Disruptive effects of brief radiofrequency noise exposure on migratory bat navigation.

A new study from the University of Oldenburg examines whether frequencies that are known to disrupt bird navigation also affect bat navigation. Previous research has shown that very weak broadband signals in the 50 to 85 MHz range disrupt the light-dependent magnetic compass of songbirds, which relies on cryptochromes in the retina. While this frequency range does not overlap with that of mobile communication technologies, it is more prevalent in urban than in wilderness areas due mostly to power lines and FM radio. Disorienting effects were observed at levels well below current ICNIRP reference levels. Although it has been clearly demonstrated that the magnetic compass of birds is based on cryptochrome 4 (CRY4), the magnetic sense in mammals has been studied less extensively. Some studies on rodents and other mammals suggest the use of cryptochrome 2 (CRY2). Therefore, it is important to understand how wild mammals respond to electromagnetic radiation that is considered safe for humans according to ICNIRP guidelines. Thus far, the effects of artificial electromagnetic fields on humans and animals have primarily been studied under conditions of direct exposure. It remains unclear whether exposure could cause behavioral effects even after the field has faded. Bats are an ideal model species for this type of research because they share many similarities with small, nocturnal migratory birds, including the distances they travel and the ecological challenges they face.

The experiments were conducted over the course of four fall migration seasons along the Baltic Sea coast from 2021 to 2024. A total of 165 adult Pipistrellus pygmaeus bats were captured at the Pape Ornithological Station in Latvia. For the experiments, the scientists selected two sites overlooking the Baltic Sea that were about 100 m apart. To prevent the bats from seeing the sky before or after treatment, the scientists carefully covered them with black linen bags, except when they were in the exposure cages. In 2021 and 2022, electromagnetic noise exposure occurred at sunset during the calibration phase. In 2023, EMF exposure took place after sunset. Thus, all bats were able to observe the sunset from the same location. In the 2024 season, EMF exposure was limited to the initial phase. The same equipment was used for EMF exposure in all four seasons. The setup remained identical during the first three seasons. The bats were placed in a non-magnetic box 1 m from a transmitting antenna (a double-turn induction loop). The transmitting antenna was connected to a signal generator that emitted electromagnetic noise (0.01–300 MHz, field strength 1 nT or 0.3 V/m). However, in the fourth season, the experimental setup was modified so that the bats were only exposed to EMF noise after being released from an orientation apparatus, a circular arena inside a converted Mongolian yurt. After placing a bat inside the yurt, the experimenter released the animal from outside. This setup allowed the experimenter to determine the bat’s orientation based on the footprints it left on a thin layer of chalk covering the arena's entire circumference. The mean orientations of all groups were analyzed using Oriana software for circular statistics. The Rayleigh test was used to assess whether any of the data sets of a tested group deviated from a uniform circular distribution. To evaluate the overall navigation of the bats, an analysis of data dispersion was conducted across all years of the study. Hypothesis: The authors expected the "sunset" and "departure" groups to exhibit impaired orientation and fly off in random directions later in the night. (Previous studies by the same research group on this species of bat revealed that the bats must calibrate their magnetic compass based on the sun’s position at sunset to ensure proper functioning.) Conversely, the authors predicted that the bats in the "after sunset" group would orient themselves similarly to the control animals because they had uninterrupted access to environmental cues during the critical phase at sunset and during their subsequent flight that night.

In the first year of the study, the bats in the control group flew northward. In the second, third, and fourth years, however, their average flight direction was southeast, southwest, and south, respectively. Significant deviations from a uniform pattern were observed in each of the control groups across the four migration seasons, but not in the treated groups. Bats that observed a natural sunset while exposed to EMF subsequently flew off in random directions in both seasons (uniform circular distribution). Contrary to the hypothesis, the bats did not exhibit oriented takeoff behavior after experiencing a sunset under natural geomagnetic field conditions followed by exposure to EMF and release within 2 to 6 hours. Their flight directions were indistinguishable from a uniform distribution. This contrasts with the control group, whose distribution resembled a symmetrical, southward-oriented pattern. A clear difference was observed between the combined control and treatment groups across all years of the study.

The experiments described here demonstrate that exposure to low-level electromagnetic noise disrupts the orientation of bats for several hours after exposure. This has far-reaching consequences and raises numerous questions. The results imply that residual effects of weak broadband radiofrequency (RF) noise influence the movement behavior and navigational decisions of animals. The fact that orientation is disrupted regardless of whether exposure occurs during the critical calibration phase or afterward suggests that the result is based on an interaction between the effect on the magnetoreceptor and the subsequent behavioral interpretation of that effect. However, the exact mechanisms by which brief exposure to electromagnetic noise can cause the magnetic compass of an animal to malfunction for several hours remain unclear. According to the simple cryptochrome-based magnetic compass model involving radical pairs, this should not be possible. Therefore, this discovery highlights a discrepancy between quantum mechanical predictions and animal behavior or between theory and experiment. The effects of such exposure on bats are largely unknown yet could play a role in phenomena such as the so-called "invasions" of pipistrelle bats. These invasions have been observed with increasing frequency in European cities since the late 20th century. The authors point out one limitation: they tested migrating bats in a flight chamber without orientation cues. In other words, the bats had no access to other navigational cues, such as the stars or natural sounds. Therefore, it is possible that additional orientation cues in a natural environment could mitigate the disorientation caused by radiofrequency radiation. Regardless of the cause of the observed carryover effect, the potential ecological consequences of the demonstrated disruption are concerning because current exposure limits apply exclusively to humans. This leaves wildlife vulnerable even within these limits.

Editor’s note:

A cryptochrome-based magnetic compass is generally only useful during the day (and in dry weather) because cryptochrome requires sunlight to activate. Whether blue or red light is needed depends on the type of cryptochrome present (CRY2 or CRY4). Some insects have a mechanism that circumvents this limitation, although the details are still unknown [1]. Since the Earth’s magnetic field has undergone catastrophic reversals throughout geological history, and since solar flares can disrupt the Earth’s natural magnetic field (and Schumann resonance) for an average of several days each year, most animals seem to possess two types of magnetic sensor: cryptochromes and magnetite crystals. Animals also use various (sensory) methods to calibrate their magnetic compass, such as the position of the sun or constellations in the night sky. This new study of bats produced results similar to those of previous studies of migratory birds, suggesting that bats, like birds, use cryptochrome (specifically CRY2, which is found in both mammals and birds, but not CRY4, which is only found in non-mammalian species, such as birds, reptiles, amphibians, and fish) to navigate using a magnetic compass. Several species, including turtles, termites [2], and humans [3, 4], possess both a cryptochrome compass and a magnetite compass. Of concern is the fact that disruption of the magnetic sense in migratory birds, as shown here in bats, occurs at very low field strengths, such as 1 nT , 0.3 V/m, and approximately 240 μW/m² for radio waves in the FM band and longer wavelengths at lower frequencies. It is also concerning that the disruption persists for several hours after EMF exposure. Further research into the magnetic sense of bats and other mammals would certainly be worthwhile, given that humans also belong to this group. (AT)

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2. Gao Y, Wen P, Cardé RT, Xu H, Huang Q (2021). In addition to cryptochrome 2, magnetic particles with olfactory co-receptor are important for magnetic orientation in termites. Communications Biology, 4, 1121. https://doi.org/10.1038/s42003-021-02661-6

3. Chae KS, Kim SC, Kwon HJ, Kim Y (2022). Human magnetic sense is mediated by a light and magnetic field resonance-dependent mechanism. Scientific Reports, 12, 8997. https://doi.org/10.1038/s41598-022-12460-6

4. Wang CX, Hilburn IA, Wu DA, Mizuhara Y, Cousté CP, Abrahams JN et al. (2019). Transduction of the geomagnetic field as evidenced from alpha-band activity in the human brain. eNeuro, 6(2), e0483-18.2019. https://doi.org/10.1523/ENEURO.0483-18.2019