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ScienceWeek
NEUROBIOLOGY: POLARIZED LIGHT AND MONARCH BUTTERFLY NAVIGATION
The following points are made by S.M. Reppert et al (Current Biology 2004 14:155):
1) During their spectacular migratory journey in the fall, North American monarch butterflies (Danaus plexippus) use a time-compensated sun compass to help them navigate to their overwintering sites in central Mexico [1-3]. One feature of the sun compass mechanism not fully explored in monarchs is the sunlight-dependent parameters used to navigate.
2) Ultraviolet (UV) light is important for the initiation of flight in monarch butterflies [3]. Once flight is attained, however, the relevant features of skylight used for actual compass orientation are unknown. Based largely on studies in honey bees and desert ants, the recognized compass signals visible to insects in the daytime sky consist of the sun itself and polarization and spectral-intensity gradients, which are generated as sunlight scatters through the atmosphere [4-5]. The skylight pattern of polarized light (the e-vector pattern) provides one of the most reliable navigational cues [4,5].
3) The authors provide data suggesting that the angle of polarized skylight (the e-vector) is a relevant orientation parameter. By placing butterflies in a flight simulator outdoors and using a linear polarizing filter, the authors demonstrate that manipulating the e-vector alters predictably the direction of oriented flight. Butterflies studied in either the morning or afternoon showed similar responses to filter rotation. Monarch butterflies possess the anatomical structure needed for polarized skylight detection, as rhabdoms in the dorsal-most row of photoreceptor cells in monarch eye show the organization characteristic of polarized-light receptors. The existence of polarized-light detection could allow migrants to accurately navigate under a variety of atmospheric conditions and reveals a critical input pathway into the sun compass mechanism.
References (abridged):
1. Perez, S.M., Taylor, O.R., and Jander, R. (1997). A sun compass in monarch butterflies. Nature 387, 29
2. Mouritsen, H. and Frost, B.J. (2002). Virtual migration in tethered flying monarch butterflies reveals their orientation mechanisms. Proc. Natl. Acad. Sci. USA 99, 10162-10166
3. Froy, O., Gotter, A.L., Casselman, A.L., and Reppert, S.M. (2003). Illuminating the circadian clock in monarch butterfly migration. Science 300, 1303-1305
4. Frisch, K.v. (1967). The Dance Language and Orientation of Bees. (Cambridge: Belknap Press of Harvard University Press)
5. Waterman, T. (1981). Polarization sensitivity. In Handbook of Sensory Physiology. Volume, V. and Autrum, H. eds. (New York: Springer-Verlag), pp. 281-469
Current Biology http://www.current-biology.com
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ZOOLOGY: A POLARIZED LIGHT COMPASS ORGAN IN SPIDERS
In all but the most primitive animals, the ability of the nervous system to respond to the internal and external environment depends on specialized sensory cells (sensory receptors) that in general can be grouped into four principal types: a) photoreceptors (sensitive to light); b) mechanoreceptors (which respond to physical pressure or movement, e.g., sound and touch); c) chemoreceptors (which detect chemical substances, e.g., odor and taste); d) thermoreceptors (which monitor temperature. In addition to the above major types, there are also sensory cells that have evolved to specifically respond to electric and magnetic fields.
In general, for most sensory modalities the sensory cell acts as an energy transducer, transforming the energy of the environmental stimulus into a change in the electrical potential difference across a biological membrane. Energy transduction is thus the fundamental physical basis of the sensory response of most biological systems to their environments, but within each of the evolved sensory modalities there exists an enormous variety of structure and function produced by evolutionary pressures. Some insects and vertebrates, for example, use the pattern of polarized light in the sky as an optical compass. In the case of bees and ants, only a small section of clear sky must be visible in order for these insects to obtain a compass bearing for accurate navigation. The receptors involved in the polarization compass are confined to a small part of the animal's retina, and the eyes are for the most part constructed for other visual tasks.
The following points are made by M. Dacke et al ((Nature 1999 401:470):
1) The authors report the discovery of a unique compass organ in the spider Drassodes cupreus, in which a pair of specialized secondary eyes cooperate to analyze skylight polarization. These eyes do not form images, but use a built-in polarization filter to determine precisely the direction of polarization. The authors report that measurements using a model eye indicate that this compass organ is best suited for navigation at dusk and dawn. Behavioral experiments demonstrate that the spiders are primarily active after sunset, and that they use polarization cues to find their way back to the nest after foraging trips.
2) The authors suggest that a similar organization of the secondary eyes in several spider families indicates that such compass organs may not be an isolated phenomenon. The authors also suggest that since this compass organ appears to be involved in no other visual tasks, it offers an interesting model for studies of the neural processing underlying polarization navigation.
Nature http://www.nature.com/nature
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MONARCH BUTTERFLY MAGNETIC COMPASS PAPER RETRACTED
This is a cautionary tale, an example of how the extraordinary complexity of biological organisms, systems whose operating variables are often unknown, can lead researchers astray. In November 1999 (Proc. Natl. Acad. Sci. US 1999 96:13845), a research team reported that fall migratory monarch butterflies, tested for their directional responses to magnetic cues under three conditions, amagnetic, normal, and reversed magnetic fields, showed three distinct patterns: In the absence of a magnetic field, monarchs lacked directionality as a group; in the normal magnetic field, monarchs oriented to the southwest with a group pattern typical for migrants; when the horizontal component of the magnetic field was reversed, the butterflies oriented to the northeast. In contrast, nonmigratory monarchs lacked directionality in the normal magnetic field. The authors suggested the results were "a direct demonstration of magnetic compass orientation in migratory insects."
Four months later, in March 2000 (Proc. Natl. Acad. Sci. 2000 97:3782), the authors retracted their paper, noting the following: "The positive response to magnetic fields in two experiments cannot be repeated. Further experiments show the false positives in these tests result from a positive [directionality of movement (taxis)] by the butterflies to the light reflected off the clothing of the observers. We therefore retract our report. We regret the inconvenience that publication of this study may have caused."
Proc. Nat. Acad. Sci. http://www.pnas.org
ScienceWeek http://scienceweek.com
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