Observations on flight behavior and ultrasonic responses in moths (Lepidoptera: Erebidae and Geometridae) from Lapland

In the quiet corners of the natural world, a silent arms race has been unfolding for millions of years between bats and moths. Bats hunt in the dark, using high-pitched squeaks that bounce off objects to create a mental map of their surroundings, a skill known as echolocation. Moths, the primary prey, have evolved their own defenses to survive these aerial hunters. Some species have developed tiny ears on their bodies to hear the approaching bat, while others, particularly the tiger moths, can produce their own ultrasonic clicks. These clicks serve a dual purpose: they can act as a warning signal to tell the bat, "I am toxic and taste bad," or they can jam the bat's sonar, creating enough static to confuse the predator and allow the moth to escape. This dynamic relationship is a classic example of coevolution, where the predator's attack drives the prey's defense, and the prey's defense drives the predator's counter-attack. But what happens when the predator simply isn't there? In the far north, where the summer sun never sets, the rules of this game change completely, offering scientists a rare chance to see what happens to a species when the pressure to fight back is removed.

In the summer of 2026, a team of researchers traveled to Finnish Lapland, a region where the sun circles the sky for more than fifty days without ever dipping below the horizon. This continuous daylight creates a unique environment where insect-eating bats, which rely on darkness to hunt, are effectively absent. The researchers, led by Simeão de Souza Moraes, set out to test a simple but profound idea: if moths do not need to defend themselves against bats, do they lose the ability to do so? They focused on two groups of moths found in the area: the tiger moths, known for their ability to make clicking sounds, and the geometrid moths, which are generally silent but rely on evasive flight. The team captured individuals of species such as Diacrisia sannio and Arctia plantaginis, along with several geometrid species, and subjected them to a series of tests. They played recordings of bat attack calls near the immobilized moths and gently brushed them with a fine tool to simulate a predator grabbing them. At the same time, they used sensitive microphones to listen for any ultrasonic clicks the moths might produce in response.

The results were strikingly clear. None of the tested moths, not even the tiger moths which are famous for their acoustic defenses, produced a single ultrasonic click in response to the bat calls or the physical touch. This suggests that in these high-latitude populations, the acoustic defense mechanism has been lost or significantly weakened. Without the constant threat of bats hunting in the dark, the energy required to maintain these complex sound-producing organs and the neural pathways to trigger them appears to have been shed by evolution. The study also observed how these moths behaved while flying. In the tropical regions where the researchers had previously worked, nocturnal moths that lack chemical defenses fly quickly and erratically during the day to avoid birds, darting into hiding at the slightest disturbance. However, in the midnight sun of Lapland, the nocturnal geometrid moths behaved quite differently. During the hours when the sun was lower in the sky, creating a "night-like" period between 21:00 and 03:00, these moths flew slowly and calmly over the vegetation. They did not seem worried or hurried, moving with a leisurely confidence that would be fatal for a similar moth in a tropical forest.

This relaxed behavior was not matched by the local predators. The researchers noted that diurnal birds, which hunt during the brighter parts of the day, were active and feeding between 03:00 and 21:00. However, during the darker hours of the "night," these birds were conspicuously inactive, even though there was still enough light to see. The birds did not seem to have adapted their schedules to hunt during this time, leaving the slow-flying moths largely unharmed. The researchers suggest that this lack of predation pressure during the night-like hours has allowed the moths to retain their slow, exposed flight style without being eaten. It appears that the evolutionary clock has not yet ticked fast enough for the birds to learn to hunt in these strange, continuous daylight conditions, nor for the moths to relearn the frantic escape tactics needed to survive a constant threat.

The study concludes that the absence of bats and the limited activity of birds during the night-like period have relaxed the selection pressure that usually forces moths to be fast, quiet, or chemically defended. While the findings are preliminary and based on a short expedition, they offer a compelling glimpse into how quickly behavior and defense mechanisms can change when the rules of survival are rewritten by the environment. The researchers emphasize that these observations need to be confirmed over multiple seasons and in other Arctic locations to fully understand the depth of this change. For now, the evidence points to a world where the midnight sun has silenced the bats, and in turn, the moths have forgotten how to click, flying slowly and openly in a landscape where the old dangers have simply faded away.