Fox S F, McCoy J K
Department of Zoology, Oklahoma State University, Stillwater, OK 74078, USA e-mail:
Oecologia. 2000 Feb;122(3):327-334. doi: 10.1007/s004420050038.
Tail autotomy is a defense against predators used by many lizard species but is associated with various costs, most of which have been measured only in the laboratory. We conducted a field experiment in which we induced tail autotomy to approximately half (58%) of a marked sample (n=326) of Uta stansburiana from western Texas in the fall, and left the other half with intact tails. The following spring we determined survival, measured growth, and brought females to the laboratory to allow them to oviposit their eggs, which we incubated until hatching. Based on past studies, we anticipated inferior survival, growth, and reproduction following tail autotomy. We also predicted that females with tail loss would be energetically compromised and would alter the sex ratio of their offspring toward more daughters (as predicted by the Trivers-Willard hypothesis). Tailless lizards experienced significantly reduced survivorship, but those that survived grew the same as their tailed counterparts. Tailed and tailless females produced clutches equivalent in number of eggs and total mass. Whereas tailed females showed a significant positive relationship between average egg mass and snout-vent length, tailless females did not. Contrary to our expectations, tailless females produced heavier hatchlings than tailed ones, and sex ratios of hatchlings were equivalent for tailed and tailless females. In this population, tail loss in subadults leads to an increased risk of death, but apparently does not impose an energetic handicap such that later growth and reproduction suffer. We suggest that because tailless females are faced with decreased reproductive value, they respond by growing as much and laying as many eggs of the same mass as tailed females, despite the fact that they are also regenerating the tail. In addition, they somehow produce larger hatchlings than tailed females. Nevertheless, tailless females probably end up with lower overall lifetime fitness than tailed females, and tail loss thus induces the conditional reproductive strategy "make the best of a bad situation". Because tailless females produce larger, not smaller, hatchlings, they do not produce more daughters as predicted; i.e., we found no evidence for the Trivers-Willard effect following tail autotomy.
自割尾巴是许多蜥蜴物种用于抵御捕食者的一种防御机制，但这一行为会带来各种代价，其中大部分代价仅在实验室环境中得到过衡量。我们进行了一项野外实验，在秋季诱导得克萨斯州西部的近一半（58%）有标记的斑纹强棱蜥（Uta stansburiana，样本数量n = 326）自割尾巴，另一半则保留完整的尾巴。次年春天，我们测定了它们的存活率，测量了生长情况，并将雌性个体带回实验室，使其产卵，之后我们对卵进行孵化直至幼体破壳。基于以往的研究，我们预计自割尾巴后个体在存活、生长和繁殖方面会表现较差。我们还预测，失去尾巴的雌性在能量获取上会受到影响，并且会改变其后代的性别比例，使其雌性后代数量增多（正如特里弗斯-威拉德假说所预测的那样）。无尾蜥蜴的存活率显著降低，但存活下来的个体与有尾的同类个体生长情况相同。有尾和无尾的雌性产出的卵在数量和总质量上相当，但有尾雌性的平均卵质量与吻肛长度之间呈现显著的正相关关系，而无尾雌性则不然。与我们的预期相反，无尾雌性产出的幼体比有尾雌性产出的更重，并且有尾和无尾雌性所产幼体的性别比例相同。在这个种群中，亚成体自割尾巴会导致死亡风险增加，但显然不会造成能量获取上的障碍，以至于影响后续的生长和繁殖。我们认为，由于无尾雌性的繁殖价值降低，尽管它们还在再生尾巴，但它们会通过尽可能多地生长以及产出与有尾雌性数量相同、质量相当的卵来做出回应。此外，它们产出的幼体比有尾雌性产出的更大。尽管如此，无尾雌性的总体终身适合度可能最终仍低于有尾雌性，因此自割尾巴促使它们采取了“在不利情况下尽力而为”的条件性繁殖策略。由于无尾雌性产出的幼体更大而非更小，所以它们并未如预期那样产出更多的雌性后代；也就是说，我们没有发现自割尾巴后存在特里弗斯-威拉德效应的证据。
