An example of phenotypic adherence to the island rule? - Anticosti gray jays are heavier but not structurally larger than mainland conspecifics.

Dan Strickland,D Ryan Norris

doi:10.1002/ece3.1557

Abstract

The island rule refers to the tendency of small vertebrates to become larger when isolated on islands and the frequent dwarfing of large forms. It implies genetic control, and a necessary linkage, of size and body-mass differences between insular and mainland populations. To examine the island rule, we compared body size and mass of gray jays (Perisoreus canadensis) on Anticosti Island, Québec, located in the Gulf of St. Lawrence, with three mainland populations (2 in Québec and 1 in Ontario). Although gray jays on Anticosti Island were ca 10% heavier, they were not structurally larger, than the three mainland populations. This suggests that Anticosti jays are not necessarily genetically distinct from mainland gray jays and that they may have achieved their greater body masses solely through packing more mass onto mainland-sized body frames. As such, they may be the first-known example of a proposed, purely phenotypic initial step in the adherence to the island rule by an insular population. Greater jay body mass is probably advantageous in Anticosti's high-density, intensely competitive social environment that may have resulted from the island's lack of mammalian nest predators.

Highlights

Small mammals established on islands tend to become structurally larger and heavier than their mainland ancestors, whereas insular populations of larger species (>100 g) often develop dwarf forms (Foster 1964), a pattern termed the “island rule” by Van Valen (1973). Case (1978) proposed that insular body-size trends might be explained by ecological release from predation, parasitism, and interspecific competition and/or by intense intraspecific competition for limited resources exacerbated by high densities of conspecifics
Structural size, as estimated from PC1 scores of tarsus, bill length, and wing chord, varied by study area but not in the direction predicted by the island rule
Given the usually assumed necessary coupling of structural size and mass (e.g., Grant 1965), the expectation is that an insular form conforming to the island rule should be both structurally larger and correspondingly heavier than the related mainland form

Summary

Introduction

Small mammals established on islands tend to become structurally larger and heavier than their mainland ancestors, whereas insular populations of larger species (>100 g) often develop dwarf forms (Foster 1964), a pattern termed the “island rule” by Van Valen (1973). Case (1978) proposed that insular body-size trends might be explained by ecological release from predation, parasitism, and interspecific competition and/or by intense intraspecific competition for limited resources exacerbated by high densities of conspecifics. Case (1978) proposed that insular body-size trends might be explained by ecological release from predation, parasitism, and interspecific competition and/or by intense intraspecific competition for limited resources exacerbated by high densities of conspecifics. Other workers expanded this perspective predicting that, following ecological release, evolution would tend towards taxon-specific optimum sizes variously estimated to be, for mammals, 100 g (Brown et al 1993), 1000 g (Damuth 1993), or, for birds, 33 g (Maurer 1998). The examples include north Atlantic island populations of the wren (Troglodytes troglodytes; Williamson 1981), the Gotland population of the coal tit (Periparus ater; Alatalo and Gustafsson 1988), the island scrub-jay (Aphelocoma insularis) of Santa Cruz Island CA, (Curry and Delaney 2002), the Capricorn white-eye

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