One of the great mysteries of the natural world is how animals navigate long distance travels on land, sea, and air. We can’t get inside their head, so we have to observe and infer.
One intriguing example is the 4000KM migration of the Monarch butterflies, which travel in their thousands South to breeding areas in Mexico. To make this flight, the insects must orient to fly south or south west, as they fly over land and see they have never visited. In the spring, they fly north and north east, to summer habitat that many individuals have never visited.
Evidence suggests that Monarchs use the position of the Sun to determine the direction to fly. This requires some kind of timekeeping, because the position of the Sun changes through the day and seasons. How can a tiny, simple insect’s brain accomplish this task?
A recent study by Eli Shlizerman and colleagues proposes a detailed account of how the Monarch’s brain processes this data.
Earlier research has documented timekeeping functions in the Monarch antenna. Cyclical chemical reactions “tick”, and light sensative reactions synchronize the ticks with light (the sun). At the same time, the Monarch’s compound eye detects the position of the sun or other light source, which is signalled by neural firing to the brain.
This group tackled the fundamental question of “how the circadian clock interacts with the changing position of the sun to form a time-compensated sun compass that directs flight.” Their approach is to construct a model neural system that encodes the necessarily logical information. “This allows us to propose a mechanism, which uses a small number of neurons, to compare the firing rate of azimuthal neurons, responding to the luminance detected by the eyes, with neurons whose firing rate shows a circadian rhythm,”
Their model starts from mechanisms observed in other animals (if not necessarily in Monarchs), which would be the logical signals for luminance and rhythm: the neural input signals. The rest of the model is based on plausible inferences about neurons and insects.
They construct a simple control circuit which responds to sun position by signaling outputs that signify ‘turn left’ or ‘turn right’. Their mechanism steers southwest and by inverting the input signals, steers northeast. The model is somewhat elegant, in that it “supports only two flight directions: SW or NE”, and is the only stable way to put together these pieces.
The group developed an abstract flight dynamic equation that appears to model the behavior of the Monarch. The model uses input from the position of the sun to generate corrections to maintain a southwest heading. They examine the model across different input cases, including morning and evening, and cloudy.
Finally, the model was compared with the behavior of tethered butterflies, which showed similar patterns of disturbance and recovery in the direction. This agreement suggests that the abstract model may well represent how the Monarch nervous system operates.
This study is an interesting example of how computational modeling can be used to understand natural systems. This careful work gives us insight into how the Monarch nervous systems works, bolstered by physiology, neural, and behavior evidence.
But, interestingly, it does not actually demonstrate that any such neural mechanism exists in a Monarch.
Of course, we certainly know what to look for now. I expect that future studies will attempt to document these neural systems are present.
- Eli Shlizerman, James Phillips-Portillo, Daniel B Forger, and Steven M Reppert, Neural Integration Underlying a Time-Compensated Sun Compass in the Migratory Monarch Butterfly. Cell Reports, http://dx.doi.org/10.1016/j.celrep.2016.03.057