TUCSON, ARIZONA--Paper wasps show behavior that bears many earmarks of sleep, a study has found. The findings help challenge the notion that insects and other invertebrates do not sleep.
Scientists who study sleep have focused on humans and sometimes other mammals, and they have defined sleep in terms of neural patterns from electroencephalograms (EEGs). Because most invertebrates have neural architecture that doesn't allow for an EEG, many sleep specialists don't consider invertebrates to truly sleep. But a handful of researchers have marshaled evidence for apparent sleep in fruit flies, scorpions, cockroaches, and honeybees. Now paper wasps have joined the club, after University of Arizona graduate student Barrett Klein took a close look at their behavior.
Raising groups of the paper wasp Polistes flavus in the lab, Klein discovered that they fell quiet for most of the night, every night. During this time, the wasps breathed less frequently and their metabolism fell. Infrared imaging showed that their thoraxes, warmed by muscle movement when the wasps are active, turned cold when quiescent. And the quiescence was reversible; that is, Klein could wake the wasps up, showing they were not hibernating or in a coma. During times of quiescence, the wasps always slung their bodies low and drooped their antennas. Klein found similar behaviors in three other species of Polistes, and he says the sleeplike behavior "could be a ubiquitous phenomenon across many groups of insects." He presented his results here yesterday at the meeting of the Ecological Society of America and the Society for Ecological Restoration.
"I love the fact that he's doing this with wasps," says Paul Shaw of The Neurosciences Institute in San Diego, who in 2000 demonstrated sleep in fruit flies (Science, 10 March 2000, p. 1834). Paper wasps are large enough for scientists to conduct infrared and metabolic research that Shaw was unable to perform on fruit flies.
Recognizing that sleep occurs in invertebrates will allow us to better understand the origin and evolution of sleep and to figure out its function, Shaw says. Klein speculates that insects sleep for the same reasons vertebrates do. Those reasons remain contentious, but hypotheses include memory consolidation, learning improvement, tissue repair, and energy conservation.