The evolution of conspicuousness in frogs: when to signal toxicity?

Many organisms use conspicuous color patterns to advertise their toxicity or unpalatability, a strategy known as aposematism. Despite the recognized benefits of this antipredator tactic, not all chemically defended species exhibit warning coloration. Here, we use a comparative approach to investigate which factors predict the evolution of conspicuousness in frogs, a group in which conspicuous coloration and toxicity have evolved multiple times. We extracted color information from dorsal and ventral photos of 594 frog species for which chemical defense information was available. Our results show that chemically defended and diurnal species have higher internal chromatic contrast, both ventrally and dorsally, than chemically undefended and/or nocturnal species. Among species that are chemically defended, conspicuous coloration is more likely to occur if species are diurnal. Our results also suggest that the evolution of conspicuous color is more likely to occur in chemically defended prey with smaller body size. We discuss potential explanations for this association and suggest that prey profitability (related to body size) could be an important force driving the macroevolution of warning signals.

许多生物通过醒目的体色模式来警示自身的毒性或不可食用性，这一策略被称为警戒色（aposematism）。尽管这一反捕食策略已被证实具有诸多益处，但并非所有具备化学防御能力的物种都演化出了警戒色。本研究采用比较研究方法，探究驱动蛙类醒目体色演化的关键因素；蛙类类群中，醒目体色与毒性均已独立演化多次。我们从594种已有化学防御数据的蛙类的背部与腹部照片中提取了体色信息。研究结果显示，与无化学防御能力且/或夜行性的蛙类相比，具备化学防御能力的昼行性蛙类的背部和腹部均具有更高的体色内部对比度。在具备化学防御能力的蛙类中，昼行性物种更易演化出醒目体色。本研究结果还表明，体型更小的具备化学防御能力的猎物更易演化出醒目体色。我们对这一关联的潜在机制进行了讨论，并提出：猎物价值（与体型大小相关）可能是驱动警戒信号宏观演化的重要动力。