Birds may not be as "bird-brained" as we thought. Zebra finches show many features of sleep, which had previously been assumed to be the sole preserve of mammals.
The finding raises new questions about the complexity of the bird brain and about the evolution of sleep as we know it.
Mammalian sleep is characterised by distinct stages: slow wave sleep (SWS), intermediate sleep (IS) and rapid-eye-movement (REM) sleep, with a progression towards more REM sleep.
EEG recordings of the brain's electrical activity also show specific landmarks called "K-complexes" and "spindles" during SWS.
Since K-complexes had only ever been observed in mammals, neuroscientists assumed that a neocortex - an area of the brain involved in higher functions such as conscious thought and language - was needed to generate them.
Philip Low at the Salk Institute for Biological Sciences in La Jolla, California, US, and his colleagues monitored five zebra finches during the night, tracking eye and body movements, and brain activity.
They found that the birds display all of these characteristics of mammalian sleep - including K-complexes - even though they lack a neocortex. "The neocortex is at best sufficient, but not necessary for the production of these mammalian patterns," says Low.
If this holds true, it could mean that mammalian-like sleep evolved earlier that previously assumed - before higher brain structures like the cortex developed.
"These very specific characteristics of sleep that are observed in humans may go back to very ancient times - to birds," says Luca Finelli, a sleep expert at Novartis in Basel, Switzerland.
Another possibility is that birds do in fact possess a cortex-like structure, but that it looks different to the mammalian one. "Perhaps they are able to perform the same computations that we can, but the structure is [different]," says Low.
Observations of crow and scrub-jay behaviour might support this, with previous studies suggesting that these birds possess "theory of mind", or the ability to estimate what another individual might be thinking or how they will respond to a given situation.
The findings raise other questions. For example, do birds use sleep the same way as mammals? "Are these signals linked with memory consolidation, and do they have anything to do with learning?" asks Low. "If there's so much structure during bird sleep, is it possible that this structure is being used in a similar way?"
Finelli agrees that the findings are surprising, but cautions against over-interpreting them at such an early stage. "We don't know if these were actually K-complexes, or something that looks like them," he says. Future research should investigate the underlying physical processes that generate these electrical signals in birds.
Meanwhile, Terry Sejnowski from Salk Institute for Biological Studies, who also contributed to the study, points out that, while there may be many similarities between bird and mammalian sleep, there are also some differences.
For example, humans cycle between SWS and REM sleep roughly every 1.5 hours, but birds appear to do it every 10-15 minutes. "The question is why?" he says.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0703452105)
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Sleep Cycle Periods
Fri Jun 27 09:22:49 BST 2008 by Migrant
"For example, humans cycle between SWS and REM sleep roughly every 1.5 hours, but birds appear to do it every 10-15 minutes. "The question is why?" he says."
Perhaps because birds have a significantly higher heartbeat rate and a correspondingly shorter life-span. Their whole metabolism might be running ~6 times faster.
Some resting birds' heartbeat rates are (in beats per minute)
Domestic Chicken 245
House Sparrow 460
Ruby-throated Hummingbird 615
Shorter Cycles = Quicker Reaction-time.
Fri Jun 27 11:42:42 BST 2008 by Captain Obvious
Quicker to wake-up, quicker to be functional to scoot if some hawk or related shows-up.
We're functionally-impaired for a full sleep-cycle of time after we wake-up, if that equivalent-duration holds with birds, shorter cycles would have been required by natural selection.
Sun Jun 29 00:33:01 BST 2008 by Matt
Also is what else is happening in human/bird body else in both phases. Is also cns in different mode? Do organs/periferies have some special instrucions while sleeping? Or these phases are mainly tokens of necessary psychical processes (memory etc). Maybe it is some system how to "do sleeping" best way.
Also may be important how many times a day they sleep, and for how long, and if dependetly on sun?
Migrant may also be true...
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