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Negative evidence

Posted by jmwild on 08 Mar 2010 at 19:40 GMT

While the attempt to characterize the magnetoreceptor is laudable, especially when it is done well and in detail, as in this paper, the ignoring of published negative evidence that bears directly on the function of the magnetoreceptor that these authors postulate and favour, is definitely not laudable. The authors suggest that the magnetoreceptor functions as a "map sense", e.g. in homing pigeons, yet three recent papers that show that severing the relevant nerve in homing pigeons has no effect whatsoever on homing performance are completely ignored by these authors. As has so often been warned in science, the trouble with looking only for positive evidence for a hypothesis is that you are likely to find it!
Gagliardo A, Ioalè P, Savini M and Wild JM (2009)
1. Navigational abilities of adult and experienced homing pigeons deprived of olfactory or trigeminally mediated magnetic information. J Exp Biol, 212:3119-3124.
Gagliardo A, Ioalè P, Savini M and Wild JM (2008)
2. Navigational abilities of homing pigeons deprived of olfactory or trigeminally mediated magnetic information when young. J. Exp. Biol., 211, 2046-2051.
3. Gagliardo A, Ioale P, Savini M and Wild JM (2006)
Having the nerve to home: olfactory versus magnetoreceptor mediation of homing in pigeons. J. Exp. Biol., 209, 2888-2892.

Competing interests declared: My competing interests ar that I am one of the ignored authors of the negative evidence alluded to.

RE: Negative evidence

fleissner replied to jmwild on 21 Mar 2010 at 14:47 GMT

Not at all “negative evidence”!

Firstly, we are well aware of the cited papers by Gagliardo et al., but they do not provide negative evidence concerning an implication of the beak sensory system as a magnetic map sense.

We did not cite the Gagliardo et al. papers as our manuscript is not dealing with orientation in terms of behavioral experiments. We exclusively describe anatomical structures which seem to function as magneto receptors in whatever behavioral context.

However, now that the issue has been brought up, we would like to remind that navigation and homing can be based on many different cues detected by various sensory systems (like magnetic fields, odours, optic or acoustic landmarks). This was stated in our paper and corresponding review articles were cited. It is not certain at all that the results reported by Gagliardo et al. can be generalized. At various locations, the relative importance of the different cues may be different and animals can, of course, find their destination even if one of the respective sensory systems (like a magnetic map sense) is not available,

Secondly, Wild’s comment is based on a more than twenty years old controversy concerning the implication of olfactory cues in navigation (in the beginning stressed by the Papi-team). Series of experiments have since been conducted – partly in Italy, Germany and the US - to verify this hypothesis or elucidate the discrepancies of the various results, but so far olfaction-based navigation of homing pigeons seems to depend on special conditions at the home loft, for example its wind exposure (Wiltschko, Wiltschko and Walcott 1987; Wiltschko and Wiltschko 1992). However, it is far beyond the scope of our manuscript to contribute to this dispute.

Thirdly, it is simply not true that we did not critically discuss the so far available information on the implication of the magnetic map sense in avian navigation. There are several papers on older and newer results from various teams (for example: Dennis et al. 2007; Thalau et al 2007; Wiltschko et al 2006; Fransson et al. 2001), which provide experimental evidence supporting an implication of magnetic field strength in bird navigation. In these manuscripts, it has been demonstrated that besides the photopigment-based magnetic compass, a magnetic map sense in the beak is present in birds (e.g., for review Wiltschko et al. 2010). It was shown that the birds find home, but a missing magnetic map sense modified the route taken. The topic of our recent manuscript in PlosOne is actually a different one. Here we show that all birds seem to be equipped with a sensory system, which is suggested to detect magnetic field parameters as a basis for magnetic map being processed somewhere in the central nervous system.

No competing interests declared.