Morphological Divergence among Populations of Xantusia riversiana, a Night Lizard Endemic to the Channel Islands of California

Morphological variation between closely related island endemics offers a unique system to study ecological and evolutionary processes. The Island Night Lizard, Xantusia riversiana (Cope, 1883), is endemic to three of the Southern Channel Islands off the coast of Southern California: Santa Barbara, San Clemente, and San Nicolas. Some authors treat the species as polytypic with the night lizards on San Nicolas (X. r. riversiana) distinct from those on Santa Barbara and San Clemente (X. r. reticulata). Previous studies failed to find strong morphological divergence, but it remains uncertain if those studies were hampered by a combination of small sample size, small number of characters, and/or the lack of modern morphometric techniques. Here we examined 172 Island Night Lizards from the three islands for nine morphometric and five meristic characters, increasing the number and types of morphological characters examined over previous studies and applying modern morphometric techniques to test for divergence associated with island and sex. We found significant differences in both body measurements and meristic characters among the nominal subspecies as well as among the three islands. We also detected significant sexual dimorphism in body and scale characteristics for both subspecies. However, assigning individuals to an island based on morphology is difficult because all three islands harbor morphologically overlapping individuals. Our study clarifies Island Night Lizard systematics, as well as informs conservation efforts for an island endemic that was until recently listed as threatened under the U.S. Endangered Species Act.