Convergent? Minds? Some questions about mental evolution.
- Matt Cartmill
- Interface focus
IN THE NEWS FOCUS STORY “SIMPLE SLEEPers” (18 July, p. 334), E. Youngsteadt highlights recent advances, derived from genetic research in fruit flies and other “simple” animal models, in our understanding of sleep. The power of this genetic approach is undeniable, but the simplicity that renders such model animals amenable to genetic manipulation necessarily limits their ability to model all aspects of sleep in mammals. Most notably, simple animal models lack the changes in brain activity that define mammalian slowwave sleep (SWS) and rapid eye movement (REM) sleep (1). Until we determine the functional relevance of the changes in brain activity that define these characteristics of mammalian sleep, our understanding of human sleep may be incomplete. We contend that birds, as the only nonmammalian taxonomic group to exhibit SWS and REM sleep, provide a largely untapped opportunity to determine the functions of these states in mammals. Reptiles and amphibians lack comparable sleep states, indicating that SWS and REM sleep evolved independently in the respective ancestors of mammals and birds (2). Consequently, traits shared only by mammals and birds may be the functional targets of these states. Interestingly, mammals and birds also independently evolved complex brains capable of orchestrating complex cognition, unlike that observed in reptiles and amphibians (3). Given that complex brains, complex cognition, and SWS and REM sleep evolved in concert, these shared traits may be functionally interrelated (2, 4). Indeed, the recent discovery of mammalian-like SWS homeostasis in birds suggests that functional hypotheses that posit a role for SWS homeostasis in maintaining adaptive brain function in mammals may also apply to birds (4). We believe that comparative studies of sleep in mammals and birds offer the promise of revealing overriding principles directly tied to the function(s) of SWS and REM sleep that might remain obscure through an exclusively mammal-based or simple animal model approach. NIELS C. RATTENBORG,* DOLORES MARTINEZ-GONZALEZ, JOHN A. LESKU, MADELEINE SCRIBA