Electronic Supporting Material from A light-dependent magnetoreception mechanism insensitive to light intensity and polarization

Billions of migratory birds navigate thousands of kilometres every year aided by a magnetic compass sense the biophysical mechanism of which is unclear. One leading hypothesis is that absorption of light by specialized photoreceptors in the retina produces short-lived chemical intermediates known as radical pairs whose chemistry is sensitive to tiny magnetic interactions. A potentially serious but largely ignored obstacle to this theory is how directional information derived from the Earth's magnetic field can be separated from the much stronger variations in the intensity and polarization of the incident light. Here we propose a simple solution in which these extraneous effects are cancelled by taking the ratio of the signals from two neighbouring populations of magnetoreceptors. Geometric and biological arguments are used to derive a set of conditions that make this possible. We argue that one likely location of the magnetoreceptor molecules would be in association with ordered opsin dimers in the membrane discs of the outer segments of double-cone photoreceptor cells

Orientation of birds at the capture and displacement site.

<p>(<i>A, C</i>) Results for sham-sectioned birds, before sham-surgery at the capture site (<i>A</i>) and after sham-surgery and translocation to the displacement site (<i>C</i>). (<i>E, G</i>) Results for V1-sectioned birds before V1-sectioning at the capture site (<i>E</i>) and after V1-sectioning surgery and translocation to the displacement site (<i>G</i>). For description of the circular diagrams and the map (<i>D</i>), see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065847#pone-0065847-g001" target="_blank">Figure 1</a>. The schemes of real V1-section (<i>F</i>) and sham section (<i>B</i>) show the approximate locations of the three branches of the trigeminal nerve. The ophthalmic branch (V1) is shown in bold. The crosses on <i>F</i> indicate the approximate locations at which the nerve was sectioned and a piece of the nerve was removed. For details about the surgeries, see Methods.</p

Differences of the geomagnetic field parameters between capture site, displacement site and a putative goal area.

<p>(<i>A</i>) Difference in total intensity; (<i>B</i>) Difference in inclination; (<i>C</i>) Difference in declination. The measured geomagnetic field parameters at Rybachy (the left dot) were the following: total intensity 50,688 nT, inclination 70.3°, declination 5.6°. The measured geomagnetic field parameters at Zvenigorod (the right dot) were the following: total intensity 52,175 nT, inclination 71.2°, declination 10.1°. As a goal site (the upper dot), the centroid of “the goal” shown on <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065847#pone-0065847-g001" target="_blank">Figure 1</a> (60° 30′N, 27° 51′E) was taken. Computation of the Earth’s magnetic field parameters for the goal site was done with the calculator of IGRF Model 11 in the website of the National Geophysical Data Center (<a href="http://www.ngdc.noaa.gov/geomag/geomag.shtml" target="_blank">http://www.ngdc.noaa.gov/geomag/geomag.shtml</a>). The calculated geomagnetic field parameters at the goal site were the following: total intensity 52,172 nT, inclination 73.7°, declination 8.9°. The charts with the isolines of the geomagnetic field parameters are taken from <a href="http://pubs.usgs.gov/sim/2007/2964" target="_blank">http://pubs.usgs.gov/sim/2007/2964</a> with modifications.</p

Results of our previous displacement study with intact Eurasian reed warblers (re-drawn after [4]).

<p>(<i>A</i>) Orientation of birds at the capture site (Rybachy). (<i>C</i>) Orientation of the same birds after the 1,000 km eastward translocation at the displacement site (Zvenigorod). On <i>A</i> and <i>C</i>, pooled data for 2004, 2005 and 2007 are shown. Each dot at the circular diagram periphery indicates the mean orientation of one individual bird. The arrows show group mean directions and vector lengths. The dashed circles indicate the length of the group mean vector needed for significance (5% and 1% level for inner and outer dashed circle, correspondingly) according to the Rayleigh test <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065847#pone.0065847-Batschelet1" target="_blank">[21]</a>. The lines flanking group mean vectors give 95% confidence intervals. gN – geographic North. On (<i>B</i>), a map of the displacement is shown. The shaded light gray zone represents the breeding range of the Eurasian reed warbler. Visual observations by local ornithologists (e.g. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065847#pone.0065847-Popelnyukh1" target="_blank">[22]</a>) confirm that there are no regular breeding populations of Eurasian reed warblers further east than indicated on the map. The black filled circle represents the single known recovery of a reed warbler ringed in Rybachy and re-captured as a breeding bird <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065847#pone.0065847-Bolshakov1" target="_blank">[23]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065847#pone.0065847-Bolshakov2" target="_blank">[24]</a>. The dashed line vector from the capture site at Rybachy shows the mean migratory direction of the given species according to our previous study (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065847#pone.0065847-Chernetsov1" target="_blank">[4]</a>, α = 42°). The solid line circle represents a proposed area where transit Eurasian reed warblers are heading to based on our previous study <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065847#pone.0065847-Chernetsov1" target="_blank">[4]</a> combined with ringing recoveries (the goal). The solid line vector from Rybachy to Zvenigorod shows the direction and distance of the displacement. The two dashed line vectors from Zvenigorod represent our expectations for V1-sectioned and sham-sectioned birds, respectively: (1): no compensation, (2): compensation towards the eastern part of the breeding range.</p

Model illustrating how a time-compensated sun compass could help passively drifted reef fish larvae to relocate their natal OTI reef.

<p>Red dots indicate the locations at ebb tide of passively dispersed particles 8 days after release from One Tree Reef according to the dispersal experiments and model calculations (Red dots drawn after Fig. 1C in reference <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066039#pone.0066039-Gerlach1" target="_blank">[2]</a>). At flood tide the dots will be displaced 5–7 km WNW. Notice that most larvae considered as passive particles for their first 8 days would be transported significantly to the NNW beyond the odor halo of OTI (idealized odor halos in decreasing intensity blue) and even beyond neighboring reefs to the NNW before they gain sustained swimming capabilities. The polar diagram shows the time-compensated sun compass orientation of just-settled (•) and pre-settlement (o) larvae (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066039#pone-0066039-g002" target="_blank">Figure 2A and 2C</a> combined: mean direction 155°, n = 21, r = 0.89, p<0.001). One-week old <i>O. doederleini</i> larvae (symbolized by the dispersed cloud of red dots) would be more likely to relocate the OTI reef if they used a sun compass to swim actively toward SSE, than if they would swim in random directions. Picture shows settling stage <i>Ostorhinchus doederleini.</i></p

Settling stage <i>Ostorhinchus doederleini</i> use a time-compensated sun compass to orient towards SSE.

<p><b>A</b>: Fourteen just-settled <i>O. doederleini</i> tested under natural sunny skies in 2011 showed a clear orientation towards SSE (mean direction: 152°, r = 0.88, n = 14, p<0.001). <b>B</b>: When five of these fish were clock-shifted 6 hours backwards, they turned their orientation by ca. 180° (mean direction: 344°, r = 0.94, n = 5, p<0.01). <b>C, D</b>: In January 2012, we repeated the experiments with pre-settlement fish and got very similar results. Seven pre-settlement <i>O. doederleini</i> tested under natural sunny skies showed a clear orientation towards SSE (<b>C</b>: mean direction: 161°, r = 0.91, n = 7, p<0.001). When all 7 fish were clock-shifted 6 hours backwards, they turned their orientation by ca. 180° (<b>D</b>: mean direction: 321°, r = 0.92, n = 7, p<0.001). Each dot at the circle periphery indicates the mean orientation chosen by each individual fish based on the second order average of all tests made with a given fish in the given condition. Arrows indicate the group mean vectors. Inner and outer dashed circles indicate the radius of the group mean vector needed for significance according to the Rayleigh Test (p<0.05 and p<0.01, respectively). Lines flanking the group mean vector indicate the 95% confidence intervals for the group mean direction. <b>E</b>: We performed all orientation tests between 20/Jan and 01/Feb. The yellow curve in <b>E</b> shows the height of the sun above the horizon at One Tree Island calculated for 25 January 2012 (90° means directly overhead, 0° means that the sun is at the horizon). The blue curve in <b>E</b> is the sun azimuth curve at One Tree Island calculated for 25 January 2012. Notice that in the morning until about 11∶15, the sun azimuth is very consistently in the East (117°–77°). Likewise, in the afternoon from 12∶45 onwards, the sun azimuth is very consistently in the West (293°–243°). In contrast, at noon between 11∶15 and 12∶45, the sun is more or less directly overhead (the sun is 78–86 degrees above the horizon, see yellow curve) and the sun azimuth changes by 139 degrees in just 90 minutes. <b>F</b> is showing how strongly oriented the individual fish were during tests in the different time intervals. The left y-axis is indicating the length of the mean vector, “r”, calculated by vector addition of the 40 observed directions during a single test of a given individual. The greater the r, the more consistently the fish oriented. The mean vector length is inversely proportional to the angular standard deviation (s = (-ln(r))½) which is indicated on the right y-axis. Figure <b>F</b> is aligned exactly under Figure <b>E</b> so that the blue dashed lines identify the time range and sun azimuth positions that contributed data to each of the six time blocks. Notice that the fish oriented very poorly during the 11∶15–12∶45 time block, when a sun compass would be very difficult to use because the sun is almost directly overhead and shows an exceptionally rapid change in azimuth (139 degrees in just 90 minutes, i.e. 1.5 degrees/minute). Accurate orientation during this time would require a very precise synchronization of the animals’ internal clock to the specific sun curve. In contrast, late in the afternoon when a sun compass would be particularly easy to use because the sun azimuth changes very slowly and because the sun is close to the horizon, the fish showed extremely directed orientation. The unusual sun curve also means that a 6 hour clock-shift where the animals wake up around midnight and are tested before noon, when they think it is afternoon, leads to an extremely consistent predicted change in orientation of 180 degrees. This is documented by the red curve in Fig. <b>E</b>, which shows the predicted clockwise shift in orientation following a 6 hour clock-shift as a function of the time of day during which the fish are tested after being clock-shifted 6 hours. The red curve was calculated as follows: the Sun azimuth at testing time - the sun azimuth 6 hours later. We tested our clock-shifted fish between 06∶45 and 11∶02 (as indicated by the dashed vertical red lines) when the expected orientation of the 6 hrs time shifted larvae predicts a 180 degree shift for this entire 4∶17-hr observation window.</p

Location of One tree Island.

<p>One Tree reef (OTI, 23°30′S, 152°06′E) is one of fourteen reefs in the Capricorn Bunker Group in the southern Great Barrier Reef, Australia. OTI is situated 90 km from the Queensland coast and 5–10 km southeast of neighboring reefs Heron and Sykes.</p

All but one pigeon significantly preferred to peck at a stationary dot that was additionally introduced into the rotating dot pattern (a). After specific training to ignore eccentric stationary dots, all of the pigeons significantly preferred pecking in the rotational centre over pecking at the additional stationary dot (b). Bars depict the group mean peck distances in cm with standard errors; open circles represent the individual median peck distances in cm.

<p>The group mean peck distances were compared by a paired t-test (* p < 0.05, *** p < 0.001).</p

Peck distances of the pigeons relative to the centre of rotation increased (r = 0.81, p < 0.0001) when the distance of the dot located closest to the rotational centre increased.

<p>Individual median peck distances relative to the rotational centre in cm are depicted by individual symbols. Note that pigeon 818 (diamonds) showed generally greater peck distances to the rotational centre than all other pigeons (multiple comparison test: p < 0.001).</p

Perceptual Strategies of Pigeons to Detect a Rotational Centre—A Hint for Star Compass Learning?

<div><p>Birds can rely on a variety of cues for orientation during migration and homing. Celestial rotation provides the key information for the development of a functioning star and/or sun compass. This celestial compass seems to be the primary reference for calibrating the other orientation systems including the magnetic compass. Thus, detection of the celestial rotational axis is crucial for bird orientation. Here, we use operant conditioning to demonstrate that homing pigeons can principally learn to detect a rotational centre in a rotating dot pattern and we examine their behavioural response strategies in a series of experiments. Initially, most pigeons applied a strategy based on local stimulus information such as movement characteristics of single dots. One pigeon seemed to immediately ignore eccentric stationary dots. After special training, all pigeons could shift their attention to more global cues, which implies that pigeons can learn the concept of a rotational axis. In our experiments, the ability to precisely locate the rotational centre was strongly dependent on the rotational velocity of the dot pattern and it crashed at velocities that were still much faster than natural celestial rotation. We therefore suggest that the axis of the very slow, natural, celestial rotation could be perceived by birds through the movement itself, but that a time-delayed pattern comparison should also be considered as a very likely alternative strategy.</p></div