Seasonality in communication and collective decision-making in ants

The ability of animals to adjust their behaviour according to seasonal changes in their ecology is crucial for their fitness. Eusocial insects display strong collective behavioural seasonality, yet the mechanisms underlying such changes are poorly understood. We show that nest preference by emigrating Temnothorax albipennis ant colonies is influenced by a season-specific modulatory pheromone that may help tune decision-making according to seasonal constraints. The modulatory pheromone triggers aversion towards low-quality nests and enhances colony cohesion in summer and autumn, but not after overwintering-in agreement with reports that field colonies split in spring and reunite in summer. Interestingly, we show that the pheromone acts by downgrading the perceived value of marked nests by informed and naive individuals. This contrasts with theories of collective intelligence, stating that accurate collective decision-making requires independent evaluation of options by individuals. The violation of independence highlighted here was accordingly shown to increase error rate during emigrations. However, this is counterbalanced by enhanced cohesion and the transmission of valuable information through the colony. Our results support recent claims that optimal decisions are not necessarily those that maximize accuracy. Other criteria-such as cohesion or reward rate-may be more relevant in animal decision-making.

Early olfactory experience induces structural changes in the primary olfactory center of an insect brain

The antennal lobe (AL) is the first olfactory center of the insect brain and is constituted of different functional units, the glomeruli. In the AL, odors are coded as spatiotemporal patterns of glomerular activity. In honeybees, olfactory learning during early adulthood modifies neural activity in the AL on a long-term scale and also enhances later memory retention. By means of behavioral experiments, we first verified that olfactory learning between the fifth and eighth day of adulthood induces better retention performances at a late adult stage than the same experience acquired before or after this period. We checked that the specificity of memory for the odorants used was improved. We then studied whether such early olfactory learning also induces long-term structural changes in the AL consistent with the formation of long-term olfactory memories. We also measured the volume of 15 identified glomeruli in the ALs of 17-day-old honeybees that either experienced an odor associated with sucrose solution between the fifth and eighth day of adulthood or were left untreated. We found that early olfactory experience induces glomerulus-selective increases in volume that were specific to the learned odor. By comparing our volumetric measures with calcium-imaging recordings from a previous study, performed in 17-day-old bees subjected to the same treatment and experimental conditions, we found that glomeruli that showed structural changes after early learning were those that exhibited a significant increase in neural activity. Our results make evident a correlation between structural and functional changes in the AL following early olfactory learning.

Configural processing enables discrimination and categorization of face-like stimuli in honeybees

We studied whether honeybees can distinguish face-like configurations by using standardized stimuli commonly employed in primate and human visual research. Furthermore, we studied whether, irrespective of their capacity to distinguish between face-like stimuli, bees learn to classify visual stimuli built up of the same elements in face-like versus non-face-like categories. We showed that bees succeeded in discriminating both face-like and non-face-like stimuli and categorized appropriately novel stimuli in these two classes. To this end, they used configural information and not just isolated features or low-level cues. Bees looked for a specific configuration in which each feature had to be located in an appropriate spatial relationship with respect to the others, thus showing sensitivity for first-order relationships between features. Although faces are biologically irrelevant stimuli for bees, the fact that they were able to integrate visual features into complex representations suggests that face-like stimulus categorization can occur even in the absence of brain regions specialized in face processing.

Early olfactory experience modifies neural activity in the antennal lobe of a social insect at the adult stage

In the antennal lobe (AL), the first olfactory centre of the insect brain, odorants are represented as spatiotemporal patterns of glomerular activity. Whether and how such patterns are modified in the long term after precocious olfactory experiences (i.e. in the first days of adulthood) remains unknown. To address this question, we used in vivo optical imaging of calcium activity in the antennal lobe of 17-day-old honeybees which either experienced an odorant associated with sucrose solution 5-8 days after emergence or were left untreated. In both cases, we imaged neural responses to the learned odor and to three novel odors varying in functional group and carbon-chain length. Two different odor concentrations were used. We also measured behavioral responses of 17-day-old honeybees, treated and untreated, to these stimuli. We show that precocious olfactory experience increased general odor-induced activity and the number of activated glomeruli in the adult AL, but also affected qualitative odor representations, which appeared shifted in the neural space of treated animals relative to control animals. Such effects were not limited to the experienced odor, but were generalized to other perceptually similar odors. A similar trend was found in behavioral experiments, in which increased responses to the learned odor extended to perceptually similar odors in treated bees. Our results show that early olfactory experiences have long-lasting effects, reflected in behavioral responses to odorants and concomitant neural activity in the adult olfactory system.

Discrimination of coloured patterns by honeybees through chromatic and achromatic cues

We investigated pattern discrimination by worker honeybees, Apis mellifera, focusing on the roles of spectral cues and the angular size of patterns. Free-flying bees were trained to discriminate concentric patterns in a Y-maze. The rewarded pattern could be composed of either a cyan and a yellow colour, which presented both different chromatic and achromatic L-receptor contrast, or an orange and a blue colour, which presented different chromatic cues, but the same L-receptor contrast. The non-rewarded alternative was either a single-coloured disc with the colour of the central disc or the surrounding ring of the pattern, a checkerboard pattern with non-resolvable squares, the reversed pattern, or the elements of the training pattern (disc or ring alone). Bees resolved and learned both colour elements in the rewarded patterns and their spatial properties. When the patterns subtended large visual angles, this discrimination used chromatic cues only. Patterns with yellow or orange central discs were generalised toward the yellow and orange colours, respectively. When the patterns subtended a visual angle close to the detection limit and L-receptor contrast was mediating discrimination, pattern perception was reduced: bees perceived only the pattern element with higher contrast.

Different functions of two alarm substances in the honeybee

In the honeybee, isopentyl acetate and 2-heptanone are described as alarm substances. We asked whether both substances have a similar role by testing the effect of their exposure on the appetitive proboscis extension reflex and on the aversive stinging reflex. In the appetitive context of sucrose stimulation no differences were found between isopentyl acetate and 2-heptanone. Small amounts of isopentyl acetate or 2-heptanone (3 microl of 1:9 dilution) yielded a response similar to that of a non-exposed control. Larger amounts of both substances (125 microl of 1:9 dilutions) as well as mixtures led to a decrease of responsiveness to sucrose. In the aversive context of electrical stimulation, significant differences between isopentyl acetate and 2-heptanone were found. Exposure to a small amount of isopentyl acetate (3 microl of 1:9 dilution) or to a large amount of 2-heptanone (125 microl of 1:9 dilution) led to an increase of responsiveness to the electric shock. Larger quantities of isopentyl acetate (125 microl of 1:9 dilution) decreased the responsiveness to the shock. 2-Heptanone never decreased the responsiveness to the shock. Our results indicate that isopentyl acetate and 2-heptanone have different functions even if both are capable of evoking deterrent responses in a defensive context.

The concepts of 'sameness' and 'difference' in an insect

Insects process and learn information flexibly to adapt to their environment. The honeybee Apis mellifera constitutes a traditional model for studying learning and memory at behavioural, cellular and molecular levels. Earlier studies focused on elementary associative and non-associative forms of learning determined by either olfactory conditioning of the proboscis extension reflex or the learning of visual stimuli in an operant context. However, research has indicated that bees are capable of cognitive performances that were thought to occur only in some vertebrate species. For example, honeybees can interpolate visual information, exhibit associative recall, categorize visual information and learn contextual information. Here we show that honeybees can form 'sameness' and 'difference' concepts. They learn to solve 'delayed matching-to-sample' tasks, in which they are required to respond to a matching stimulus, and 'delayed non-matching-to-sample' tasks, in which they are required to respond to a different stimulus; they can also transfer the learned rules to new stimuli of the same or a different sensory modality. Thus, not only can bees learn specific objects and their physical parameters, but they can also master abstract inter-relationships, such as sameness and difference.

Detection of coloured patterns by honeybees through chromatic and achromatic cues

We asked whether the detection range of two-coloured centre-surround patterns differs from that of single-coloured targets. Honeybees Apis mellifera were trained to distinguish between the presence and absence of a single-coloured disc or a coloured pattern at different visual angles. The patterns presented colours which were either different in chromatic and L-receptor contrasts to the background, equal in chromatic but different in L-receptor contrasts, or vice-versa. Patterns with colours presenting only chromatic contrast were also tested. Patterns with higher L-receptor contrast in its outer than in its inner element were better detected than patterns with a reversed L-contrast distribution. However, both were detected worse than single-coloured discs of the respective colours. When the L-receptor contrast was the same for both elements, the detection range of the two-coloured and single-coloured targets was the same. Patterns whose colours lacked L-receptor contrast were detected just as single-coloured targets of the same colours. These results demonstrate that both chromatic and L-receptor contrasts mediate the detection of coloured patterns and that particular distributions of L-receptor contrast within a target are better detected than others. This finding is consistent with the intervention of neurons with centre-surround receptive fields in the detection of coloured patterns.

Configural olfactory learning in honeybees: negative and positive patterning discrimination

In an appetitive context, honeybees (Apis mellifera) learn to associate odors with a reward of sucrose solution. If an odor is presented immediately before the sucrose, an elemental association is formed that enables the odor to release the proboscis extension response (PER). Olfactory conditioning of PER was used to study whether, beyond elemental associations, honeybees are able to process configural associations. Bees were trained in a positive and anegative patterning discrimination problem. In the first problem, single odorants were nonreinforced whereas the compound was reinforced. In the second problem, single odorants were reinforced whereas the compound was nonreinforced. We studied whether bees can solve these problems and whether the ratio between the number of presentations of the reinforced stimuli and the number of presentations of the nonreinforced stimuli affects discrimination. Honeybees differentiated reinforced and nonreinforced stimuli in positive and negative patterning discriminations. They thus can process configural associations. The variation of the ratio of reinforced to nonreinforced stimuli modulated the amount of differentiation. The assignment of singular codes to complex odor blends could be implemented at the neural level: When bees are stimulated with odor mixtures, the activation patterns evoked at the primary olfactory neuropile, the antennal lobe, may be combinations of the single odorant responses that are not necessarily fully additive.

Cognitive architecture of a mini-brain: the honeybee

Honeybees have small brains, but their behavioural repertoire is impressive. In this article we focus on the extent to which adaptive behaviour in honeybees exceeds elementary forms of learning. We use the concept of modularity of cognitive functions to characterize levels of complexity in the honeybee brain. We show that behavioural complexity in the honeybee cannot be explained by independent functions of vertically arranged, domain-specific processing modules, but requires horizontal integration in a central state, and we identify neural mechanisms that may underlie domain-specific processing and central integration. The honeybee may serve as a useful model for the study of intermediate levels of complexity in cognitive functions and the search for their neural substrates.

Detection of bright and dim colours by honeybees

Honeybees, Apis mellifera, were trained to detect coloured disks with either a strong or a weak intensity difference against the background. Green, blue, ultraviolet-reflecting white and grey papers were reciprocally combined as targets or backgrounds, providing strong chromatic and/or achromatic cues. The behavioural performance of the honeybees was always symmetrical for both reciprocal target/background combinations of a colour pair, thus showing that target detection is independent of whether the colour is presented as a background or as a target in combination with the other colour. Bright targets against dim backgrounds and vice versa were detected more reliably than dim target/background combinations. This result favours the general assumption that the detectability of a coloured stimulus increases with increasing intensity.

Interaction of visual and olfactory cues in the aggregation behaviour of the haematophagous bug Triatoma infestans

We analysed how the assembling behaviour of Triatoma infestans is modulated by the convergence of chemical cues released by their faeces and the spectral quality of the light associated with refuges. Second-instar larvae were confronted with refuges associated with a visual stimulus (either blue, green or red lights having the same intensity, or darkness) and a chemical cue (presence or absence of faeces). In this context, faeces constitute a major attractant for bugs. In the absence of faeces, bugs always assembled in dark places. Green light was always rejected despite the presence of faeces, i.e. the assembling behaviour was controlled by a photonegative reaction to this light. In the presence of red light, orientation towards the chemical cue dominated over the bugs' photonegative reaction to this light. Such a light was avoided in the absence of faeces but not in their presence. Also, negative phototaxis to blue light could be counteracted by the presence of the chemical cue. Thus, a concrete interaction between visual and olfactory cues occurred in these experiments: the bugs' response changed depending on the specific combination of spectral light and faeces. Finally, bugs responded differentially to blue, red and green lights of the same intensity. They assembled preferentially on red, followed by blue and then by green. Thus, discrimination between lights of different spectral quality is possible, probably through an achromatic mechanism.

Floral Symmetry and Its Role in Plant-Pollinator Systems

Floral symmetry has a relevant status in the study of both pollination biology and animal behavior. In this work, a brief review and classification of symmetry types in flowers is provided as a basis for understanding the role of floral symmetry in pollination phenomena. We focus on insects as a fundamental group of pollinators, and we discuss symmetry from the perspective of insect perception. We conclude that symmetry is a specific cue with a signal value that is perceived by insect pollinators. A simple nervous system, such as that of honeybees, is capable of an extremely flexible and adaptive processing of symmetry. Performances consistent with categorization and concept building may be observed, provided that appropriate learning paradigms are employed. Perfectly symmetrical flowers might signal a high quality and/or quantity of nectar or pollen to pollinators that, in turn, might exert strong selection pressure on symmetric features. However, coadaptation arguments in the strict sense are not adequate because it is impossible to determine whether the insect's capacity to perceive symmetry is younger or older than is the origin of flower symmetry.

Olfactory discrimination ability and odor structure-activity relationships in honeybees

Using the training procedure introduced by von Frisch in 1919, we tested the ability of free-flying honeybees to discriminate a conditioning odor from an array of 44 simultaneously presented substances. The stimuli included homologous series of aliphatic alcohols, aldehydes and ketones, isomeric forms of some of these substances, as well as several terpenes and odor mixtures, and thus comprised stimuli of varying degrees of structural similarity to any conditioning odor. We found (i) that the honeybees significantly distinguished between 97.0% of the 1848 odor pairs tested, thus showing an excellent discrimination performance when tested in a free-flying situation with an array of structurally related substances; (ii) a significant negative correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length with all aliphatic substance classes tested; (iii) that both the position and type of a functional group also affected discriminability of odorants in a substance class-specific manner; and (iv) striking similarities in odor structure-activity relationships between honeybees and human and nonhuman primates tested previously on a subset of substances employed here. Our findings demonstrate that the similarities found in the structural organization of the olfactory systems of insects and vertebrates are paralleled by striking similarities in relative discrimination abilities. This strongly suggests that similar mechanisms of odor coding and discrimination may underlie olfaction in vertebrates and insects.

Pattern learning by honeybees: conditioning procedure and recognition strategy

In recognizing a pattern, honeybees Apis mellifera, may focus either on its ventral frontal part, or on the whole frontal image. We asked whether the conditioning procedure used to train the bees to a pattern determines the recognition strategy employed. Bees were trained with the same patterns presented vertically on the back walls of a Y maze. Conditioning was either absolute, that is, bees should learn to choose a rewarded pattern when there is no alternative, or differential, that is, bees should learn to choose a rewarded pattern that is paired with a different, nonrewarded one. Bees used different pattern recognition strategies depending on the conditioning procedure: absolute conditioning restricted recognition to the lower half whilst differential conditioning extended it to the whole pattern. Bees trained with absolute conditioning saw and learned the features of the upper part of the trained patterns, but assigned more weight to the lower part. Bees trained with differential conditioning learned not only the features of the reinforced stimulus in an excitatory way, but also those of the nonreinforced one in an inhibitory way. Thus, conditioning tasks that involve not only excitatory acquisition of the conditioned stimulus per se, but also discrimination of nonreinforced stimuli, result in an increase in the visual field assigned to the recognition task. Conditioning tasks that involve only excitatory acquisition of the rewarded stimulus result in a higher weighting of the lower pattern half and thus in a more reduced field assigned to the recognition task. This difference may reflect that existing between a conditioned and an incidental behavioural modification. Copyright 1999 The Association for the Study of Animal Behaviour.

Alarm pheromone induces stress analgesia via an opioid system in the honeybee

Changes of the stinging response threshold of Apis mellifera scutellata were measured on foragers fixed on a holder and stimulated with an electric shock as a noxious stimulus. The threshold of responsiveness to the noxious stimulus increased when bees were previously stimulated with isopentyl acetate, which is a main component of the alarm pheromone of the sting chamber. This effect is antagonised by previous injection of naloxone-hydrochloride (Endo Laboratories Inc.). Results suggest that in the honeybee an endogenous opioid system activated by isopentyl acetate is responsible for modulation of perception for nociceptive stimuli. The resulting stress-induced analgesia in the defender bee would reduce its probability of withdrawal thus increasing its efficiency against enemies.

Insect visual perception: complex abilities of simple nervous systems

Despite their relatively simple nervous systems, insects display a rich behavioural repertoire, in which vision plays a major role. In the past two years, much knowledge has been gained about how insects are capable of a variety of flexible, visually guided tasks that involve a high level of complexity. From long-range navigation to median-range orientation and close-up recognition, insects apply different strategies that complement each other, that are used sequentially during their approach flight towards their goals, and that may replace each other, depending on the salience of, and the attention towards, particular visual cues.