快猫短视频

Sleep is thought to be vital for the health of animals including us, but one kind of cavefish can somehow manage without it. Yet the cavefish appear to be healthy despite this lack of sleep, with normal lifespans.

at Texas A&M University first observed more than a decade ago that some fish from cave-dwelling populations of the Mexican tetra (Astyanax mexicanus) seem to barely sleep. 鈥淪ome sleep a little bit,鈥� says Keene. 鈥淎 lot of the fish sleep zero hours.鈥�

One issue with these previous studies is that they involved fish kept in the lab. Now, with the help of researchers in Mexico, Keene has shown that these cavefish barely sleep in the wild, too.

Most A. mexicanus live in rivers outside caves and have normal vision and some colouration. There are also more than 30 populations living in caves that have partly or completely lost their eyes and colour.

Keene鈥檚 team caught fish from four populations of surface-dwelling fish, six populations of cavefish and five populations of intermediate forms. The surface fish were found to sleep between 3 and 6 hours per day, based on their activity levels, whereas the cavefish and intermediate forms barely slept.

The team then confined some surface and cavefish in netted enclosures in the wild, and saw that each of these groups exhibited the same behaviour as before. The study also shows that this trait has evolved independently on at least three occasions.

鈥淭he next step is clearly to record brain activity and look at how that鈥檚 different between surface fish and cavefish,鈥� says Keene.

Brainwave recordings have shown that some birds sleep briefly during long flights. Sometimes only half of their brain sleeps at a time, as in some whales.

So the cave tetra may be swimming while asleep, says Keene, but it isn鈥檛 clear why they would need to when surface fish don鈥檛. Yet the fact that the sleeplessness trait has evolved independently on at least three occasions suggests there is an important reason for it.

鈥淭here has to be something about these caves that is pushing the animals to rapidly adapt not to sleep,鈥� says Keene. One idea is that they have to spend more time looking for food.

Another possibility is that they have evolved the equivalent of such sensitive hearing that even the slightest noise keeps them awake. Fish have lines along their side that can detect vibrations, and Keene鈥檚 team has previously shown that, in cavefish, these lateral lines are extremely sensitive and that their output appears to be involved in suppressing sleep.

It is a wonderful study, says at St. John鈥檚 University in New York, who also studies A. mexicanus. 鈥淚t provides strong evidence that near-total sleep loss is a common and convergent cave adaptation,鈥� says Kozol.