Agalychnis callidryas and A. spurrelli egg masses

Syntopic populations of species provide a unique opportunity to examine divergence of adaptive traits that suggest character displacement when compared with allopatric populations of those species. Past work on syntopic populations of tree frogs, Agalychnis callidryas and A spurrelli documented differences in embryonic risk responses. Due to a serendipitous opportunity, we sampled egg masses from these species oviposited during a breeding aggregation at a forested wetland during July 2017 in the Puntarenas Province of Costa Rica. We collected data on egg masses of the two species including oviposition site and evidence of predation. Both species oviposited on vegetation around the perimeter of the wetland. Additionally, we found that A. callidryas, the species that was previously documented as hatching quickly in response to snake predation took advantage of canopy vegetation overhanging the wetland for oviposition, sites more available to arboreal snakes. This canopy vegetation was not used by A spurrelli. Conversely, A spurrelli, the species shown to respond more strongly to submergence by hatching early, oviposited on vegetation directly emerging from the wetland, sites not used by A callidryas. This divergence of reproductive and ecological traits is not seen in allopatric populations of the same species. These findings suggest that character displacement in oviposition site may have caused differences in selection pressure on embryonic behavior.

物种的同域共存种群（syntopic populations）为研究适应性性状的分化提供了独特契机——相较于该物种的异域种群（allopatric populations），这类分化可反映特征替代（character displacement）现象。过往针对红眼树蛙（Agalychnis callidryas）与斯氏红眼树蛙（A. spurrelli）的同域共存种群研究，已记录到二者在胚胎风险响应（embryonic risk responses）上的差异。得益于一次偶然契机，我们于2017年7月在哥斯达黎加蓬塔雷纳斯省的一处森林湿地的繁殖聚集期，采集了这两个物种的卵块样本。我们收集了两个物种卵块的相关数据，包括产卵位点（oviposition site）与捕食痕迹。两个物种均将卵产在湿地周边的植被上。此外我们发现，此前被记录到会针对蛇类捕食快速孵化的红眼树蛙（A. callidryas），会利用湿地上方的冠层植被作为产卵位点——这类位点更易被树栖蛇类（arboreal snakes）利用。而斯氏红眼树蛙（A. spurrelli）并未使用这类冠层植被。与之相反，此前被证实会通过提早孵化以应对淹没胁迫的斯氏红眼树蛙，会将卵产在直接从湿地中长出的植被上，而这类位点并非红眼树蛙的产卵选择。这类繁殖与生态性状的分化，在上述物种的异域种群中并未被观测到。本研究结果表明，产卵位点的特征替代可能对胚胎行为施加了差异化的选择压力。