Evolution in the dark
In the limestone massifs of Picos de Europa National Park, a real-time evolutionary process is unfolding. The fire salamander, Salamandra salamandra, a species typically known for its bright black and yellow warning coloration, has found its way into the deep, lightless cave systems. Specifically, populations of the local subspecies, Salamandra salamandra bernardezi, have become isolated in these subterranean habitats. This separation from the surface world has triggered a series of remarkable adaptations to a life of perpetual darkness.
These karst environments present a consistent, challenging habitat. The temperature remains stable and cool, and the humidity is near 100 percent. In the absence of light, the selective pressures that shaped their surface-dwelling relatives have vanished. The salamanders' bright, aposematic coloring, which warns predators of their toxic skin secretions, is useless here. Similarly, functional eyes, which are metabolically expensive to develop and maintain, offer no advantage. As a result, these cave-dwelling populations are exhibiting troglomorphism—the evolution of traits specific to cave life. This includes a gradual loss of skin pigmentation and the slow regression of their eyes, which become smaller and less functional with each passing generation. It is an example of regressive evolution, where complex features are lost when they no longer provide a survival advantage.
A different way of life
Adapting to the dark means relying on other senses. While their sight diminishes, these salamanders likely develop heightened senses of smell and touch to navigate and hunt for invertebrates like insects, slugs, and earthworms in the darkness. Some populations of S. s. bernardezi also display a notable reproductive shift. While most fire salamanders give birth to aquatic larvae (larviparity), some of these Asturian salamanders are pueriparous, giving birth to fully-formed, terrestrial juveniles—an effective strategy in an environment where suitable pools for larvae may be scarce.
The Picos de Europa National Park, established in 1918, is Spain's first national park and spans 646 square kilometers across Asturias, Cantabria, and Castile and León. Its limestone peaks, including the 2,648-meter Torre de Cerredo, are riddled with deep gorges and cave systems carved by water over millennia. While the park is a haven for famous Iberian wildlife like brown bears and wolves, the salamanders in its hidden caves represent a more subtle, but equally fascinating, story of life's adaptability. These creatures are not a separate species but a localized example of a species in transition, providing scientists with a natural laboratory for studying the mechanisms of evolution.