Undervalued habitat or impoverished guild? Explaining the scarcity of living semiaquatic sigmodontine rodents (Cricetidae)

Abstract

Sigmodontines (Rodentia: Cricetidae), the most diverse extant radiation of Neotropical rodents (91 genera, 507 species), include only a small fraction of taxa exhibiting morphological and ecological specializations associated with a semiaquatic mode of life. These specializations are unevenly distributed within the subfamily, being largely restricted to Ichthyomyini and to a limited number of mostly large-bodied species within Oryzomyini. Here, we examine the taxonomic, phylogenetic, and geographic distribution of semiaquatic sigmodontines within a historical framework. Phylogenetic analyses and ancestral-state reconstructions indicate that amphibious habits evolved convergently in these two lineages from a terrestrial ancestor. The resulting patterns reveal a marked phylogenetic clustering and a pronounced geographic asymmetry, with semiaquatic forms largely absent from extensive lowland and southern freshwater systems. These patterns are consistent with a scenario in which ecological constraints and historical processes jointly shaped the current distribution of semiaquatic taxa. In particular, predation pressure within continental freshwater environments may have limited successful colonization by small mammals, while occupation of predator-poor habitats may have facilitated localized diversification. In addition, paleontological and biogeographic evidence suggests that semiaquatic sigmodontines were more diverse and geographically widespread in the past, with subsequent reductions in diversity. In this context, the contemporary scarcity of semiaquatic sigmodontine rodents can be interpreted as consistent with a combination of ecological filtering associated with freshwater habitats and historical reductions in diversity through extinction. Comparison with murid rodents suggests that continental freshwater environments may represent a challenging ecological domain for small muroids more broadly.