Phillips John B, Borland S Chris, Freake Michael J, Brassart Jacques, Kirschvink Joseph L
Biology Department, Virginia Tech University, Blacksburg 24061, USA.
J Exp Biol. 2002 Dec;205(Pt 24):3903-14. doi: 10.1242/jeb.205.24.3903.
Experiments were carried out to investigate the earlier prediction that prolonged exposure to long-wavelength (>500 nm) light would eliminate homing orientation by male Eastern red-spotted newts Notophthalmus viridescens. As in previous experiments, controls held in outdoor tanks under natural lighting conditions and tested in a visually uniform indoor arena under full-spectrum light were homeward oriented. As predicted, however, newts held under long-wavelength light and tested under either full-spectrum or long-wavelength light (>500 nm) failed to show consistent homeward orientation. The newts also did not orient with respect to the shore directions in the outdoor tanks in which they were held prior to testing. Unexpectedly, however, the newts exhibited bimodal orientation along a more-or-less 'fixed' north-northeast-south-southwest magnetic axis. The orientation exhibited by newts tested under full-spectrum light was indistinguishable from that of newts tested under long-wavelength light, although these two wavelength conditions have previously been shown to differentially affect both shoreward compass orientation and homing orientation. To investigate the possibility that the 'fixed-axis' response of the newts was mediated by a magnetoreception mechanism involving single-domain particles of magnetite, natural remanent magnetism (NRM) was measured from a subset of the newts. The distribution of NRM alignments with respect to the head-body axis of the newts was indistinguishable from random. Furthermore, there was no consistent relationship between the NRM of individual newts and their directional response in the overall sample. However, under full-spectrum, but not long-wavelength, light, the alignment of the NRM when the newts reached the 20 cm radius criterion circle in the indoor testing arena (estimated by adding the NRM alignment measured from each newt to its magnetic bearing) was non-randomly distributed. These findings are consistent with the earlier suggestion that homing newts use the light-dependent magnetic compass to align a magnetite-based 'map detector' when obtaining the precise measurements necessary to derive map information from the magnetic field. However, aligning the putative map detector does not explain the fixed-axis response of newts tested under long-wavelength light. Preliminary evidence suggests that, in the absence of reliable directional information from the magnetic compass (caused by the 90 degrees rotation of the response of the magnetic compass under long-wavelength light), newts may resort to a systematic sampling strategy to identify alignment(s) of the map detector that yields reliable magnetic field measurements.
开展了实验，以研究早期的一项预测，即长时间暴露于长波长（>500纳米）光下会消除雄性东部红腹蝾螈（Notophthalmus viridescens）的归巢定向。与之前的实验一样，在自然光照条件下饲养于室外水箱中，并在全光谱光下的视觉均匀室内场地进行测试的对照组蝾螈具有归巢定向。然而，正如预测的那样，在长波长光下饲养并在全光谱或长波长（>500纳米）光下进行测试的蝾螈未能表现出一致的归巢定向。这些蝾螈在测试前饲养它们的室外水箱中也没有相对于岸边方向进行定向。然而，出乎意料的是，蝾螈沿着或多或少“固定”的北北东 - 南南西磁轴表现出双峰定向。在全光谱光下测试的蝾螈所表现出的定向与在长波长光下测试的蝾螈的定向没有区别，尽管此前已表明这两种波长条件对向岸罗盘定向和归巢定向有不同影响。为了研究蝾螈的“固定轴”反应是否由涉及磁铁矿单畴颗粒的磁感受机制介导，对一部分蝾螈测量了天然剩磁（NRM）。NRM相对于蝾螈头 - 体轴的排列分布与随机分布没有区别。此外，在整个样本中，单个蝾螈的NRM与其定向反应之间没有一致的关系。然而，在全光谱光下（而非长波长光下），当蝾螈在室内测试场地到达20厘米半径的标准圆时（通过将从每个蝾螈测量的NRM排列与其磁方位相加来估计），NRM的排列是非随机分布的。这些发现与早期的观点一致，即归巢蝾螈在从磁场中获取推导地图信息所需的精确测量时，利用光依赖磁罗盘来校准基于磁铁矿的“地图探测器”。然而，校准假定地图探测器并不能解释在长波长光下测试的蝾螈的固定轴反应。初步证据表明，在缺乏来自磁罗盘的可靠方向信息时（由长波长光下磁罗盘反应的90度旋转引起），蝾螈可能会采用系统采样策略来识别能产生可靠磁场测量的地图探测器的校准。
