Space-time patterns of body size variation in island bovids: The key role of predatory release

Aim: I provide the first comprehensive study on body size evolution of extinct and living insular bovids, exploring the causal biotic and abiotic selective factors for observed patterns. Location: Islands worldwide. Methods: I assembled data on the geographic characteristics of 13 focal islands (area, isolation, latitude, net primary productivity) and on the ecological and morphological characteristics of 32 insular bovid taxa (number of predators and competitors, richness of large mammals, body mass of mainland relatives). I used linear regressions and machine learning methods (regression trees and random forest analyses) to examine the hypothesized contextual importance of these factors in explaining variation in the body size of island bovids. I also calculated evolutionary rates of body size divergence of focal taxa in order to assess whether this phenomenon is influenced by time in isolation. Results: The results of regression, regression tree and random forest analyses were in agreement with predictions based on a hypothesis of ecological release (more pronounced body size divergence on islands with the fewest competitors and predators). Only limited support for a resource limitation hypothesis (more pronounced body size divergence for the larger taxa on smaller islands) was found. Main conclusions: The majority of insular bovids, as large mammals, do follow the main prediction of the island rule, exhibiting body size reduction, and predator diversity is the main factor influencing body size evolution of these taxa. Results obtained highlighted the crucial role of time in isolation, with body size reduction becoming more pronounced for bovids with longer residence times on the islands.