A monarch butterfly flying on a fixed bearing relative to a green LED in the flight arena. Photo credit: Myriam Franzke.
While the logistics of international travel remain off-putting as the Covid-19 pandemic ebbs and flows, creatures that routinely circumnavigate oceans and continents have continued undaunted. Arctic terns are still smashing long distance records while wildebeest move en masse in colossal herds. But the sovereign of the multigenerational migration has to be the monarch butterfly. Each spring, the original pioneers depart their overwintering grounds in Mexico for their Canadian summer home, yet it is their great-great-great-grandchildren that return ∼8 months later. ‘To keep a constant direction over this enormous distance, these animals rely on the sun for orientation’, says Myriam Franzke from the University of Wuerzburg, Germany. However, when the insects near their final destination, they may switch to other forms of guidance to select their ultimate landing site. Yet, no one had tested whether monarch butterflies can use other forms of navigation, beyond being directed by the sun, so Franzke, Christian Kraus, Maria Gayler, Keram Pfeiffer and Basil el Jundi, from the University of Wuerzburg, with David Dreyer from Lund University, Sweden, teamed up to test out the insect's homing instinct.
‘We ordered the butterfly pupae from Costa Rica where they were raised in captivity’ says el Jundi, recalling that the adults began emerging from their chrysalises 3 days after their arrival in Germany. ‘It is always a remarkable moment when the small chrysalises turn into colourful butterflies with large wings’, says Franzke. Next, the team gently tethered individual butterflies at the centre of a cylindrical flight arena, allowing the insects to swivel in any direction as they took to the wing. Then, the scientists tested which directions the insects selected when they illuminated LEDs lining the wall of the arena, providing the insects with different views.
Switching off three LED columns to show a 4 cm wide vertical dark band against a bright background, the team filmed the butterflies’ manoeuvres for 8 min, switching the position of the bar to the opposite side of the arena from time to time to make sure that the insects were still paying attention. Although the butterflies took hardly any notice of the bar initially, swivelling continually in different directions, by the end they were occasionally holding a steady direction toward one edge of the bar, before veering off again.
In contrast, the monarchs seemed far more fixated by a bright vertical bar against a dark background. Some flew directly toward the light for the full 8 min, only switching direction when the bar shifted spontaneously to the other side of the arena: they were definitely attracted to the bright object. But how would the insects fare when the team presented them with a single illuminated green LED, which the butterflies mistake for the sun?
This time, the insects set a bearing relative to the fake sun and tended to keep to that path, rotating through 180 deg each time the simulated sun jumped to the other side of the arena. They were using the bright light as a compass guiding their choice of direction, as if they were dispersing toward Canada or returning to Mexico to overwinter. And when the researchers alternated the bright vertical bar with the simulated sun, the butterflies flew persistently toward the bar, but switched direction and set a bearing when the fake sun flashed into view.
‘[Monarch] butterflies can switch between compass orientation and attraction’, says el Jundi, and the team suspects that the insects depend on specialised guidance circuits in the brain when navigating over long distances by the sun, but have the ability to switch with ease and simply gravitate toward an object when close to home.
