Dinosaurs are known for their large body size. Sauropod dinosaurs (Sauropodomorpha) had an especially large body size; some species reached 30 m long and 50 tons. Many hypotheses have been proposed to explain this phenomenon. In this study we examined this question using the life history theory. We constructed a simple model of life history with the following assumptions: the body size of immature individuals increases following a logistic equation. A higher quality and availability of food plants make the initial growth rate faster and the final saturating size larger. The increase in body size stops once reproduction starts. Fertility increases with adult body size and food-plant quality. Mortality due to predation is mitigated by a larger body size. We calculated the optimal body size at maturity that would maximize the lifetime reproductive success or fitness. The analysis showed that adult body size increased with food-plant quality and availability but decreased with higher mortality due to predators and other factors. This conclusion is consistent with geological studies that suggest a high quality and availability of food plants in the Mesozoic era, efficient air-sac breathing, and the lightweight bones of sauropod dinosaurs, allowing rapid growth of small individuals.

恐龙以体型大而著称。蜥脚类恐龙（Sauropodomorpha）的体型特别大。一些物种长30 m，重50吨。已经提出了许多假设来解释这种现象。在这项研究中，我们使用生活史理论研究了这个问题。我们根据以下假设构建了一个简单的生活史模型：未成熟个体的身体大小遵循逻辑方程式增加。食用植物的更高质量和可用性使得初始生长速度更快，最终饱和尺寸更大。一旦开始繁殖，身体大小的增加就停止了。生育力随着成年体重和食用植物的质量而增加。更大的体型减轻了被捕食导致的死亡率。我们计算了成熟时的最佳体重，这将使一生的生殖成功或适应能力最大化。分析表明，由于食肉动物和其他因素的影响，成虫的体型随食用植物的质量和可获得性而增加，但随着死亡率的增加而减小。该结论与地质研究一致，该研究表明中生代时期食用植物的质量和可用性高，气囊呼吸效率高，蜥脚类恐龙的骨骼轻巧，可以使小个体快速生长。