Layne John E, Barnes W Jon P, Duncan Lindsey M J
Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK.
J Exp Biol. 2003 Dec;206(Pt 24):4425-42. doi: 10.1242/jeb.00661.
Fiddler crabs Uca rapax are central-place foragers, making feeding excursions of up to several meters from their burrows. This study investigates the sources of directional and distance information used by these crabs when returning to their burrows. We tested the spatial frame of reference (egocentric or exocentric), and the source of spatial information (idiothetic or allothetic) used during homing. We also tested which components of their locomotion they integrated (only voluntary, or voluntary plus reflexive). Fiddler crabs in their natural mudflat habitat were passively rotated during normal foraging behavior using experimenter-controlled disks, before they returned home. Crabs resisted passive rotations on the disk by counter-rotating when the disk turned, which was a compensatory response to unintended movement. Crabs were usually situated eccentrically on the disk, and therefore were also subjected to a translation when the disk rotated. No crab actively compensated for this translation. Crabs that fully compensated for disk rotation made no directional homing error. Crabs that did not fully compensate homed in a direction that reflected their new body orientation. In other words, if we succeeded in reorienting a crab (i.e. it undercompensated for disk rotation), its homing error was equal to the angle by which it had been reoriented, regardless of the magnitude of the optomotor compensation. Computer-modelled crabs, each equipped with a path integrator utilizing different combinations of external (allothetic) and path-related (idiothetic) input, traversed the digitized paths of the real crabs. The home vector computed by the model crab was then compared to the homing direction observed in the real crab. The model home vector that most closely matched that of the real crab was taken to comprise the path integration mechanism employed by fiddler crabs. The model that best matched the real crab gained direction and distance idiothetically (from internal sources such as proprioceptors), and integrated only voluntary locomotory information. Crabs were also made to run home across a patch of wet acetate, on which they slipped and were thus forced to take more steps on the homeward path than theoretically required by the home vector. Crabs whose running velocity across the patch was unusually low also stopped short of their burrow before finding it. Crabs whose running velocity was not impeded by the patch did not stop short, but ran straight to the burrow entrance, as did control crabs that ran home with no slippery patch. We interpret this to mean that the velocity of some crabs was impeded because of slipping, and these therefore stopped short of their burrow after having run out their homing vector. This is positive evidence in support of the hypothesis that path integration is mediated either by leg proprioceptors or by efferent commands, but our data do not allow us to distinguish between these two possibilities.
招潮蟹（Uca rapax）是中心地觅食者，会从洞穴出发进行长达数米的觅食之旅。本研究调查了这些螃蟹返回洞穴时所使用的方向和距离信息的来源。我们测试了归巢过程中使用的空间参照系（自我中心或非自我中心）以及空间信息的来源（自身运动感知或外部感知）。我们还测试了它们在运动中整合了哪些组成部分（仅自主运动，还是自主运动加反射运动）。在自然泥滩栖息地的招潮蟹在正常觅食行为期间，利用实验者控制的圆盘进行被动旋转，然后让它们回家。当圆盘转动时，螃蟹会通过反向旋转来抵抗在圆盘上的被动旋转，这是对意外运动的一种补偿反应。螃蟹通常偏心地位于圆盘上，因此当圆盘旋转时它们也会发生平移。没有螃蟹主动补偿这种平移。完全补偿圆盘旋转的螃蟹没有方向归巢误差。没有完全补偿的螃蟹归巢的方向反映了它们新的身体方向。换句话说，如果我们成功地使螃蟹重新定向（即它对圆盘旋转的补偿不足），其归巢误差就等于它被重新定向的角度，而与视动补偿的大小无关。计算机模拟的螃蟹，每只都配备了一个路径积分器，利用不同组合的外部（外部感知）和路径相关（自身运动感知）输入，遍历真实螃蟹的数字化路径。然后将模型螃蟹计算出的归巢向量与在真实螃蟹中观察到的归巢方向进行比较。与真实螃蟹最匹配的模型归巢向量被认为构成了招潮蟹所采用的路径积分机制。最能匹配真实螃蟹的模型从内部来源（如本体感受器）以自身运动感知的方式获取方向和距离，并仅整合自主运动信息。还让螃蟹穿过一片湿的醋酸盐区域跑回家，在这片区域上它们会滑倒，因此在回家的路上被迫比归巢向量理论上所需的步数更多。在这片区域上奔跑速度异常低的螃蟹在找到洞穴之前也会在洞穴前停下。奔跑速度未受这片区域阻碍的螃蟹没有提前停下，而是径直跑到洞穴入口，就像没有湿滑区域直接跑回家的对照螃蟹一样。我们将此解释为意味着一些螃蟹的速度因滑倒而受到阻碍，因此在耗尽归巢向量后在洞穴前停下。这是支持路径积分由腿部本体感受器或传出指令介导这一假设的有力证据，但我们的数据无法让我们区分这两种可能性。
