Receiver bias for exaggerated signals in honeybees and its implications for the evolution of floral displays

Mimicking orchids lure bees from afar with exaggerated ultraviolet signals

Flowers have many traits to appeal to pollinators, including ultraviolet (UV) absorbing markings, which are well-known for attracting bees at close proximity (e.g., <1 m). While striking UV signals have been thought to attract pollinators also from far away, if these signals impact the plant pollinia removal over distance remains unknown. Here, we report the case of the Australian orchid Diuris brumalis, a nonrewarding species, pollinated by bees via mimicry of the rewarding pea plant Daviesia decurrens. When distant from the pea plant, Diuris was hypothesized to enhance pollinator attraction by exaggeratedly mimicking the floral ultraviolet (UV) reflecting patterns of its model. By experimentally modulating floral UV reflectance with a UV screening solution, we quantified the orchid pollinia removal at a variable distance from the model pea plants. We demonstrate that the deceptive orchid Diuris attracts bee pollinators by emphasizing the visual stimuli, which mimic the floral UV signaling of the rewarding model Daviesia. Moreover, the exaggerated UV reflectance of Diuris flowers impacted pollinators' visitation at an optimal distance from Da. decurrens, and the effect decreased when orchids were too close or too far away from the model. Our findings support the hypothesis that salient UV flower signaling plays a functional role in visual floral mimicry, likely exploiting perceptual gaps in bee neural coding, and mediates the plant pollinia removal at much greater spatial scales than previously expected. The ruse works most effectively at an optimal distance of several meters revealing the importance of salient visual stimuli when mimicry is imperfect.

Asymmetrical reproductive barriers in sympatric jewelflowers: are floral isolation, genetic incompatibilities and floral trait displacement connected?

Floral visitors influence reproductive interactions among sympatric plant species, either by facilitating assortative mating and contributing to reproductive isolation, or by promoting heterospecific pollen transfer, potentially leading to reproductive interference or hybridization. We assessed preference and constancy of floral visitors on two co-occurring jewelflowers [Streptanthus breweri and Streptanthus hesperidis (Brassicaceae)] using field arrays, and quantified two floral rewards potentially important to foraging choice – pollen production and nectar sugar concentration – in a greenhouse common garden. Floral visitors made an abundance of conspecific transitions between S. breweri individuals, which thus experienced minimal opportunities for heterospecific pollen transfer from S. hesperidis. In contrast, behavioural isolation for S. hesperidis was essentially absent due to pollinator inconstancy. This pattern emerged across multiple biotic environments and was unrelated to local density dependence. S. breweri populations that were sympatric with S. hesperidis had higher nectar sugar concentrations than their sympatric congeners, as well as allopatric conspecifics. Previous work shows that S. breweri suffers a greater cost to hybridization than S. hesperidis, and here we find that it also shows asymmetrical floral isolation and floral trait displacement in sympatry. These findings suggest that trait divergence may reduce negative reproductive interactions between sympatric but genetically incompatible relatives.

Visuo-motor biases in buff-tailed bumblebees (Bombus terrestris)

Bees provide a good model to investigate the evolution of lateralization. So far, most studies focused on olfactory learning and memories in tethered bees. This study investigated possible behavioural biases in free-flying buff-tailed bumblebees (Bombus terrestris) by analysing their turning decisions in a T-maze. Bees of various size were trained to associate a syrup reward with a blue target placed at the centre of the T-maze. The bees were then tested over 16 trials by presenting them with blue targets at the end of the maze's arms. The maze was rotated 180° after the first 8 trials to control for environmental factors. The number of turnings to the left and right arms were analysed. The bees sampled exhibited a population-level rightward turning bias. As bumblebees vary significantly in size with large bees being better learners than smaller ones, we measured the thorax width to identify a possible relationship between size and bias. No significant correlation was identified. This study shows that bees present lateralization in a visuo-motor task that mimics their foraging behaviour, indicating a possible specialization of the right side of the nervous system in routine tasks.

Processing bias: extending sensory drive to include efficacy and efficiency in information processing

Communication signals often comprise an array of colours, lines, spots, notes or odours that are arranged in complex patterns, melodies or blends. Receiver perception is assumed to influence preference and thus the evolution of signal design, but evolutionary biologists still struggle to understand how perception, preference and signal design are mechanistically linked. In parallel, the field of empirical aesthetics aims to understand why people like some designs more than others. The model of processing bias discussed here is rooted in empirical aesthetics, which posits that preferences are influenced by the emotional system as it monitors the dynamics of information processing and that attractive signals have effective designs that maximize information transmission, efficient designs that allow information processing at low metabolic cost, or both. We refer to the causal link between preference and the emotionally rewarding experience of effective and efficient information processing as the processing bias, and we apply it to the evolutionary model of sensory drive. A sensory drive model that incorporates processing bias hypothesizes a causal chain of relationships between the environment, perception, pleasure, preference and ultimately the evolution of signal design, both simple and complex.

Innate Receiver Bias: Its Role in the Ecology and Evolution of Plant–Animal Interactions

Receiver bias in plant–animal interactions is here defined as “selection mediated by behavioral responses of animals, where those responses have evolved in a context outside the interactions.” As a consequence, the responses are not necessarily linked to fitness gains in interacting animals. Thus, receiver bias can help explain seemingly maladaptive patterns of behavior in interacting animals and the evolution of plant traits that trigger such behavior. In this review, I discuss principles of receiver bias, show its overlap with mimicry and how it differs from mimicry, and outline examples in different plant–animal interactions. The most numerous and best documented examples of receiver bias occur within plant–pollinator interactions. I elaborate on the ability of some plants to heat up their flowers (i.e., floral thermogenesis) and argue that this trait likely evolved under receiver bias, especially in pollination systems with oviposition mimicry. Further examples include signals in insect-mediated seed dispersal and plant defense through repellence of aphids. These examples show that receiver bias is widespread in different plant–animal interactions. For a broader understanding of the role of receiver bias in those interactions, we need more data on how animals respond to plant signals, the context and evolutionary history of those behaviors, and the evolutionary patterns of plant signals.

Signal Diversity, Sexual Selection, and Speciation

Communication is ubiquitous. Developing a framework for the diversity of signals has important consequences for understanding alternative models of sexual selection and the processes contributing to speciation. In this article we review how models of neutral evolution in the perceptual space of signal perceivers provide a first step toward constructing a framework for signal diversity. We discuss how the distinction between additive and multiplicative effects of multimodal signaling represents a second step. We then assess how signal efficiency, reliability, and the aesthetics of perceivers provide distinct mechanisms for signals to be effective, thereby partly explaining signal diversity. Understanding the relative contribution of each of these mechanisms to the effectiveness of mate choice signals unravels the relative importance of alternative models of sexual selection. It can also help to distinguish whether divergence of communication is a driver or a consequence of speciation. Throughout the review we emphasize the importance of verification and learning in repeated interactions for understanding variation in signals.

FLOBOTS: ROBOTIC FLOWERS FOR BEE BEHAVIOUR EXPERIMENTS

Studies of pollinator foraging behaviour often require artificial flowers that can refill themselves, allowing pollinators to forage for long periods of time under experimental conditions. Here I describe a design for inexpensive flowers that can refill themselves upon demand and that are easy enough to set up and clean that they can be used in arrays of 30 or more flowers. I also summarize of a variety of artificial flower designs developed by other researchers.

Psychophysics and the evolution of behavior

Sensory information allows animals to interpret their environment and make decisions. The ways in which animals perceive and measure stimuli from the social and physical environment guide nearly every decision they make. Thus, sensory perception and associated cognitive processing have a strong impact on behavioral evolution. Research in this area often focuses on the unique properties of the sensory system of an individual species, yet certain relevant features of perception and cognition generally hold across taxa. One such general feature is the proportionally based translation of physical stimulus magnitude into perceived stimulus magnitude. This process has been recognized for over a century, but recent studies have begun to consider how a law of proportional psychophysics, Weber's law, exerts selective force in behavioral evolution.

Conceptualization of relative size by honeybees

The ability to process visual information using relational rules allows for decisions independent of the specific physical attributes of individual stimuli. Until recently, the manipulation of relational concepts was considered as a prerogative of large mammalian brains. Here we show that individual free flying honeybees can learn to use size relationship rules to choose either the larger or smaller stimulus as the correct solution in a given context, and subsequently apply the learnt rule to novel colors and shapes providing that there is sufficient input to the long wavelength (green) photoreceptor channel. Our results add a novel, size-based conceptual rule to the set of relational concepts that honeybees have been shown to master and underline the value of bees as an animal model for studying the emergence of conceptualization abilities.

Pollinator-mediated evolution of floral signals

Because most plants rely on animals for pollination, insights from animal sensory ecology and behavior are essential for understanding the evolution of flowers. In this review, we compare and contrast three main types of pollinator responses to floral signals--receiver bias, 'adaptive' innate preferences, and associative learning--and discuss how they can shape selection on floral signals. We show that pollinator-mediated selection on floral signals can be strong and that the molecular bases of floral signal variation are often surprisingly simple. These new empirical and conceptual insights into pollinator-mediated evolution provide a framework for understanding patterns of both convergent (pollination syndromes) and advergent (floral mimicry) floral signal evolution.

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.

Pollinators' mating rendezvous and the evolution of floral advertisement

Successful cross-fertilization in plant species that rely on animal pollinators depends not just on the number of pollinator visits, but also on these visits' duration. Furthermore, in non-deceptive pollination, a visit's duration depends on the magnitude of the reward provided to the pollinator. Accordingly, plants that rely on biotic pollination have to partition their investment in cross-fertilization assurance between attracting pollinator visits - advertisement, and rewarding visitors to assure that the visit is of productive duration. Here we analyze these processes by a combination of optimality methods and game theoretical modeling. Our results indicate that the optimality in such allocation of resources depends on the types of reward offered to the pollinators. More precisely, we show that plants that offer both food reward and mating rendezvous to pollinators will evolve to allocate a higher proportion of their cross-fertilization assurance budget to advertisement than plants that offer only food reward. That is, our results indicate that pollinators' mating habits may play a role in floral evolution.

From foraging to operant conditioning: a new computer-controlled Skinner box to study free-flying nectar gathering behavior in bees

The experimental study of nectar foraging behavior in free-flying bees requires the use of automated devices to control solution delivery and measure dependent variables associated with nectar gathering. We describe a new computer-controlled artificial flower and provide calibration data to measure the precision of the apparatus. Our device is similar to a "Skinner box" and we present data of an experiment where various amounts of a 50% sugar solution are presented randomly to individual bees. These data show large individual variations among subjects across several dependent variables. Finally, we discuss possible applications of our device to problems in behavioral sciences.

Do flowers wave to attract pollinators? A case study with Silene maritima

To answer the question whether flowers wave to attract pollinators, we determine: (1) the heritability of floral mobility; (2) whether wavy flowers attract more insects; (3) does the duration of pollination affect seed set; and (4) the relationship between seed set and floral mobility. The pollination ecology of Silene maritima was investigated. Flowers on stalks of different waviness were used to investigate the effect of floral movement on pollinator visits. There is heritable variation in both direct and indirect estimates of floral mobility. The highest insect total visitation times were associated with medium length thin stalks that were visited more frequently and by more insect species. Although mean individual visit durations were less than those of less mobile flowers, this was compensated for by increased visits. Observations of controlled pollinations show that when the visit times are low, so is seed set and therefore low and high mobility flowers might suffer from reduced fitness. Combining these observations provides a mechanism that could be driving stabilizing selection for flower stalk traits, with a trade-off applying between waving to attract pollinators and not being too mobile as to prevent effective pollination. Further evidence for stabilizing selection is provided by the relationship observed in the field between seed set and floral mobility where the highest levels of fitness was associated with intermediate levels of floral waving.