Report of the March meeting.
What’s on the Mind of Pollen Gatherers? A talk by Dr. Natalie Hempel de Ibarra from the University of Exeter.
Natalie and colleagues have been researching the abilities and behaviours of bees when pollen gathering. Relevant questions they are trying to answer are:
- Do bees foraging for pollen have preferences for certain types of pollen?
- Do they know what they need to collect?
- Can they discriminate between pollen from different plants in favour of their nutritional requirements?
The plant’s point of view:
For the vast majority of flowering plants the goal is cross pollination, which will maintain or improve the gene pool and improve the species’ chances of survival. Many animals will pollinate by transfering pollen from flower to flower but pollen removal for feeding comes at a cost, both in terms of energy and lost reproductive potential.
In evolutionary terms pollen has been an insect food source for 100-120 million years. Nectaries appeared about 70 million years ago to attract pollinating insects and cut down on the energy-expensive production of pollen. This strategy was hugely successful as part of the expansion in new species around that time.
The bee’s point of view:
Bees are 100% dependent on flowers for their nutritional requirements, nectar and pollen.
When feeding, insects should:
- Make the lowest effort for the highest benefit and
- Reduce their exposure to toxins
Enormous variation in pollen nutritional quality exists within species and between populations, so how selective should bees be? The benefits of generalised foraging behaviour are:
- Wider diversity of nutrients
- More flexibility to collect sufficient pollen
- Adaptability to change
- Dilution of toxin intake
So, can bees distinguish pollen varieties by taste? The answer is ‘Probably yes’, although they have only 10 taste receptors (we have 5, bumble bees have 23). The bee brain is equipped for learning smell and taste so knowledge and experience of pollen taste could well be a driving force for efficient pollen collection, although the primary driver appears to be quantity of pollen collected. This may partially answer the first question of preferences for certain types of pollen.
To answer the second question, experiments in the past have focussed on whether foraging bees can determine the quality of pollen from different plant species, but without a great deal of success. More recent experiments have adopted a novel experimental approach testing the learning ability of bees with pollen rewards.
Bees possess receptors on their antennae, so individual bees in cages can be trained to extend their proboscis for a reward at the same time as the antennae are exposed to an odour (see image below). This conditioning failed to elicit a response with a variety of dry pollen samples. However experiments conducted with bumblebees and wet samples of varying concentration showed the bees’ ability to distinguish between pollen and pollen-surrogate differing in protein content. Bees preferred a particular pollen type, but this was not always the most concentrated sample. Only when presented with the biggest difference in concentration did the bees prefer the higher concentration.
Methods for experimental testing of pollen collection and pollen-rewarded learning in bees.
(a) When stimulated with pollen bees spontaneously respond with a proboscis extension (PER). The typical sucrose reward was substituted with pollen in an attempt to train honeybees to associate an unfamiliar odour with pollen reward.
(b) Bees accept pollen presented in Petri dishes, which can be presented on various coloured rings to test learned responses.
In conclusion it seems bees will use all the cues available to form their experience of pollen collection. These cues include taste, colour, ease of collection, abundance and possibly freedom from toxins.
- Do they have a preference for certain types of pollen? Yes
- Do they know what they need? No good evidence that they do, or indeed that they would have to.
- Can they discriminate nutritionally? To some extent, perhaps, but it is more important to collect large quantities and avoid toxins.
Images of experiments courtesy Natalie Hempel de IbarraCentre for Research in Animal Behaviour,
University of Exeter,
Exeter EX4 4QG