Effective population size associated with sky-island distribution in the summit rat, Rattus baluensis, a mountain Bornean endemic.

Tropical mountains are hotspots of biodiversity, and they can harbor species with sky-island distributions on mountaintops. The summit rat, Rattus baluensis, is a Bornean endemic previously known from two populations in the Kinabalu range. Here, we report the discovery of a third, genetically distinct population on Mt. Trusmadi, Sabah, Borneo. Using 27 nuclear introns genotyped in 44 individuals, we show that this population is genetically isolated and more differentiated than those in the Kinabalu range. ABC analyses reveal the three populations likely became isolated in mountain tops during the Holocene. Genetic diversity and effective population size across the three populations correlate with the area of high-elevation habitat on each mountain. Despite relatively large population sizes and well-preserved habitats, the species' strict association with montane forest and confinement to three mountaintops make it particularly vulnerable to climate change, stochastic events, and localized anthropogenic impacts.