COMPARATIVE POLLINATION BIOLOGY OF VENEZUELAN COLUMNAR CACTI AND THE ROLE OF NECTAR-FEEDING BATS IN THEIR SEXUAL REPRODUCTION

The floral biology, reproductive system, and visitation behavior of po llinators of four species of columnar cacti, Stenocereus griseus, Pilo socereus moritzianus, Subpilocerreus repandus, and Subpilocereus horri spinus, were studied in two arid zones in the north of Venezuela. Our results support the hypothesis that Venezuelan species of columnar cac ti have evolved toward specialization on bat pollination. Additional i nformation on the floral biology of a fifth species, Pilosocereus lanu ginosus, was also included. All species showed the typical traits that characterize tile pollination syndrome of chiropterophily. All specie s but Pilosocereus moritzianus were obligate outcrossers. Nectar and p ollen were restricted to nocturnal floral visitors. Two species of nec tar-feeding bats, Leptonycteris curasoae Miller and Glossophaga longir ostris Miller, were responsible for practically all the fruit set in t hese cacti. Frequency of bat visitation per flower per night was highl y variable within and between species of cactus, with average frequenc ies varying between 27 and 78 visits . flower(-1) . night(-1). In gene ral terms, the pattern of floral visitation through the night was sign ificantly correlated with the pattern of nectar production and nectar sugar concentration for all species of cactus. Under natural pollinati on, fruit:flower ratios varied from 0.46 in Subpilocereus repandus to 0.76 in Stenocereus griseus. The efficiency of bat pollination in term s of seed:ovule ratio was high in all species, varying between 0.70 an d 0.94.