14713659_265453713851843_739080250392396373_nSet your meetings, phone calls and emails aside, at least for the next several minutes. That’s because today you’re a bee.

It’s time to leave your hive, or your underground burrow, and forage for pollen. Pollen is the stuff that flowers use to reproduce. But it’s also essential grub for you, other bees in your hive and your larvae. Once you’ve gathered pollen to take home, you or another bee will mix it with water and flower nectar that other bees have gathered and stored in the hive. But how do you decide which flowers to approach? What draws you in?

In a review published last week in the journal Functional Ecology, researchers asked: What is a flower like from a bee’s perspective, and what does the pollinator experience as it gathers pollen? And that’s why we’re talking to you in the second person: to help you understand how bees like you, while hunting for pollen, use all of your senses — taste, touch, smell and more — to decide what to pick up and bring home.

Maybe you’re ready to go find some pollen. But do you even know where to look?
Assessment of pollen rewards by foraging bees
Elizabeth Nicholls
and Natalie Hempel de Ibarra*
School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK; and
Centre for Research in Animal
Behaviour, Psychology, University of Exeter, Perry Road, Exeter EX4 4QG, UK
1. The removal of pollen by flower-visiting insects is costly to plants , not only in terms of pro-
duction, but also via lost reproductive potential. Modern angiosperms have evolved various
reward strategies to limit these costs, yet many plant species still offer pollen as a sole or major
reward for pollinati ng insects.
2. The benefits plants gain by offering pollen as a reward for pollinating are defined by the
behaviour of their pollinators, some of which feed on the pollen at the flower, while others col-
lect pollen to pro vision offspring.
3. We explore how pollen impacts on the behaviour and foraging decisions of pollen-collecting
bees, drawing compari sons with what is known for nectar rewards. This question is of particu-
lar interest since foraging bees typically do not eat pollen during collection, meaning the sen-
sory pathways involved in evaluat ing this resource are not imm ediately obvious.
4. Previous research has focussed on whether foraging bees can determine the quality of pollen
sources offered by different plant species, and attempted to infer the mechanisms underpinning
such evaluations, mainly through observations of collection preferences in the field
5. More recently exp erimental research has started to ask whether pollen itself can mediate the
detection of, and learning about, pollen sources and associated floral cues.
6. We review advancements in the understanding of how bees forage for pollen and respond
to variation in pollen quality, and discuss future directions for studying how this an cestral flo-
ral food reward sh apes the behaviour of pollinating insects.
Key-words: bees, behaviour, learning, pollen, pollination, sensory
Insect pollination is considered the oldest form of pollen
transfer (Labandeira & Currano 2013), and the vast major-
ity of modern angiosperms benefit from visitation by insects
(Ollerton, Winfree & Tarrant 2011), investing heavily in
attractive floral displays and rewards for pollinators.
Despite a widespread switch during angiosperm evolution
from rewarding with pollen to the provision of nectar for
insect visitors, pollen nevertheless remains an important
food resource for consumption and collection by flower-vis-
iting insects. While insects wish to maximize the amount of
pollen they consume or collect during a flower visit, for
plants, pollen removal also comes at a cost, both energetic
and in terms of lost reproductive potential (Westerkamp
1997; Hargreaves, Harder & Johnson 2009). Compared to
pollen, nectar is considered to be a more convenient pollina-
tor reward for the plant to produce (Simpson & Neff 1983;
Heil 2011). From an insect’s perspective, harvesting nectar
requires fewer morphological and behavioural adaptations
than pollen collection (Thorp 1979) and is easier to digest
(Huber & Mathison 1976). In addition, nectar often con-
tains solutes such as amino acids, meaning pollinators are
able to meet a range of nutritional demands with this
reward (for reviews, see Nicolson 2011; Nepi 2014).
The emergence of nectar-producing organs during the
late Cretaceous period, a time characterized by a fast suc-
cession of radiation bouts in both plants and the insects
that pollinate them (Grimaldi 1999), likely led to the
recruitment of novel pollinator clades. However, the man-
ner in which pollinator behaviour may have changed in
response to this new floral reward is rarely discussed. Most
probably, behavioural changes exerted new selective pres-
sures that resulted in further co-evolutionary changes in
both flowers and insects. One idea that has received little
attention is that due to the relative ease with which nutri-
tional quality can be assessed, nectar may be more effective
at rewarding learning than pollen and thus may exert
*Correspondence author. E-mail: n.hempel@exeter.ac.uk
© 2016 The Authors. Functional Ecology © 2016 British Ecological Society
Functional Ecology 2016 doi: 10.1111/1365-2435.12778
greater control over the behaviour of pollinators. If true,
then nectar may also better promote constancy to the flow-
ers visited by insects, enhancing outcrossing potential. In
order to compare, we need to know how each reward type
affects movement patterns, learning and foraging decisions,
and whether this varies between reward types, leading to
differential effects on plant–pollinator relationships. The
assessment of pollen rewards by pollinators is not yet fully
understood, but with recent advances in research concern-
ing pollen-foraging behaviour, sensory processing and
learning, it is becoming ever more feasible to evaluate the
influence of reward type in shaping plant–pollinator inter-
actions. In this review, we will largely focus on bees,
including examples and references to work with both social
and solitary species which have thus far provided most of
the relevant facts and insights.
For bees and many other flower visitors, pollen is an
important source of nutrition for larval development, adult
maintenance and sexual maturation. The dietary needs of
these insects and their various life stages are diverse, as is
the nutritional ‘quality’ of pollen provided by different
plant families, species and even individual plants within a
population (reviewed by Roulston & Cane 2000). Bee spe-
cies differ in their ability to digest different pollen types
and to cope with the presence of toxins or protective com-
pounds. Pollen type has been shown to dramatically affect
both the development and survival of young bees and lar-
vae (e.g. Standifer 1967; Schmidt, Thoenes & Levin 1987;
Schmidt et al. 1995; Genissel et al. 2002; Roulston & Cane
2002; Tasei & Aupinel 2008; Sedivy, M

uller & Dorn 2011;
Di Pasquale et al. 2013), and so it has often been postu-
lated that bees would stand to benefit by being selective in
the pollen they choose to collect.
In the case of nectar foraging, it is well-established that
bees evaluate the nutritional value of this reward instanta-
neously and over the duration of the foraging trip, accu-
rately assessing the flow rate and sugar content of nectar
provided by flowers (N

nez 1970). Pollen is diverse in
form and the proportions of key nutrients vary consider-
ably, which is likely to make foraging choices and the
assessment of profitability a more complex task. One solu-
tion would be to establish foraging selectivity by specializ-
ing on pollen of particular plants or plant families, and
indeed the majority of early bees were oligolectic (Michez
et al. 2008; Wappler et al. 2015). However, among modern
bees, only a few truly monolectic species remain, as over
evolutionary time increases in the breadth of pollen diets
have been more common than restrictions (Mü ller 1996;
Danforth, Conway & Ji 2003). More generalist collection
strategies ensure that bees consume a diverse range of
nutrients while also diluting their intake of plant protec-
tion products and toxins (Eckhardt et al. 2014). Yet even
in highly polylectic species, such as honeybees and bumble-
bees, selectivity seems to persist and bees from these spe-
cies do not collect pollen from all plant species available.
Rather, individual foragers concentrate their foraging
efforts on a selection of plant species, showing preferences
for one pollen type over another (e.g. Schmidt 1982;

uller 1995; Cook et al. 2003; Requier et al. 2015; Vaudo
et al. 2016) and a capacity for flower constancy during
pollen collection (e.g. Heinrich 1979; Minckley & Roulston
2006). However, whether such preferences are based on
individual foragers’ assessment of nutritional differences
between pollen rewards remains a major outstanding ques-
So far studies attempting to address this issue have
yielded mixed results. Many are correlational, relating bees’
foraging preferences in the field to the levels of a particular
nutrient(s) found in the pollen provided by different plant
species (Robertson et al. 1999; Hanley et al. 2008; Leon-
hardt & Bl

uthgen 2012; Somme et al. 2015). Since pollen is
the major source of protein for bees, levels of this macronu-
trient and/or the relative abundance of amino acids have
frequently been proposed as cues likely to be relevant to
bees, but results are not consistent, and there appears to be
no simple relationship between collection preferences and
the nitrogen content of pollen (Levin & Bohart 1955;
Schmidt 1982, 1984; Schmidt & Johnson 1984; van der
Moezel et al. 1987; Pernal & Currie 2002). For example,
when honeybees were offered a source of protein in the
form of defatted soya bean flour, diluted to varying degrees
with alpha-cellulose, a non-nutritional, inert powder,
Pernal & Currie (2002) observed no difference in the weight
of pollen loads collected by foragers, suggesting they did
not discriminate between pollen samples on the basis of
protein content alone. Similarly, Roulston & Cane (2002)
reported that when offered pollen sources enriched with
protein via the addition of soya bean meal, sweat bee
foragers did not vary the volume of pollen they provisioned,
even though pollen protein content was shown to affect
offspring body size. As such, evidence is lacking for the
bees’ ability to discriminate between floral pollen on the
basis of crude protein content alone, particularly within the
range of naturally occurring variation. Further studies have
suggested that other macronutrients such as lipids are either
equally or even more important (Singh, Saini & Jain 1999;
Schmidt & Hanna 2006; Avni et al. 2014; Vaudo et al.
2016), or that bees may be guided by the presence of toxins
or distasteful compounds (Sedivy, M

uller & Dorn 2011).
The lack of consensus among the aforementioned studies
likely arises from the method of investigation. In the first
instance, pollen is a complex substance, varying between
species and individual plants in a multitude of respects.
Though sometimes acknowledged, this is frequently unac-
counted for in field studies. This is perhaps not surprising
however, given that it is impossible to simultaneously con-
trol all the dimensions along which pollen varies without
the use of artificial pollen surrogates. Furthermore, accu-
rate measurements of the chemical composition of pollen
are hampered by methodological limitations arising from
the use of fresh plant samples or bee-collected pollen that
has been altered through the addition of nectar by foraging
corbiculate bees (Roulston & Cane 2000; Campos et al.
2008; Nicolson 2011). Finally, such studies often do not
© 2016 The Authors. Functional Ecology © 2016 British Ecological Society, Functional Ecology
2 E. Nicholls & N. Hempel de Ibarra
consider the sensory experience of an individual forager, as
well as their prior experience and other floral cues and envi-
ronmental factors which may play a role in guiding collec-
tion preferences. We argue that in order to determine which
component(s) of the pollen reward may be guiding bees’
foraging preferences, it is important to consider pollen col-
lection from a behavioural perspective. In this review, we
examine current evidence regarding what bees can sense
during pollen collection, considering which cues are salient
and what role learning, prior experience and, in the case of
social bees, feedback from the nest might play in determin-
ing preferences. We also evaluate to what extent current
experimental evidence, and comparisons with nectar-fora-
ging behaviour, might explain the factors that guide pollen
collection and the formation of associations between floral
cues and pollen rewards. We hypothesize that rather than
simply detecting and basing foraging decisions on the pres-
ence or concentration of particular nutrients, pollen-collect-
ing bees are likely to make an overall sensory assessment
during foraging, utilizing a suite of cues and the recall of
prior experience.
Do foraging bees taste pollen?
Pollen-collecting bees typically do not eat pollen at the
flower. Instead they transport it back to the nest via their
corbiculae or specialized body hairs, to be consumed by
their offspring, or in the case of social bees, the colony as a
whole. Nevertheless, foraging bees may have ample oppor-
tunity to sample grains pre-ingestively with their main gus-
tatory organs, the mouthparts and antennae, which
frequently come into contact with pollen during collection.
Bees often probe flowers with the antennae (Ribbands
1949; Lunau 2000) and in some cases, grasp and scrape pol-
len from the anthers with their mandibles (Thorp 1979).
Some species even have specialized hairs on the mouthparts,
designed for collecting pollen from flowers with protected
anthers (Parker & Tepedino 1982; M

uller 1995). To facili-
tate adherence of the pollen grains to each other and the
pollen baskets, corbiculate bees add regurgitated fluids to
the grains, thus potentially providing further opportunities
for gustatory sampling through contact between the pollen-
covered body and the mouthparts. But what can bees taste?
Compared to what is known about both vision and
olfaction, the gustatory sense of bees is still poorly under-
stood. Honeybees possess only 10 intact gustatory receptor
genes (Robertson & Wanner 2006; Jung et al. 2015); bum-
blebees have 23 (Sadd et al. 2015). This is substantially
fewer than found in other insects [60 genes encoding 68
receptor proteins in fruit flies (Liman, Zhang & Montell
2014); 52 genes for 76 receptors in mosquitoes (Hill et al.
2002)], and has been taken as an indication of bees’ limited
ability to detect gustatory compounds in their environ-
ment. Taste responses are recorded extracellularly at the
tip of sensory sensillae and assigned to functional classes
of gustatory receptor (GRN). ‘Sweet’ and ‘bitter’ recep-
tors, genes and pathways (in analogy to the human sense
of taste) are well described in Drosophila, as well as recep-
tors that respond to salt, water and carbonation
(Yarmolinsky, Zuker & Ryba 2009). [Correction added
after online publication on 21 November 2016: ‘are well
described for 76 receptors in Drosophila’ was changed to
‘are well described in Drosophila’] Drosophila are quite
insensitive to amino acids and proteins in their food, which
occur only at low concentrations in their diet. However, to
date, the Drosophila gustatory system is the best under-
stood among insects, and work with this species has shown
that taste perception arises from the combined activity of
different GRN. Sugar-sensitive GRN are found on the
antennae, mouthparts and distal segment (tarsi) of the
forelegs in honeybees (Whitehead & Larsen 1976). Some
honeybee GRN respond to salts or particular toxins, either
when presented alone or in combination with sucrose
(Wright et al. 2010; de Brito Sanchez 2011; Kessler et al.
2015). Honeybees could possess a receptor type that
mediates responses to protein or amino acids, as in flies
(Dethier 1961; Shiraishi & Kuwabara 1970), but this is yet
to be tested at the physiological level in bees. In the hover-
fly Eristalis tenax, a pollinator which consumes pollen at
the flower, extracts of pollen diluted in water stimulate the
labellar salt receptor cells but not sugar receptors (Wacht,
Lunau & Hansen 2000). More studies characterizing the
response profiles of gustatory receptors and neural path-
ways in bees and other pollen-collecting insects are
certainly much needed.
Behavioural experiments have provided further insights
into the gustatory pathways that could be relevant to the
assessment of pollen. For example, bees have been shown
to be sensitive to the presence of amino acids in nectar.
When offered the choice, bees preferentially imbibe those
solutions containing amino acids over pure sucrose solu-
tions, presumably differentiating between the two rewards
through pre-ingestive mechanisms (e.g. Inouye & Waller
1984; Simcock, Gray & Wright 2014). In restrained bees,
when the antennae of unsatiated bees are touched with
nectar or artificial sucrose solution, a reflexive extension of
the proboscis (PER) is observed, a behaviour characterized
as an unconditioned, appetitive response to stimulation
with a food reward (Bitterman et al. 1983). Such a
response can be elicited following a single or few repeated
pairings with olfactory, visual or tactile stimuli, and is fre-
quently utilized as a paradigm for studying learning with
sucrose rewards (PER conditioning). Reflexive PER
responses have also been observed in honeybees stimulated
at the antennae with hand-collected almond pollen (Schei-
ner, Page & Erber 2004) and bee-collected pollen (Gr

Arenas & Farina 2008; Nicholls & Hempel de Ibarra
2013), supporting the idea that pre-ingestive gustatory
pathways are involved in the assessment of pollen rewards.
Very few individuals respond with PER to inert alpha-
cellulose powder, frequently used to dilute pollen in experi-
ments or as a pollen surrogate, which suggests that bees
are able to detect phagostimulatory compounds in pollen
through the antennae. The presence of additional sugars in
© 2016 The Authors. Functional Ecology © 2016 British Ecological Society, Functional Ecology
Assessment of pollen rewards by foraging bees 3
dry honeybee-collected pollen does not seem to be per-
ceived by honeybees, at least not at the level of the anten-
nae, the most sucrose-sensitive sensory organ. When
pollen was delivered to the antennae of honeybees with a
small sponge during an attempt to condition the pollen-
PER to an odour (Nicholls & Hempel de Ibarra 2013),
bees failed to form an association between the odour and
reward, responding no differently from a control group
that was stimulated with a clean sponge (Fig. 1a). Since
bees readily form an association between this same odour
and sugars presented in solution with water, this suggests
that any sugar present in the dry pollen was not detected
by bees, given that no association was formed.
More recently, Ruedenauer, Spaethe & Leonhardt
(2016) trained bumblebees in a different PER conditioning
paradigm, in which pollen and a pollen surrogate were
paired with a sucrose reward. Pollen or casein (mammalian
milk protein) were mixed in various concentrations with
cellulose and water to form a sticky paste that was pre-
sented on a small copper plate which bees touched with
their antennae. The sucrose reward was delivered to one of
the antenna whilst it was still in contact with the humid
paste. Using chemo-tactile cues, bees learnt to distinguish
between pollen and pollen-surrogate stimuli differing in
absolute protein concentration, though only when the con-
centration differences between the two stimuli were
sufficiently large. Though it is unclear how these differences
might compare to naturally occurring variation in crude
protein between pollen species (2–60% protein, Roulston
& Cane 2000), the study provides new methods and
insights, and ultimately yet another demonstration of the
rich sensory capabilities of bees and the multisensory nat-
ure of the information extracted from pollen rewards.
Furthermore, the numerous controls that were conducted
alongside these experiments reflect the difficulties that
experimenters face when trying to reliably separate tactile
and chemical stimulation (Scheiner, Erber & Page 1999;
Giurfa & Malun 2004; Nicholls & Hempel de Ibarra 2013).
The importance of olfactory cues
Pollen is fragrant and often also conspicuously coloured,
providing additional, potentially highly salient, cues to
bees alongside those provided by the flower itself. It has
been suggested that in early angiosperms, prior to the
appearance of a well-developed perianth, the androecium
itself may have served as the original advertisement for
attracting pollinating insects (Faegri & Pijl 1971; Crepet
et al. 1991). In general, floral odours provide important
cues that can guide pollinator foraging decisions (Raguso
2008; Wright & Schiestl 2009) and are undoubtedly impor-
tant sensory stimuli for bees. Renowned for their
(b) (c) (d)
Fig. 1. 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). In the olfactory PER conditioning paradigm, the typical sucrose reward was
substituted with pollen in an attempt to train honeybees to associate an unfamiliar odour with pollen reward (Nicholls & Hempel de
Ibarra 2013). Small cosmetic sponges were dusted in dry pollen and frequently replaced during conditioning. Bees in the control group
were trained to the same unfamiliar odour but ‘rewarded’ with a clean sponge that was attached to a pollen-coated sponge to provide pol-
len scent. (b) Bees accept pollen presented in Petri dishes, which can be presented with a coloured ring surrounding them (Nicholls &
Hempel de Ibarra 2014). (c) Sophisticated pollen feeders, where the pollen is dusted onto small chenille brushes (Muth, Papaj & Leonard
2016). The brushes are placed inside of differently shaped artificial flowers or attached to a coloured base to form anther-like structures
(photographs courtesy of A. Russel; from Russell & Papaj 2016). (d) Honeybees can collected sucrose or pollen rewards inside of dark
boxes. One colour marked the entry tube that led to the inside of the reward box (Nicholls, Ehrendreich & Hempel de Ibarra 2015). The
entrance marked by the alternative, unrewarded colour was blocked at the end with mesh, allowing pollen odour to diffuse.
© 2016 The Authors. Functional Ecology © 2016 British Ecological Society, Functional Ecology
4 E. Nicholls & N. Hempel de Ibarra
extraordinary ability to detect, discriminate and learn
odours (e.g. Laska et al. 1999), bees are nonetheless poor
at detecting the odour of amino acids, which as previously
discussed, are considered an important nutritional compo-
nent of the pollen reward (Linander, Hempel de Ibarra &
Laska 2012). Most likely insects learn and rely on the
overall olfactory signature of pollen-rewarding flowers.
For example, bees have been shown to be capable of dis-
tinguishing pollen odours from that of the whole flower
(von Aufsess 1960; Dobson, Danielson & Wesep 1999;
Carr et al. 2015), perhaps unsurprising given pollen, par-
ticularly the outer pollenkitt layer, emits odour bouquets
that differ strikingly in their composition from other floral
odours (Dobson & Bergstr

om 2000). Bees in controlled
choice experiments have been found to be guided by the
presence of previously experienced pollen odours (Hoh-
mann 1970; Pernal & Currie 2002; Konzmann & Lunau
2014; Beekman, Preece & Schaerf 2016), preferring pollen-
containing samples that are rich in odour over odour-poor
surrogates, or learning the odour bouquets of different pol-
len species when rewarded with sucrose (von Aufsess 1960;
Cook et al. 2005; Ruedenauer, Spaethe & Leonhardt
In natural settings, it is more difficult to measure how
pollinators respond to variation in odour concentrations
and to test the significance of pollen odour cues for finding
flowers or predicting the amount of pollen available (Gal-
izia et al. 2005; Raguso 2008; Carr et al. 2015), especially
considering that at the flower, pollen odours are presented
simultaneously alongside other strong sensory cues in the
form of floral odour bouquets, colours or patterns. In
experimental tests, we found that pollen-foraging bumble-
bees did not utilize a considerable contrast in odour con-
centration to distinguish between pollen samples and
instead based their choices on differences in visual appear-
ance (Nicholls & Hempel de Ibarra 2014).
Studies testing olfactory learning where pollen itself
serves as the reward can provide further insights. Arenas
& Farina (2012) concluded from their field experiments
with scented pollen feeders, that honeybees learn to associ-
ate a particular odour with the presence of pollen, though
it cannot be fully ruled out that the preferences observed
were not determined by bees’ earlier olfactory experiences
(Arenas & Farina 2014). To demonstrate if and what bees
learn when pollen alone serves as the reward, it may be
more informative to train less experienced foragers and
test bees under more controlled conditions.
The PER conditioning paradigm offers the advantage of
controlled pollen application to specific sensory organs in
order to condition bees to an unfamiliar odour under
highly controlled conditions. As previously mentioned, the
PER paradigm has proven extremely valuable for examin-
ing the sensory and neural pathways underlying sucrose-
rewarded learning in bees and other insects (e.g. Hammer
& Menzel 1995; Burke & Waddell 2011). Pollen elicits
reflexive proboscis extensions when applied to the anten-
nae, as required for the paradigm; however, multiple
pairings of odour and pollen presentation do not result in
a conditioned response to the odour. This suggests bees
are not able to form an association between an odour and
a pollen reward under these conditions (Nicholls & Hem-
pel de Ibarra 2013). An earlier study by Gr

uter, Arenas &
Farina (2008) prematurely reported that honeybees could
learn to associate a reward mixture of pollen and water
(70% pollen w:w) with an odour following three PER
training trials. However, without certain indispensable
controls, it is not possible to conclude that an observed
increase in responsiveness to the conditioned odour is truly
the result of bees learning a predictive relationship between
the odour and pollen reward. Instead such a response may
potentially be caused by other factors, such as an increase
in sensitivity due to repeated antennal stimulation or clog-
ging of the antennae with a sticky substance.
Pollen-rewarded learning of visual cues
While PER conditioning paradigms permit tight control
over the delivery of conditioned and contextual odour
stimuli and rewards, as demonstrated by some of the
aforementioned studies, it can be challenging to select
appropriate stimuli and obtain necessary controls, espe-
cially when both the conditioned stimulus and the reward
provide cues in the same sensory modality. Furthermore,
bees are restrained in these experiments, which may nega-
tively impact on the learning process. Visual conditioning
of freely behaving bees thus appears to be a more suitable
method for examining the reward properties of pollen in
associative learning.
Bees and most other pollinating insects have excellent
visual capabilities, even though their eyes are small and
have low spatial resolution (von Frisch 1967; Kevan &
Baker 1983; Hempel de Ibarra, Vorobyev & Menzel 2014).
When pollen is displayed openly by the flower, it often
contributes to flower patterns, though as with pollen
odour, visual cues cannot be perceived from a distance
and are resolved only once a pollinator has arrived at the
flower (reviewed by Hempel de Ibarra, Langridge & Voro-
byev 2015). Whenever visual cues are learnt, it is neverthe-
less most likely that foragers are largely guided by the
whole display of the flower and/or by joint displays of
inflorescences and across colocated plants.
In learning experiments we showed that na

ıve bumble-
bees (Bombus terrrestris) not only learn the colour of pol-
len samples, but are also able to form an association
between the pollen reward and a coloured stimulus sur-
rounding it (Nicholls & Hempel de Ibarra 2014). Bees were
offered two colours in combination with two pollen sam-
ples that differed in pollen concentration (Fig. 1b). After a
short training period, they shifted their initial preference
for the coloured stimulus paired with the low concentra-
tion of pollen towards the alternative colour associated
with the more concentrated pollen mixture. This was
demonstrated for different colour pairings, suggesting that
bees’ ability to learn about pollen rewards are not limited
© 2016 The Authors. Functional Ecology © 2016 British Ecological Society, Functional Ecology
Assessment of pollen rewards by foraging bees 5
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