Transgressive niche across a salamander hybrid zone revealed by microhabitat analyses

Aim: Hybrid populations can have intermediate, conserved or transgressive niches, compared to the parental species. Fine-scale analyses can improve our understanding of niche evolution, but information on microhabitat differences between parental species and hybrids is extremely scarce for animal populations. We assessed the pattern and role of niche variation along a hybrid zone, by investigating differences in microhabitat selection between terrestrial cave salamander species (Hydromantes ambrosii and Hydromantes italicus) and their hybrids. We tested whether introgressed populations show intermediate, conserved or transgressive niche, compared to parental species. Furthermore, we compared body condition index (BCI) among individuals observed in different microhabitats (different areas of caves), to assess the potential advantages of exploiting specific microhabitats. Location: We surveyed ~700 cave sectors in Italy, across the whole range of the two parental species and along their contact zone. Methods: We recorded salamander distribution and measured microhabitat features influencing salamander distribution: humidity, temperature and incident light. We tested niche differences between parental species and hybrids, and analysed spatial variation of body condition. Results: We detected significant niche shifts between parental species and hybrids. Introgressed populations showed a transgressive niche and exploited sectors with warmer temperature, lower humidity and more light, compared to the non-introgressed ones. Introgressed salamanders were better able to exploit the microhabitat conditions that are frequently found nearby cave entrance. For males, BCI was significantly higher in individuals found close to the cave entrance. Main conclusions: The transgressive niche of hybrids between H. ambrosii and H. italicus indicates niche expansion towards harsh environmental conditions, and such transgressive segregation allows exploiting environments with higher food availability. The ability to exploit dry and warm sectors can provide important advantages to hybrid populations that can better persist under stressful conditions. Transgressive niche can be important for long-term persistence and evolution within hybrid zones.