Data from: Oviposition site choice under conflicting risks demonstrates that aquatic predators drive terrestrial egg-laying

Laying eggs out of water was crucial to the transition to land and has evolved repeatedly in multiple animal phyla. However, testing hypotheses about this transition has been difficult because extant species only breed in one environment. The pantless treefrog, Dendropsophus ebraccatus, makes such tests possible because they lay both aquatic and arboreal eggs. Here, we test the oviposition site choices of D. ebraccatus under conflicting risks of arboreal egg desiccation and aquatic egg predation, thereby estimating the relative importance of each selective agent on reproduction. We also measured discrimination between habitats with and without predators and development of naturally laid aquatic and arboreal eggs. Aquatic embryos in nature developed faster than arboreal embryos, implying no cost to aquatic egg laying. In choice tests, D. ebraccatus avoided habitats with fish, showing that they can detect aquatic egg predators. Most importantly, D. ebraccatus laid most eggs in the water when faced with only desiccation risk, but switched to laying eggs arboreally when desiccation risk and aquatic predators were both present. This provides the first experimental evidence to our knowledge that aquatic predation risk influences non-aquatic oviposition and strongly supports the hypothesis that it was a driver of the evolution of terrestrial reproduction.

脱离水体产卵是动物登陆演化过程中的关键事件，且该繁殖策略已在多个动物门中独立演化多次。然而，针对该演化过渡的相关假说一直难以验证，原因在于现存物种仅能在单一环境中完成繁殖。露胯树蛙（Dendropsophus ebraccatus）为这类验证提供了可行路径，因其既能在水生环境产卵，也能在树栖环境产卵。本研究中，我们针对露胯树蛙在树栖卵脱水风险与水生卵捕食风险这两类冲突性选择压力下的产卵位点选择行为开展实验，以此评估两种选择因子对繁殖的相对重要性。我们同时检测了该物种对有无捕食者的生境的辨别能力，并观测了自然产下的水生卵与树栖卵的发育进程。野外环境中，水生胚胎的发育速度显著快于树栖胚胎，这表明在水生环境产卵并未带来适合度成本。在产卵选择实验中，露胯树蛙会主动避开存在鱼类的生境，这表明其能够识别水生卵捕食者。最为关键的实验结果是：当仅面临脱水风险时，露胯树蛙会将绝大多数卵产在水中；而当同时面临脱水风险与水生捕食者威胁时，其产卵位点会转向树栖环境。据我们所知，本研究首次通过实验证实，水生捕食压力会影响非水生环境的产卵行为，且有力支持了‘水生捕食压力是陆地繁殖演化的关键驱动因素’这一假说。