@phdthesis{Grob2022,
  author    = {Grob, Robin},
  title     = {The Function of Learning Walks of \({Cataglyphis Ants}\): Behavioral and Neuronal Analyses},
  doi       = {10.25972/OPUS-29017},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-290173},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Humans and animals alike use the sun, the moon, and the stars to guide their ways. However, the position of celestial cues changes depending on daytime, season, and place on earth. To use these celestial cues for reliable navigation, the rotation of the sky has to be compensated. While humans invented complicated mechanisms like the Antikythera mechanism to keep track of celestial movements, animals can only rely on their brains. The desert ant Cataglyphis is a prime example of an animal using celestial cues for navigation. Using the sun and the related skylight polarization pattern as a compass, and a step integrator for distance measurements, it can determine a vector always pointing homewards. This mechanism is called path integration. Since the sun's position and, therefore, also the polarization pattern changes throughout the day, Cataglyphis have to correct this movement. If they did not compensate for time, the ants' compass would direct them in different directions in the morning and the evening. Thus, the ants have to learn the solar ephemeris before their far-reaching foraging trips. To do so, Cataglyphis ants perform a well-structured learning-walk behavior during the transition phase from indoor worker to outdoor forager. While walking in small loops around the nest entrance, the ants repeatedly stop their forward movements to perform turns. These can be small walked circles (voltes) or tight turns about the ants' body axes (pirouettes). During pirouettes, the ants gaze back to their nest entrance during stopping phases. These look backs provide a behavioral read-out for the state of the path integrator. The ants "tell" the observer where they think their nest is, by looking back to it. Pirouettes are only performed by Cataglyphis ants inhabiting an environment with a prominent visual panorama. This indicates, that pirouettes are performed to learn the visual panorama. Voltes, on the other hand, might be used for calibrating the celestial compass of the ants. In my doctoral thesis, I employed a wide range of state-of-the-art techniques from different disciplines in biology to gain a deeper understanding of how navigational information is acquired, memorized, used, and calibrated during the transition phase from interior worker to outdoor forager. I could show, that celestial orientation cues that provide the main compass during foraging, do not guide the ants during the look-backbehavior of initial learning walks. Instead Cataglyphis nodus relies on the earth's magnetic field as a compass during this early learning phase. While not guiding the ants during their first walks outside of the nest, excluding the ants from perceiving the natural polarization pattern of the skylight has significant consequences on learning-related plasticity in the ants' brain. Only if the ants are able to perform their learning-walk behavior under a skylight polarization pattern that changes throughout the day, plastic neuronal changes in high-order integration centers are induced. Especially the mushroom bogy collar, a center for learning and memory, and the central complex, a center for orientation and motor control, showed an increase in volume after learning walks. This underlines the importance of learning walks for calibrating the celestial compass. The magnetic compass might provide the necessary stable reference system for the ants to calibrate their celestial compass and learn the position of landmark information. In the ant brain, visual information from the polarization-sensitive ocelli converge in tight apposition with neuronal afferents of the mechanosensitive Johnston's organ in the ant's antennae. This makes the ants' antennae an interesting candidate for studying the sensory bases of compass calibration in Cataglyphis ants. The brain of the desert navigators is well adapted to successfully accomplish their navigational needs. Females (gynes and workers) have voluminous mushroom bodies, and the synaptic complexity to store large amount of view-based navigational information, which they acquire during initial learning walks. The male Cataglyphis brain is better suited for innate behaviors that support finding a mate. The results of my thesis show that the well adapted brain of C. nodus ants undergoes massive structural changes during leaning walks, dependent on a changing celestial polarization pattern. This underlies the essential role of learning walks in the calibration of orientation systems in desert ants.},
  subject      = {Cataglyphis},
  language  = {en}
}
@phdthesis{Frank2019,
  author    = {Frank, Erik Thomas},
  title     = {Behavioral adaptations in the foraging behaviour of \(Megaponera\) \(analis\)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-156544},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {An efficient foraging strategy is one of the most important traits for the fitness of animals. The theory of optimal foraging tries to predict foraging behaviour through the overarching question: how animals should forage so as to minimize costs while maximizing profits? Social insects, having occupied nearly every natural niche through widely different strategies, offer themselves as an ideal group to study how well optimal foraging theory can explain their behaviour and success. Specialization often leads to unique adaptations in morphology and behaviour. I therefore decided to investigate the behaviour of Megaponera analis. This ponerine ant species is specialized on hunting only termites of the subfamily Macrotermitinae at their foraging sites. Their foraging behaviour is regulated by a handful of individual scouts (10-20) that search for termite foraging sites before returning to the nest to recruit a large number of nestmates (200-500 ants). These ants then follow the scout in a column formation to the termites and after the hunt return together to the nest, these raids occur two to five times per day. Predators of highly defensive prey likely develop cost reducing adaptations. The evolutionary arms race between termites and ants led to various defensive mechanisms in termites, e.g. a caste specialized in fighting predators. As M. analis incurs high injury/mortality risks when preying on termites, some risk mitigating adaptations have evolved. I show that a unique rescue behaviour in M. analis, consisting of injured nestmates being carried back to the nest, reduces combat mortality. These injured ants "call for help" with pheromones present in their mandibular gland reservoirs. A model accounting for this rescue behaviour identifies the drivers favouring its evolution and estimates that rescuing allows for maintaining a 29\% larger colony size. Heavily injured ants that lost too many legs during the fight on the other hand are not helped. Interestingly, this was regulated not by the helper but by the uncooperativeness of the injured ant. I further observed treatment of the injury by nestmates inside the nest through intense allogrooming directly at the wound. Lack of treatment increased mortality from 10\% to 80\% within 24 hours, with the cause of death most likely being infections. Collective decision-making is one of the main mechanisms in social insects through which foraging is regulated. However, individual decision-making can also play an important role, depending on the type of foraging behaviour. In M. analis only a handful of individuals (the scouts) hold all the valuable information about foraging sites. I therefore looked at predictions made by optimal foraging theory to better understand the interplay between collective and individual decision-making in this obligate group-raiding predator. I found a clear positive relation between raid size and termite abundance at the foraging site. Furthermore, selectivity of the food source increased with distance. The confirmation of optimal foraging theory suggests that individual scouts must be the main driver behind raid size, choice and raiding behaviour. Therefore most central place foraging behaviours in M. analis were not achieved by collective decisions but rather by individual decisions of scout ants. Thus, 1\% of the colony (10-20 scouts) decided the fate and foraging efficiency of the remaining 99\%. Division of labour is one of the main reasons for the success of social insects. Worker polymorphism, age polyethism and work division in more primitive ants, like the ponerines, remain mostly unexplored though. Since M. analis specializes on a defensive prey, adaptations to reduce their foraging costs can be expected. I found that the work division, task allocation and column-formation during the hunt were much more sophisticated than was previously thought. The column-formation was remarkably stable, with the same ants resuming similar positions in subsequent raids and front ants even returning to their positions if displaced in the same raid. Most of the raid tasks were not executed by predetermined members of the raid but were filled out as need arose during the hunt, with a clear preference for larger ants to conduct most tasks. I show that specialization towards a highly defensive prey can lead to very unique adaptations in the foraging behaviour of a species. I explored experimentally the adaptive value of rescue behaviour focused on injured nestmates in social insects. This was not only limited to selective rescuing of lightly injured individuals by carrying them back (thus reducing predation risk) but moreover includes a differentiated treatment inside the nest. These observations will help to improve our understanding of the evolution of rescue behaviour in animals. I further show that most optimal foraging predictions are fulfilled and regulated by a handful of individuals in M. analis. Lastly, I propose that the continuous allometric size polymorphism in M. analis allows for greater flexibility in task allocation, necessary due to the unpredictability of task requirements in an irregular system such as hunting termites in groups. All of my observations help to further understand how a group-hunting predator should forage so as to minimize costs while maximizing profits.},
  subject      = {Stechameisen},
  language  = {en}
}
@phdthesis{Saverschek2010,
  author    = {Saverschek, Nicole},
  title     = {The influence of the symbiotic fungus on foraging decisions in leaf-cutting ants - Individual behavior and collective patterns},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-52087},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {Foraging behavior is a particularly fascinating topic within the studies of social insects. Decisions made by individuals have effects not only on the individual level, but on the colony level as well. Social information available through foraging in a group modulates individual preferences and shapes the foraging pattern of a colony. Identifying parameters influencing foraging behavior in leaf-cutting ants is especially intriguing because they do not harvest for themselves, but for their symbiotic fungus which in turn influences their plant preferences after the incorporation of the substrate. To learn about the substrates' unsuitability for the fungus, ants need to be able to identify the incorporated substrate and associate it with detrimental effects on the fungus. Odor is an important plant characteristic known to be used as recognition key outside the nest in the context of foraging. Chapter 1 shows that foragers are able to recall information about the unsuitability of a substrate through odor alone and consequently reject the substrate, which leads to the conclusion that inside the nest, odor might be enough to indentify incorporated substrate. Identification of plant species is a key factor in the foraging success of leaf-cutting ants as they harvest a multitude of different plant species in a diverse environment and host plant availability and suitability changes throughout the year. Fixed plant preferences of individuals through innate tendencies are therefore only one factor influencing foraging decisions. On the individual as well as the colony level, foraging patterns are flexible and a result of an intricate interplay between the different members involved in the harvesting process: foragers, gardeners and the symbiotic fungus. In chapter 2 I identified several conditions necessary for na{\"i}ve foragers to learn about the unsuitability of substrate inside the nest. In order to exchange of information about the unsuitability of a substrate, the plant in question must be present in the fungus garden. Foragers can learn without own foraging experience and even without experiencing the effects of the substrate on the fungus, solely through the presence of experienced gardeners. The presence of experienced foragers alone on the other hand is not enough to lower the acceptance of substrate by na{\"i}ve foragers in the presence of na{\"i}ve gardeners, even if experienced foragers make up the majority of the workforce inside the nest. Experienced foragers are also able to reverse their previous negative experience and start accepting the substrate again. The individual behavior of foragers and gardeners with different experiential backgrounds in the presence of suitable or unsuitable substrate inside the fungus chamber was investigated in chapter 3 to shed some light on possible mechanisms involved in the flow of information about substrate suitability from the fungus to the ants. Gardeners as well as foragers are involved in the leaf processing and treatment of the applied leaf patches on the fungus. If the plant material is unsuitable, significantly more ants treat the plant patches, but foragers are less active overall. Contacts between workers initiated by either gardeners or foragers occur significantly more frequent and last longer if the substrate is unsuitable. Even though experienced gardeners increase na{\"i}ve foragers' contact rates and duration with other workers in the presence of suitable plant patches, na{\"i}ve foragers show no differences in the handling of the plant patches. This suggests that foragers gain information about plant suitability not only indirectly through the gardening workers, but might also be able to directly evaluate the effects of the substrate on the fungus themselves. Outside the nest, foragers influence each other the trail (chapter 4). Foraging in a group and the presence of social information is a decisive factor in the substrate choice of the individual and leads to a distinct and consentaneous colony response when encountering unfamiliar or unsuitable substrates. As leaf-cutting ants harvest different plant species simultaneously on several trails, foragers gain individual experiences concerning potential host plants. Preferences might vary among individuals of the same colony to the degree that foragers on the same trail perceive a certain substrate as either suitable or unsuitable. If the majority of foragers on the trail perceives one of the currently harvested substrates as unsuitable, na{\"i}ve foragers lower their acceptance within 4 hours. In the absence of a cue in the fungus, na{\"i}ve foragers harvesting by themselves still eventually (within 6 hours) reject the substrate as they encounter experienced gardeners during visits to the nest within foraging bouts. As foraging trails can be up to 100 m long and foragers spend a considerable amount of time away from the nest, learning indirectly from experienced foragers on the trail accelerates the distribution of information about substrate suitability. The level of rejection of a formerly unsuitable substrate after eight hours of foraging by na{\"i}ve foragers correlates with the average percentage of unladen experienced foragers active on the trail. This suggests that unladen experienced foragers might actively contact laden na{\"i}ve workers transmitting information about the unsuitability of the load they carry. Results from experiments were I observed individual laden foragers on their way back to the nest backed up this assumption as individuals were antennated and received bites into the leaf disk they carried. Individuals were contacted significantly more often by nestmates that perceived the carried leaf disk as unsuitable due to previous experience than by nestmates without this experience (chapter 6). Leaf-cutting ants constantly evaluate, learn and re-evaluate the suitability of harvested substrate and adjust their foraging activity accordingly. The importance of the different sources of information within the colony and their effect on the foraging pattern of the colony depend on the presence or absence of each of them as e.g. experienced foragers have a bigger influence on the plant preferences of na{\"i}ve foragers in the absence of a cue in the fungus garden.},
  subject      = {Blattschneiderameisen},
  language  = {en}
}
@phdthesis{Berghoff2002,
  author    = {Berghoff, Stefanie M.},
  title     = {Sociobiology of the hypogaeic army ant Dorylus (Dichthadia) laevigatus Fr. Smith},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-5005},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2002},
  abstract  = {Originally renowned for their spectacular epigaeic raids, army ants have captured scientific attention for almost two centuries. They now belong to one of the best studied group of ants. However, most of our knowledge about army ants was derived from the study of the minority of specialized, epigaeicly active species. These species evolved probably rather recently from hypogaeic ancestors. The majority of army ant species still leads a hypogaeic life and is almost completely unknown in its entire sociobiology. It thus remained speculative, whether the assumed 'general' characteristics of army ants represent an adaptation to epigaeic activity or apply also to the majority of hypogaeic species. Based on the recent observation that the hypogaeic Asian army ant Dorylus (Dichthadia) laevigatus recruits predictably to palm oil baits, I developed and tested an oil-baiting method for the study of hypogaeic (army)ants. Prior to my study, nothing was known about the sociobiology of the assumed rare D. laevigatus. Throughout my work, I showed D. laevigatus to be very common and abundant in a wide range of habitats in West-Malaysia and on Borneo. Investigating its foraging behavior, I revealed D. laevigatus to differ from epigaeicly active species in several ways. Never demonstrated for any of the epigaeic species, D. laevigatus established stable trunk trail systems. Such a trail system contradicted the perception of army ant foraging, which was believed to be characterized by raids with constantly alternating trail directions. The trunk trail system further enabled a near omnipresence of D. laevigatus within its foraging area, which was also believed to be atypical for an army ant. Raids differed in structure and composition of participating workers from those of epigaeic species. Also, bulky food sources could be exploited over long periods of time. The foraging system of D. laevigatus resembled in several ways that of e.g. leaf-cutter and harvester ants. Likewise contrary to the assumptions, D. laevigatus had a wide food spectrum and showed only little effect on local arthropod communities, even falling itself prey to other ants. Strong aggressive behavior was observed only towards ant species with similar lifestyles, enabling me to provide the first detailed documentation of interspecific fights between two sympatric Dorylus species. Similar to foraging habits or ecological impact, nothing was known about colony size and composition, nesting habits, or worker polymorphism for D. laevigatus or any other hypogaeic Dorylus species prior to my work. By observing and eventually excavating a colony, I showed D. laevigatus to have a much smaller colony size and to lack the large sized workers of epigaeic Dorylus species. Similar to epigaeic Dorylinae, I showed D. laevigatus to have a non-phasic brood production, to emigrate rarely, and to alter its nest form along with habitat conditions. Detailed morphological and geographical descriptions give an impression of the Asian Dorylus species and are expected to aid other researchers in the difficult species identification. The genetic analysis of a male collected at a light trap demonstrated its relation to D. laevigatus. Confirming the male and queen associations, D. laevigatus is now one of five Dorylus species (out of a total of 61), for which all castes are known. In cooperation with D. Kistner, I provide a morphological and taxonomical description of nine Coleopteran beetles associated with D. laevigatus. Behavioral observations indicated the degree of their integration into the colony. The taxonomic position of the beetles further indicated that D. laevigatus emigrated from Africa to Asia, and was accompanied by the majority of associated beetles. The diversity of D. laevigatus guests, which included a number of unidentified mites, was rather low compared to that of epigaeic species. Overall, I demonstrated the developed baiting containers to effectively enable the study of hypogaeic ants. I showed several other hypogaeic ant species to be undersampled by other methods. Furthermore, the method enabled me to documented a second hypogaeic Dorylus species on Borneo. A detailed description of this species' morphology, ecology, and interactions with D. laevigatus is provided. My study indicated D. laevigatus to be an ecologically important species, able to influence soil structure and organisms of tropical regions in many ways. Relating the observed traits of D. laevigatus to epigaeicly active species, I conclude that our assumption of 'general' army ant behavior is erroneous in several aspects and needs to be changed. The oil-baiting method finally provides a tool enabling the location and study of hypogaeic (army)ant species. This opens a broad field for future studies on this cryptic but nonetheless important group of ants.},
  subject      = {Borneo},
  language  = {en}
}
@phdthesis{Thom2002,
  author    = {Thom, Corinna},
  title     = {Dynamics and Communication Structures of Nectar Foraging in Honey Bees (Apis mellifera)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-3601},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2002},
  abstract  = {In this thesis, I examined honey bee nectar foraging with emphasis on the communication system. To document how a honey bee colony adjusts its daily nectar foraging effort, I observed a random sample of individually marked workers during the entire day, and then estimated the number and activity of all nectar foragers in the colony. The total number of active nectar foragers in a colony changed frequently between days. Foraging activity did not usually change between days. A honey bee colony adjusts its daily foraging effort by changing the number of its nectar foragers rather than their activity. I tested whether volatiles produced by a foraging colony activated nectar foragers of a non-foraging colony by connecting with a glass tube two colonies. Each colony had access to a different green house. In 50\% of all experiments, volatile substances from the foraging colony stimulated nectar foragers of the non-foraging colony to fly to an empty feeder. The results of this study show that honey bees can produce a chemical signal or cue that activates nectar foragers. However, more experiments are needed to establish the significance of the activating volatiles for the foraging communication system. The brief piping signal of nectar foragers inhibits forager recruitment by stopping waggle dances (Nieh 1993, Kirchner 1993). However, I observed that many piping signals (approximately 43\%) were produced off the dance floor, a restricted area in the hive where most waggle dances are performed. If the inhibition of waggle dances would be the only function of the brief piping signal, tremble dancers should produce piping signals mainly on the dance floor, where the probability to encounter waggle dancers is highest. To therefore investigate the piping signal in more detail, I experimentally established the foraging context of the brief piping signal, characterized its acoustic properties, and documented for the first time the unique behavior of piping nectar foragers by observing foragers throughout their entire stay in the hive. Piping nectar foragers usually began to tremble dance immediately upon their return into the hive, spent more time in the hive, more time dancing, had longer unloading latencies, and were the only foragers that sometimes unloaded their nectar directly into cells instead of giving it to a nectar receiver bee. Most of the brief piping signals (approximately 99\%) were produced by tremble dancers, yet not all tremble dancers (approximately 48\%) piped. This suggests that piping and tremble dancing have related, but not identical functions in the foraging system. Thus, the brief piping signals may not only inhibit forager recruitment, but have an additional function both on and off the dance floor. In particular, the piping signal might function 1. to stop the recruitment of additional nectar foragers, and 2. as a modulatory signal to alter the response threshold of signal receivers to the tremble dance. The observation that piping tremble dancers often did not experience long unloading delays before they started to dance gave rise to a question. A forager's unloading delay provides reliable information about the relative work capacities of nectar foragers and nectar receivers, because each returning forager unloads her nectar to a nectar receiver before she takes off for the next foraging trip. Queuing delays for either foragers or receivers lower foraging efficiency and can be eliminated by recruiting workers to the group in shortage. Short unloading delays indicate to the nectar forager a shortage of foragers and stimulate waggle dancing which recruits nectar foragers. Long unloading delays indicate a shortage of nectar receivers and stimulate tremble dancing which recruits nectar receivers (Seeley 1992, Seeley et al. 1996). Because the short unloading delays of piping tremble dancers indicated that tremble dancing can be elicited by other factors than long unloading delays, I tested whether a hive-external stimulus, the density of foragers at the food source, stimulated tremble dancing directly. The experiments show that tremble dancing can be caused directly by a high density of foragers at the food source and suggest that tremble dancing can be elicited by a decrease of foraging efficiency either inside (e.g. shortage of receiver bees) or outside (e.g. difficulty of loading nectar) the hive. Tremble dancing as a reaction to hive-external stimuli seems to occur under natural conditions and can thus be expected to have some adaptive significance. The results imply that if the hive-external factors that elicit tremble dancing do not indicate a shortage of nectar receiver bees in the hive, the function of the tremble dance may not be restricted to the recruitment of additional nectar receivers, but might be the inhibition or re-organization of nectar foraging.},
  subject      = {Bienen <Familie>},
  language  = {en}
}
@phdthesis{Dornhaus2002,
  author    = {Dornhaus, Anna},
  title     = {The role of communication in the foraging process of social bees},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-3468},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2002},
  abstract  = {In the various groups of social bees, different systems of communication about food sources occur. These communication systems are different solutions to a common problem of social insects: efficiently allocating the necessary number of workers first to the task of foraging and second to the most profitable food sources. The solution chosen by each species depends on the particular ecological circumstances as well as the evolutionary history of that species. For example, the outstanding difference between the bumble bee and the honey bee system is that honey bees can communicate the location of profitable food sources to nestmates, which bumble bees cannot. To identify possible selection pressures that could explain this difference, I have quantified the benefits of communicating location in honey bees. I show that these strongly depend on the habitat, and that communicating location might not benefit bees in temperate habitats. This could be due to the differing spatial distributions of resources in different habitats, in particular between temperate and tropical regions. These distributions may be the reason why the mostly temperate-living bumble bees have never evolved a communication system that allows them to transfer information on location of food sources, whereas most tropical social bees (all honey bees and many stingless bees) are able to recruit nestmates to specific points in their foraging range. Nevertheless, I show that in bumble bees the allocation of workers to foraging is also regulated by communication. Successful foragers distribute in the nest a pheromone which alerts other bees to the presence of food. This pheromone stems from a tergite gland, the function of which had not been identified previously. Usage of a pheromone in the nest to alert other individuals to forage has not been described in other social insects, and might constitute a new mode of communicating about food sources. The signal might be modulated depending on the quality of the food source. Bees in the nest sample the nectar that has been brought into the nest. Their decision whether to go out and forage depends not only on the pheromone signal, but also on the quality of the nectar they have sampled. In this way, foraging activity of a bumble bee colony is adjusted to foraging conditions, which means most bees are allocated to foraging only if high-quality food sources are available. In addition, foraging activity is adjusted to the amount of food already stored. In a colony with full honeypots, no new bees are allocated to foraging. These results help us understand how the allocation of workers to the task of food collection is regulated according to external and internal nest conditions in bumble bees.},
  subject      = {Hummel},
  language  = {en}
}
@phdthesis{Spaethe2001,
  author    = {Spaethe, Johannes},
  title     = {Sensory Ecology of Foraging in Bumblebees},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-1179692},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2001},
  abstract  = {Pollinating insects exhibit a complex behavior while foraging for nectar and pollen. Many studies have focused on ultimate mechanisms of this behavior, however, the sensory-perceptual processes that constrain such behavior have rarely been considered. In the present study I used bumblebees (Bombus terrestris), an important pollinating insect, to investigate possible sensory constraints on foraging behavior. Additionally, I survey inter-individual variation in the sensory capabilities and behavior of bumblebees caused by the pronounced size polymorphism among members of a single colony. In the first chapter I have focused on the sensory-perceptual processes that constrain the search for flowers. I measured search time for artificial flowers of various sizes and colors, a key variable defining the value of a prey type in optimal foraging theory. When flowers were large, search times correlate well with the color contrast of the targets with their green foliage-type background, as predicted by a model of color opponent coding using inputs from the bee's UV, blue, and green receptors. Targets which made poor color contrast with their backdrop, such as white, UV-reflecting ones, or red flowers, take longest to detect, even though brightness contrast with the background is pronounced. When searching for small targets, bumblebees change their strategy in several ways. They fly significantly slower and closer to the ground, so increasing the minimum detectable area subtended by an object on the ground. In addition they use a different neuronal channel for flower detection: instead of color contrast, they now employ only the green receptor signal for detection. I related these findings to temporal and spatial limitations of different neuronal channels involved in stimulus detection and recognition. Bumblebees do not only possess species-specific sensory capacities but they also exhibit inter-individual differences due to size. Therefore, in the next two chapters I have examined size-related effects on the visual and olfactory system of Bombus terrestris. Chapter two deals with the effect of scaling on eye architecture and spatial resolving power of workers. Foraging efficiency in bees is strongly affected by proficiency of detecting flowers. Both floral display size and bee spatial vision limit flower detection. In chapter one I have shown that search times for flowers strongly increases with decreasing floral display size. The second factor, bee spatial vision, is mainly limited by two properties of compound eyes: (a) the interommatidial angle {\c{C}}{\aa} and (b) the ommatidial acceptance angle {\c{C}}{\´a}. When a pollinator strives to increase the resolving power of its eyes, it is forced to increase both features simultaneously. Bumblebees show a large variation in body size. I found that larger workers with larger eyes possess more ommatidia and larger facet diameters. Large workers with twice the size of small workers (thorax width) have about 50 per cent more ommatidia, and a 1.5 fold enlarged facet diameter. In a behavioral test, large and small workers were trained to detect the presence of a colored stimulus in a Y-maze apparatus. The stimulus was associated with a sucrose reward and was presented in one arm, the other arm contained neither stimulus nor reward. The minimum visual angle a bee is able to detect was estimated by testing the bee at different stimuli sizes subtending angles between 30° and 3° on the bee's eye. Minimum visual detection angles range from 3.4° to 7.0° among tested workers. Larger bumblebees are able to detect objects subtending smaller visual angles, i.e. they are able to detect smaller objects than their small conspecifics. Thus morphological and behavioral findings indicate an improved visual system in larger bees. Beside vision, olfaction is the most important sensory modality while foraging in bees. Bumblebees utilize species-specific odors for detecting and identifying nectar and pollen rich flowers. In chapter three I have investigated the olfactory system of Bombus terrestris and the effect of scaling on antennal olfactory sensilla and the first olfactory neuropil in the bumblebee brain, the antennal lobes. I found that the pronounced size polymorphism exhibited by bumblebees also effects their olfactory system. Sensilla number (I measured the most common olfactory sensilla type, s. placodea), sensilla density, volume of antennal lobe neuropil and volume of single identified glomeruli correlate significantly with worker's size. The enlarged volume of the first olfactory neuropil in large individuals is caused by an increase in glomeruli volume and coarse neuropil volume. Additionally, beside an overall increase of brain volume with scaling I found that the olfactory neuropil increases disproportionately compared to a higher order neuropil, the central body. The data predict a higher odor sensitivity in larger bumblebee workers. In the last chapter I have addressed the question if scaling alters foraging behavior and rate in freely foraging bumblebees. I observed two freely foraging B. terrestris colonies and measured i) trip number, ii) trip time, iii) proportion of nectar trips, and iv) nectar foraging rate of different sized foragers. In all observation periods large foragers exhibit a significantly higher foraging rate than small foragers. None of the other three foraging parameters is affected by workers' size. Thus, large foragers contribute disproportionately more to the current nectar influx of their colony. To summarize, this study shows that understanding the mechanisms of visual information processing and additionally comprising inter-individual differences of sensory capabilities is crucial to interpret foraging behavior of bees.},
  subject      = {Hummeln},
  language  = {en}
}
@phdthesis{Roeschard2002,
  author    = {R{\"o}schard, Jacqueline},
  title     = {Cutter, carriers and bucket brigades ...},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-2240},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2002},
  abstract  = {This study investigates the foraging behaviour of grass-cutting ants, Atta vollenweideri, with specific consideration of the following issues: (a) cutting behaviour and the determination of fragment size, (b) the effect of load size on transport economics, (c) division of labour and task-partitioning. Grass-cutting ants, Atta vollenweideri, harvest grass fragments that serve as substrate for the cultivation of a symbiotic fungus. Foragers were observed to cut grass fragments across the blade, thus resulting in longish, rectangular-shaped fragments in contrast to the semicircular fragments of leaf-cutting ants. Cutting was very time-consuming: In tough grasses like the typical grassland species Paspallum intermedium and Cyperus entrerrianus, cutting times lasted up to more than 20 minutes per fragment and roughly half of all initiated cutting attempts were given up by the ants. Foragers harvesting the softer grass Leersia hexandra were smaller than those foraging on the hard grasses. Fragment size determination and the extent of size-matching between ant body size and fragment size was investigated regarding possible effects of tissue toughness on decision-making and as a function of the distance from the nest. Tissue toughness affected decision-making such that fragment width correlated with ant body mass for the hard grass but not for the soft one, suggesting that when cutting is difficult, larger ants tend to select wider grasses to initiate cutting. The length of the fragments cut out of the two grass species differed statistically, but showed a large overlap in their distribution. Distance from the nest affected load size as well as the extent of size-matching: Fragments collected directly after cutting were significantly larger than those carried on the trail. This indicates that fragments were cut once again on their way to the nest. Size-matching depended on the trail sector considered, and was stronger in ants sampled closer to the nest, suggesting that carriers either cut fragments in sizes corresponding to their body mass prior transport, or transferred them to nestmates of different size after a short carrying distance. During transport, a worker takes a fragment with its mandibles at one end and carries it in a more or less vertical position. Thus, load length might particularly affect maneuverability, because of the marked displacement of the gravitational center. Conversely, based on the energetic of cutting, workers might maximise their individual harvesting rate by cutting long grass fragments, since the longer a grass fragment, the larger is the amount of material harvested per unit cutting effort. I therefore investigated the economics of load transport by focusing on the effects of load size (mass and length) on gross material transport rate to the nest. When controlling for fragment mass, both running speed of foragers and gross material transport rate was observed to be higher for short fragments. In contrast, if fragment mass was doubled and length maintained, running speed differed according to the mass of the loads, with the heavier fragments being transported at the lower pace. For the sizes tested, heavy fragments yielded a higher transport rate in spite of the lower speed of transport, as they did not slow down foragers so much that it counterbalanced the positive effects of fragment mass on material transport rate. The sizes of the fragments cut by grass-cutting ants under natural conditions therefore may represent the outcome of an evolutionary trade-off between maximising harvesting rate at the cutting site and minimising the effects of fragment size on material transport rates. I investigated division of labour and task partitioning during foraging by recording the behaviour of marked ants while cutting, and by monitoring the transport of fragments from the cutting until they reached the nest. A. vollenweideri foragers showed division of labour between cutting and carrying, with larger workers cutting the fragments, and smaller ones transporting them. This division was absent for food sources very close to the nest, when no physical trail was present. Along the trail, the transport of fragment was a partitioned task, i.e., workers formed bucket brigades composed of 2 to 5 carriers. This sequential load transport occurred more often on long than on short trails. The first carriers of a bucket brigade covered only short distances before dropping their fragments, turned back and continued foraging at the same food source. The last carriers covered the longest distance. There was no particular location on the trail for load dropping , i.e., fragments were not cached. I tested the predictions of two hypotheses about the causes of bucket brigades: First, bucket brigades might occur because of load-carriage effects: A load that is too big for an ant to be carried is dropped and carried further by nestmates. Second, fragments carried by bucket brigades might reach the nest quicker than if they are transported by a single carrier. Third, bucket brigades might enhance information flow among foragers: By transferring the load a worker may return earlier back to the foraging site and be able to reinforce the chemical trail, thus recruitment. In addition, the dropped fragment itself may contain information for unladen foragers about currently harvested sources and may enable them to choose between sources of different quality. I investigated load-carriage effects and possible time-saving by presenting ants with fragments of different but defined sizes. Load size did not affect frequency of load dropping nor the distance the first carrier covered before dropping, and transport time by bucket brigades was significantly longer than by single carriers. In order to study the information transfer hypothesis, I presented ants with fragments of different attractivity but constant size. Ants carrying high-quality fragments would be expected to drop them more often than workers transporting low-quality fragments, thus increasing the frequency of bucket brigades. My results show that increasing load quality increased the frequency of bucket brigades as well as it decreased the carrying distance of the first carrier. In other words, more attractive loads were dropped more frequently and after a shorter distance than less attractive ones with the first carriers returning to the foraging site to continue foraging. Summing up, neither load-carriage effects nor time-saving caused the occurrence of bucket brigades. Rather, the benefit might be found at colony level in an enhanced information flow.},
  subject      = {Atta},
  language  = {en}
}