The monarch butterfly (Danaus plexippus) is one of the most well-known species of butterfly in the world (San Diego Zoo). Other than their iconic orange and black coloration, what makes them so special is their yearly migration across the United States. Every fall season, millions of monarch butterflies embark on a massive expedition to find a warmer place to live and outlast the winter. Some trips extend from the northwestern U.S. to California, with the longer and more common route extending from the northeastern U.S. and Canada to central Mexico. This mass exodus extends one generation and can cover upwards of 2500 miles (World Wildlife Fund)! Excluding the enormous distance this migration covers, what makes it so special is that no other species of butterfly makes a two-way trip like this, being comparable to migrations of many bird species, who travel at similar times for similar reasons (U.S. Forest Service). What is more amazing is how these butterflies go about finding their way to these warmer microclimates in California and central Mexico.
In order to know where they are going, they position themselves relative to the sun. Using their antennae, they can accurately determine the time of day, and position themselves accordingly. Based on these antennal clocks, in the mornings, they keep the sun on their left, at noon there is a specific range of 47-56º that they align the sun in, and in the afternoon, they fly with the sun on their left as they travel south (Parlin, Adam F., et al., 2022). But there is a huge problem. Human light pollution confuses monarchs, making them travel off-course. This is especially problematic at night, when the butterflies are tricked into traveling when they should be resting, being sent in the wrong direction, and messing up their circadian clocks (Parlin, Adam F., et al., 2022). This is such a big issue because these monarchs are wasting precious time and energy that they need to race against the coming cold temperatures. In fact, the efficient utilization of energy is so important to them that all the monarchs do during the day is eat and fly south. They even refrain from mating during these migrations because they need all the energy they can get to cover such vast distances (Villa, Scott M., et al., 2022).
Researchers began testing the effects of light pollution on migrating fall monarchs (Parlin, Adam F., et al., 2022). For their experiments, they used a dark room with a single halogen bulb, a common culprit for nighttime light pollution (NLP). They would then place butterflies in the room to see if they used the light to orient themselves for migration. Sure enough, the monarchs flew based on the orientation and elevation of the bulb, similarly to the way they use the sun (Figure S4, Parlin, Adam F., et al., 2022). They found that in the dark, the butterflies were easily influenced by the light and flew as if it was the morning. Moreover, with this new “nightly” exposure to light, the antennal clocks of the monarchs had false readings. As a result of this, their circadian rhythms became compromised, and they were making proteins and catalyzing other bodily functions to prepare their bodies for another day of constant flying. Although, it was not the day, and the monarchs, should they be released from these experiments, would likely get lost among the streetlights and other sources of NLP. This ongoing struggle for accurate navigation by monarch butterflies is just one of the many anthropogenic issues affecting animals. And, like other problems caused by humans, they lead to drastic decline in population sizes and/or behavior. The monarch butterfly is no exception, their numbers being in stray decline for decades (San Diego Zoo). Hopefully, future efforts to reduce light pollution may help increase their numbers by providing safer and less perplexing traveling experiences.
Works Cited
“Monarch Butterfly.” San Diego Zoo Wildlife Alliance Animals and Plants, https://animals.sandiegozoo.org/animals/monarch-butterfly.
Parlin, Adam F., et al. “Oriented Migratory Flight at Night: Consequences of Nighttime Light Pollution for Monarch Butterflies.” IScience, Elsevier, 27 Apr. 2022, www.sciencedirect.com/science/article/pii/S2589004222005818.
“U.S. Forest Service.” Forest Service Shield, www.fs.usda.gov/wildflowers/pollinators/Monarch_Butterfly/migration/index.shtml
Villa, Scott M., et al. “Lack of Inbreeding Avoidance during Mate Selection in Migratory Monarch Butterflies.” Behavioural Processes, Elsevier, 2 Apr. 2022, www.sciencedirect.com/science/article/pii/S037663572200047X?via%3Dihub.
Watt, Liz. “The Great Monarch Migration.” WWF, World Wildlife Fund, 16 Mar. 2021, www.worldwildlife.org/stories/the-great-monarch-migration
