Colour vision in pollinators : conclusions from two species beyond the Apis Mellifera model : a behavioural approach

[EN]In the present thesis, composed of four chapters, we studied different aspects of the visual system of two pollinator species: Bombus terrestris (bumblebee) and Macroglossum stellatarum (hummingbird hawkmoth). Some studies have already investigated the visual system, behaviour and cognition of both species at different levels. Yet, much information is missing. In the four chapters of this thesis we studied the behaviour of both species, with differences concerning the sensorial system and the applied methodology. In the first two chapters we investigated the behaviour of Bombus terrestris while performing tasks related with discrimination and detection of artificial flowers. In in the last two chapters we studied basic and fundamental aspects of the visual system of Macroglossum stellatarum: the spectral sensitivity and the colour discrimination capacity. Despite the fact that bumblebees (Bombus terrestris) and honeybees (Apis mellifera) are related species, comparative studies have shown differences in many aspects of their behaviour related with the use of visual information. Flowers are diverse and so is the foraging behaviour of pollinators. These can employ different strategies and modulate their behaviour to select, detect and discriminate the most rewarding flowers. Macroglossum stellatarum has been considered as an important pollinator of many plants in Europe; yet, basic information about its visual system remained unknown until the development of this thesisFloral visitors are often required to make comparative judgments between flowers, based on, mainly, visual differences between them. Models of colour vision attempt to predict the ease with which animals can discriminate between colours with different spectral properties. The predictions of models are based on the perceptual distance between two colours, where small perceptual distances indicate that discrimination will be difficult, while large perceptual distances mean that discrimination will be easy. There is, in principle, no obvious reason to use one model over the other, but due to models’ formulation and assumptions, predictions can diverge. In the first chapter, we focused on the predictions of different colour vision models and tested how well they adjust to the bumblebee behaviour during a flower discrimination task. We selected four pairs of artificial flowers differing in their perceptual distances (chromatic information) according to two models, while being similar according to a third one. Achromatic contrasts (brightness and green contrast) between flowers were also measured and incorporated into the analyses. Bumblebees were divided between pairs of artificial flowers and trained to discriminate between the two colours of the pair. One of the colours carried the reward (sucrose solution), while the other a punishment (quinine). We measured the time bumblebees took to find the rewarding flowers, as well as the accuracy (measured as the proportion of correct choices). The time that bees required to select a flower and the proportion of correct choices differed between pairs: decision times decreased as achromatic contrast increased, and the proportion of correct choices increased with achromatic contrast and perceptual distance. First, these results suggest that both chromatic and achromatic contrasts affected the discriminability of colour pairs and second, that model predictions are not always in agreement with the animal behaviour. Since flower colour affects the foraging choices of bees and foraging choices affect the reproductive success of plants, a better understanding of which model is more accurate is required to predict bee behaviour and the ecological implications of flower choice and colour.As well as colour, the size of the flowers constrains bumblebee behaviour during the foraging activity when flowers are presented in homogeneous backgrounds. When flowers are big, search time correlates with the colour contrast between flowers and background. When colour contrast or flowers are small, bees take longer to detect these flowers against a homogeneous green background and adjust their behaviour in several ways. Besides vision, olfaction is another important sensory modality that bees use during the foraging activity. Therefore, in the second chapter we studied the effect of colour, size, training and the presence or absence of a second cue (odour) on bumblebee behaviour while detecting and discriminating flowers presented against a complex background. Bumblebees looking for blue flowers were more accurate, flew faster, followed more direct paths between flowers (travelling less distances) and needed less time to find them, than bumblebees looking for red flowers. When exploiting red flowers they made more errors and required more time to find flowers, although performance increased with experience and with the presence of odour. The size of flowers alone did not directly affect the bumblebee behaviour; however, when combined with colour it had an effect. Bumblebees took longer to find big red flowers. These flowers, had the smallest chromatic contrast with the background, and although the achromatic contrast exceeded that of the blue flowers, the efficiency in detection is not comparable between channels (chromatic and achromatic). The perception of stimuli was mainly affected by the chromatic contrast between the flowers and the complex background, but the difficulty imposed by this combination could be compensated by the presence of a second cue (odour), or the amount of received training. Our results differ from those found in a similar experiment using homogeneous green background and we present possible reasons in the discussion of the chapter.In recent years many studies have been done on the visual system of some floral visitors (especially bees and to some extent butterflies and Syrphids), raising the understanding about mechanisms and functions involved in visual perception. Nevertheless, for most species many basic aspects remain unclear. Macroglossum stellatarum is a diurnal floral visitor – of the otherwise mainly crepuscular Sphingidae family – with demonstrated learning abilities. Macroglossum stellatarum can easily be raised in laboratory, which makes this moth an excellent candidate for studies in environments under controlled conditions. Previous experiments have suggested that M. stellatarum possesses true colour vision, with receptors sensitive in the ultraviolet (UV), blue and green areas of the spectrum. The moth has demonstrated innate preferences and broad colour discrimination capacity over some ranges of the hypothetical visible spectrum. However, the spectral sensitivity of the retinal photoreceptors remains unknown, as well as their peak absorbance. If we aim to understand an animal’s behaviour by carrying out colour vision experiments, we first have to comprehend how animals perceive their environment and distinguish different visual stimuli. In the third chapter we therefore characterised the spectral sensitivity of the hummingbird hawkmoth Macroglossum stellatarum and established the peak absorbance of its photoreceptors, using behavioural tests and electroretinogram (ERGs) analyses. By measuring ERGs we determined that M. stellatarum has ultraviolet (UV), blue and green receptors maximally sensitive at 349, 440 and 521 nm. To determine the behavioural spectral sensitivity (action spectrum), we trained moths to associate an illuminated disk with a food reward, and a dark disk with no reward. While the spectral positions of sensitivity maxima found in behavioural tests agree with model predictions based on the ERG data, the sensitivity to blue light was 30 times higher than expected. We relate this higher sensitivity with the moth’s innate preference for blue colour and present a general discussion about results within the chapter.Despite the innate preference for particular colours, M. stellatarum feeds from a variety of flower species and prefers colour over scent when given the opportunity to choose. Given the importance of the spectral properties of flowers for this moth, and knowing the spectral sensitivity of its photoreceptors, the moth’s capacity to discriminate between similar wavelengths is of interesting. Wavelength discrimination is defined as the ability of an organism to discriminate between perceptually close monochromatic stimuli. In the context of foraging, this capacity can result advantageous, especially for insects demanding high energy intakes such as M. stellatarum due to its hovering behaviour while feeding. In the fourth chapter we tested the capacity of M. stellatarum to discriminate between similar wavelengths. We trained moths to associate specific monochromatic lights with a reward. Wavelength selection was based on the moth’s photoreceptors peaks and overlapping areas, determined in the chapter three. Once a monochromatic light was associated with a reward, moths were tested in a dual choice experiment, in which the rewarding wavelength was presented together with an unrewarding wavelength, initially 20 nm longer or shorter than the trained wavelength. Besides two expected pronounced minima (where photoreceptors overlap), M. stelattarum presented a third minimum between the peak of blue receptor and the inflexion curve of the green receptor. We used a colour vision model to predict the discrimination capabilities of the moth and to compare it with the behavioural data. Moth´s behavioural minima were similar to those predicted by the colour vision model, but the model did not predict a third less pronounced minimum of discrimination found during the behavioural tests. Minima of wavelength discrimination in M. stellatarum were smaller than those found for the honeybees (Apis mellifera), approaching values found for a tetrachromatic butterfly (Papilio xuthus). In that chapter, we discuss the differences in the discrimination capacity of those floral visitors tested so far, while integrating new perspectives for this type of experiments: the use of achromatic information.The sensory capacity of floral visitors has helped to shape and increase the variability of the flower signals. However, for many floral visitors, the sensorial system capacity and also its limits are not well understood, and for many others, it remains largely unexplored. It results difficult to interpret the behaviour of a floral visitor within a plant community if we do not properly understand the mechanisms modulating this relationship. The results of the present thesis raise the understanding of the visual systems of two important pollinator species in Europe, the bumblebee (Bombus terrestris) and the diurnal hummingbird hawkmoth (Macroglossum stellatarum). For years, experiments using the honeybee helped to build an understanding of insect vision, but similar information for the vast majority of floral visitors remains unknown. The present thesis shows that the visual capacity of each species is unique. The evolutionary processes that have shaped the sensory ability of floral visitors are unclear, but the importance of behavioural studies in the plant-pollinator context is essential, not only because of the economic value of the pollination service, but also to properly understand the mechanisms linking the plant-pollinator relationship.Esta tesis ha sido posible gracias a la concesión de una beca del Programa “Junta para la Ampliación de Estudios” JAE-Predoc (JAEPre033) del Consejo Superior de Investigaciones Científicas de España (CSIC), cofinanciada por el Fondo Social Europeo (FSE), y a los fondos del Ministerio de Ciencia e Innovación/FEDER (Proyecto CGL2010-16795). La tesis ha sido desarrollada en el Departamento de Ecología Funcional y Evolutiva de la Estación Experimental de Zonas Áridas (EEZA - Almería), perteneciente al Consejo Superior de Investigaciones Científicas (CSIC).Peer reviewe

Colour vision in pollinators: Conclusions from two species beyond the Apis mellifera model

In the present thesis, composed of four chapters, we studied different aspects of the visual system of two pollinator species: Bombus terrestris (bumblebee) and Macroglossum stellatarum (hummingbird hawkmoth). Some studies have already investigated the visual system, behaviour and cognition of both species at different levels. Yet, much information is missing. In the four chapters of this thesis we studied the behaviour of both species, with differences concerning the sensorial system and the applied methodology. In the first two chapters we investigated the behaviour of Bombus terrestris while performing tasks related with discrimination and detection of artificial flowers. In in the last two chapters we studied basic and fundamental aspects of the visual system of Macroglossum stellatarum: the spectral sensitivity and the colour discrimination capacity. Despite the fact that bumblebees (Bombus terrestris) and honeybees (Apis mellifera) are related species, comparative studies have shown differences in many aspects of their behaviour related with the use of visual information. Flowers are diverse and so is the foraging behaviour of pollinators. These can employ different strategies and modulate their behaviour to select, detect and discriminate the most rewarding flowers. Macroglossum stellatarum has been considered as an important pollinator of many plants in Europe; yet, basic information about its visual system remained unknown until the development of this thesis. The results of the present thesis raise the understanding of the visual systems of two important pollinator species in Europe, the bumblebee (Bombus terrestris) and the diurnal hummingbird hawkmoth (Macroglossum stellatarum). For years, experiments using the honeybee helped to build an understanding of insect vision, but similar information for the vast majority of floral visitors remains unknown. The present thesis shows that the visual capacity of each species is unique. The evolutionary processes that have shaped the sensory ability of floral visitors are unclear, but the importance of behavioural studies in the plant-pollinator context is essential, not only because of the economic value of the pollination service, but also to properly understand the mechanisms linking the plant-pollinator relationship.Tesis Univ. Granada. Programa Oficial de Doctorado en: Biología Fundamental y de SistemasPrograma “Junta para la Ampliación de Estudios” JAE-Predoc (JAEPre033) del Consejo Superior de Investigaciones Científicas de España (CSIC), cofinanciada por el Fondo Social Europeo (FSE), y a los fondos del Ministerio de Ciencia e Innovación/FEDER (Proyecto CGL2010-16795)

Colour vision in pollinators: Conclusions from two species beyond the Apis mellifera model

In the present thesis, composed of four chapters, we studied different aspects of the visual system of two pollinator species: Bombus terrestris (bumblebee) and Macroglossum stellatarum (hummingbird hawkmoth). Some studies have already investigated the visual system, behaviour and cognition of both species at different levels. Yet, much information is missing. In the four chapters of this thesis we studied the behaviour of both species, with differences concerning the sensorial system and the applied methodology. In the first two chapters we investigated the behaviour of Bombus terrestris while performing tasks related with discrimination and detection of artificial flowers. In in the last two chapters we studied basic and fundamental aspects of the visual system of Macroglossum stellatarum: the spectral sensitivity and the colour discrimination capacity. Despite the fact that bumblebees (Bombus terrestris) and honeybees (Apis mellifera) are related species, comparative studies have shown differences in many aspects of their behaviour related with the use of visual information. Flowers are diverse and so is the foraging behaviour of pollinators. These can employ different strategies and modulate their behaviour to select, detect and discriminate the most rewarding flowers. Macroglossum stellatarum has been considered as an important pollinator of many plants in Europe; yet, basic information about its visual system remained unknown until the development of this thesis. The results of the present thesis raise the understanding of the visual systems of two important pollinator species in Europe, the bumblebee (Bombus terrestris) and the diurnal hummingbird hawkmoth (Macroglossum stellatarum). For years, experiments using the honeybee helped to build an understanding of insect vision, but similar information for the vast majority of floral visitors remains unknown. The present thesis shows that the visual capacity of each species is unique. The evolutionary processes that have shaped the sensory ability of floral visitors are unclear, but the importance of behavioural studies in the plant-pollinator context is essential, not only because of the economic value of the pollination service, but also to properly understand the mechanisms linking the plant-pollinator relationship.Tesis Univ. Granada. Programa Oficial de Doctorado en: Biología Fundamental y de SistemasPrograma “Junta para la Ampliación de Estudios” JAE-Predoc (JAEPre033) del Consejo Superior de Investigaciones Científicas de España (CSIC), cofinanciada por el Fondo Social Europeo (FSE), y a los fondos del Ministerio de Ciencia e Innovación/FEDER (Proyecto CGL2010-16795)