans) miss too much of it.
Yet as important as sleep is, some animals have lifestyles that make it very challenging. One of those is a large seabird called the great frigatebird.
The great frigatebird is an unusual animal in many ways. It spends weeks or months at sea but lacks webbed feet and waterproof feathers. Lacking those things means it cannot take off from the water, and thus cannot land on the water either. If it did, it might never get airborne again. All of which leads to one other extraordinary thing about frigatebirds: they stay in the air continuously when at sea.
These birds cover thousands of kilometers of ocean without landing. One study looked at a population of frigatebirds based on Europa Island in the Mozambique channel, off the southeast coast of Africa. Over a period of months, they ranged all the way across the equatorial Indian Ocean. They would sometimes stop on islands for a rest, but spent weeks continuously in the air, typically covering more than 250 miles (400 km) in a day. Newly independent juveniles ranged particularly far, in some cases going all the way to Indonesia.
Evolution and continuous flight
Frigatebirds have evolved for efficient soaring. They have the lowest known wing loading (weight per unit area of wing) of any bird. And they also have extraordinary behavioral adaptations.
In order to stay aloft for weeks, they are masters at judging wind conditions. The ocean around the equator contains a region of little wind known as the doldrums. Frigatebirds in the Indian Ocean avoid this region, spending most of their time either north or south of it in a belt of wind that can be found around the edges of the doldrums. There they catch thermals and have been observed to ascend hundreds of feet without flapping their wings. One favorite technique is to enter cumulus clouds where the updrafts can be as strong as 15 feet per second.
Continuous flight presents behavioral challenges. To feed, frigatebirds must seize their prey, mostly fish, without getting in the water. This is only possible if the prey are near the surface, something that happens most frequently when they are driven there by aquatic predators. In order to maximize their odds of finding such opportunities, frigatebirds need sharp eyes, and the ability to recognize subtle environmental cues. Some of these cues are features of the ocean itself. Frigatebirds have been observed to track ocean eddies in order to find places where schools of fish are forced to the surface by whales and tuna.
But how do they sleep?
Researchers have long thought that frigatebirds must sleep on the wing. Many suspected they might sleep one cerebral hemisphere at a time, as has been observed in other birds and whales. But there wasn’t any direct evidence for this.
To learn more about how frigatebirds sleep, one research group put miniature 55 gram EEG rigs on them. EEG (electroencephalography) tracks electrical activity in the brain and is a common method in human sleep studies. Different types of sleep show distinct EEG waveforms, so EEG can reveal whether and what type of sleep a human (or animal) is engaging in.
The researchers tracked brainwave patterns in the frigatebirds over a period of days. When flying, frigatebirds displayed a variety of different kinds of sleep. Sometimes they slept with one cerebral hemisphere at a time, sometimes with both together. And like other vertebrates they had two types of sleep: a more quiescent slow wave type, as well as a rapid eye movement (REM) waveform.
But the most striking thing about their sleep was how little they did while flying.
On land, frigatebirds spend more than half their time sleeping. However, in the air, they averaged less than one hour per day. The duration of sleep episodes in the air was also very short, averaging 10 seconds at a time. Contrary to what people had been assuming, it seems that frigatebirds cannot go long stretches with one hemisphere asleep. Apparently, the demands of keeping in the air and finding food require more alertness.
Power naps are key
The fact that these birds can last several weeks on such small amounts of sleep is remarkable. This is not possible for humans and most other animals. But while frigatebirds can go with small amounts of sleep for a while, they still need it. When they return to land, they sleep more in the period just after coming back, as though they are working off a deficit.
So how do they delay the demands of sleep when flying?
One possibility is it has to do with the sleep they do get. It seems they’ve evolved to use their “power naps” to hold off the physiological need for sleep until they can land and get some proper rest. If so, studying how may help us understand the physiological purpose of sleep.
