Hilltopping Behavior In Tiger Moths

Naturalists have long marveled at the beauty of butterflies fluttering about verdant fields, blossoming gardens, and urban green spaces. Watching a brightly-colored butterfly alight on an equally brilliant flower and unfurl its long proboscis to get a drink of sweet nectar is an experience shared by people across all walks of life. Simply enjoying such visually appealing displays of nature is as far as most people engage in lepidoptery in modern times.  At the turn of the 19^th century, however, butterfly collecting was a common leisure activity.

Avid collectors quickly recognized that hilltops and ridges were often associated with high abundances of butterflies and, consequently, a good place to capture them for future display. Curiously, butterflies found within these aggregations were predominately male. The prevailing hypothesis posited that updrafts would passively cause butterflies to aggregate around these elevated areas. While seemingly reasonable, this hypothesis failed to explain the disproportionate number of males found within aggregations.  This hypothesis was challenged by Oakley Shields in the 1960s.

Through careful observations and manipulative studies, including those where Shields captured, marked, released, and subsequently recaptured individual butterflies, he gathered evidence to support an alternative hypothesis: butterflies navigate to and aggregate on hilltops, thereby increasing their likelihood of encountering mates; males  generally remain at these sites and copulate with several females, while females return to low-lying areas to locate suitable host plants on which to lay eggs and consequently skew the sex ratio. This mate-locating strategy generally referred to as “hilltopping” is now widely accepted and described in numerous insect taxa. In effect, hilltops are the nightclubs of the insect world.

For humans, finding nightclubs, pubs, and other gathering places of the opposite sex is as easy as using Google Maps and Uber.  But how do insects navigate to summits and know which summits afford the highest chance of finding a mate in a topographically complex landscape?

To answer this question, Guy Pe’er and colleagues followed released butterflies and observed their behavior. They found that butterflies used topographical cues to gain elevation; they would follow an elevational gradient, continually turning to ensure they moved uphill until they reached a summit. They hypothesized that simple movement rules in response to elevational changes would lead to lead to spatial distribution patterns like the aggregating of individuals on summits, a spatial pattern seen in hilltopping species.

To test this hypothesis, researchers programmed simulated butterflies using these movement rules and released them in a virtual landscape. As expected, the simulated butterflies navigated to and aggregated around a few, relatively high elevation summits.  This work demonstrated that a seemingly complicated mating behavior, which leads to complex collective movement, was achievable with a few simple behavioral responses to a limited number of parameters.

Based on these simulations and seminal empirical experiments, we developed and tested four predictions regarding movement behavior and the resulting spatial distribution of summit aggregations using a day-flying tiger moth in Sonoma County, California. We caught, marked, and released over 1000 moths, recapturing them over several weeks to track their movement. In agreement with our predictions, we found the highest densities of moths were associated with a just few, high-elevation summits. Individuals within these aggregations were predominately male and post-mated females spent less time within aggregations than their male counterparts.

Interestingly, we found no movement between summit aggregations, suggesting that once an individual arrived at an aggregation it remained until death or leaving to lay eggs.  The most interesting finding was that the density of moths at an aggregation was better explained by summit elevation than proximity to larval patches. Showing population-level preferences for aggregation site characteristics, such as elevation, likely increases mating success. Additionally, finding evidence of hilltop preferences further demonstrates that movement is not random, which has important implications for population dynamics and regional persistence.

To reduce complexity, many spatial population models assume that animal movement such as dispersal is a random process. However, field studies have demonstrated that this assumption is often violated; food, competitors, and predators can influence behavioral decisions associated with aspects of dispersal, including departure, flight path, and settlement. Because most models ignore individual behavior, the prevalence of nonrandom, behaviorally based dispersal and its consequences on population dynamics across a region of is not well understood.

Because the movement exhibited by some hilltopping species is predictable, it is gaining attention as a model system to study nonrandom dispersal. Incorporating realistic dispersal behavior into population models may increase their overall predictability and consequently, their utility for conservation management.

These findings are described in the article entitled Testing predictions of movement behaviour in a hilltopping moth, recently published in the journal Animal Behaviour. This work was conducted by Patrick Grof-Tisza, Zack Steel, Marcel Holyoak, and Richard Karban from the University of California, Davis, and Esther M. Cole from Stanford University.