Anne S. Leonard

Multisensory integration of colors and scents: insights from bees and flowers

Karl von Frisch’s studies of bees’ color vision and chemical senses opened a window into the perceptual world of a species other than our own. A century of subsequent research on bees’ visual and olfactory systems has developed along two productive but independent trajectories, leaving the questions of how and why bees use these two senses in concert largely unexplored. Given current interest in multimodal communication and recently discovered interplay between olfaction and vision in humans and Drosophila, understanding multisensory integration in bees is an opportunity to advance knowledge across fields. Using a classic ethological framework, we formulate proximate and ultimate perspectives on bees’ use of multisensory stimuli. We discuss interactions between scent and color in the context of bee cognition and perception, focusing on mechanistic and functional approaches, and we highlight opportunities to further explore the development and evolution of multisensory integration. We argue that although the visual and olfactory worlds of bees are perhaps the best-studied of any non-human species, research focusing on the interactions between these two sensory modalities is vitally needed.

Floral signal complexity as a possible adaptation to environmental variability: a test using nectar-foraging bumblebees, Bombus impatiens

Floral signals are typically emitted across multiple sensory modalities, although why they are multimodal is unclear. One possible explanation is that multimodal signalling ensures that at least one signal component will be transmitted effectively under varying environmental conditions (the ‘efficacy backup’ hypothesis). For example, by transmitting both component A and B, a signaller can communicate under environmental conditions where transmission of component A is reduced; component B ‘backs up’ A. To test this hypothesis, we determined whether a floral scent could back up a floral colour signal when light levels were low. We trained nectar-foraging bumblebees to discriminate rewarding and unrewarding targets that differed in colour, scent, or both colour and scent, and then presented the targets at different levels of illumination. We measured bees’ accuracy at distinguishing the two targets and their rate of visits to the trained target. Performance on both measures declined under low light when targets were unscented. The presence of scent reduced the loss of accuracy under low light, supporting the efficacy backup hypothesis, but this effect depended upon the colour of the previously rewarded target. In contrast, the presence of scent did not affect the overall rate of correct visits under low light (correct visits/foraging time). A backup mechanism that maintains accuracy, but not rate of nectar collection, does not necessarily benefit the pollinator. However, it most likely benefits the plant through reduced pollen wastage. In short, multimodal floral signals may benefit the plant by improving pollen transfer, while not benefiting the pollinator.

Floral Nectar Guide Patterns Discourage Nectar Robbing by Bumble Bees

Floral displays are under selection to both attract pollinators and deter antagonists. Here we show that a common floral trait, a nectar guide pattern, alters the behavior of bees that can act opportunistically as both pollinators and as antagonists. Generally, bees access nectar via the floral limb, transporting pollen through contact with the plant’s reproductive structures; however bees sometimes extract nectar from a hole in the side of the flower that they or other floral visitors create. This behavior is called “nectar robbing” because bees may acquire the nectar without transporting pollen. We asked whether the presence of a symmetric floral nectar guide pattern on artificial flowers affected bumble bees’ (Bombus impatiens) propensity to rob or access nectar “legitimately.” We discovered that nectar guides made legitimate visits more efficient for bees than robbing, and increased the relative frequency of legitimate visits, compared to flowers lacking nectar guides. This study is the first to show that beyond speeding nectar discovery, a nectar guide pattern can influence bees’ flower handling in a way that could benefit the plant.

Colour learning when foraging for nectar and pollen: bees learn two colours at once

Bees are model organisms for the study of learning and memory, yet nearly all such research to date has used a single reward, nectar. Many bees collect both nectar (carbohydrates) and pollen (protein) on a single foraging bout, sometimes from different plant species. We tested whether individual bumblebees could learn colour associations with nectar and pollen rewards simultaneously in a foraging scenario where one floral type offered only nectar and the other only pollen. We found that bees readily learned multiple reward–colour associations, and when presented with novel floral targets generalized to colours similar to those trained for each reward type. These results expand the ecological significance of work on bee learning and raise new questions regarding the cognitive ecology of pollination.

Evolution of Plant-Pollinator Relationships: Why are floral signals complex? An outline of functional hypotheses

Plants produce a remarkable variety of displays to attract animals that transfer pollen. These floral displays are usually complex, broadcasting various combinations of visual, olfactory, gustatory, tactile, and thermal stimuli (Raguso 2004a). Even acoustic stimuli may be involved, as in the case of structural nectar guides used by echolocating flower-feeding bats (von Helversen and von Helversen 1999). Yet these sensorially complex advertisements likely evolved from an ancestor that primarily transmitted only chemicals, serving a defensive function (Pellmyr and Thein 1986). The subsequent amplification and elaboration of floral stimuli therefore offers an intriguing opportunity to study signal evolution. However, at present, we know surprisingly little about why floral displays consist of so many elements. This contrasts with progress in other areas: recently, researchers studying topics as diverse as sexual selection, warning displays, animal learning, and

Plasticity of the Worker Bumblebee Brain in Relation to Age and Rearing Environment

The environment experienced during development can dramatically affect the brain, with possible implications for sensory processing, learning, and memory. Although the effects of single sensory modalities on brain development have been repeatedly explored, the additive or interactive effects of multiple modalities have been less thoroughly investigated. We asked how experience with multisensory stimuli affected brain development in the bumblebee Bombus impatiens. First, to establish the timeline of brain development during early adulthood, we estimated regional brain volumes across a range of ages. We discovered significant age-related volume changes in nearly every region of the brain. Next, to determine whether these changes were dependent upon certain environmental stimuli, we manipulated the visual and olfactory stimuli available to newly emerged bumblebee workers in a factorial manner. Newly emerged bumblebees were maintained in the presence or absence of supplemental visual and/or olfactory stimuli for 7 days, after which the volumes of several brain regions were estimated. We found that the volumes of the mushroom body lobes and calyces were larger in the absence of visual stimuli. Additionally, visual deprivation was associated with the expression of larger antennal lobes, the primary olfactory processing regions of the brain. In contrast, exposure to plant-derived olfactory stimuli did not have a significant effect on brain region volumes. This study is the first to explore the separate and interactive effects of visual and olfactory stimuli on bee brain development. Assessing the timing and sensitivity of brain development is a first step toward understanding how different rearing environments differentially affect regional brain volumes in this species. Our findings suggest that environmental factors experienced during the first week of adulthood can modify bumblebee brain development in many subtle ways.

Concealed floral rewards and the role of experience in floral sonication by bees

Pollinators frequently use complex motor routines to find and extract floral rewards. Studies of pollinators foraging for nectar rewards indicate these routines are typically learned, and that constraints associated with learning and memory give pollinators incentive to continue foraging on these flowers. However, plants offer rewards besides nectar, including pollen, lipids and essential oils. In particular, bees use a complex motor routine termed floral sonication to extract pollen, their primary source of protein, from the more than 6% of flowering plant species (>22 000 species) that conceal pollen rewards within tube-like poricidal anthers. If floral sonication requires learning, this pollen extraction behaviour could contribute to floral fidelity. However, no studies have quantified the effect of experience on flower handling for bees extracting pollen from poricidal species. We therefore examined the degree to which floral sonication behaviour was modified by experience. We found that the key elements of the sonication motor routine appeared in full-blown form in a flower-naive bees first visit to a flower. We additionally found consistent, albeit modest, effects of experience on certain aspects of sonication behaviour. The latency to sonicate slightly decreased with experience. Bees also adjusted the length and amplitude of their sonication buzzes in response to pollen receipt. We conclude that the role of experience in foraging for concealed pollen rewards is different from that reported for nectar rewards. We offer an alternative explanation for its function in sonication. Finally, we discuss alternative hypotheses for the function of poricidal anthers and for how pollen-bearing plants may ensure floral fidelity even in the absence of a significant impact of experience on pollen extraction behaviour.

Plant–animal communication: past, present and future

Communication between plants and their animal partners underlies some of the planet’s most ecologically and economically important mutualisms. Study of communication in this context offers many opportunities to address fundamental questions about the costs and benefits of signal production, signal honesty, and receiver cognition. In this special issue, contributors highlight several key areas of current research, including how multiple receivers affect floral signaling, and how signaling may be related across different phases of reproduction. Visual signals are a particular emphasis, including how learning can mediate pollinator preferences, and the evolution of conspicuousness. In light of these focal areas, we summarize current trends towards the study of greater complexity both in terms of floral phenotypes and signaling/interaction networks.

A novel protocol for studying bee cognition in the wild

Summary 1.Understanding how animals perceive, learn and remember stimuli is critical for understanding both how cognition is shaped by natural selection, and how ecological factors impact behaviour. However, the majority of studies on cognition involve captive animals in laboratory settings. While controlled settings are required to accurately measure aspects of cognition, they may not yield realistic estimates of learning performance in natural environments. Wild bees offer a useful system in which to study cognitive ecology as well as comparative cognition more broadly: they encompass around 20,000 species globally, varying in characteristics such as life history strategy, degree of sociality, and dietary specialization. Yet the limited number of protocols currently available for studying insect cognition has restricted research to a few commercially available bee species, in almost exclusively lab settings. 2.We present a protocol (Free-Moving Proboscis Extension Response [FMPER]) to measure wild bees’ colour preferences, learning performance, and memory. 3.We first used lab-reared bumblebees Bombus impatiens to establish that FMPER yielded results consistent with learning theory. We then successfully tested wild honeybees Apis mellifera in the lab, and Bombus vosnesenskii at field sites. 4.FMPER is straightforward to implement, low-cost, and may be readily adapted to other flower-visiting insects. We believe it will be useful to a broad range of evolutionary biologists, behavioural ecologists, and pollination ecologists interested in measuring cognitive performance in the wild and across a broader range of species. This article is protected by copyright. All rights reserved.

A pollen fatty acid enhances learning and survival in bumblebees

Learning associations between food-related stimuli and nutrients allows foragers to collect resources efficiently. In turn, the nutrients that foragers consume can themselves affect learning performance, through innate preferences for pre-ingestive stimuli, as well as post-ingestive reinforcement. Bees are insect models of learning and memory, yet the vast majority of this research concerns nectar (carbohydrate) rather than pollen (protein/lipid) rewards, despite the fact that many bees collect both simultaneously. We asked how one component of pollen surface chemistry, a free fatty acid (oleic acid), affected bees’ performance in a nectar-learning task. We found that ingestion of oleic acid enhanced visual learning, likely through positive post-ingestive reinforcement. This was supported by our finding that although bees did not prefer to consume the oleic acid solution, its ingestion both decreased motor activity and increased survival. These results are a step towards understanding how nutritionally complex floral rewards may affect cognitive processes that underlie pollination mutualisms.