A considerable number of plant species are known whose flowers have moveable parts that have to be actively handled by insects for pollination to take place. Such is the case, among others, of species of Antirrhinum
; Brantjes and De Vos, 1981
; Cocucci, 1989
; Sérsic, 1991
; Edwards et al., 2005
; Claßen-Bockhoff et al., 2004b
) and, most notably, of papilionoid Fabaceae. It has been suggested previously that the weight of pollinators is not enough to trigger such a mechanism and that additional muscular power exerted by them is needed (Faegri and van der Pijl, 1966
). Herein, a flower mechanism will be termed ‘forcible’ when an obstacle has to be actively moved by a visitor to access the flower rewards and, consequently, to perform pollination. We will call the mechanical strength needed to open such a forcible floral mechanism the ‘operative strength’. The above-mentioned plants all have flowers with forcible mechanims but can differ greatly in operative strength. For example, operative strength measured for the forcible mechanisms of Salvia pratensis
and Phlomis fruticosa
are 2·98 and 49 mN, respectively (Brantjes, 1981a
; Claßen-Bockhoff et al., 2004a
). In most legumes with papilionate flowers, i.e. those with the corolla architecture consisting of one dorsal petal representing the flag, two lateral petals representing the wings and two ventrally attached petals representing the keel, fertile organs are hidden by the keel and pollinators mostly access rewards by pushing down the lateral and ventral petals, which are moveable, causing them to descend. Only in that way do rewards and fertile organs become exposed and make contact with the ventral side of the body of the pollinator.
Though the study of the strength needed to operate moveable parts should allow us to shed light on relevant aspects of the functional morphology of flowers, very little has been published in this respect after seminal contributions (Brantjes, 1981a
; Brantjes and De Vos, 1981
), and nothing on legumes, a plant family where forcible flower mechanisms are widespread. Two relevant ecological and evolutionary aspects immediately arise with regard to the relationship of floral operative strength with either functional specialization in plant–pollinator interaction or morphological specialization in floral phenotype.
In regard to functional specialization, similar to operative length (Moré et al., 2007
), a flower's operative strength may serve to filter out visitors that are unsuitable for efficient pollination. Operative strength would prevent weak visitors gaining access to rewards and admit stronger ones which, for some reason correlated with strength, could be better pollinators. This is suggested by the finding of a correlation between a flower's operative strength and pollinator mass (Brantjes, 1981b
) among several species of Lamiaceae, Polygalaceae and Orchidaceae, in addition to the observations by Edwards et al. (2005)
showing that only large insects could trip the flower mechanism of Cornus
. Both observations assume that pollinator mass or size is correlated with the strength they can exert when operating a floral mechanism, which is known only for very few flower visitors (see, however, Claßen-Bockhoff et al., 2004a
; Muchhala and Thomson, 2009
). In addition, since recent studies in sage do not support the previous view (Claßen-Bockhoff et al., 2004b
) the pollinator-filtering explanation remains only weakly supported.
In regards to the morphological specialization in floral phenotype, the interaction through a floral mechanism of plants with animal pollen vectors should explain a great part of this structural complexity, since effective pollen delivery and reception requires that several traits individually, or in an integrated manner, adjust to the pollinator morphology and behaviour (Stebbins, 1970
). There has been recent effort to determine whether or not combinations of traits explain the association of plants with different kinds of pollinators (Martén-Rodríguez et al., 2009
; Ollerton et al., 2009
). Also information is beginning to accumulate showing that trait combinations may respond to pollinator-driven natural selection (O'Connell and Johnston, 1998
; Maad, 2000
; Benítez-Vieyra et al., 2006
; Nattero et al., 2010
). However, the premise that flower phenotype should work and evolve as an integrated whole has received very limited support (Herrera et al., 2002
; Ordano et al., 2008
). It has been argued that flowers actually have a parcellated phenotype and are, consequently, not integrated as a whole because several functional modules are nested within a single flower with strong intramodular and weak intermodular correlations (Ordano et al., 2008
). In spite of this interesting pattern, there have been difficulties in identifying which combinations of traits are expected to act coordinately for a given flower function.
In this study, we address these above aspects by testing the following hypotheses: (1) a flower's operative strength has a visitor-filtering property by allowing access to floral rewards to pollinators strong enough to trip the floral forcible mechanism; and (2) the operative strength of a flower is given by a combination of traits that form a functional flower module.
The first hypothesis predicts that plant species with forcible floral mechanisms will differ in the operative strength of their flowers and this will be correlated to the strength that pollinators are capable of exerting. Previously (Brantjes, 1981b
), correlations were studied with body mass, whose correlation with strength is not certain. Consequently, in addition to explicitly testing the operative strength and pollinator strength, we also studied correlation between body mass and the strength of the pollinators to determine how well body mass predicts pollinator strength. In addition, and relative to this aspect, we wanted to prove if the forcible mechanism was stable during successive visits or if it was stronger or weaker at different visits.
The second hypothesis predicts that morphological traits that are significantly correlated with operative strength should have higher and more significant correlations between them than with traits not related to operative strength and that this correlation could involve traits belonging to different developmental modules, i.e. the types of petal (flag, wing or keel).