Abstract Synchronous hatching is widespread among oviparous taxa. However, the adaptive significance of this phenomenon is unclear, as are the cues mediating hatching synchrony. We took a comparative approach and experimentally tested for synchronous hatching in two freshwater turtles with different life histories ( Apalone spinifera and Graptemys geographica ). We also aimed to disentangle the cues and mechanisms facilitating any synchronisation and explored its potential costs. For each species, we incubated eggs of different ages and from different mothers in two conditions—in direct contact with other eggs, or physically isolated but able to receive acoustic or chemical cues. We found evidence of hatching synchrony in A. spinifera , but not in G. geographica . Apalone spinifera eggs of different ages that developed in-contact hatched together, implicating mechanical cues. Younger eggs that were incubated in isolation also synchronised with in-contact eggs, which also implicates acoustic or chemosensory cues. Hatchling yolk sac size and mass were similar among treatments. Overall, A. spinifera exhibits hatching synchrony and there was no evidence of developmental costs. The lack of hatching synchrony in G. geographica may reflect their different life-history strategy, as this species can overwinter in the nest after hatching. Clutch effects explained a large and significant proportion of variance in both hatching date and incubation duration in both turtle species. Future research on hatching synchrony should control for these potentially confounding clutch effects. Variation in hatching or emergence synchrony among freshwater turtles raises questions about the selective forces that favour evolution of this behaviour. Significance statement Many freshwater turtles hatch from their eggs together—but how do they coordinate? We identified that if eggs of Spiny Softshell turtles develop in contact with each other, the eggs hatch together. But, Northern Map turtles do not synchronise hatching, possibly because of an important difference in their ecology—Northern Map turtles overwinter within their nest. Variable occurrence of hatching synchrony across turtle species may reflect different egg structures and/or overwintering behaviour. Research testing for the presence of hatching synchrony typically incubates less and more advanced siblings together. But we found that clutch effects explained a large and significant proportion of variance in both hatching date and incubation duration, which highlights the need to consider and control for these effects moving forward. Our study provides a promising experimental framework to study hatching synchrony that controls for clutch effects.

中文翻译：

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现在都在一起了！淡水龟的孵化同步

摘要 同步孵化在卵生类群中普遍存在。然而，这种现象的适应性意义尚不清楚，调解孵化同步的线索也是如此。我们采用了比较方法并通过实验测试了两种具有不同生活史的淡水龟（Apalone spinifera 和 Graptemys geoca）的同步孵化。我们还旨在解开促进任何同步的线索和机制，并探索其潜在成本。对于每个物种，我们在两种条件下孵化不同年龄和不同母亲的卵——与其他卵直接接触，或物理隔离但能够接收声学或化学线索。我们在 A. spinifera 中发现了孵化同步的证据，但在 G. georia 中没有发现。不同年龄的Apalone spinifera 卵以接触方式孵化在一起，暗示机械线索。单独孵化的年轻鸡蛋也与接触鸡蛋同步，这也涉及声学或化学感应线索。各处理间的孵化卵黄囊大小和质量相似。总体而言，A. spinifera 表现出孵化同步性，并且没有发育成本的证据。G. georia 缺乏孵化同步可能反映了它们不同的生活史策略，因为该物种孵化后可以在巢中越冬。离合器效应解释了两种龟类在孵化日期和孵化持续时间上的巨大差异。孵化同步的未来研究应该控制这些潜在的混淆离合器效应。淡水龟孵化或出现同步的变化引发了关于有利于这种行为进化的选择性力量的问题。意义声明 许多淡水龟是从它们的蛋中孵化出来的——但它们是如何协调的？我们发现，如果多刺软壳龟的卵相互接触发育，卵就会一起孵化。但是，北方地图龟不会同步孵化，这可能是因为它们的生态系统存在重要差异——北方地图龟在它们的巢穴中越冬。海龟物种孵化同步的不同发生可能反映了不同的卵结构和/或越冬行为。对孵化同步性存在的研究测试通常会将越来越少和更先进的兄弟姐妹一起孵化。但是我们发现离合器效应解释了孵化日期和孵化持续时间的很大一部分差异，这突出了考虑和控制这些影响向前发展的必要性。我们的研究提供了一个有前途的实验框架来研究控制离合器效应的孵化同步。