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FLOWERS UP, BATS IN THE AREA! THE ROLE OF FLORAL HEIGHT ALONG THE SHOOT AXIS IN BAT-POLLINATION SYSTEMS
Bat-pollination is an important system in terms of occurrence and distribution in the Neotropics, but our understanding surrounding its ecology is still being consolidated. Thus, the role of particular flower traits in this interaction is still uncertain. Flower height along the shoot axis and its relation to flower exposure in this system has long been deemed a key-trait because it might increase accessibility and conspicuity to large and echolocating pollinators such as bats. Nonetheless, its effect on fitness has never been effectively tested. We aimed to validate its role, based on a series of assumptions related to bat ecology and behavior, and to the typical morphology of bat-pollinated plants. We propose the hypothesis that taller flowers attain higher fitness because they are visited more frequently and thus receive larger pollen loads. We investigated the effect of flower height on bat visiting rates to Crescentia cujete (Bignoniaceae), a model chiropterophilous species with a marked gradient of flower height along the shoot axis due to cauliflory. Additionally, we sampled seed:ovule ratio measurements from varying heights along the shoot axis from seven other species of different families and along a herb-tree stature gradient. Hypothesis testing was done through a series of mixed-effects linear models, where flower height was employed as the explanatory fixed variable and fitness measurements as response variables. We recorded 949 bat visits to 25 flowers of C. cujete, which varied positively in function of flower height along the shoot axis. The relationship, however, was significant only for the subset of flower located on the trunk and closer to the ground in comparison to flower located within the crown. Similarly, out of 318 seed:ovule ratio measurements, only those from the three species with the shortest statures along the height gradient and the shortest average plant heights were positively correlated to flower height. A significant effect was not found for tall bushes and trees. Flower height along the shoot axis presents thus a relative role in the pollination by bats. The outcomes suggest that the proximity to the ground, associated to the proximity to herbaceous or bushy surrounding vegetation, may be an obstacle to the foraging and resource detection for nectar bats, which in turn should explain the convergence of typical inflorescence structures of short-statured bat-pollinated plants, such as tall peduncles or stalks exposed well above the vegetation. Our work brought a novel numeric perspective to the role of an unconsolidated trait in bat-pollination, and elucidated some aspects of the adaptive importance of flower height based on limitations imposed by ecologically complex pollinators.
Chiropterophily, cauliflory, fitness, flagelliflory, floral traits, mixed-effect models, styliflory.
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), grant numbers: U.M.D - 157052/2017-6, I.C.S.M - 311021/2014-0 and 459485/2014-8.
Fundação de Apoio à Pesquisa do Estado de Pernambuco (FACEPE), gant number: A.D.M - IBPG-0550-2.03/14.
Ugo Mendes Diniz, Arthur Domingos-Melo, Isabel Cristina Sobreira Machado