Writer: Dilvin Lacin
Migration is a phenomenon observed in various animals, including vertebrates, insects, and marine invertebrates. Animals of all sizes migrate and make up a significant portion of the world's animal population. Although some species make just regional movement, great distances such as across continents and even hemispheres are traveled by different species through swimming, flying, running, or walking. Butterflies which fascinate all of us are one of these migratory species. In this article, we are going to discuss the monarch butterflies and their sense of navigation. (Chowdhury et al al., 2021)
Migration
Migration is a vital process that allows animals to travel to more sustainable habitats before or during the deterioration of their present habitat. This synchronizes with the regular schedule. Animals possess morphological, physiological, and behavioral traits that make possible migration. For instance, many migratory animals have an internal “biological clock” that schedules the various components of migration. To find their species- or population-specific seasonal areas, migrating animals need to combine temporal and spatial information elements, known as a spatiotemporal migration program. (Chowdhury et al al., 2021; Brower et al., 1995; Reppert et al., 2018; Reppert et al., 2010)
Butterflies Migration Movements
Butterflies are relatively large insects that are often colorful and active during the day. They are more easily observed and recorded than other insect groups. While some species of butterflies are highly sedentary, others are known for their long-distance migrations. For example, Euphydryas checkerspot butterflies are usually confined to specific colonies with only short-distance dispersal, while monarch butterflies travel about 5000-6000 km annually over 3-5 generations. Painted ladies migrate even further, covering up to about 15,000 km in round-trip migrations from Northern Europe to West Africa over at least 6-7 generations. Movement patterns can vary significantly among species and populations of the same species. (Chowdhury et al al., 2021)
Butterflies can be categorized into two main types based on their movements: Flying within their local habitats and traversing their local habitats or other defined areas, which is equivalent to the home range of vertebrates. Butterflies that exhibit the second type of behavior are often considered migrants. The primary difference between migrant and non-migrant butterflies is that the former generally do not get distracted by appetitive signals such as food or sexual signals. However, migratory butterflies often pause their journey to collect nectar and then resume their flight towards their destination. In vertebrates, migration usually implies an outward and return journey by the same individual. However, due to the short lifespan of butterflies, the typical model involves successive generations executing long-distance directional movements resulting in an overall change in the distribution of a population in response to an environmental cue. (Chowdhury et al al., 2021)
Navigation sense of Monarch butterflies
Monarch butterflies are known for their impressive navigation abilities despite having a relatively small brain. Migratory animals are classified as either true navigators or compass navigators. True navigators, such as white-crowned sparrows, loggerhead sea turtles, and spiny lobsters, know both the direction of travel and their geographical locations about their goals. Compass navigators, on the other hand, use compasses to orient themselves towards their destinations. Millions of monarch butterflies use compass navigation during their long-distance migrations to their wintering grounds, utilizing predictable environmental cues available throughout the day during migration. Monarchs have two compass systems for proper orientation—one primarily based on skylight cues and a clock, while the other utilizes a magnetic cue. However, it is still unknown whether monarchs possess a true “map” sense. How migratory individuals find their wintering sites from year to year remains a fascinating migration mystery. One interpretation of tagging studies conducted over the last 50 years suggests that fall migrants may indeed employ a map sense. Monarchs from different breeding areas consistently exhibit distinct southward orientation directions. Generally, those from the Northeastern U.S. orient southwest, while those from the Midwest/West are more likely to orient due south or even southeast. However, a study reported that migrants longitudinally displaced west by 2,500 km across Canada do not adjust their orientation to compensate for the displacement, sparking debate among researchers about the experimental design. More extensive displacement studies are needed. (Brower et al., 1995; Reppert et al., 2018; Reppert et al., 2010; Froy et al., 2003; Nguyen et al., 2021)
Using a Time-Compensated Sun Compass
Monarch butterflies use a time-compensated sun compass as their primary compass system for orientation during both the southward fall migration and the northward spring remigration. The sun's position is the dominant skylight cue used for orientation. Due to the Earth's daily rotation, the sun's azimuthal (horizontal) position appears to move across the horizon from East to West. For an animal to use the sun's position for a fixed, daily heading, a circadian clock has evolved as a timing mechanism for adjusting or compensating for the sun's movement. Therefore, the sun compass is time-compensated. On partly cloudy days, when the sun is obscured but the blue sky is still visible, scattered sunlight generates the skylight polarization pattern, which may provide a backup directional cue. The monarch butterfly's genetically determined, unrelenting directional orientation, day in and day out, has made it an ideal system for studying the mechanistic details of this compass system. (Reppert et al., 2018; Reppert et al., 2010; Froy et al., 2003; Guerra et al., 2012; Nguyen et al., 2021)
Conclusion
In this article, we discussed the general migration movement of butterflies and the navigation sense of monarch butterflies. Insect migration is much less well studied than migration in birds and other vertebrates, despite its potentially huge importance to ecological resource flows and ecosystem services globally. I hope that the number of case studies is going to be improved.
References
1- Chowdhury, S., Fuller, R. A., Dingle, H., Chapman, J. W., & Zalucki, M. P. (2021). Migration in butterflies: a global overview. Biological Reviews, 96(4), 1462-1483
2- Brower, L. P. (1995). Understanding and misunderstanding the migration of the monarch butterfly (Nymphalidae) in North America: 1857–1995. Journal of the Lepidopterists' Society, 49, 304
3- Reppert, S. M., & de Roode, J. C. (2018). Demystifying monarch butterfly migration. Current Biology, 28(17), R1009-R1022
4- Reppert, S. M., Gegear, R. J., & Merlin, C. (2010). Navigational mechanisms of migrating monarch butterflies. Trends in Neurosciences, 33(9), 399-406. https://doi.org/10.1016/j.tins.2010.04.004
5- Froy, O., Gotter, A. L., Casselman, A. L., & Reppert, S. M. (2003). Illuminating the Circadian Clock in Monarch Butterfly Migration. Science, 300(5623), 1303-1305
6- Guerra, P. A., Merlin, C., Gegear, R. J., & Reppert, S. M. (2012). Discordant timing between antennae disrupts sun compass orientation in migratory monarch butterflies. Nature Communications, 3(1), 958
7- Nguyen, T. A. T., Beetz, M. J., Merlin, C., & El Jundi, B. (2021). Sun compass neurons are tuned to migratory orientation in monarch butterflies. Proceedings of the Royal Society B, 288(1945), 20202988