PHENOTYPIC VARIATION IN MOOSE: THE ISLAND RULE AND THE MOOSE OF ISLE ROYALE

Abstract

There is strong empirical support for the island rule, whereby body size in insular populations of animals tends toward gigantism in small-bodied species and dwarfism in large-bodied species (>10 kg).  For large-bodied species, underlying reasons for dwarfism in insular populations include lack of predation and resource limitation.  We found that metatarsal length of moose (Alces alces) from Isle Royale National Park in Lake Superior (N. America) was significantly shorter than that of mainland moose in Minnesota and Michigan (which were similar in size).  On Isle Royale, moose body size was inversely related to moose density at the time of birth, illustrating the resource limitation that is influential where average moose density is 5-10 times higher than on the mainland.  Reduced body size probably developed in Isle Royale moose within a half century of their establishment, prior to the arrival of wolves, and subsequently body size should be shaped by the countervailing influences of resource limitation and predation by wolves (Canis lupus).  Bones provide an excellent basis for spatio-temporal comparisons of body size among moose populations.  With additional data on metatarsus length from moose in Alaska and Sweden, we illustrate important considerations such as sample size, sex differences, and biases arising from source of bone collections.