草乌花蜜产量的梯度分布及熊蜂自下而上的访花行为

收益降低假说(declining reward hypothesis)认为熊蜂自下而上的访花顺序是对花蜜产量的直接响应,先访问下部花蜜产量高的花可以获得更多的收益;花开口方向假说认为自下而上访花是因为熊蜂更容易看见其上部的花朵。为了验证上述两个假说,我们于2008年8月在北京小龙门国家森林公园调查了红光熊蜂(Bombus ignitus)访问草乌(Aconitum kusnezoffii)直立和倒立顶生花序的访问顺序,测量了直立花序下部雌性阶段花和上部雄性阶段花花蜜的糖浓度、体积,计算了花蜜中的糖含量。结果表明,红光熊蜂在直立花序和倒立花序内均以向上运动为主,分别占总运动次数的62.77%和68.35%;直立花序下部雌性阶段花花蜜糖浓度比上部雄性阶段花低1.44%,但是花蜜体积和花蜜中的糖含量都显著高于雄性阶段的花。由于熊蜂访问倒立花序时先访问的是下部的低回报雄性阶段的花,然后再访问上部高回报的雌性阶段的花,这与收益降低假说矛盾,表明红光熊蜂自下而上访问草乌直立花序可能不是受到花蜜产量的调节。

Flight patterns of bumblebees (Bombus ignitus) on vertical inflorescences of Aconitum kusnezoffii

Nectar-collecting bumblebees usually move upward while foraging in consecutive flowers on vertical inflorescences.This tendency to successively move upwards has been suggested to be a direct response to available nectar rewards.Therefore,bumblebees starting at bottom flowers will visit the most rewarding flowers first.An alternative explanation is that bumblebees have a better view of the flowers above than those below and they fly to those they can most readily see while maintaining an upright orientation.To evaluate these two hypotheses,we recorded flight patterns of bumblebee(Bombus ignitus) nectar foraging behaviors on upward and downward vertical inflorescences of monkshood(Aconitum kusnezoffii) and measured the difference in the reward for bumblebee from nectar between lower(female phase) and upper(male phase) flowers.We found significant support for the hypothesis explaining bottom-to-up flights while visiting upward vertical inflorescences.Nectar analysis indicated that both nectar volume and sugar content in lower female-phase flowers were significantly higher than those in upper male-phase flowers.Our findings suggest that B.ignitus forage from more to less rewarding flowers and depart when gain of reward is low.We also noted bottom-to-up foraging behaviors for both the downward and upward vertical inflorescences.However,pollinators started from less rewarding male-phase flowers,which was inconsistent with the declining reward hypothesis.Our results suggest that gender-biased nectar production towards the female phase does not directly regulate bumblebee foraging sequences,but rather attract bumblebees starting from the bottom female-phase flowers.