Newswise — Immerse yourself within the exoskeleton of a bumblebee momentarily: your existence would encompass a plethora of hues and fragrances, both pivotal to navigate your quest for pollen and nectar. Bumblebees possess exceptional sight: boasting a duo of compound eyes adept at discerning UV and nearly all hues except crimson, alongside three supplementary uncomplicated eyes designed for perceiving polarized light. Their olfactory prowess eclipses ours: roughly 100 times more acute, capable of detecting illicit substances or explosive materials at airports, affirming gestation in women, or identifying malignancies and diabetes in patients at an early stage.
Presently, scientists have demonstrated that bumblebees can additionally employ their olfactory capabilities to locate their dwelling. This becomes particularly significant when the surroundings undergo an abrupt transformation, such as when familiar visual landmarks are obliterated by gusts of wind. The findings of this research have been published in Frontiers in Behavioral Neuroscience.
"Within this study, we demonstrate that bumblebees heavily depend on their own scent markings, which they leave at the entrance of their nest before embarking on a foraging expedition, in order to navigate back to their abode when visual cues prove to be inadequately reliable," stated Sonja Eckel, a PhD student at the Department of Neurobiology at Bielefeld University in Germany, and the first author of the published research.
Visual landmarks
Eckel and her colleagues conducted a study on the homing behavior of the buff-tailed bumblebee, Bombus terrestris, within a laboratory setting. In its natural habitat, these bumblebees construct their nests in abandoned mouseholes, concealed beneath grass or leaves. In the experiment, the foraging bumblebees were trained to locate their nest entrance by visually orienting themselves using two distinct sets of landmarks within a circular enclosed flight arena measuring 150cm in diameter and 85cm in height. The first set of landmarks comprised three black vertical stripes, each spanning 12cm in width and 85cm in length, positioned against the white background of the arena walls. The second set consisted of three cylinders arranged in a triangular formation around the entrance hole, with each cylinder measuring 2.5cm in width and 15cm in height.
The floor of the arena lacked any visual cues as it was adorned with a randomized pattern of red and white, appearing as black and white to the bumblebees. With practice, the worker bumblebees developed the ability to fly directly back to the nest entrance upon their return from the outer foraging chamber, where they had access to pollen and nectar.
Bumblebees possess numerous scent glands that release chemicals throughout their bodies. Prior studies have indicated that whenever they come into contact with a surface, such as their nest entrance, they inadvertently leave behind scent marks. To capture these scent marks, the researchers implemented a glass ring around the nest entrance. Foragers would often traverse this ring while departing from or returning to the nest, allowing the researchers to collect and analyze the scent marks left by the bumblebees.
Tricking the bumblebees
Next, Eckel and her colleagues devised an experiment to deceive the bumblebees by suddenly altering the positions of both sets of visual landmarks, completely independent of one another. Consequently, the modified landmarks provided conflicting information regarding the location of the nest entrance, with neither set accurately indicating the correct spot. The true entrance was concealed and effectively blocked, resulting in none of the returning foragers being able to locate it. The researchers achieved success in preventing the bumblebees from finding the hidden entrance.
The researchers conducted measurements to determine the duration and proximity of hovering behavior exhibited by the returning foragers around each false location for the entrance, taking into account the conflicting visual landmarks. They hypothesized that the longer a forager spent hovering around a particular spot, and the shorter the average flight distance to that spot, the more likely it was that the forager considered that spot as the most promising candidate for the location of the entrance. These measurements were used as indicators of the foragers' focus and attention towards a specific spot.
In general, the foragers displayed comparable levels of focus on both false locations, indicating their attempt to rely on both sets of visual landmarks in their quest to relocate the nest, albeit unsuccessfully. However, a significant shift in behavior was observed when the researchers introduced the glass ring, which carried bumblebee scent marks, around one of the locations. In this scenario, the foragers exhibited a pronounced and overwhelming preference for the false location indicated by the scent marks. The presence of the scent marks strongly influenced the foragers' perception and led them to concentrate their attention on the location associated with the scent marks, despite it being a misleading indication of the nest entrance.
The scientists determined that foragers employ both their olfactory sense and eyesight to navigate back to their dwelling, particularly when confronted with conflicting visual cues.
Bouquet of scents
"According to Eckel, visual cues are effective in guiding bumblebees towards the general vicinity of the nest, covering greater distances. However, scent markers play a crucial role in precisely locating the nest entrance at close proximity. Eckel suggests that physical contact is likely required for scent identification."
"Our chemical examination revealed that this aroma consists of a combination of hydrocarbons, fatty acids, and various compounds like esters and alcohols. Numerous of these substances are recognized for their utilization by bumblebees in diverse behavioral scenarios, as well as by other insect species."
"In our subsequent study, our objective is to explore the process by which bumblebees acquire and integrate diverse sensory cues, enabling them to differentiate between various behaviorally significant objectives, such as locating the nest entrance and food sources."
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