Spatial reorientation by geometry with freestanding objects and extended surfaces: a unifying view

The macroscopic, three-dimensional surface layout geometry of an enclosure apparently provides a different contribution for spatial reorientation than the geometric cues associated with freestanding objects arranged in arrays with similar geometric shape. Here, we showed that a unitary spatial representation can account for the capability of animals to reorient both by extended surfaces and discrete objects in a small-scale spatial task. We trained domestic chicks to locate a food-reward from an opening on isolated cylinders arranged either in a geometrically uninformative (square-shaped) or informative (rectangular-shaped) arrays. The arrays were located centrally within a rectangular-shaped enclosure. Chicks trained to access the reward from a fixed position of openings proved able to reorient according to the geometric cues specified by the shape of the enclosure in all conditions. Chicks trained in a fixed position of opening with geometric cues provided both by the arena and the array proved able to reorient according to each shape separately. However, chicks trained to access the reward from a variable position of openings failed to reorient. The results suggest that the physical constrains associated with the presence of obstacles in a scene, rather than their apparent visual extension, are crucial for spatial reorientation.     

Trapline foraging by bumblebees: I. Persistence of flight-path geometry

By setting out arrays of potted plants of Penstemon strictus,I tested whether freely foraging bumblebee (Bombus spp. ) workerswould establish regular foraging routes that reflected the geometryof the array. They did, passing through an asymmetrical arrayin a pattern that minimized interplant flight distances. Afterthe array was changed to a symmetrical pattern, however, theexperienced bees continued to show their previous asymmetricalflight patterns. New bees without experience on the asymmetricalarray showed no asymmetry on the symmetrical array. I term thispersistence of flight-path geometry "trapline holdover, " anddiscuss its implications for the study of animals' learningand foraging behavior.     

Spatial distribution of bumblebees foraging on two cultivars of tomato in a commercial greenhouse

The spatial distribution of foraging bumblebees, Bombus terrestris L. (Hymenoptera: Apidae), was studied in a greenhouse planted with two cultivars of tomato, Lycopersicum esculentum Mill. (Solanaceae), in two patches. In both patches, bumblebee densities per square meter were measured on plots, and the results showed that their densities were nearly similar. The densities of available flowers, their pollen production, and availability also were measured. Our results showed that, although the cultivars greatly differed in flower density, flower morphology, and pollen production, their pollen availability (i.e., pollen actually collected by bumblebees per square meter) was approximately the same. Therefore, the mean quantities of pollen collected per bumblebee were similar in each patch. Knowing that bumblebees do not visit different varieties randomly, our results suggest that the major factor affecting the bumblebee distribution among patches was the density of available resource. Results are discussed both from an applied point of view and in relation to the assumptions of the ideal free distribution theory.     

Optimal foraging in bumblebees: Hunting by expectation

The foraging behaviour literature contains three hypotheses concerned with hunting by expectation. These suggest possible rules animals use to decide when to leave particular feeding sites and search in other places for food. Predictions of the three hypotheses were tested experimentally by varying the quality of plants (amount and distribution of nectar) encountered by bumblebees (Bombus appositus). Results support only a rate expectation hypothesis. Bees left multiflowered plants when the amount of nectar found in the first flower was below a threshold volume. Bees stayed on plants if they received greater than the threshold volume. This threshold nectar volume is close to the amount predicted if a bee forages to maximize its rate of net energy intake.     

Spatial encoding in mountain chickadees: features overshadow geometry

Encoding the global geometric shape of an enclosed environment is a principal means of orientation in human and non-human animals. Animals spontaneously encode the geometry of an enclosure even when featural information is available. Although features can be used, they typically do not overshadow geometry. However, all previously tested organisms have been reared in human-made environments with salient geometrical cues. Here, we show that wild-caught mountain chickadees (Poecile gambeli) do not spontaneously encode the geometry of an enclosure when salient features are present near the goal. However, chickadees trained without salient features encode geometric information, but this encoding is overshadowed by features.     

Size-related selection of food plants by bumblebees

Abstract. 1. A positive correlation between the tongue length of conspecific workers collecting nectar from seven plant species and the corolla length of the flowers probed was found for B.lapidarius and B.pascuomm but not B.terrestris . No simple relationship was found between the volume, sugar weight or concentration of nectar in flowers and the tongue or wing length of probing bees.
2. B.terrestris workers collecting pollen from four plant species producing pollen only, were found to differ in size according to the type of pollen presentation mechanism and the pollen content per flower. Body size variation was also related to the foraging of pollen plus nectar from two other plant species.     

Remote perception of floral nectar by bumblebees

Summary On both artificial flowers in the laboratory and certain plant species in the field, bumblebees often closely approached flowers and then departed without probing for nectar. In laboratory experiments where nectar rewards were associated with subtle visual or olfactory cues, bumblebees approached and avoided non-rewarding flowers. Flowers that bees entered and probed for nectar contained rewards much more frequently than predicted by chance alone. When there were no external cues associated with nectar content, bees visited rewarding flowers by chance alone, provided rewarding flowers were not spatially clumped. In the field, bumblebees approached and rejected a large proportion of dogbane flowers and red clover inflorescences. On both species, flowers or inflorescences probed by bees contained more nectar than those rejected by bees or those that I chose at random. On fireweed and monkshood, bees rarely or never approached and rejected healthy-looking flowers. Predictions generated by an optimal foraging model were tested on data from four bumblebee species foraging on red clover. The model was highly successful in qualitatively predicting the relationship between handling time and proportion of inflorescences rejected by individual bees, and the relationship between threshold nectar content for acceptance by bees and average resource availability. Thus, bees appeared to use remotely perceived cues to maximize their rates of nectar intake.     

Geotaxis by motile spermatozoa hydrodynamic reorientation

This paper studies the tendency of a motile human spermatozoon to reorient in the direction of an applied gravitational field. The dynamics of this process are formulated from first principles. The hydrodynamic forces and moments acting upon the spermatozoon are determined by means of resistive force theory. The solution of the dynamical equations of motion indicates that if the center of mass of the spermatozoon lies along its geometrical symmetry axis, then the angular reorientation rate is proportional to the sine of the angle of inclination to gravity. For flagellar beat shapes typical of human spermatozoa this simple relationship is not strictly true. However it is approximately valid, and for practical purposes useful, in the time-average sense. On the basis of this theory, possible geotactic distinctions between X- and Y-bearing human spermatozoa are considered.     

Optimal body size in bumblebees

Summary It is hypothesized that the body size of a bumblebee will be that size which maximizes its average net rate of energy intake while collecting nectar. A mathematical model is developed with the result that the net rate of energy intake of a nectar-collecting bumblebee is expressed as a function of the body size of the bumblebee. From this model the body size which maximizes the net rate of energy intake (i.e., optimal body size) is found (as the solution of an implicit equation). In this situation the advantage of large size is that larger bumblebees fly faster and hence take less flight time than smaller bumblebees. The disadvantage of larger size is greater energetic costs.The parameters of the model are estimated using data obtained from the foraging behavior of bumblebees on monkshood (Aconitum columbianum). The optimal body size is then calculated for workers of Bombus appositus which obtained almost all their nectar from monkshood. The observed and expected (i.e., optimal) body size are found to be close and not significantly different.The model also predicts that, from the bumblebee's point of view, there should be a positive correlation between the size of the bumblebee and the average amount of nectar obtained per flower. Evidence of this correlation is presented and the possible significance of the correlation from the plant's point of view is discussed. A possible extension of the model to general relationships between predator body size, prey size and prey density is discussed.     

Spatial dynamics of invasion: the geometry of introduced species

Many exotic species combine low probability of establishment at each introduction with rapid population growth once introduction does succeed. To analyse this phenomenon, we note that invaders often cluster spatially when rare, and consequently an introduced exotic's population dynamics should depend on locally structured interactions. Ecological theory for spatially structured invasion relies on deterministic approximations, and determinism does not address the observed uncertainty of the exotic-introduction process. We take a new approach to the population dynamics of invasion and, by extension, to the general question of invasibility in any spatial ecology. We apply the physical theory for nucleation of spatial systems to a lattice-based model of competition between plant species, a resident and an invader, and the analysis reaches conclusions that differ qualitatively from the standard ecological theories. Nucleation theory distinguishes between dynamics of single- and multi-cluster invasion. Low introduction rates and small system size produce single-cluster dynamics, where success or failure of introduction is inherently stochastic. Single-cluster invasion occurs only if the cluster reaches a critical size, typically preceded by a number of failed attempts. For this case, we identify the functional form of the probability distribution of time elapsing until invasion succeeds. Although multi-cluster invasion for sufficiently large systems exhibits spatial averaging and almost-deterministic dynamics of the global densities, an analytical approximation from nucleation theory, known as Avrami's law, describes our simulation results far better than standard ecological approximations.     

Conspecific and heterospecific information use in bumblebees

Heterospecific social learning has been understudied in comparison to interactions between members of the same species. However, the learning mechanisms behind such information use can allow animals to be flexible in the cues that are used. This raises the question of whether conspecific cues are inherently more influential than cues provided by heterospecifics, or whether animals can simply use any cue that predicts fitness enhancing conditions, including those provided by heterospecifics. To determine how freely social information travels across species boundaries, we trained bumblebees (Bombus terrestris) to learn to use cues provided by conspecifics and heterospecific honey bees (Apis mellifera) to locate valuable floral resources. We found that heterospecific demonstrators did not differ from conspecifics in the extent to which they guided observers'' choices, whereas various types of inorganic visual cues were consistently less effective than conspecifics. This was also true in a transfer test where bees were confronted with a novel flower type. However, in the transfer test, conspecifics were slightly more effective than heterospecific demonstrators. We then repeated the experiment with entirely naïve bees that had never foraged alongside conspecifics before. In this case, heterospecific demonstrators were equally efficient as conspecifics both in the initial learning task and the transfer test. Our findings demonstrate that social learning is not a unique process limited to conspecifics and that through associative learning, interspecifically sourced information can be just as valuable as that provided by conspecific individuals. Furthermore the results of this study highlight potential implications for understanding competition within natural pollinator communities.     

The binding of visual patterns in bumblebees

Bees navigating between their nests and foraging sites rely on their ability to learn and to recall many complex visual patterns [1-4]. How are the elements that make up one of these patterns bound together so that the whole pattern can be recalled when it is required? Consider the sentence: 'Dons nod off.' The words in it can be distinguished by the pattern of elements or letters that they contain. Words may contain the same elements arranged in different orders (don, nod), or contain elements of different types, or vary in both these respects (nod, off). We show here that bumblebees (Bombus terrestris) can learn to group the elements of a pattern together, such that different identifiable patterns contain the same elements in different combinations--analogous to the grouping of letters found in words. Our results suggest that pattern binding in bees is achieved in part by linking pattern elements directly together and in part by associating the elements with cues that are related to the context in which the pattern is seen.     

Wing flexibility enhances load-lifting capacity in bumblebees

The effect of wing flexibility on aerodynamic force production has emerged as a central question in insect flight research. However, physical and computational models have yielded conflicting results regarding whether wing deformations enhance or diminish flight forces. By experimentally stiffening the wings of live bumblebees, we demonstrate that wing flexibility affects aerodynamic force production in a natural behavioural context. Bumblebee wings were artificially stiffened in vivo by applying a micro-splint to a single flexible vein joint, and the bees were subjected to load-lifting tests. Bees with stiffened wings showed an 8.6 per cent reduction in maximum vertical aerodynamic force production, which cannot be accounted for by changes in gross wing kinematics, as stroke amplitude and flapping frequency were unchanged. Our results reveal that flexible wing design and the resulting passive deformations enhance vertical force production and load-lifting capacity in bumblebees, locomotory traits with important ecological implications.     

Endoparasitic flies,pollen-collection by bumblebees and a potential host-parasite conflict

Summary Conopid flies (Conopidae, Diptera) are common larval parasites of bumblebees. The larva develops inside the abdomen of workers, queens and males. Development is completed within 10–12 days after oviposition when the host is killed and the parasite pupates in situ. Development results in parasitised bees becoming unable to carry large loads of nectar, as the conopid larvae reside where the honey crop is normally located. Furthermore, an addition to the bee's unloaden body mass is likely (average larval weight reached at pupation by the common parasite species Sicus ferrugineus: ±SD 36.3±12.3 mg, n=59; by Physocephala rufipes: 55.8±16.9 mg, n=108). We here asked whether the propensity of workers of the bumblebee Bombus pascuorum to collect nectar rather than pollen is related to the presence of conopid larvae. For samples of bees (n=2254 workers) collected over 3 years of field studies in northwestern Switzerland, there was no difference in the frequency of bees caught as pollen collectors among parasitised (38.1% of cases, n=210) as compared to non-parastised bees (43.9%, n=360) (²=1.83, n.s.). However, compared to the non-parasitised bees (n=360), those hosts containing a third (last) instar larva (n=9) were less likely to collect pollen than expected by chance ²=6.91, P=0.003. Similarly, hosts with short survival time between capture and being killed by the developing larva (which hence must have harboured a late instar parasite at time of capture) were less likely to collect pollen (8%, n=25) than those found not parasitised (37.6%, n=891 ²=9.16, P<0.001). Late instar larvae grow so big that they fill the entire abdomen. Although there was also a tendency for presumably older bees to collect less pollen, this is unlikely to explain the observations. We also discuss whether these changes in foraging behaviour of bumblebees may reflect a host-parasite conflict over the type of resource to be collected.