Cavefish lose sleep
Cave-dwelling fish sleep less than their open-water relatives, challenging the idea that variation in sleep patterns is driven by cognition and brain function
Why animals sleep has long puzzled scientists, although many studies suggest benefits to brain function, memory and learning. But linkurl:research;http://www.cell.com/current-biology/abstract/S0960-9822(11)00292-2 published online today in Current Biology
reports that three distinct populations of cavefish sleep dramatically less than their open-water relatives, suggesting that sleep is not a cognitive necessity but an ecological adaptation to the specific environment of a population.
|A cave variety in front of two open water varieties of the Mexican tetra, Astyanax mexicanus|
Image: Courtesy of Richard Borowsky
"This study is the first to show that cavefish may have dramatic reductions in sleep," wrote linkurl:William Jeffery,;http://www.life.umd.edu/labs/jeffery/ an evolutionary and developmental biologist at the University of Maryland who was not involved in the research. The findings suggest that sleeplessness has "significance for survival in the cave environment."
The Mexican tetra is a freshwater fish known mostly for its blind cave variety, although populations also exist in open water. The blind cavefish have evolved many adaptations to cope with their dark environment, including increased sensory perception, and have lost traits that are presumably only beneficial in lit environments, such as sight (and even eyes) and pigmentation. These evolutionary trends have been identified in thirty distinct populations of cave-dwelling tetras, suggesting that the changes are directly linked to the unusual environment.
Evolutionary neurobiologist Erik Duboué of New York University and his colleagues generated sleep profiles of three distinct populations of cavefish and one population of open-water fish to quantify how long and when the fish slept. While open-water fish averaged over 800 minutes of sleep each day, the cavefish populations only slept for 110 to 250 minutes. "I was not surprised that they slept less, but I was surprised that the phenotype was so robust," said Duboué.
By breeding the cave populations with the surface populations, the researchers were able to explore the genetic basis for this difference in sleep patterns. Unlike genes underlying other cave-specific features, sleeplessness appeared to be a dominant trait determined by only a few genes.
"[It's] an unusual genetic pattern," said paper author and evolutionary biologist linkurl:Richard Borowsky;http://as.nyu.edu/object/RichardBorowsky.html of New York University. "It shows that selection is extremely powerful for loss of sleep when you're in a cave."
The selective pressures that drive cavefish to sleep less, however, are still unclear, though the new findings seem to suggest that the behavioral change has little to do with brain function, said linkurl:Jerry Siegel,;http://www.semel.ucla.edu/sleepresearch the director of the University of Los Angeles's Center for Sleep Research. The fact that these three populations evolved this sleepless behavior independently supports the idea the change in their environment is the driving force. "Evolution can really fine-tune sleep for ecological need," Siegel said.
"This doesn't mean that sleep doesn't have other functions," he added. Sleep may very well affect cognition, but that may not be the dominant function driving its evolution. He considers sleep as kind of a daily hibernation, a way to significantly reduce energy use without risking being eaten or not getting enough to eat -- and cavefish may not have that luxury.
Cavefish are the top predators in their habitats, so sleeping would not make them more vulnerable to predation. But the relative paucity of food resources in caves, where there is no photosynthesis, could be at the root of the fish's stunted sleep patterns, Borowsky said. "If you're asleep when this little morsel of food floats by, you don't get to eat it," he explained. When food is really scarce, "an awake fish will have higher fitness."
E.R. Duboué et al., "Evolutionary Convergence on Sleep Loss in Cavefish Populations," Current Biology, 21:8, 2011.
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