Andre J. Riveros

Olfactory learning and memory in the bumblebee Bombus occidentalis.

In many respects, the behavior of bumblebees is similar to that of the closely related honeybees, a long-standing model system for learning and memory research. Living in smaller and less regulated colonies, bumblebees are physiologically more robust and thus have advantages in particular for indoor experiments. Here, we report results on Pavlovian odor conditioning of bumblebees using the proboscis extension reflex (PER) that has been successfully used in honeybee learning research. We examine the effect of age, body size, and experience on learning and memory performance. We find that age does not affect learning and memory ability, while body size positively correlates with memory performance. Foraging experience seems not to be necessary for learning to occur, but it may contribute to learning performance as bumblebees with more foraging experience on average were better learners. The PER represents a reliable tool for learning and memory research in bumblebees and allows examining interspecific similarities and differences of honeybee and bumblebee behavior, which we discuss in the context of social organization.

Brain allometry and neural plasticity in the bumblebee Bombus occidentalis.

Brain plasticity is a common phenomenon across animals and in many cases it is associated with behavioral transitions. In social insects, such as bees, wasps and ants, plasticity in a particular brain compartment involved in multisensory integration (the mushroom body) has been associated with transitions between tasks differing in cognitive demands. However, in most of these cases, transitions between tasks are age-related, requiring the experimental manipulation of the age structure in the studied colonies to distinguish age and experience-dependent effects. To better understand the interplay between brain plasticity and behavioral performance it would therefore be advantageous to study species whose division of labor is not age-dependent. Here, we focus on brain plasticity in the bumblebee Bombus occidentalis, in which division of labor is strongly affected by the individual’s body size instead of age. We show that, like in vertebrates, body size strongly correlates with brain size. We also show that foraging experience, but not age, significantly correlates with the increase in the size of the mushroom body, and in particular one of its components, the medial calyx. Our results support previous findings from other social insects suggesting that the mushroom body plays a key role in experience-based decision making. We also discuss the use of bumblebees as models to analyze neural plasticity and the association between brain size and behavioral performance.

Metabolic scaling in insects supports the predictions of the WBE model.

The functional association between body size and metabolic rate (BS-MR) is one of the most intriguing issues in ecological physiology. An average scaling exponent of 3/4 is broadly observed across animal and plant taxa. The numerical value of 3/4 is theoretically predicted under the optimized version of West, Brown, and Enquists vascular resource supply network model. Insects, however, have recently been proposed to express a numerically different scaling exponent and thus application of the WBE network model to insects has been rejected. Here, we re-analyze whether such variation is indeed supported by a global deviation across all insect taxa at the order and family levels to assess if specific taxa influence insect metabolic scaling. We show that a previous reported deviation is largely due to the effect of a single insect family (Termitidae). We conclude that the BS-MR relationship in insects broadly supports the core predictions of the WBE model. We suggest that the deviation observed within the termites warrants further investigation and may be due to either difficulty in accurately measuring termite metabolism and/or particularities of their life history. Future work on allometric scaling should assess the nature of variation around the central tendencies in scaling exponents in order to test if this variation is consistent with core assumptions and predictions of the WBE model that stem by relaxing its secondary optimizing assumptions that lead to the 3/4 exponent.

Decision-making and associative color learning in harnessed bumblebees (Bombus impatiens)

In honeybees, the conditioning of the proboscis extension response (PER) has provided a powerful tool to explore the mechanisms underlying olfactory learning and memory. Unfortunately, PER conditioning does not work well for visual stimuli in intact honeybees, and performance is improved only after antennal amputation, thus limiting the analysis of visual learning and multimodal integration. Here, we study visual learning using the PER protocol in harnessed bumblebees, which exhibit high levels of odor learning under restrained conditions. We trained bumblebees in a differential task in which two colors differed in their rewarding values. We recorded learning performance as well as response latency and accuracy. Bumblebees rapidly learned the task and discriminated the colors within the first two trials. However, performance varied between combinations of colors and was higher when blue or violet was associated with a high reward. Overall, accuracy and speed were negatively associated, but both components increased during acquisition. We conclude that PER conditioning is a good tool to study visual learning, using Bombus impatiens as a model, opening new possibilities to analyze the proximate mechanisms of visual learning and memory, as well as the process of multimodal integration and decision-making.

Evolution of brain size in class-based societies of fungus-growing ants (Attini)

A social lifestyle is often assumed to be more complex than a solitary one, due to social demands that may require increased cognitive capabilities. These nested assumptions underlie hypotheses to explain a correlation between brain size and group size in social vertebrates, using group size and accumulation of social traits, as alternative proxies for social complexity. Eusocial insects challenge the generality of the hypothesis that social complexity relies on increased cognitive capabilities of individuals. We used data from previously published studies to test for an association between sociality and brain size across 18 species (nine genera) of fungus-growing ants (Attini), which range from basal taxa with fewer than 100 monomorphic individuals, to derived colonies containing several million polymorphic, highly specialized individuals. Among monomorphic species, increased colony size was associated with decreased relative brain size and increased olfactory lobe size, although the latter result was sensitive to both the exclusion of potential outliers and whether phylogenetically independent contrasts were used. Within leafcutters (Atta), the relative size of the antennal lobes was also associated with group size, but may also reflect ecological foraging specialization, which may be a confounding variable. Comparisons between class- and individual-based societies highlight the general problem of increasing social structure in proportion to group size and show that there are alternative solutions to this problem: one alternative involves increasing behavioural specialization of individuals and evolved rules; the other involves increased diversification of individual behaviour, social norms and ultimately institutions.

Sensory allometry, foraging task specialization and resource exploitation in honeybees

Insect societies are important models for evolutionary biology and sociobiology. The complexity of some eusocial insect societies appears to arise from self-organized task allocation and group cohesion. One of the best-supported models explaining self-organized task allocation in social insects is the response threshold model, which predicts specialization due to inter-individual variability in sensitivity to task-associated stimuli. The model explains foraging task specialization among honeybee workers, but the factors underlying the differences in individual sensitivity remain elusive. Here, we propose that in honeybees, sensory sensitivity correlates with individual differences in the number of sensory structures, as it does in solitary species. Examining European and Africanized honeybees, we introduce and test the hypothesis that body size and/or sensory allometry is associated with foraging task preferences and resource exploitation. We focus on common morphological measures and on the size and number of structures associated with olfactory sensitivity. We show that the number of olfactory sensilla is greater in pollen and water foragers, which are known to exhibit higher sensory sensitivity, compared to nectar foragers. These differences are independent of the distribution of size within a colony. Our data also suggest that body mass and number of olfactory sensilla correlate with the concentration of nectar gathered by workers, and with the size of pollen loads they carry. We conclude that sensory allometry, but not necessarily body size, is associated with resource exploitation in honeybees and that the differences in number of sensilla may underlie the observed differences in sensitivity between bees specialized on water, pollen and nectar collection.

Learning from learning and memory in bumblebees.

The difficulty to simultaneously record neural activity and behavior presents a considerable limitation for studying mechanisms of insect learning and memory. The challenge is finding a model suitable for the use of behavioral paradigms under the restrained conditions necessary for neural recording. In honeybees, Pavlovian conditioning relying on the proboscis extension reflex (PER) has been used with great success to study different aspects of insect cognition. However, it is desirable to combine the advantages of the PER with a more robust model that allows simultaneous electrical or optical recording of neural activity. Here, we briefly discuss the potential use of bumblebees as models for the study of learning and memory under restrained conditions. We base our arguments on the well-known cognitive abilities of bumblebees, their social organization and phylogenetic proximity to honeybees, our recent success using Pavlovian conditioning to study learning in two bumblebee species, and on the recently demonstrated robustness of bumblebees under conditions suitable for electrophysiological recording.

Color-dependent learning in restrained Africanized honey bees

Associative color learning has been demonstrated to be very poor using restrained European honey bees unless the antennae are amputated. Consequently, our understanding of proximate mechanisms in visual information processing is handicapped. Here we test learning performance of Africanized honey bees under restrained conditions with visual and olfactory stimulation using the proboscis extension response (PER) protocol. Restrained individuals were trained to learn an association between a color stimulus and a sugar–water reward. We evaluated performance for ‘absolute’ learning (learned association between a stimulus and a reward) and ‘discriminant’ learning (discrimination between two stimuli). Restrained Africanized honey bees (AHBs) readily learned the association of color stimulus for both blue and green LED stimuli in absolute and discriminatory learning tasks within seven presentations, but not with violet as the rewarded color. Additionally, 24-h memory improved considerably during the discrimination task, compared with absolute association (15–55%). We found that antennal amputation was unnecessary and reduced performance in AHBs. Thus color learning can now be studied using the PER protocol with intact AHBs. This finding opens the way towards investigating visual and multimodal learning with application of neural techniques commonly used in restrained honey bees.

Magnetic Compasses in Insects

The use of magnetic information for orientation and navigation is a widespread phenomenon in animals. In contrast to our knowledge of navigational systems in vertebrates, our understanding of the mechanisms underlying the insect magnetic perception and use of the information is at an early stage. Some insects use magnetic information for simple body alignment or homing. Also, insects might use the Earth’s magnetic field to orient during long-distance migrations. In most known cases, insects use a polarity compass, orienting by the North–South axis of the Earth’s magnetic field. However, recent studies have also pointed to a role for magnetic inclination in insect orientation. Also, magnetic information is coupled with other navigation compasses or cues, such as the sun or significant landmarks. The use of traditional insect models will be critical to increasing our knowledge of the proximal mechanisms. Nevertheless, the study of new species is necessary for the solution of specific questions regarding perception, processing, and use of magnetic information in insects. In this article, our current knowledge on the use of magnetic information for orientation and navigation in insects is broadly reviewed, from the nature of the magnetic compass to the diversity of its uses. Important directions for future research are also discussed.

Do leaf-cutter ants Atta colombica obtain their magnetic sensors from soil?

How animals sense, process, and use magnetic information remains elusive. In insects, magnetic particles are candidates for a magnetic sensor. Recent studies suggest that the ant Pachycondyla marginata incorporates iron-containing particles from soil. We used leaf-cutter ants Atta colombica to test whether soil contact is necessary for developing a functional magnetic compass. A. colombica is the only invertebrate known to calculate a path-integrated home vector using a magnetic compass. Here, we show that A. colombica requires contact with soil to incorporate magnetic particles that can be used as a magnetic compass; yet, we also show that ants can biosynthesize magnetic particles. Workers from a soil-free colony ignored a 90° shift in the horizontal component of the geomagnetic field, yet oriented homeward despite the occlusion of any geocentric cues. In contrast, workers from a soil-exposed colony oriented to an intermediate direction between their true and subjective home in the shifted field. Homeward orientations under shifted fields suggest that ants calculated a path-integrated vector using proprioceptive information. Strikingly, ants from the soil-free colony also had magnetic particles; yet, as observed by ferromagnetic resonance, these particles differed from those in soil-exposed ants and were not associated with a magnetic compass sensitive to this experimental manipulation.