Ecological character displacement in Plethodon: Biomechanical differences found from a geometric morphometric study

Ecological character displacement describes a pattern where morphological d ifferences between sympatric species are enhanced through interspecific com petition. Although widely considered a pervasive force in evolutionary ecol ogy, few clear-cut examples have been documented. Here we report a case of ecological character displacement between two salamander species, Plethodon cinereus and Plethodon hoffmani. Morphology was quantified by using linear measurements and landmark-based geometric morphometric methods for specime ns from allopatric and sympatric populations from two geographic transects in south-central Pennsylvania, and stomach contents were assayed to quantif y food resource use. Morphological variation was also assessed in 13 additi onal allopatric populations. In both transects, we found significant morpho logical differentiation between sympatric populations that was associated w ith a reduction in prey consumption in sympatry and a segregation of prey a ccording to prey size. No trophic morphological or resource use differences were found between allopatric populations, and comparisons of sympatric po pulations with randomly paired allopatric populations revealed that the obs erved sympatric morphological differentiation was greater than expected by chance. The major trophic anatomical differences between sympatric populati ons relates to functional and biomechanical differences in jaw closure: sym patric P, hoffmani have a faster closing jaw, whereas sympatric P. cinereus have a slower, stronger jaw. Because salamanders immobilize prey of differ ent sizes in different ways, and because the observed sympatric biomechanic al differences in jaw closure are associated with the differences in prey c onsumption, the observed character displacement has a functional ecological correlate, and we can link changes in form with changes in function in thi s apparent example of character displacement.