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Along with the migrant songbirds (the recent tanager, a female black-headed grosbeak, and a couple of yellow warblers, among others), I’ve been seeing a steady trickle of monarch butterflies in the neighborhood. Whether western monarchs actually migrate or not is a point of controversy. UC Santa Barbara emeritus professor Adrian Wenner has argued that what we have here is more of a range expansion and contraction between wintering roosts along the Central Coast and interior breeding areas. Unlike eastern monarchs, the western population doesn’t all wind up in the same place. 

As Art Sharpiro, UC Davis butterfly maven and author of a notable recent field guide, points out, it’s curious that there are no records of wintering monarchs in California before the twentieth century. You’d think someone would have noticed those huge aggregations. But the absence of evidence (although not evidence of absence) supports the idea that the monarch may be a fairly recent colonist along the West Coast. It also seems that no one has found genetic markers that reliably differentiate western and eastern monarchs, so they may still constitute one big gene pool. 

The journey of the eastern monarch is a different story. From all over temperate North America, these butterflies find their way each winter to a small patch of oyamel fir forest in the Mexican state of Michoacan. They do this with no prior experience of the journey and with a brain the size of a poppy seed. This navigational feat has challenged scientists for decades.  

Researchers concluded some time ago that monarchs orient themselves by the sun’s position in the sky. But the sun is a moving target; the butterflies need to keep recalibrating their solar fix based on the time of day. It was assumed that all of that happened within the organic microcircuitry of the brain. 

But there was one bit of anecdotal evidence that suggested something else might be going on. Here I should warn readers that the research I’m about to describe involves what some might consider physical cruelty to insect experimental subjects. The highly empathetic may want to stop here. The pain perception of butterflies (or lobsters, for that matter) is a can of worms I’m not about to open here. And I’m not necessarily endorsing the methodology in question, just reporting the results. 

Anyway, sometime in the 1950s entomologist Fred Urquhart clipped off the antennae of a group of monarchs. When they were released, they appeared disoriented and flew in random directions. 

Fifty-odd years later, Steven Reppert of the University of Massachusetts School of Medicine decided to follow this up. 

Reppert and his group had been working with a flight-simulation device, gluing monarchs to wires and suspending them in open barrels where they could see the sun but no other landmarks. Wire or no, they attempted to fly, and a monitoring device recorded their heading. Intact butterflies oriented toward the south or southeast, on the proper course for Mexico. 

When Reppert clipped his subjects’ antennae, they still flew in a single direction—but a random one, unrelated to the sun’s position. They appeared to have lost their time-compensation ability. Not completely, though: the clock genes in the butterflies’ brains continued to manufacture the protein that governs the recalibration. 

So the group took two more sets of monarchs. In one, the subjects had their antennae painted with black paint; in the other, with a clear enamel. The clear-enamel group oriented properly in the flight simulator, indicating that just having their antennae messed with didn’t throw them off. But the black-paint group tried to fly north to northwest. That suggested to Reppert that the onboard clocks were still functioning, but were off by a few hours because they hadn’t compensated for the sun’s movement. 

This wasn’t the first time an antennal clock had been discovered in insects. Previous studies had found that a similar mechanism enhanced the sensitivity of moths and fruit flies to pheromones at certain times of the day. Could an older system inherited from some remote common ancestor have been repurposed as a navigational tool? There are hints that honeybees may also use a time-compensated sun compass in wayfinding, although it’s not clear that the antennae are involved. 

But how on earth does it work? Reppert’s team found a light-sensing protein in the monarch’s antennal clock that had previously been identified as a magnetic field sensor in fruit flies. 

Like homing pigeons and sea turtles, monarch butterflies may be able to tune in to the earth’s magnetic field. It will be interesting to see where this research leads.