A new study examines how migratory monarch butterflies use an internal compass and skylight cues to navigate from eastern North America to Mexico each fall.
“In general, this sun compass mechanism proposes that skylight cues providing directional information are sensed by the eyes and that this sensory information is then transmitted to a sun compass system in the brain,” said Dr. Steven Reppert from the University of Massachusetts Medical School.
“There, information from both eyes is integrated and time compensated for the sun’s movement by a circadian clock so that flight direction is constantly adjusted to maintain a southerly bearing over the day.”
“The pattern of linearly polarized skylight is arranged as concentric circles of electric field vectors (E-vectors) around the sun, and they can indicate the sun’s position, even when the sun itself is covered with clouds,” he said.
“However, the symmetrical nature of the polarized skylight pattern leads to directional uncertainty unless the pattern is integrated with the horizontal position of the sun, called the solar azimuth.”
Dr. Stanley Heinze compared the neuronal organization of the monarch brain sun compass network to that of the well-characterized desert locust and found it to be remarkably similar.
Then he showed that individual neurons in the sun compass were tuned to specific E-vector angles of polarized light, as well as azimuth-dependent responses to unpolarized light.
“Our results reveal the general layout of the neuronal machinery for sun compass navigation in the monarch brain and provide insights into a possible mechanism of integrating polarized skylight information and solar azimuth,” concluded the authors.
“More generally, our results address a fundamental problem of sensory processing by showing how seemingly contradictory skylight signals are integrated into a consistent, neural representation of the environment.”