Among vertebrates, nearly all oviparous animals are considered to have either obligate aquatic or terrestrial oviposition, with eggs that are specialized for developing in those environments. The terrestrial environment has considerably more oxygen but is dry and thus presents both opportunities and challenges for developing embryos, particularly those adapted for aquatic development. Here, we present evidence from field experiments examining egg-laying behavior, egg size, and egg jelly function of 13 species of Central and South American treefrogs in the genus Dendropsophus, which demonstrates that flexible oviposition (individuals laying eggs both in and out of water) and eggs capable of both aquatic and terrestrial development are the likely factors which enable the transition from aquatic to terrestrial reproduction. Nearly half of the species we studied had previously undescribed degrees of flexible oviposition. Species with obligate terrestrial reproduction have larger eggs than species with aquatic reproduction, and species with flexible reproduction have eggs of intermediate sizes. Obligate terrestrial breeding frogs also have egg masses that absorb water more quickly than those with flexible oviposition. We also examined eight populations of a single species, Dendropsophus ebraccatus , and document substantial intraspecific variation in terrestrial oviposition; populations in rainy, stable climates lay fewer eggs in water than those in drier areas. However, no differences in egg size were found, supporting the idea that the behavioral component of oviposition evolves before other adaptations associated with obligate terrestrial reproduction. Collectively, these data demonstrate the key role that behavior can have in facilitating major evolutionary transitions.

中文翻译：

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灵活的产卵行为促进了陆地繁殖的进化


在脊椎动物中，几乎所有卵生动物都被认为具有专性水生或陆生产卵，其卵专门用于在这些环境中发育。陆地环境的氧气含量要高得多，但干燥，因此为胚胎发育提供了机遇和挑战，特别是那些适应水生发育的胚胎。在这里，我们提供了现场实验的证据，该实验检查了该属 13 种中美洲和南美洲树蛙的产卵行为、卵大小和卵果冻功能树栖属，这表明灵活的产卵（个体在水中和水外产卵）和能够在水生和陆地发育的卵是实现从水生繁殖向陆生繁殖过渡的可能因素。我们研究的近一半物种具有先前未描述过的灵活产卵程度。专性陆地繁殖的物种比水生繁殖的物种拥有更大的卵，而灵活繁殖的物种则拥有中等大小的卵。专性陆生蛙类的卵块吸收水分的速度也比灵活产卵的蛙类更快。我们还检查了一个物种的八个种群，黑斑树蝽，并记录陆地产卵的重大种内变异；多雨、气候稳定的种群在水中产卵的数量比干旱地区的种群要少。然而，没有发现卵大小的差异，这支持了这样的观点，即产卵的行为成分在与专性陆地繁殖相关的其他适应之前进化。 总的来说，这些数据证明了行为在促进重大进化转变方面可以发挥的关键作用。