Of mice and mammoths: generality and antiquity of the island rule

Abstract

AimWe assessed the generality of the island rule in a database comprising 1593 populations of insular mammals (439 species, including 63 species of fossil mammals), and tested whether observed patterns differed among taxonomic and functional groups.Location Islands world-wide. MethodsWe measured museum specimens (fossil mammals) and reviewed the literature to compile a database of insular animal body size (S i = mean mass of individuals from an insular population divided by that of individuals from an ancestral or mainland population, M). We used linear regressions to investigate the relationship between S i and M, and ANCOVA to compare trends among taxonomic and functional groups.Results S i was significantly and negatively related to the mass of the ancestral or mainland population across all mammals and within all orders of extant mammals analysed, and across palaeo-insular (considered separately) mammals as well. Insular body size was significantly smaller for bats and insectivores than for the other orders studied here, but significantly larger for mammals that utilized aquatic prey than for those restricted to terrestrial prey. Main conclusionsThe island rule appears to be a pervasive pattern, exhibited by mammals from a broad range of orders, functional groups and time periods. There remains, however, much scatter about the general trend; this residual variation may be highly informative as it appears consistent with differences among species, islands and environmental characteristics hypothesized to influence body size evolution in general. The more pronounced gigantism and dwarfism of palaeo-insular mammals, in particular, is consistent with a hypothesis that emphasizes the importance of ecological interactions (time in isolation from mammalian predators and competitors was 0.1 to > 1.0 Myr for palaeo-insular mammals, but < 0.01 Myr for extant populations of insular mammals). While ecological displacement may be a major force driving diversification in body size in high-diversity biotas, ecological release in species-poor biotas often results in the convergence of insular mammals on the size of intermediate but absent species.

How do seemingly non-vagile clades accomplish trans-marine dispersal? Trait and dispersal evolution in the landfowl (Aves: Galliformes)

“…In theory, island gigantism (i.e. organism body size increase on islands) is due to multiple factors including ecological release from mainland predation and character displacement from insular competitors [59,60]. Whether or not predation is a credible factor in phasianoid body size evolution is difficult to ascertain because of little direct evidence.…”

Mortality in a Predator-free Insular Environment: the Dwarf Deer of Crete

“…Some consequences are more obvious and amenable to interpretation than others, and for that reason have been widely studied. Thus, the island "rule" predicts that, in insular contexts, large mammals generally become smaller (dwarfism), while small mammals generally become larger (gigantism) (for a recent overview comprising extant as well as extinct forms, see Lomolino et al, 2013). Factors such as ancestral body mass, trophic strategy, competition, predation, dispersal abilities, bauplan, relative isolation, and island area all contribute to the speed and direction of body size evolution in island contexts (Lomolino, 1985(Lomolino, , 2005Lomolino et al, 2012;van der Geer et al, 2013a), and in some sense every case is unique to a certain degree.…”

Morphometrics of a wild Asian elephant exhibiting disproportionate dwarfism

“…Insular dwar sm' refers to the phenomenon by which terrestrial island species exhibit size reduction in comparison to mainland sister species through some evolutionary process [11,12]. The pygmy hippopota- …”