Flowering plants generally lure pollinators (hereafter ‘bees’) to perform pollination by offering them food (or sex, or physical warmth or protection or other things in some cases, but generally, food) in the form of nectar and/or surplus pollen.
Bees transfer the pollen between plants, basically helping the flowers to have sex. All good so far.
How do the plants persuade the bees to visit other plants of their own species in succession? - It’s a waste of time getting your pollen transferred somewhere else, or receiving pollen from another, different species of plant.
The naive answer is ‘by offering or promising the greatest reward’ - the most copious or sugary nectar, but it can’t just be that - I’ve been watching the bees (in particular, Bombus hortorum - the garden bumblebee) and I noticed that any individual bumble bee will visit all or most of the flowers on one toadflax plant, then fly to the another toadflax plant (ignoring the foxgloves, the cranesbill, the bistort, the woundwort) and repeat, only collecting from toadflax, until it presumably is full up and it flies off back to its nest.
Meanwhile, other individual bumble bees of the same species are visiting exclusively foxgloves, ignoring all the other species. Other individuals, again, same species, are only visiting woundwort. So it’s not a case of different species of bee for different plants (although I’m sure that also happens).
So how is an individual bee influenced to decide to collect nectar from just one species source? Obviously it’s to the advantage of those plants that this happens, but how does it happen?
One method (among many) is for all the plants of a particular species to bloom at the same time in a relatively short window. This vastly increases the odds of a bee visiting the same species of flower in succession.
Another method is to have a specialized flower structure that only a specific kind of pollinator can get at. This increases the odds that they will go to another flower of the same kind, since they’re the only ones who can get nectar out of it.
Sure, but what I’m describing is not any kind of permanent specialisation. All 5 species of plant I mentioned blooming copiously at the same time, intermingled, all being visited by the same species of bee, but any individual bee, when you follow it, is passing over 4 out of the 5 species of flowers, to exclusively collect from the fifth species, whilst other individuals of the same bee species are doing the same, but for a different choice of flower.
I’ve been watching them closely for a couple of days, following individual bees - and it’s very consistent.
The only thing I don’t know is how long the preference persists - whether an individual bee chooses a flower species per outing, per day, per lifetime, or what. I’d have to mark individual bees to measure that.
My guess is that the plant is not influencing the bees in this behavior; this behavior evolved in the bees because it benefits the bees themselves. If a bee visits a toadflax first thing in the morning and gets a good haul, there is a good chance that other toadflax are also paying off - so it’s energy efficient to stick to toadflax instead of exploring other flowers which may be less productive. If the same bee had first landed on a good foxglove, it might stick to foxglove all day. The fact that this bee behavior benefits the flowers as well is just good luck for the flowers.
There may, though, be a second-level evolutionary force involved. Imagine that in some little valley a colony of bees evolved the ability to sense the best-producing flowers and so hopped around between species. The flowers would loose out on the benefit of bee pollination and would decline in number. This super-bee population itself would then decline and maybe die out. Also, the flowers may end up evolving the ability to thwart the bees’ detection ability - to successfully hide their nectar/pollen output so that the bees must fall back on their original stick-to-one strategy.
Could be, but it’s such a huge differential of benefit for the flowers; basically the difference between pollination working vs not working - I’d have thought there has to be something more than just luck involved.
One thing I haven’t done is to get up before the bees and track individuals first thing - to see if they are exploring different options on their first foray.
Something that only just occurred to me; of course we’re familiar with honey bees communicating inside the hive about the location of abundant nectar sources, and it’s easy to think honey bee-> hive, bumble bee → solitary, but they’re not; an established, successful bumble bee nest can contain hundreds of individual bees (dozens is probably more normal).
The bees may be communicating with each other in terms of their plans for collecting; there is no point in two bees from the same nest going to the same location, then, once they get there, competing for the same source of nectar - deciding that various individual bees will each only visit one species of flower (but collectively, all of the good sources in an area) would be a sensible way to organise that non-competition. Not sure if that’s what they actually do, but it’s a potential explanation.
I still think it’s likely that the flowers are in on it somehow, just because it is such a huge advantage to them, if the bees stick to the same species.
There might be an element of taste to it - this bee prefers the taste of foxglove, that one prefers the taste of toadflax. You’d need to mark bees different colours to check if the same bees return to the same species of flower on successive days, or if a toadflax bee today is a foxglove bee tomorrow.
Maybe there is some chemical trigger in the nectar or flower scent that tells the bee ‘that was actually the best nectar in the world, you want more of this specific thing’
I suspect your bees are cheating somewhat, maybe by visiting other flowers on their way back to the nest (that you are not witnessing/privy to). My thinking is the polinators (generally, not ones co-evolving with specific flowers/plants) end up with pollen from various plants they are visiting all over their bodies, and whenever they visit a flower, there is a good chance some pollen from the same species of plant falls off along with a bunch of unusable pollen from other plants. All it takes is a spec of the right pollen amongst the noise to pollenate a flower. Individual bees may have preferences for specific flowers, but among an abundance of food I suspect they are not going to be very selective.
Well I haven’t watched all the bees, and once they fly off away from the area, it’s only my assumption that they are returning to the nest, but what I have witnessed, with very great consistency, is individual bees visiting only one species of flower, passing right over/through adjacent plants of a different species, to get to the next plant of the same species. Meanwhile, other individual bees of the exact same species visiting plants of a different species, exclusively, passing over the other plants.
If they were going to ‘cheat’ on the way home, why do you think they’re not doing that while I’m watching?
I don’t doubt your observation, but perhaps no one has done any research on this before. We don’t knnow everything about any animal species, which means there’s always new things to study about them. This is perhaps one of those things.
I suspect it’s because it’s easier for the bees. Not just because they know that the toadflax is good today. But also, each flower is approached slightly differently. The bee already learned how to get at the nectar in toadflax. Why spend the energy to work out the best way to harvest foxglove if there is still plentiful toadflax around.
You could test this hypothesis by seeing how often flowers, of the same shape and appearance but of different species, flower simultaneously in a mature ecosystem.
And flowers can reinforce this by being different from the other flowers in the area.
I haven’t thought of any mechanism by which plants could keep bees from visiting flowers of other species. The flower-selection algorithm is all on the bee side. It certainly makes sense for bees to stick to a single species, either because that they learned that those flowers are rewarding or because that have already figured out how to access them. It would also make sense for the bees to occasionally sample other types to see if they have bigger payoffs. Whatever the bee’s algorithm is, the plants will evolve to game it: to get the most visits for the least nector cost. I not sure if it would be advantageous for plants to hide their nectar payoff; it seems like it might be if the other plants are doing the same.
Interesting question with more nuance than I expected from the title.
Turns out the term is “flower constancy” and is the pollinator behavior of returning to the same flower type. Bees can distinguish different plant species, and even tell if a flower has recently been visited by another bee.
The Wikipedia article says it is a observed behavior in several pollinating insects, and it is obvious what is in it for the plant, but not so much for the bee.
There are several hypothesis, but I won’t retype them, because it is a short article people can read themselves.
As I was reading that, I was thinking: maybe their tiny brains just aren’t big enough to hold more than one flower type at a time. And indeed that’s more or less one of the explanations.
Though I’m not sure I’d call that “memory” as opposed to a single-purpose pattern recognizer. They surely have some specialized flower-detector in their brains, but it may just not have the capacity to handle more than one flower.
Really, humans have the same problem. Ask someone to count all the red Toyotas on the road. That’s a way easier problem than counting all the red Toyotas, and blue Hyundais, and white Fords. Paying attention to all those things simultaneously stresses our recognition circuits.
That does make sense - given that different species of flowers have different markings to guide the bees (some of which markings are only visible to organisms that can see in UV - including bees) - the flowers also need to be approached in specific ways - toadflax, for example, has little ‘snapdragon’ flowers that the bees must pry open in order to get at the nectar inside (the action of prying them open also triggers the flower to deposit pollen on the bee’s head).
So plants may have engineered this constancy by making the instructions sufficiently complex that a bee will just figure out one version and stick to that.
Or it could be that, plus simple reinforcement learning; following the instructions to get nectar from flower X resulted in an immediate reward to the bee - which is an incentive to repeat that exact nectar-obtainment ritual.
Just revisiting this thread and I want to recommend watching some bees at work sometime - apart from just being a wonderfully therapeutic thing to do, it pretty quickly becomes clear that there are some quite complex goings-on. For example in the Woundwort, the anthers (bearing the pollen) are hidden in the top part of the flower and they are spring-loaded - when the bee reaches inside the flower to sip nectar, the anther snaps down and precisely deposits a dab of pollen on the head of the bee; the bee can’t do much about this on exiting the flower as that exit move tends to be rather like just falling off, but when the bee visits the next flower, the first thing it does is to comb its hair - in a forward direction (because that’s how bee legs work) - some of the combed pollen is stored by the bee in pollen-baskets on its legs, but the combing action of the pollen off the head just happens to be in the exact right place for some of it to be brushed onto the waiting stigma of the flower, pollinating it.
There probably are flowers where pollination is a bit more like seasoning the bees all over and hoping for the best, but in a lot of cases, it’s really deliberate and precise.
I got footage of it which I published here - at about the 8:12 mark, you can see the flower deposit pollen on the bee’s head; at 8:18 you can see the bee combing it off, on the next flower and at 8:26 there is a really clear view of the next flower booping some more white pollen onto the bee’s head.
Of course the above doesn’t necessarily result in pollination at every event - if the plant is not self-fertile, it only counts when the pollen is placed by one individual plant and is combed off on the next, but the flowers make that happen by applying pollen to the bees quite surgically.
The thing that also occurred to me reading this thread - why do flowers smell? Presumably as an attractant. So the scent is another clue along with appearance pattern to alert bees etc. that “pick me! Pick me!”.
Bumble bees are fascinating - the missing link between solitary and hive bees. The “queen” starts out in the spring after witer hibernation as solitary, builds a nest, lays a few eggs. She takes on the whole work of feeding a few larva until they emerge as underfed stunted infertile workers who them help expant the nest//hive for more eggs laid by the queen. The hive keeps growing with progressively bigger “workers” as they are better fed, until by the end of the summer, the latest round of bees emerge fully fed and full size and fertile, and they can mate and hibernate until spring and start their own nest/hive.
Once in a while, especially in early spring, you can see what look like bumble bees but are less than half the size of what we expect as bumble bees.