Body size evolution of palaeo-insular mammals: temporal variations and interspecific interactions

Aim We investigated the hypothesis that body size evolution of mammals is strongly influenced by ecological interactions, resulting in evolutionary divergence in body size in species-rich (e.g. mainland) biotas, and convergence on the size of intermediate but absent species in species-poor (e.g. insular) biotas. Location Mediterranean palaeo-islands. Methods We assembled data on temporal variation in body size of palaeo-insular mammals and associated variation in ecological characteristics (colonization or extirpation of mammalian competitors and predators) for 19 species of fossil, non-volant mammals across four large (>3640km(2)) islands ranging between the late Miocene and Holocene. These are the only fossil species for which fine-detailed time series are available at present. Results Our results are consistent with predictions based on an ecological interactions hypothesis of body size evolution. Following colonization (or first appearance in the insular fossil record) small mammals (such as mice, shrews and pikas) tended to increase in body size. These trends, however, ceased or were reversed following colonization of the focal islands by mammalian predators or competitors. Main conclusions While body size evolution is likely to be influenced by a variety of characteristics of the focal islands (e.g. climate, area, isolation and habitat diversity) and species (e.g. diet, resource requirements and dispersal abilities), temporal trends for palaeo-insular mammals indicated that the observed trends for any particular species, island and climatic regime may be strongly influenced by interactions among species. Ultimately, invasion of a competitor often leads to the extinction of the native, insular species.