Pollination biology of two chiropterophilous agaves in Arizona

I studied the pollination biology of two closely related species of agave, Agave palmeri and A. chrysantha (Agavaceae), which exhibit several chiropte rophilous (bat-pollinated) traits. Floral studies, floral visitor observati ons, and pollination studies were conducted over four summers at six differ ent sites to examine floral traits and determine the relative importance of diurnal vs. nocturnal pollinators. Agave chrysantha appears to have develo ped minor shifts in several floral characters that enhance diurnal pollinat ion. Although floral shifts towards diurnal pollination were fewer in A. pa lmeri, stigmas were diurnally receptive and copious floral rewards were ava ilable in the morning, indicating that some adaptations exist to allow for multiple pollinators. Differences in fruit and seed set between naturally d ay- and night-pollinated umbels for both species were either not significan t or significantly higher in day-pollinated plants. Bats were not important pollinators of A. chrysantha, and the mutualistic relationship between A. palmeri and the lesser long-nosed bat was found to be asymmetric. "Bat-adap ted" floral traits appear to be flexible enough to respond to the climatic and pollinator unpredictability experienced by agaves at the northern edge of their distribution. This variability may be a more important factor affe cting evolution of floral characters than a particular pollinator.