Size and shape in floral advertisement: Measurement, concepts and implications

The importance of the size and shape of flowers in the attraction of pollinators has long been recognized. It is thus not surprising that many different methods have been used to quantify floral characteristics. Similarly, many hypotheses have been proposed regarding the role of flower size and shape in floral advertisement for potential pollinators. Here we review a number of these methods and hypotheses. We find that some commonly used measures of flower size are inappropriate for studies of floral attraction. In some cases, the measures are not pertinent to floral advertisement, while in others they provide biased estimates of size, especially among flowers with different shapes. En face planar projection from the angle of approach of the visitor is recommended as the best measure of floral advertisement. Quantification of floral form is still in a rudimentary state. At present, contour density (i.e., perimeter/area) is the only measure of shape that is comparable for flowers of different forms (e.g., bell-shaped vs. bilabial flowers). The advantages and disadvantages of various experimental approaches for evaluating the role of flower shape and size are presented. We conclude that our understanding of the pollination process would benefit from placing less emphasis on flowers as morphological units and more emphasis on their functional role in the overall pollination process of each species. More field experiments testing the learned as well as the learned responses of a wide range of pollinating species are needed.