A surprisingly large number and diversity of bee species – 74 out of 96 tested – have magnetic properties, according to research my colleagues and I recently published in the journal Science Advances.
Some animals are able to use iron-based magnetic compounds such as magnetite to detect and navigate via the Earth’s magnetic field – a sense called magnetoreception. We considered magnetism in the insects we tested to be a proxy for which species might be magnetoreceptive.
For decades, biologists have known that social, cavity-nesting honeybees exhibit magnetoreception. Most researchers assumed that this internal compass was tied to living in a colony; honeybees communicate the location of floral resources to other colony members through a dance that indicates direction relative to the position of the Sun and the geomagnetic field.
Our study had two goals: to compare magnetism between bee species that live in groups versus on their own, and to track down the evolutionary origin of magnetoreception in bees.
Finding magnetism wherever we looked
To test magnetic responses, we collected bee specimens from across the bee family Apidae, which includes social species such as honeybees along with solitary species such as chimney bees. We ground dried dead bees into a powder, then measured how magnetic this powder was in a magnetometer.
To our surprise, we found that the magnetic response was strong in both bees that live in groups and those that live alone. This result forced us to reject our initial hypothesis that magnetism was necessary only for social bee species.
Even more unexpected, a bee from a small social species in the family Halictidae was also strongly magnetic. We then broadened our search to include bees from across the bee evolutionary tree, suspecting that the evolutionary origin of magnetism could be found in older bee lineages.
We identified some trends regarding the strength of the magnetic response of the bees in our study. Larger bees tested as more magnetic. Social bees did tend to be more magnetic than solitary bees. And cavity-nesting bees tended to be more magnetic than ground-nesting bees.
Overall, though, we detected magnetism across all the different families of bees, for social and solitary bees, in nocturnal bees, and in bees that live in nests in the ground as well as those that live above ground in hives. Insects from other groups we examined for comparison, including beetles, wasps and flies, were also magnetic.
We again had to reject our hypothesis; this time, we demonstrated that magnetism probably predates the evolutionary origin of bees. We concluded that magnetism is probably an ancient, well-conserved trait.
What still isn’t known
Our work leaves a lot of unanswered questions.
For one, although we assume that the magnetic response is a proxy for magnetoreception, it is notoriously difficult to demonstrate this because it requires experiments with live organisms removed from their natural environment.
Magnetoreception is one of the most controversial animal senses. While there is good evidence that some organisms have the ability to detect and navigate along the magnetic fields of the Earth, it is probably not the primary sense used, even for organisms that do have magnetoreception. That makes it a challenging sense to isolate and study. Even in bumblebees, which biologists believe are magnetoreceptive, there remain many questions and doubts about their use of this sense.
Scientists are more certain that honeybees are magnetoreceptive – researchers have even trained them to discriminate between local magnetic anomalies. So we made the assumption that insects we tested that had stronger magnetic responses than honeybees are also magnetoreceptive. But we cannot prove it. Moreover, our work does not explain the function of magnetism, nor the mechanism behind magnetoreception.
And while the strength of the magnetic signal varied among body parts, it was never restricted to a single body part in the bees we tested. This means that some of the hypotheses for how magnetoreception operates – for instance, through light-sensitive cryptochromes in the eyes – are not well supported by our results.
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