The angular range of achromatic target detection by honey bees

Honeybees Apis mellifera were trained to enter a Y-maze and choose the arm with a rewarded disc presented against a grey background. The alternative arm displayed the unrewarded grey background alone. Training and testing were performed with the rewarding disc subtending different visual angles. The training disc was either achromatic and provided green contrast, or chromatic and provided the same amount of green contrast as the achromatic one. The bee-achromatic disc could be learned and detected by the bees whenever it subtended 5° or 10°, but not if it subtended 30°. The chromatic disc was learned well and detected at all three visual angles. However, at 5° the maximum level of correct choices was ca. 75% with the achromatic disc whilst it was ca. 90% with the chromatic one. Thus, the presence of chromatic contrast enhances considerably the level of correct choices for the same amount of green contrast. The lower threshold of achromatic target detection lies between 3.7° and 5°; the upper threshold between 15° and 10°. At the upper threshold, detection switches from chromatic-based to achromatic-based. Thus, in the context of target detection, the achromatic green contrast channel specialises in the detection of objects of reduced angular size, whilst the chromatic channels are specialised for objects of large angular size. We suggest that achromatic detectors with a centre-surround organisation are involved in the task of detecting achromatic targets. Accepted: 23 February 1998     

Discrimination of complex textures by bees

A problem confronted by visual systems is that of discriminating textures. It appears that a recently described class of orientation-tuned neurones in the bee brain embody properties of mechanisms used by humans to discriminate complex textures. In particular these mechanisms would permit bees to discriminate a large range of textures by giving bees access to information related to higher-order correlations between texture elements. To determine if bees can exploit such textural information we have conducted behavioural experiments employing iso-dipole textures, that statistically speaking, differ from binary noise textures, and each other, only in their third-order correlation functions. While these textures are not themselves of any ethological significance their special properties permit us to show that bees can potentially use a very large palette of textures to classify textured objects. In electrophysiological experiments we demonstrate the requisite contrast sign invariance (rectification) of the orientation-selective neurones' responses and discuss other similarities of these neurones' responses to models accounting for human texture discrimination. Accepted: 7 October 1998     

Effects of perceived danger on flower choice by bees

Studies on animal–flower interactions have mostly neglected the third trophic level of pollinators' predators, even though antipredatory behaviour of pollinators may affect patterns of pollinator visitation, pollen transfer and floral traits. In three experiments, it was found that honeybees showed sensitivity to perceived danger at flowers by preferring apparently safe flowers over equally rewarding alternatives harbouring either a dead bee or a spider, and avoiding revisitation of a site where the bees had escaped a simulated predation attempt. These results suggest that bees, like other animals, take antipredatory measures, which may have far reaching effects on animal–flower interactions.     

The collection of pollen by bees

Bees require pollen for their reproduction and pollen comprises the basic larval food for bees. Most bees acquire pollen passively during flower visitation, but female bees may also collect pollen actively with the aid of various structural and behavioral adaptations. Most bees have evolved adaptations to concentrate pollen into discrete loads and transport pollen back to their nests. The various structural and behavioral adaptations of female bees for acquiring and transporting pollen are the basis of this review.     

Flower colour variation across a hybrid zone in Antirrhinum as perceived by bumblebee pollinators

To assess if pollinators’ behaviour could explain the maintenance of hybrid zones between different flower colour morphs, we analyzed flower colour variation in an Antirrhinum hybrid zone using spectrometry and a model of bee perception. Some colours generated by hybridization were not observed in any Antirrhinum species and even appeared to be rare among angiosperms. Variation in flower colours within the hybrid zone was continuous; the most similar colours were predicted not to be discriminated from one another in natural foraging situations. However, when compared at a scale corresponding to bees’ foraging range, some flower colours could be discriminated from all colours displayed by neighbouring plants. This could affect pollinator behaviour and explain lower visitation rates within the centre of the hybrid zone. Behavioural studies involving bumblebees and plant mixtures of parental and hybrid flower colours carefully characterized with appropriate visual models will be necessary to test this hypothesis. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: Lars Chittka     

Habitat structure and animal movement: the behaviour of bumble bees in uniform and random spatial resource distributions

Foraging organisms (like bumble bees) move between resource points (like flowers) whose natural distributions vary enormously: from hyperdispersed to random to clumped. These differences in habitat structure may significantly influence the fitness of both plant and pollinator. To examine the effect of habitat structure on pollinator movement and fitness, we observed captive worker bumble bees collecting nectar from artificial flowers containing equal volumes of reward and arranged in two spatial configurations: a hexagonal array with constant distances between flowers (“constant”), and an “exploded hexagonal” array, with variable distances between flowers (“variable”). The mean nearest-neighbour distance was the same in both arrays, as was the general hexagonal appearance. The experiment therefore compares how resource dispersion, independent of nearest-neighbour distance, influences bee behaviour. Bees in the variable array showed decreased directionality, higher revisitation frequencies, and greater inter-flower flight distances than shown in the constant array. As a consequence, bees in the variable array had a 19% lower gross rate of nectar collection. Our results suggest that wild-foraging bees should prefer regularly spaced flowers (when all else, including mean nearest-neighbour distance, is equal), and that plants can decrease self-pollination by regular spacing between flowers, inflorescences, or individuals. Received: 16 January 1996 / Accepted: 30 June 1997     

Ecogeographical patterns in the incidence of brood parasitism in bees

The percentage of cleptoparasitic species in a fauna of Apoidea decreases from 25–30% at high latitudes to 10–15% in the subtropics. We argue that this pattern should not be explained by an allegedly stronger seasonality at higher latitudes (that would enable parasites to better synchronize with their hosts), but rather by the unpredictability of the climate, and concomitantly host phenology and population size, in (sub)arid climates.     

The visual ecology of Puerto Rican anoline lizards: habitat light and spectral sensitivity

The visual ecology of six closely related species of Puerto Rican anoline lizards was investigated and they were found to occupy four distinct habitat types in terms of light conditions: “full shade”, “partial shade”, “no shade”, and “forest canopy.”The habitats differed substantially in total radiance and irradiance as well as in the shape of the irradiance spectrum. The shape of the radiance spectrum was similar in all of the habitats. We used electroretinogram (ERG) flicker photometry to measure spectral sensitivity and found the curves for all six species to be similar. The spectral sensitivity peaked in the range 550–560 nm, which matched the peak in spectral radiance for all of the habitats. The shape of the spectral-sensitivity curve was similar to those of a number of other terrestrial vertebrates. We suggest that the convergence of the shape of the photopic ERG-determined spectral-sensitivity curve in many terrestrial vertebrates may, in part, be due to the fact that the background radiance of many terrestrial habitats is dominated by the reflectance spectrum of green vegetation which peaks at 550 nm. Accepted: 14 May 1997     

Colour-dependent target detection by bees

The distance over which an object is detected by bees depends on the subtended visual angle and on spectral cues. At large angular subtenses detection is mediated only by chromatic cues. Achromatic targets, however, are also detectable. We investigated how chromatic and achromatic cues interact in detecting large-size targets. Coloured targets were used, with varied chromatic contrast that either did or did not present L-receptor contrast. Better detection correlated with higher chromatic contrast. Adding L-receptor contrast did not affect detection. It did allow the detection of achromatic targets, but at a lower level than most coloured ones, which indicates that the input from the achromatic system is negligible due to low sensitivity.     

Flower morphology, phenology and visitor patterns in an alpine community on Mt Olympos, Greece

Alpine vegetation, restricted to the top of high mountains, is among the vegetation types most endangered by global warming, currently predicted to raise temperatures from 1.1 to 6.4 °C, by the end of the century. Nevertheless, background information allowing evaluation of impacts is rather scarce for some geographic zones. Our study of an alpine community on the Plateau of Muses (2600–2750 m a.s.l.) of Mt Olympos, the highest mountain of Greece, conducted in 1993–1994, can provide such background information for the Mediterranean region. We studied features relating to phenology of flowering, floral morphology, distribution and abundance, and flower visitors of plant species that exhibit a biotic pollination syndrome. We identified dominant patterns and we further (i) explored the relative contribution of the plant features and abiotic factors studied in explaining the activity patterns of flower visitors, (ii) examined if flower and visitor traits of the alpine community match each other according to the classical pollination syndromes, and (iii) investigated whether the responses of individual plant species to the yearly climatic variability result into phenological patterns that characterize the whole community. The common strategy of the alpine community was for early flowering and long flower life span; consistently early flowering species were twice as many as late flowering ones, whereas floral longevity (estimated for 36 species) averaged 5.2 days. Duration of flowering (estimated for 57 species) averaged 18.2 days. Climatic variability affected onset of flowering; all late flowering species delayed their flowering during the year characterized by a humid and cold summer. Duration of flowering and floral longevity did not change in a consistent way. Hymenoptera (Aculeates) were the dominant flower visitors. They accounted for 43.3% of the visits recorded, with bumblebees making a little less than half. Diptera followed making 37.5% of the visits (most made by syrphid flies). There was a mismatch between flower-morphology and flower visitor traits; the alpine community had predominantly non-specialized, pale-colour flowers, which are traits assumed to correspond to Diptera dominance and absence of social bees. Visitation was influenced by flower abundance and duration of flowering; proportionately more Diptera, and proportionately less Hymenoptera visited species with short flowering periods and few flowers present in the field. In a number of cases, the phenological and flower visitor patterns of the community of Mt Olympos deviated from those observed in other alpine environments suggesting a mediterranean influence even at high altitudes.     

Ground-nesting bees determine the location of their nest relative to a landmark by other than angular size cues

Bees and wasps acquire a visual representation of their nest's environment and use it to locate their nest when they return from foraging trips. This representation contains among other features cues to the distance of near-by landmarks. We worked with two species of ground-nesting bees, Lasioglossum malachurum (Hymenoptera: Halictidae), Dasypoda hirtipes (Hymenoptera: Melittidae) and asked which cues to landmark distance they use during homing. Bees learned to associate a single cylindrical landmark with their nest's location. We subsequently tested returning bees with landmarks of different sizes and thus introduced large discrepancies between the angular size of the landmark as seen from the nest during training and its distance from the nest. The bees' search behaviour and their choice of dummy nest entrances show that both species of ground-nesting bees consistently search for their nest at the learned distance from landmarks. The influence of the apparent size of landmarks on the bees' search and choice behaviour is comparatively weak. We suggest that the bees exploit cues derived from the apparent speed of the landmark's image at their retina for distance evaluation.     

Tetragonisca guard bees interpret expanding and contracting patterns as unintended displacement in space

Guard bees of the stingless bee Tetragonisca angustula (Apidae: Meliponinae) hover in stable positions in front of the nest to protect the flight corridor leading to the nest entrance against insect intruders. To unravel the visual control of station keeping, we exposed these hovering guards to expanding and contracting patterns at the nest front. The bees fly away from an expanding pattern and towards the centre of a contracting pattern along a line connecting their initial position and the centre of expansion regardless of where in the visual field they view the pattern. The response of bees to a spinning radial pattern is different: they fly parallel to the pattern, up and down or forward and backward depending on whether they initially hover to the side, above or below the centre of rotation. The bees respond to horizontal and to vertical expansion and contraction. They also adjust their distance relative to a rotating spiral which produces a realistic flow field and thus allowed us to test to what extent the bees minimize image motion speed. We find that guard bees indeed move in the appropriate direction to minimize the image motion speed they experience. A comparison of bees hovering at different distances from the nestfront at the onset of pattern motion and experiencing very different image velocities shows that the dynamics of the reaction is quite uniform. At the pattern velocities tested, we did not find evidence that guard bees use image motion to control their flight speed. The bees' response rather suggests that the underlying mechanism might be insensitive to the size of motion vectors. Accepted: 2 April 1997     

PCR diagnostic methods for Ascosphaera infections in bees

Fungi in the genus Ascosphaera are the causative agents of chalkbrood, a major disease affecting bee larval viability. Identification of individual Ascosphaera species based on morphological features has been difficult due to a lack of distinguishing characteristics. Most identifications are based on the size and shape of the ascomata, spore balls and conidia. Unfortunately, much overlap occurs in the size of these structures, and some Ascosphaera species will not produce sexual structures in vitro. We report a quick and reliable diagnostic method for identifying Ascosphaera infections in Megachile bees (leafcutting bees) using PCR markers that employ genus-specific primers for Ascosphaera, and species-specific primers for species known to be associated with Megachile spp. Using these methods, species identifications can be performed directly on bees, including asymptomatic individuals. Furthermore, the PCR markers can detect co-infections of multiple Ascosphaera species in a single host. We also identified a marker for Ascosphaera apis, the predominant cause of chalkbrood in Apis mellifera, the honey bee. Our diagnostic methods eliminate the need for culturing samples, and could be used to process a large number of field collected bee larvae.     

Visual search and the importance of time in complex decision making by bees

Psychophysicists studying decision making in animals have overwhelmingly focused on choice accuracy, not speed. Results from human visual search, however, show that there might be a tight link between the two. Here we review both visual-sensory and cognitive mechanisms that affect decision speed in flower visiting bees. We show that decision times are affected by contrast of targets and background, by similarity between targets and distractors, numbers of distractors present in a scene, illuminating light intensity, presence or absence of punishment, and complexity of tasks. We explore between-individual and within-individual speed-accuracy tradeoffs, and show that bees resort to highly dynamic strategies when solving visual search tasks. Where possible, we attempt to link the observed search behaviour to the temporal and spatial properties of neuronal circuits underlying visual object detection. We demonstrate that natural foraging speed may not only be limited by factors such as food item density, flight energetics and scramble competition, as often implied. Our results show that understanding the behavioural ecology of foraging can substantially gain from knowledge about mechanisms of visual information processing.     

A model of visual detection of angular speed for bees

A fly or bee's responses to widefield image motion depend on two basic parameters: temporal frequency and angular speed. Rotational optic flow is monitored using temporal frequency analysers, whereas translational optic flow seems to be monitored in terms of angular speed. Here we present a possible model of an angular speed detector which processes input signals through two parallel channels. The output of the detector is taken as the ratio of the two channels’ outputs. This operation amplifies angular speed sensitivity and depresses temporal frequency tuning. We analyse the behaviour of two versions of this model with different filtering properties in response to a variety of input signals. We then embody the detector in a simulated agent's visual system and explore its behaviour in experiments on speed control and odometry. The latter leads us to suggest a new algorithm for optic flow driven odometry.     

Flower color change accelerated by bee pollination in Tibouchina (Melastomataceae)

Floral color changes are common among Melastomataceae and have been interpreted as a warning mechanism for bees to avoid old flowers, albeit increasing long-distance flower display. Here the reproductive systems of Tibouchina pulchra and T. sellowiana were investigated by controlled pollinations. Their pollinators were identified, and experiments on floral color and fragrance changes were conduced to verify if those changes affect the floral visitation. Both Tibouchina species are self compatible. The flowers lasted three days or more, and the floral color changed from white in the 1st day to pink in the following days. Pollen deposition on stigma induced floral color change. The effectiveness of the pollination is dependent on bees’ size; only large bees were regarded as effective pollinators. In experimental tests, the bees in T. pulchra preferred the natural white flowers while the visitors of T. sellowiana were attracted by both natural and mimetic 1st-day flowers (2nd-day flowers with experimentally attached 1st-day flower petals). During the experiments on floral fragrance, the bees visited both natural and mimetic 1st-day flowers (2nd-day flowers with 1st-day flower scents). In both experiments, the bees avoided natural 2nd-day flowers, but seldom visited modified 2nd-day flowers. The attractiveness of T. pulchra and T. sellowiana flowers cannot be attributed exclusively to the color or the fragrance separately, both factors seemingly act together.     

Consequences of flower heliotropism for reproduction in an alpine buttercup (Ranunculus adoneus)

Summary The flowers of the alpine snow buttercup Ranunculus adoneus track the sun's movement from early morning until mid-afternoon. Individual blooms last up to a week: younger female stage flowers show greater solar tracking fidelity than older hermaphrodite or dehisced flowers. Flowers aligned parallel to the sun's rays reach mean internal temperatures several degrees Celsius above ambient air temperature. As a flower's angle of deviation from the sun increases beyond 45 degrees, internal flower temperature is significantly reduced. Fly pollinators are seen disproportionately often on flowers aligned with the sun; this is due, in part, to their greater residence time on tracking flowers. Fly visitation is important to fecundity. When flies were excluded from flowers, some selfed seeds were matured, but total seed production was much less than in either handoutcrossed or open-pollinated flowers. Flowers that were tethered at random angles to prevent solar tracking set fewer, smaller seeds than unmanipulated control flowers. Reductions in seed weight were statistically consistent at different times in the season, but tethering influenced seed number per flower most strongly in the early season. Effects of tethering on maternal reproduction could be due to either pollinator diserimination, post-pollination developmental processes, or both. In a second series of experiments using fluorescent dye particles as pollen analogs, tethered donor flowers dispersed dye to as many recipients as paired control flowers, suggesting that solar tracking fidelity may have little effect upon this component of male reproduction.     

Pelagic seabird flight patterns are consistent with a reliance on olfactory maps for oceanic navigation

Homing studies have provided tantalizing evidence that the remarkable ability of shearwaters (Procellariiformes) to pinpoint their breeding colony after crossing vast expanses of featureless open ocean can be attributed to their assembling cognitive maps of wind-borne odours but crucially, it has not been tested whether olfactory cues are actually used as a system for navigation. Obtaining statistically important samples of wild birds for use in experimental approaches is, however, impossible because of invasive sensory manipulation. Using an innovative non-invasive approach, we provide strong evidence that shearwaters rely on olfactory cues for oceanic navigation. We tested for compliance with olfactory-cued navigation in the flight patterns of 210 shearwaters of three species (Cory''s shearwaters, Calonectris borealis, North Atlantic Ocean, Scopoli''s shearwaters, C. diomedea Mediterranean Sea, and Cape Verde shearwaters, C. edwardsii, Central Atlantic Ocean) tagged with high-resolution GPS loggers during both incubation and chick rearing. We found that most (69%) birds displayed exponentially truncated scale-free (Lévy-flight like) displacements, which we show are consistent with olfactory-cued navigation in the presence of atmospheric turbulence. Our analysis provides the strongest evidence yet for cognitive odour map navigation in wild birds. Thus, we may reconcile two highly disputed questions in movement ecology, by mechanistically connecting Lévy displacements and olfactory navigation. Our approach can be applied to any species which can be tracked at sufficient spatial resolution, using a GPS logger.     

Perception of ultraviolet light by crab spiders and its role in selection of hunting sites

The perception of ultraviolet (UV) light by spiders has so far been only demonstrated in salticids. Crab spiders (Thomisidae) hunt mostly on flowers and need to find appropriate hunting sites. Previous studies have shown that some crab spiders that reflect UV light use UV contrast to enhance prey capture. The high UV contrast can be obtained either by modulation of body colouration or active selection of appropriate backgrounds for foraging. We show that crab spiders (Thomisus sp.) hunting on Spathiphyllum plants use chromatic contrast, especially UV contrast, to make themselves attractive to hymenopteran prey. Apart from that, they are able to achieve high UV contrast by active selection of non-UV reflecting surfaces when given a choice of UV-reflecting and non-UV reflecting surfaces in the absence of odour cues. Honeybees (Apis cerana) approached Spathiphyllum plants bearing crab spiders on which the spiders were high UV-contrast targets with greater frequency than those plants on which the UV contrast of the spiders was low. Thus, crab spiders can perceive UV and may use it to choose appropriate backgrounds to enhance prey capture, by exploiting the attraction of prey such as honeybees to UV.