MIGRATION

Skylight Cues

Migrating monarch butterflies use the sun or some aspect of the daylight sky as a guidepost for sun compass orientation. The angle of plane polarized light (the electric [E]-vector) is detected by photoreceptors in the dorsal rim area of the compound eye, and then processed and translated by the sun compass structure in central brain into actionable information about the butterfly’s orientation. The sun or spectral gradients are also used, and are detected by the main retina. We are working to decipher exactly how the monarch’s sun compass informs its southerly migration.

Side view of monarch eye, adapted from Monica R., flickr.com

For most of their journey to Mexico, monarch butterflies use the horizontal position of the sun itself in the sky (the solar azimuth) to steer a steady course to the south. When the sun is obscured by clouds or mountains, polarized light provides a backup system to guide the butterflies, as long as patches of blue sky can be observed.

Skylight is polarized to different degrees depending on its location in relation to the sun. This means that the sky has distinctive E-vector patterns, much like the distinctive ridges of a fingerprint. Skylight polarization patterns change over the course of the day. Skylight is most polarized at a 90-degree angle from the sun. At noon, when the sun is high in the sky, the most polarization occurs at the horizons. At sunrise and sunset, when the sun is near the horizon, the most polarization occurs at the zenith.

Some insects, including monarchs, have specialized photoreceptors in the dorsal rim area of the eye, as mentioned above, that can detect skylight E-vectors. By sensing the shifting pattern of polarized skylight, an insect can locate the sun’s position throughout the day, even if the sun itself is obscured.

Using flight simulator experiments, we have demonstrated that by manipulating the pattern of polarized light, we can manipulate monarchs’ ability to steer using the time-compensated sun compass (Sauman et al., 2005). This flight simulator video captures this phenomenon. The monarch is viewed from a camera at the bottom of the flight simulator barrel; the donut-shaped object visible above the butterfly is a linear polarizing filter. The barrel is sized and the experiment is timed so that the sun itself is not visible to the butterfly.

First, the polarizer is placed parallel to the sky’s polarization plane at the zenith. The monarch  orients to the southwest, just as it would during migration. Then the filter is rotated 90 degrees so that it is perpendicular to the zenith polarization plane; the monarch adjusts its orientation accordingly. Next, the filter is rotated another 90 degrees back to parallel but this time a UV-interference filter blocks any ultraviolet light from entering the simulator (polarized light sensitivity is mediated by UV-sensitive photoreceptors). Without any information about skylight polarization, the monarch is disoriented and can’t compensate for the polarization filter rotation. When the UV-interference filter is removed, the monarch adjusts its orientation in accordance with the polarization filter.