One warm but very windy day, a friend and I were perched on top of Gold Ridge in a small swale that provided some shelter from the wind. The alpine meadow was dotted with the purple flowers of monkshood. I had forgotten the structure of monkshood flowers, so I spent a few minutes opening one and examining the arrangement of parts inside.
The structure is really quite odd—very different from most other flowers in our area—although there are dozens of other species of monkshood (genus Aconitum) elsewhere in the world, which suggests that this odd structure is one that works well. Monkshood flowers are considered to be pollinated chiefly by bumblebees, although other animals may also visit the flowers occasionally. As I examined my specimen flower, it occurred to me that perhaps it would be interesting to learn how bumblebees visit and exploit the nectar while (potentially) depositing or exporting pollen.
Monkshood flowers are slightly complex, requiring a visiting bee to enter and move in a particular way. Most bumblebee species are generalists, capable of exploiting several kinds of flower. Naïve, inexperienced bees have no trouble figuring out simple, open flowers such as those of buttercups or roses, but they have to learn how to exploit more complicated flowers such as lupine or monkshood, and it may take a number of tries before the bee succeeds in getting to the nectar—indeed, these failed foragers often just give up. One species of bumblebee is considered to be a specialist on monkshood (elsewhere) and this species has a very short learning period, quickly getting to the nectar.
To explain what a bee has to do in a monkshood flower, I first need to describe a typical monkshood flower so an interested reader can then visualize a bee’s activity. In most other flowers, there are colorful petals that are backed by protective green sepals, but in monkshood, the purple exterior of the flower is composed of sepals that have been transformed to function like petals. There are two small sepals at the lower edge of the flower and two large, lateral sepals. Most conspicuously, there is a large, expanded sepal that forms a hood on the top of the flower (botanists perversely call this a ‘helmet’). The hood is reminiscent of the cowl of a medieval monk—hence the common name.
Inside the flower, at the base of the sepals, lie the working parts: lots of short stamens offering pollen in their anthers, to be picked up and exported by a flower visitor (male function) surrounding a few stubby pistils that will become fruits if pollen is deposited on their receptive stigmas (female function). Inside the expanded hood lie two true, narrow petals that bear nectaries tucked way up into the top of the hood. To get to the nectar, a bumblebee has to reach or crawl inside the flower, passing over the sexual parts as it does so, picking up or depositing pollen.
As is so often the case, our local species of Aconitum (A. delphiniifolium) has not been studied, so research on other monkshood species may be used to shed light on the local species.
Japanese researchers experimented with the flowers of their monkshood species: they removed one or more sepals and watched the behavior of bumbles on the manipulated flowers, recording the amount of pollen picked up and deposited by the visiting bees. Removal of the large lateral sepals deprived a bee of her usual platform for standing in the flower while she reached up to the nectaries. This meant that her body did not contact the anthers or stigma properly, so pollen pick-up and deposition (and fruit set) was reduced. Removal of the small lower sepals had little effect except that a bee had some trouble entering the flower. Taking off the hood of the flower changed the look of the flower greatly, but bees still visited. However, sometimes the bee extended her tongue into the air instead of inserting it into the nectary, so although the pollination effectiveness of the visit was adequate, the bee often got no reward—and that would mean that the rewardless bees would be less likely to visit other such flowers. The researchers suggest that the function of the hood is to guide a bee’s tongue to the nectaries and perhaps also to maintain the concentration of sugars therein, thus keeping the bees’ interest in visiting.
In at least some species of monkshood, there is considerable variation among populations in the depth of the nectary and hence in the distance a bee has to reach in order to get nectar. A long nectary can only be reached by a long-tongued bumblebee species; short-tongued bumblebees have trouble reaching the nectar in the normal way and may become nectar-robbers, by chewing a hole in the hood and reaching into the nectary that way. Nectar robbing may reduce visits by good pollinators and has the potential to reduce both pollen export and deposition.