草场上没有了羊群

<正>这里的羊已全部被淘汰,只因为牛羊争水,每天都有几只羊被顶死拜访才代的翌日,我们来到了采日玛乡的麦果尔大队,村干部告诉我们,这个大队的8个队中有5个队严重缺水,麦果尔3队更是个出了名的缺水队,在他们那片远离河流的贫瘠草场上,牧民们为了适应极端缺水的环境,甚至改变了畜群的类型,把队上所有的羊都淘汰了。     

Bird biodiversity

Endemic Bird Areas of the World. Priorities for Biodiversity Conservation by A.J. Stattersfield, M.J. Crosby, A.J. Long and D.C. Wege, Birdlife International, 1997. £37.00 pbk (860 pages) ISBN 0946 888 33 7CDROM: Birds of the World—A Multimedia Encyclopedia, Ransom, 1997. £14.99 ISBN 1 863 89 1748     

鸟类的飞翔

本文综述了目前对鸟类飞翔的研究 ,包括飞翔机制、运动学和形态适应等问题。首先讨论动力产生的基础包括翼型运动和气体旋涡以及飞翔需要的动力 ,而后论述振翅飞翔和姿势变换的运动学原理及量化飞翔强度 ,阐明了翼部和尾部外形对飞翔的重要作用 ,并分析了始祖鸟的飞翔能力     

The coordinated aerobatics of dunlin flocks

Flocks of dunlin (Calidris alpina) were filmed during rapid aerial manoeuvres. Two types of manoeuvres were identified: (a) course-change movements in which the entire flock altered course, and (b) rotation movements in which individual birds rotated on their sagittal axes without necessarily changing direction. Either manoeuvre could make a flock's overall appearance shift suddenly from light to dark (or vice versa) as the birds oriented first with their light ventral, then with their dark dorsal, plumage toward the observer. Slow-motion analysis revealed that the average time required by a majority of birds in camera view to change orientations during a manoeuvre was 196 ms (sd=137 ms). ‘Self-generated synchrony’ is a possible mechanism for this highly coordinated group behaviour.     

Bird sex determination

During the evolution, sex determination occurred early. Sex determining factors were progressively isolated from other genes in sexual chromosomes, or gonosomes. Among vertebrates, evolution took two opposite pathways : in mammals, the system of XX:XY sex determination, with Y chromosome, induces male differentiation. In contrast, in birds, the system ZZ:ZW, with the W chromosome, induces female differentiation. But comparative studies show that the two pathways are not so simple. In the chicken as in the lower vertebrates, estrogens play a central role in gonadal sex differentiation. Several genes, show to be critical for mammalian determination, are also expressed in the chicken but their expression pattern differs, indicating functional plasticity. The W-linked female determinants remains still unknown. But comparative studies of the two pathways, with conserved and divergent elements, are broadening our understanding of sex determination.     

猛禽中的捕蛇者

古今中外,雕在人类的以上中往往与天界和神祗的权利联系起来,象征着力量、骄傲和权力。人们认为雕是神的信使,因为它强壮有力,飞的很高,古希腊人相信,神祗是不是会伤害自己的信使的,于是雕被捆在庙宇的屋顶上,以防止闪电击毁建筑人。希腊人和罗马人认为雕有治愈创     

Detection of prey flocks by predators

The model presented by Vine (1971) to demonstrate a possible advantage of flocking behaviour in reducing predation risk made the unrealistic assumption that flocking would not affect the maximum distance at which a predator could just detect that an animal or animals were present. If flocking does reduce this distance it might yield a compensating disadvantage to prey animals, or an advantage to the predator. While quantification of these possibilities remains problematic in general, empirical data suggests that detection range depends on a simple power function of the number of individuals in a tight circular flock and their physical size. It is concluded that small flocks are likely to be advantageous to the predator and quite possibly disadvantageous to the prey, although the “selfish” advantage of flocking is likely to remain high for prey animals in medium or large sized flocks.