Pravosudov Vladimir V, Sanford Kirsten, Hahn Thomas P
Department of Biology, University of Nevada, Reno.
Anim Behav. 2007 Mar;73(3):535-539. doi: 10.1016/j.anbehav.2006.10.005.
Migratory birds appear to have relatively smaller brain size compared to sedentary species. It has been hypothesized that initial differences in brain size underlying behavioural flexibility drove the evolution of migratory behaviour; birds with relatively large brains evolved sedentary habits and those with relatively small brains evolved migratory behaviour (migratory precursor hypothesis). Alternative hypotheses suggest that changes in brain size might follow different behavioural strategies and that sedentary species might have evolved larger brains because of differences in selection pressures on brain size in migratory and nonmigratory species. Here we present the first evidence arguing against the migratory precursor hypothesis. We compared relative brain volume of three subspecies of the white-crowned sparrow: sedentary Zonotrichia leucophrys nuttalli and migratory Z. l. gambelii and Z. l. oriantha. Within the five subspecies of the white-crowned sparrow, only Z. l. nuttalli is strictly sedentary. The sedentary behaviour of Z. l. nuttalli is probably a derived trait, because Z. l. nuttalli appears to be the most recent subspecies and because all species ancestral to Zonotrichia as well as all older subspecies of Z. leucophrys are migratory. Compared to migratory Z. l. gambelii and Z. l. oriantha, we found that sedentary Z. l. nuttalli had a significantly larger relative brain volume, suggesting that the larger brain of Z. l. nuttalli evolved after a switch to sedentary behaviour. Thus, in this group, brain size does not appear to be a precursor to the evolution of migratory or sedentary behaviour but rather an evolutionary consequence of a change in migratory strategy.
与定居性鸟类相比，候鸟的脑容量似乎相对较小。有假说认为，行为灵活性背后脑容量的初始差异推动了迁徙行为的进化；脑容量相对较大的鸟类进化出了定居习性，而脑容量相对较小的鸟类则进化出了迁徙行为（迁徙前体假说）。另一些假说则认为，脑容量的变化可能遵循不同的行为策略，而定居性物种可能由于迁徙和非迁徙物种在脑容量选择压力上的差异而进化出了更大的脑容量。在此，我们首次提出了反对迁徙前体假说的证据。我们比较了白冠雀三个亚种的相对脑容量：定居的白冠雀纳氏亚种（Zonotrichia leucophrys nuttalli）以及迁徙的甘氏亚种（Z. l. gambelii）和奥氏亚种（Z. l. oriantha）。在白冠雀的五个亚种中，只有纳氏亚种是严格定居的。纳氏亚种的定居行为可能是一种衍生特征，因为纳氏亚种似乎是最新的亚种，而且白冠雀属的所有祖先物种以及白冠雀的所有较古老亚种都是迁徙性的。与迁徙的甘氏亚种和奥氏亚种相比，我们发现定居的纳氏亚种具有显著更大的相对脑容量，这表明纳氏亚种较大的脑容量是在转变为定居行为之后进化而来的。因此，在这个群体中，脑容量似乎不是迁徙或定居行为进化的前体，而是迁徙策略变化的进化结果。
