A global assessment of the ‘island rule’ in bats based on functionally distinct measures of body size

Abstract

AbstractAimThe ‘island rule’ postulates that on islands large continental animals become smaller, and small continental animals become larger. Its generality has been supported by some studies but contested by others. Bats are cosmopolitan and speciose mammals thus are ideal to test ecogeographic patterns. Based on three functionally distinct measures of body size, I compare mainland and island variation in bats at the order to subspecies level.LocationGlobally.TaxonChiroptera.MethodsI compiled information from literature on the skull length, forearm length, and body mass of 251 bat species in 840 locations. For conspecific bats from different locations, I calculated size divergences, and categorized them as within‐mainland (M–M), mainland‐to‐island (M–I), or island‐to‐island (I–I). I used a modified size ratio to quantify divergences, analysis of variance, non‐parametric tests, and effect sizes to complement statistical significances.ResultsOn islands, cranial size increases, wing length becomes more uniform, and body mass decreases more often than not. The first trend would support the island rule, but had a small effect size. Families differ considerably in how they vary geographically. Species are ranked very differently based on functionally distinct size measures. Divergences are uniform across M–M, M–I, and I–I ranges, and large between bats of different subspecies.Main ConclusionsLikely owing to limitations imposed by flight and echolocation, bats do not follow the island rule. The absence of correlation between how bats are ranked based on them indicates that functionally distinct size measures are not necessarily coadaptive. High vagilities may lead to low local differentiation, but interfamilial variation shows this differentiation to also depend on other factors. Body sizes converge to species‐specific optima, not to supraspecific optima as predicted by theoretical studies. Size differences are reflected in taxonomy more often if observed across M–I ranges.