Department of Ecology, Zoological Institute, University of Mainz, Mainz, Germany.
PLoS One. 2011;6(12):e28442. doi: 10.1371/journal.pone.0028442. Epub 2011 Dec 14.
Janis and Carrano (1992) suggested that large dinosaurs might have faced a lower risk of extinction under ecological changes than similar-sized mammals because large dinosaurs had a higher potential reproductive output than similar-sized mammals (JC hypothesis). First, we tested the assumption underlying the JC hypothesis. We therefore analysed the potential reproductive output (reflected in clutch/litter size and annual offspring number) of extant terrestrial mammals and birds (as "dinosaur analogs") and of extinct dinosaurs. With the exception of rodents, the differences in the reproductive output of similar-sized birds and mammals proposed by Janis and Carrano (1992) existed even at the level of single orders. Fossil dinosaur clutches were larger than litters of similar-sized mammals, and dinosaur clutch sizes were comparable to those of similar-sized birds. Because the extinction risk of extant species often correlates with a low reproductive output, the latter difference suggests a lower risk of population extinction in dinosaurs than in mammals. Second, we present a very simple, mathematical model that demonstrates the advantage of a high reproductive output underlying the JC hypothesis. It predicts that a species with a high reproductive output that usually faces very high juvenile mortalities will benefit more strongly in terms of population size from reduced juvenile mortalities (e.g., resulting from a stochastic reduction in population size) than a species with a low reproductive output that usually comprises low juvenile mortalities. Based on our results, we suggest that reproductive strategy could have contributed to the evolution of the exceptional gigantism seen in dinosaurs that does not exist in extant terrestrial mammals. Large dinosaurs, e.g., the sauropods, may have easily sustained populations of very large-bodied species over evolutionary time.
詹尼斯和卡兰诺（1992）提出，大型恐龙在生态变化中面临的灭绝风险可能比类似大小的哺乳动物低，因为大型恐龙的潜在繁殖产量高于类似大小的哺乳动物（JC 假说）。首先，我们检验了 JC 假说的基本假设。因此，我们分析了现存陆地哺乳动物和鸟类（作为“恐龙模拟物”）和已灭绝恐龙的潜在繁殖产量（反映在卵/窝大小和年后代数量上）。除了啮齿动物，詹尼斯和卡兰诺（1992）提出的类似大小的鸟类和哺乳动物的繁殖产量差异甚至在单一目级也存在。化石恐龙卵窝比类似大小的哺乳动物的窝大，而恐龙卵窝大小与类似大小的鸟类相当。由于现存物种的灭绝风险通常与低繁殖产量相关，后一差异表明恐龙的种群灭绝风险低于哺乳动物。其次，我们提出了一个非常简单的数学模型，该模型证明了 JC 假说中高繁殖产量的优势。它预测，具有高繁殖产量的物种通常面临非常高的幼体死亡率，从减少幼体死亡率（例如，由于种群规模的随机减少）中获益将比繁殖产量低的物种更强，而繁殖产量低的物种通常幼体死亡率低。基于我们的结果，我们认为繁殖策略可能有助于恐龙中异常巨型化的进化，而这种巨型化在现存的陆地哺乳动物中并不存在。例如，蜥脚类恐龙可能很容易维持非常大型物种的种群在进化时间内。
