Congdon Justin D, Nagle Roy D, Dunham Arthur E, Beck Chirstopher W, Kinney Owen M, Yeomans S Rebecca
Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802, USA e-mail:
Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA, , , , , , US.
Oecologia. 1999 Nov;121(2):224-235. doi: 10.1007/s004420050924.
In many organisms, body size is positively correlated with traits that are presumably related to fitness. If directional selection frequently favors larger offspring (the "bigger is better" hypothesis), the results of such selection should be detectable with field experiments. We tested the "bigger is better" hypothesis in hatchling snapping turtles (Chelydra serpentina) by conducting one long-term and three short-term experiments on the University of Michigan E.S. George Reserve in southeastern Michigan. In the fall of 1995 and 1996, we released hatchlings at artificial nests separated from the nearest wetland by fences. We recorded the proportion of hatchlings recaptured, the time it took hatchlings to move to fences from artificial nests 45, 55, and 80 m away, and dispersion along the fence. We determined whether the response variables and probability of recapture at fences were associated with hatchling body size. During 1995, average travel times of hatchlings from the experimental nests were not related to distance from the fence; however, time to recapture was positively correlated with dispersion from the zero point on the fence, and the maximum time to reach the fence was almost twice as long for hatchlings from the 80-m nest compared to those from the 45-m nest. Sixty-seven percent of the hatchlings reached the fence and the proportions doing so from each nest were not different. Body size was not significantly related to probability of recapture in either of the 1995 experiments. In 1996, 59% of released hatchlings were recaptured. Time to recapture was not related to dispersion from the zero point or to body size. Cubic spline analysis suggested stabilizing selection on body size. We also conducted a set of long-term hatchling release experiments between 1980-1993 to compare the survival of hatchlings released at nest sites to that of hatchlings released directly into marshes, and we looked for relationships between survivorship and hatchling body size. During 7 years in which more than 30 hatchlings were released, 413 hatchlings were released directly into the marsh and 262 were released at nests: their probability of survival did not differ. Over all years, for both release groups combined and for each group separately, survival was not related to body size. In 1983 alone, survival was also not related to body size for either group or for both groups combined. In our three short-term experiments and one long-term experiment, we found no evidence to support the "bigger is better" hypothesis. When selection on body size did occur, selection was stabilizing, not directional for larger size.
在许多生物体中，体型大小与可能与适合度相关的性状呈正相关。如果定向选择经常有利于较大的后代（“越大越好”假说），那么这种选择的结果应该可以通过野外实验检测到。我们在密歇根大学位于密歇根州东南部的E.S.乔治保护区，对刚孵化出的鳄龟（蛇鳄龟）进行了一项长期实验和三项短期实验，以检验“越大越好”假说。在1995年和1996年秋季，我们在与最近湿地用围栏隔开的人工巢穴中放生了刚孵化的鳄龟。我们记录了被重新捕获的刚孵化鳄龟的比例、刚孵化鳄龟从距离45米、55米和80米远的人工巢穴移动到围栏所需的时间，以及沿着围栏的扩散情况。我们确定了响应变量和在围栏处被重新捕获的概率是否与刚孵化鳄龟的体型大小相关。在1995年期间，刚孵化鳄龟从实验巢穴的平均移动时间与到围栏的距离无关；然而，被重新捕获的时间与在围栏上从零点开始的扩散呈正相关，并且与来自45米巢穴的刚孵化鳄龟相比，来自80米巢穴的刚孵化鳄龟到达围栏的最长时间几乎是其两倍。67%的刚孵化鳄龟到达了围栏，并且每个巢穴到达围栏的比例没有差异。在1995年的任何一项实验中，体型大小与被重新捕获的概率均无显著关联。在1996年，59%被放生的刚孵化鳄龟被重新捕获。被重新捕获的时间与从零点开始的扩散或体型大小均无关。三次样条分析表明对体型大小存在稳定选择。我们还在1980年至1993年期间进行了一系列长期的刚孵化鳄龟放生实验，以比较在巢穴地点放生的刚孵化鳄龟与直接放入沼泽地的刚孵化鳄龟的存活率，并且我们寻找存活率与刚孵化鳄龟体型大小之间的关系。在7年中，放生了超过30只刚孵化鳄龟，其中413只刚孵化鳄龟被直接放入沼泽地，262只在巢穴放生：它们的存活概率没有差异。在所有年份中，对于两个放生组合并以及每个组单独而言，存活率与体型大小均无关。仅在1983年，无论是单个组还是两个组合并，存活率与体型大小也均无关。在我们的三项短期实验和一项长期实验中，我们没有发现支持“越大越好”假说的证据。当确实发生对体型大小的选择时，选择是稳定的，而非朝着更大体型的定向选择。
