Auditory Neuroscience: Recalibration of Space Perception Requires Cortical Feedback

feeding rhythm during the Arctic polar day and night [14], and the emperor penguin (Aptenodytes forsteri) loses its melatonin rhythm during the Antarctic mid-winter [15]; on the other hand, several Arctic rodents and carnivores have been reported to exhibit 24-h activity rhythms during the polar day and/or in constant conditions [16–19]. Clearly, further comparative studies are needed to determine the species characteristics that lead to differential dependence on circadian control at the poles. Importantly, circadian arrhythmia in reindeer does not mean complete arrhythmia. Indeed, as Lu et al. [1] point out, they display precisely timed annual reproductive cycles, perhaps involving a distinct ‘circannual’ clock. They also exhibit robust ultradian activity rhythms (with periods <24 h) throughout the year, which may represent the frequency best-suited to the energy and digestive requirements of a large Arctic herbivore [3]. Suppression of circadian and enhancement of ultradian rhythmicity is also a known feature of a non-Arctic rodent, the common vole (Microtus arvalis), and laboratory experiments have begun to dissect the underlying mechanisms [20]. Animals everywhere are confronted by environments that demand specialized behavioral and metabolic responses; for those of us intent on understanding the adaptive significance of clocks and rhythms, the premier experimental resource remains the richness and diversity of the natural world. References 1. Lu, W., Meng, Q.J., Tyler, N., Stokkan, K.A., and Loudon, A. (2010). A circadian clock is not required in an Arctic mammal. Curr. Biol. 20, 533–537. 2. van Oort, B.E., Tyler, N.J., Gerkema, M.P., Folkow, L., Blix, A.S., and Stokkan, K.A. (2005). Circadian organization in reindeer. Nature 438, 1095–1096. 3. van Oort, B.E., Tyler, N.J., Gerkema, M.P., Folkow, L., and Stokkan, K.A. (2007). Where clocks are redundant: weak circadian mechanisms in reindeer living under polar photic conditions. Naturwissenschaften 94, 183–194. 4. Simonneaux, V., and Ribelayga, C. (2003). Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters. Pharmacol. Rev. 55, 325–395. 5. Paul, M.J., Zucker, I., and Schwartz, W.J. (2008). Tracking the seasons: the internal calendars of vertebrates. Philos. Trans. R. Soc. Lond. B Biol. Sci. 363, 341–361. 6. Eloranta, E., Timisjarvi, J., Nieminen, M., Ojutkangas, V., Leppaluoto, J., and Vakkuri, O. (1992). Seasonal and daily patterns in melatonin secretion in female reindeer and their calves. Endocrinology 130, 1645–1652. 7. Stokkan, K.A., Tyler, N.J., and Reiter, R.J. (1994). The pineal gland signals autumn to reindeer (Rangifer tarandus tarandus) exposed to the continuous daylight of the Arctic summer. Can. J. Zool. 72, 904–909. 8. Eloranta, E., Timisjarvi, J., Nieminen, M., Leppaluoto, J., and Vakkuri, O. (1995). Seasonal onset and disappearance of diurnal rhythmicity in melatonin secretion in female reindeer. Am. Zool. 35, 203–214. 9. Stokkan, K.A., van Oort, B.E., Tyler, N.J., and Loudon, A.S. (2007). Adaptations for life in the Arctic: evidence that melatonin rhythms in reindeer are not driven by a circadian oscillator but remain acutely sensitive to environmental photoperiod. J. Pineal Res. 43, 289–293. 10. Stillman, B., Stewart, D., and Grodzicker, T., eds. (2007). Clocks and Rhythms: Cold Spring Harbor Symposia on Quantitative Biology LXXII (Cold Spring Harbor Laboratory Press). 11. Nagoshi, E., Saini, C., Bauer, C., Laroche, T., Naef, F., and Schibler, U. (2004). Circadian gene expression in individual fibroblasts: cell-autonomous and self-sustained oscillators pass time to daughter cells. Cell 119, 693–705. 12. Welsh, D.K., Yoo, S.H., Liu, A.C., Takahashi, J.S., and Kay, S.A. (2004). Bioluminescence imaging of individual fibroblasts reveals persistent, independently phased circadian rhythms of clock gene expression. Curr. Biol. 14, 2289–2295. 13. Liu, A.C., Welsh, D.K., Ko, C.H., Tran, H.G., Zhang, E.E., Priest, A.A., Buhr, E.D., Singer, O., Meeker, K., Verma, I.M., et al. (2007). Intercellular coupling confers robustness against mutations in the SCN circadian clock network. Cell 129, 605–616. 14. Stokkan, K.A., Mortensen, A., and Blix, A.S. (1986). Food intake, feeding rhythm, and body mass regulation in Svalbard rock ptarmigan. Am. J. Physiol. 251, R264–R267. 15. Miche, F., Vivien-Roels, B., Pevet, P., Spehner, C., Robin, J.P., and Le Maho, Y. (1991). Daily pattern of melatonin secretion in an antarctic bird, the emperor penguin, Aptenodytes forsteri: seasonal variations, effect of constant illumination and of administration of isoproterenol or propranolol. Gen. Comp. Endocrinol. 84, 249–263. 16. Folk, G.E. (1964). Daily physiological rhythms of carnivores exposed to extreme changes in Arctic daylight. Fed. Proc. 23, 1221–1228. 17. Swade, R.H., and Pittendrigh, C.S. (1967). Circadian locomotor rhythms of rodents in the Arctic. Am. Nat. 101, 431–466. 18. Semenov, Y., Ramousse, R., Le Berre, M., Vassiliev, V., and Solomonov, N. (2001). Aboveground activity rhythm in Arctic blackcapped marmot (Marmota camtschatica bungei Katschenko 1901) under polar day conditions. Acta. Oecologica 22, 99–107. 19. Folk, G.E., Thrift, D.L., Zimmerman, M.B., and Reimann, P.C. (2006). Mammalian activity-rest rhythms in Arctic continuous daylight. Biol. Rhythm Res. 37, 455–469. 20. van der Veen, D.R., Minh, N.L., Gos, P., Arneric, M., Gerkema, M.P., and Schibler, U. (2006). Impact of behavior on central and peripheral circadian clocks in the common vole Microtus arvalis, a mammal with ultradian rhythms. Proc. Natl. Acad. Sci. USA 103, 3393–3398.