Guild composition and mustelid morphology – character displacement but no character release

Aim  Competition is hypothesized to lead to the evolution of reduced morphological overlap between competitors. Within guilds, this is believed to lead to overdispersed size ratios of the trophic apparatus of adjacent-sized species. In allopatry, conditions are believed to favour character release. We examined whether character displacement is prevalent in mainland musteline guilds and sought evidence for character release in adjacent island guilds.
Location  Holarctic, from Japan through western Europe to Pacific North America.
Methods  We measured skulls and canines of members of the mustelid subfamily Mustelinae and examined whether size ratios tend towards equality. We then examined whether insular guilds are characterized by larger size ratios and a higher degree of sexual size dimorphism than mainland guilds, and whether this reflects evolution towards the size of missing guild members.
Results  We found equal ratios between skull lengths but not canine diameters in all mainland guilds. Few insular guilds showed equal ratios for either trait. There was scant evidence for character release: insular mustelines do not evolve towards the size of a missing guild member, nor is sexual size dimorphism greater on islands. There was no evidence for a lower limit on the size similarity of co-existing mustelids.
Main conclusions  We propose that different guild compositions in different localities produce different evolutionary trajectories. Similar sizes on islands and mainlands can be explained by similar prey sizes in both settings. Morphological evidence suggests that competition is probably not a ubiquitous force in the assembly and evolution of musteline guilds.