Andean rain shadow effect drives phenotypic variation in a widely distributed Austral rodent

Abstract

Aim: Mountains cover approximately 22% of the planet's terrestrial surface and have dramatic effects on climate and biodiversity. The rain shadow effect is a common feature on mountain ranges worldwide and its effects on ecology and evolution of species are incompletely known. Patterns of spatial variation in morphology associated with rain shadows have been rarely studied. Our aim in this contribution is to identify the correlates that best describe morphological variation along a pronounced rain shadow gradient. Location: Patagonia and Tierra del Fuego (southern South America), including Valdivian and Magellanic forests and steppes. Taxa: Shaggy soft-haired mouse Abrothrix hirta (Order Rodentia, Family Cricetidae, Subfamily Sigmodontinae, Tribe Abrotrichini). Methods: We measured 450 skulls of adult specimens of A. hirta from 67 localities between 35°S and 54°S and from the Pacific to the Atlantic coasts. We analysed geographic differences in skull morphology using 17 linear measurements of skull traits. Discriminant function analysis revealed clear evidence for sexual dimorphism; thus, analyses were conducted for the entire dataset, and separately for males and females. We implemented regression tree analysis to test the environmental correlates that best describe morphological variation along this gradient. Results: Size variation in shaggy soft-haired mice does not follow Bergmann's rule; however, latitude was the second node for PC1 of all samples. Regression tree analyses showed that the variables that best explained size for the pooled sample, and for males and females separately, were longitude, precipitation of the coldest quarter and temperature seasonality, respectively. Longitude, which appeared in eight of the nine regression tree analyses, is in southern South America a robust proxy for the Andean rain shadow effect, primary productivity and potentially other environmental variables. We find that organisms attain greater sizes in the western side of the Andes where there is no rain shadow effect, below 500 m of elevation. Animals get smaller to the east of the Andes as the rain shadows effect is to reduce precipitation and primary productivity.