Do vertebral-line patterns in two horned lizards (Phrynosoma spp.) mimic plant-stem shadows and stem litter?

Abstract

All species of the genus Phrynosoma are cryptically colored and patterned, but two species have bold black or white vertebral-line patterns on their dorsal surfaces. Phrynosoma mcallii lives on aeolian-sand substrates and often rests below shrubs. It has a black vertebral line which is hypothesized to mimic dark shadows of stems falling on the smooth, litter-free sand beneath shrubs. Phrynosoma cornutum inhabitats grasslands and arid shrub-habitats in which below-shrub sites are littered with bleached plant stems. The white middorsal stripe of this species is thought to mimic white stems of the under-shrub litter. Although crypticity in horned lizards has been extensively studied, the two examples of apparent object mimicry described herein have not been reported previously.