The relationship between altitudinal gradients, diversity, and body size in a dung beetle (Coleoptera: Scarabaeinae: Onthophagus) model system

Mountainous regions represent an excellent model to test ecological hypotheses encompassing assemblage diversity and body traits of species. Among insects, there is no uniform body size pattern across temperature gradients, suggesting that processes controlling body size may differ among species. The aim of this study was to explore diversity and body size patterns of dung beetle species of the genus Onthophagus Latreille, 1802 across altitudinal gradients at two mountains in Mexico. Tropical mountain species were sampled from 2200 to 3400 m a.s.l. In both mountains, there was a decrease of richness and abundance of Onthophagus with increasing altitude. There were contrasting relationships between beetle body size and altitude, which varied depending on the data set analyzed. Furthermore, interspecific variations of body size were not related to the number of altitudinal bands recorded by each species. Species from high altitudes were also recorded at lower mountain altitudes, suggesting that diversity at the highest altitudes represents a subgroup of lowland diversity. This may indicate that the fauna inhabiting higher elevations could be the most eurytopic one. Here we present an example of an assemblage that partially contradicts Bergmann's rule, with contrasting effects of altitude on dung beetle body size.