Island Living Indirectly Affects Hair Glucocorticoid Levels in a Small Mammal

AbstractIsland vertebrates that are small on the mainland tend to be larger and exhibit tamer behavior than their mainland conspecifics-a combined set of characteristics known as "island syndrome." Such island-specific traits are often attributed to lower predation pressure on islands than on the mainland. While the morphology and behavior of island vertebrates has received significant attention, relatively few studies have compared physiological traits between island and mainland populations. Given that hormones such as glucocorticoids are involved in responding to predation, it is reasonable to test whether island populations that have demonstrated characteristics of island syndrome also have different glucocorticoid levels than their mainland conspecifics. Here, we used a long-term museum collection of deer mice (<i>Peromyscus maniculatus</i>) obtained from two archipelagos and two mainland regions to test the hypothesis that island syndrome includes changes in time-averaged corticosterone levels, as measured in hair. As predicted by island syndrome, deer mice from islands were structurally larger and heavier for their given structural size than their mainland conspecifics. When we compared size-matched individuals (i.e., holding size constant), hair corticosterone levels did not differ between island and mainland mice. However, corticosterone levels scaled positively with body mass and condition across our sample population. This led to a relative increase in corticosterone levels among median-sized mice from islands relative to median-sized mice from mainland populations. We conclude that island syndrome does include effects on deer mouse stress physiology but only indirectly through the evolution of larger body size in island populations.