Body size evolution and the co-occurrence of stream-dwelling frogs in the Western Ghats of India

The evolution of body size and its role in structuring communities has long been of interest. Here, we investigated the relationship between body size, phylogeny, and spatial patterns of distribution in three aquatic frog genera that have undergone extensive diversification in the Western Ghats of India. We used a comprehensive sampling protocol designed to uncover fine-scale divergence between allopatric and parapatric lineages. First, we found that body size differences between sympatrically distributed lineages were significantly greater than expected by chance in the stream-adapted genera, Nyctibatrachus and Micrixalus, with few assemblages across the entire Western Ghats containing two lineages of similar size. This pattern was not seen in Hylarana where sympatric species typically occupied different habitats, such as streams and ponds. Body size showed significant phylogenetic signal, but we found little overall evidence of phylogenetic clustering or overdispersion in all the genera. We also show that body size evolution in all three frog groups was not strongly affected by character displacement across closely related lineages. Taken together, this provides striking evidence for the potential role of species sorting, where competition leads to spatial assortment of body size, structuring aquatic frog assemblages in this tropical biodiversity hotspot.