Part 1 of 2 in the fence lizard fire ant saga: Rapid evolution of fence lizards (Sceloporus undulatus) in response to selective pressures imposed by red imported fire ants (Solenopsis invicta).
Dr. Travis R. Robbins is a postdoctoral research fellow in the Langkilde Lab who studies the ecological mechanisms that result in evolution. His interests range from the evolution of life histories in response to climate change to behavioral evolution in response to invasive species to the evolutionary significance of culture. Most of his research, however, is on Sceloporus lizards (AKA Spiny lizards or Swifts), focusing on their genetic and plastic responses to environmental change and the underlying interactions between physiological (e.g. hormonal), behavioral (e.g. resource use and niche construction), and epigenetic mechanisms. His research endeavors have brought him to Costa Rica, Panama, Mexico, the subtropics of Florida, and inside Biosphere 2 in the Arizona desert, but he is currently focusing on lizard evolution in the Southeastern US, which brings us to the current blog post.
For the past three years I have been studying how fence lizards change their behavior and morphology after red imported fire ants invade the fence lizard habitat. This amazing study system that Dr. Tracy Langkilde fostered almost a decade ago reveals more exciting ecology with every research project! Tracy found an interesting trend across fence lizard populations that were invaded by fire ants at varying times in the past. The longer fence lizard populations coexist with fire ants, the more fence lizards in each population begin to respond to agonistic encounters with fire ants.
Usually this lizard species uses crypsis to avoid predation, so it is not prone to moving when something, that is usually harmless (i.e. not a fire ant), crawls over it. The lizards respond to fire ants, however, by dancing (twitching) and running away! And they evolve longer hind limbs so they can be really efficient at it!
Most of our data collection has been about how fence lizards respond to fire ants when they find themselves being attacked on top of a fire ant mound. Fire ants are quite aggressive when they find someone knocking on their door. Unfortunately, especially for those of you that live with fire ants in your yard, fire ants spend a lot of time away from the mound ubiquitously foraging and roaming around the habitats they invade. Lizards surely encounter fire ants when they have the displeasure of accidentally knocking, but most of the time lizards are basking in the sun or foraging for food somewhere other than fire ant mounds. Thus, we wondered how often a fence lizard would encounter a fire ant away from a fire ant mound, so we conducted an experiment. We placed lizards 4 meters away from a fire ant mound (and fire ant mounds are approximately 10 meters apart where abundant, so this almost as far as you can get from one!) and observed them to measure how long it would take for a fire ant to find the lizard. We also measured the behavioral response of the lizard and its effectiveness in avoiding an attack.
We call the first fire ant to find a lizard a “scout”, and this single ant is not much of a threat to a fence lizard. However, that scout tells his buddies where to find the lizard, and a bunch of ants start heading toward the lizard to attack. We call this “recruitment”, and this higher number of ants attacking is potentially dangerous. It only takes 12 ants to immobilize an adult fence lizard in 60 seconds. But, don’t worry, we never let this happen during our trials. We hypothesized that fence lizards that grew up with fire ants would enact their dance and run technique (twitch and flee behavior) whereas naïve fence lizards would not. We also hypothesized that the dance and run would be effective at curtailing the recruitment. If fence lizards responded to the scout before the scout could bring back recruits, the recruits would come to an empty spot, and the lizard in its new spot would no longer be threatened by an attack.
Our results suggested that this was indeed the case! Experienced lizards (those caught in the field at the invaded site) danced and ran when they encountered the scout.
We found that experience with fire ants (lizards from the invaded site) affected only adults, however, because juvenile lizards from all populations were scaredy-cats, running away quickly. We also found that dancing and running in response to a scout was an effective strategy to escape the danger of an attack by recruits.
Our results suggest that when lizards grow up with fire ants they change their behavior in an adaptive way that likely increases their biological fitness by avoiding attacks by stinging fire ants (likely keeping them alive and in better moods). Overall, we have found that the longer a population has coexisted with fire ants, the more fence lizards in the population exhibit the changes, suggesting that these behaviors and morphologies are evolving to help fence lizards adapt to deal with the pesky, painful, and potentially portentous fire ants. We are currently examining whether or not these behaviors are inherited by comparing behaviors of mothers to their offspring once they become adults.
Stay tuned . . .