The Eyes Have It: How Butterflies Navigate to Suitable Habitat
By Andrew Porterfield
For populations of any animal, the ability of far-flung members of a species to find the right habitat can be crucial to that species’ survival. How animals do this is an important question in ecology, and butterflies have proven a valuable model for studying how species members can individually find suitable living and feeding space.
But how butterflies detect and navigate their way to the right habitat remains something of a mystery. Past studies have released insects at various distances from a target habitat and studied flight behaviors, assuming certain limits to perception of these habitats. These studies have not examined how each type of sense—such as vision, olfaction, or wind perception—contribute to navigation, especially through unsuitable habitat.
To help answer this question, Zachary MacDonald, a Ph.D. student at the University of Alberta in Edmonton, Canada, led a research team that examined visual perception and navigation over water (an unsuitable habitat) to an island (a suitable habitat) in Ontario, Canada. The team published its results in June in the open-access Journal of Insect Science.
MacDonald and his team found that, in the butterfly species Speyeria cybele and Speyeria atlantis (sometimes known as the great spangled fritillary and the Atlantis fritillary, respectively), visual perception was the primary sense used to navigate to habitats. In addition, his team found no “outer” distance beyond which the butterflies could not find their way home; instead, their homing ability depended on several environmental and other factors.
The team’s experiments consisted of two parts:
First, the researchers collected 41 S. cybele and 54 S. atlantis butterflies and released them from a motorboat, at various distances from a 2.5-acre target island in Lake of the Woods, Ontario. The release distances were 30, 40, 50, or 60 meters. Each insect was visually tracked until it reached the target island or flew out of range.
Then, to determine the role of vision in the butterflies’ trek for habitat, the researchers induced flash blindness, exposing a select group of butterflies’ compound eyes to intense photographic flashes before they were released. The flash bleached rhodopsin compounds in the insect eyes, effectively blinding them.
Unflashed butterflies overall reached the target island, but their navigation ability decreased as their release distance increased. At 60 meters, half of the S. cybeles reached their destination, while at 50, 40, and 30 meters, 54.5 percent, 85.7 percent and 80 percent succeeded, respectively. In comparison, no S. atlantis insects succeeded from 60 meters. Their success was 16.7 percent at 50 meters, and half at 40 and 30 meters.
For flash-blinded butterflies, only 11 percent of S. cybeles and no S. atlantis could navigate to the island.
The decrease in navigation ability with increased distance from the island was not a surprising finding. But the effectiveness of flash blindness on navigation was unexpected, MacDonald says. “I was surprised at how effective the flash method was at inducing flash blindness and rendering butterflies unable to detect the nearby habitat patch. The effect was so obvious that, at times, it felt silly using statistics to draw our most basic inferences—a simple bar plot shows it all! After we induced flash blindness, butterflies could still fly effectively (i.e., stay above the lake surface and maintain elevation), but couldn’t find a nearby patch of habitat, even if it was within 20 meters of them. By contrast, unflashed butterflies navigated successfully to the patch almost every time.”
While MacDonald’s team studied navigation in just two species, he believes the findings could apply to other butterflies. “The further away a butterfly is from a habitat patch, the less likely it will be to successfully navigate to the habitat patch. However, specific distances at which individuals might consistently navigate is likely contingent on many variables, unique to the environmental, landscape, and possibly even the individual,” he says.
The study, while indicating visual perception is a very strong factor in successful navigation, may not entirely rule out the role of other butterfly senses.
“It is certainly possible that butterflies employ other senses for long-range detection of particular resources, including nectar and host plants, so long as the attractive odor is in sufficient quantity and the individual is able to quantify the direction of the wind,” MacDonald says. “Based on our work, I would say that butterflies may use olfaction in certain instances to gain approximate information on the location of nectar and host plant resources but that their actual navigation of fragmented landscape (at scales they are most likely to encounter) is likely accomplished with use of visual senses.”
Journal of Insect Science
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.