If You Thought that Brown Anoles Bully Green Anoles, You Were Right

Interactions between native Anolis carolinensis (green anoles) and invasive Anolis sagrei (brown anoles) in the United States are discussed often here on Anole Annals. Most recently, this blog featured a local news broadcast from Louisiana and newspaper article from Florida, both of which describe a pattern that is repeated across the southern United States: When brown anoles invade a habitat, green anoles begin perching higher off the ground and thus become more difficult for anole enthusiasts to find.

Why do green anoles and brown anoles tend to occupy different perch heights in areas where they co-occur? By far the most popular explanation is that these species partition space as a means of partitioning resources, namely arthropod food. In simpler terms, they are competitors. But competition itself is not always simple. To better understand and study competition, biologists often classify competition as one of two types. Species can compete directly via aggressive encounters (termed “interference competition”) or indirectly through their shared use of a limited resource (termed “exploitative competition”). We know that green and brown anoles eat similar prey, suggesting that their competition is at least partly exploitative. Do they also engage in direct interference?

In a recently published paper in Oecologia, Katherine Culbertson (Harvard ESPP ‘18, former undergrad researcher in the Losos lab) and I tested the hypothesis that interference competition between native green anoles and invasive brown anoles occurs in the field. More specifically, we wondered if an asymmetry in interference competition might contribute to the vertical displacement of green anoles by brown anoles. To test for competitive asymmetries between the species, we used a classic method in behavioral ecology: tethered intruder trials. We presented adult male intruders to previously undisturbed focal individuals of the opposite species and videotaped the interactions. Intruders were tied around the waist with string at the end of a fishing pole with enough slack to move freely. We analyzed several aspects of the behavior of the focal lizards to evaluate asymmetries in interspecific aggression between the species: how often they attacked, how often they displayed (throat fan extensions, headbobs, and pushups), how often they retreated, and in what direction they retreated. (Disclaimer: Whenever an attack occurred, we ended the trial immediately so no lizards were harmed.)

As anticipated, we found that interference competition is asymmetric in favor of brown anoles, which are more likely to display and less likely to retreat from interactions than green anoles. In line with their arboreal tendencies, male green anoles also trend toward retreating upward more often than expected by chance. Somewhat surprisingly, these asymmetries are prevalent despite the almost complete absence of physical attacks (there were only two attacks in nearly one hundred trials, both by brown anoles). All told, our results suggest that signaling between the species and avoidance behavior by green anoles resolve most potential conflicts before they escalate to combat.

Figure 2 from our paper, which displays posterior predictions for the (a) probability of display, (b) display rate, and (c) probability of retreat of male Anolis sagrei (brown anoles, “SA”) and male Anolis carolinensis (green anoles, “CA”) when presented with a male intruder of the opposite species. Brown anoles were more likely to display and less likely to retreat than green anoles.

Many Floridians I’ve met in the course of my fieldwork have described brown anoles as bullies. Although anecdotal observations of animal behavior do not always reliably represent biological truths, in this case, the collective of observations made the residents I’ve spoken with are concordant with our data. Kudos to the many local naturalists who’ve shared their stories!

In closing, I’ll attempt to refine the metaphor of brown anoles as bullies, in acknowledgement that metaphors are often imperfect and with apologies to those who bristle at any attempt to anthropomorphize non-human animals. First, what makes a bully effective? On the playground, a bully might gain a reputation as such by initiating and winning a fight. Afterward, the mere threat of physical combat is often enough for the bully to exert his or her will on others. At our study sites, where green anoles and brown anoles have co-existed for several generations, brown anoles tend to dominate interactions with green anoles without attacking them. Perhaps physical combat is more common during the incipient stages of brown anole invasion, a hypothesis which could be tested by applying our methods across sites that vary in their invasion history.

Second, what’s the best way to deal with a bully? Many children learn to ignore bullies, a strategy rendered possible by the existence of alternative space to play or activities to engage in. Green anoles appear to find refuge in the canopy, where brown anoles seldom venture. Anecdotally, areas where no such canopy exists (i.e. areas with few plants or with only short, shrubby vegetation) are the areas where green anoles are most likely to disappear entirely following brown anole invasion. This hypothesis deserves a formal test.

Special thanks to the Aquatic Preserve Program run by the Florida Department of Environmental Protection for making this work possible. Check out the paper to learn more about our methods, results, and the implications of our findings.

Katherine Culbertson marking the location of a captured green anole in the field.

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7 Comments

  1. Stephanie Florio

    I have been studying behavior Patterns I have observing them for three years I start training them when they are new hatchlings With a clicking sound I make and I’ve been able to get up close to the adults. The hatchlings come on my hand without chasing them and they Sit on my fingers or the palm of my hand and they don’t want to jump Ion a leaf they come back to my hand I I do it all the time and I have a full grown male and now he lives in my screen house all the time and it is differentiated by his dewlap

  2. James Duquesnel, Adjunct Instructor

    “When brown anoles invade a habitat, green anoles begin perching higher off the ground.” Granted. “We know that green and brown anoles eat similar prey.” Well, “similar prey” is not “the same” prey, so… grudgingly granted. But, note the differences in their skull (head) shape, general build and behavior. Small differences, maybe, but all likely reflect and contribute to prey preference.
    Paul Moler once told me that “the arrival of the Brown Anole was the single best thing that has ever happened to Black Racers.” Imagine, a lizard that attempts to evade snakes (without shoulders or pelvis), by hiding under things. Yum.

  3. I recently moved to Florida back in May 2020 I am with one of my plant pots came a green Anole Hatchling. I didn’t realize this right away.
    That is all I ever had in Louisiana with a green a no I did not have any brown ones. All I have here in my new home are brown this one is very outnumbered and seems to Treat a lot to the pot It came in and won’t get out of its pot very often. I’m going to go buy some small crickets for it. It looks rather thin and not so healthy. What are your thoughts on getting a few Green Anole Lizards from Petsmart. Introducing him to my garden where my other is. I would rather have those than the brown ones. He seems awful lonely and not sure he’s eating well because he seems stressed. I really love my little lizard I want to try to help him or her not sure. Appreciate any helpful knowledge on this. Thank you Terry

  4. I just read my post and there were so many errors I thought I would re-post it correctly. I had Green Anole hatchling come in one of my planter pots when I moved from Louisiana. It is the only green Anole lizard that is here at my house in Florida I only have brown ones. It seems very stressed and won’t get out of the planter pot that it came in very often. And doesn’t look very healthy I think the brown ones bully him or us keep it from coming out much. It seems rather lonely would it be a good idea if I buy several green Anoles at Petsmart they have them all the time and introduce them to the garden where my other one lives. I would much rather have those than the brown ones. I’m thinking about trapping the brown ones and relocating them to the lake area where there’s so many. Would love to hear your thoughts on this because I’m going to get some small crickets today to feed to this one to get it healthier. Thank you In advance for any suggestions Terry

  5. Wytchwood

    Personally I wish the brown anoles would go back where they came from. I enjoy seeing the green anoles and I’ve never had a problem with green anoles crapping all over my patio like the brown ones do. I’m in Southeast Louisiana and this is the 2nd year I’ve noticed the brown lizards everywhere.

  6. Kim Kerigan

    I am also wondering, here in Florida where we lost massive amounts of trees, if I should buy anoles at Pet Smart to join my one male that has been on my porch for a year.

    • I have lived in north Florida for over fifty years. The first
      time l saw a brown anoles was around 1985. In the subsequent
      years they have become far and away the most common
      wildlife to see a round my yard. I still have a population of
      green anoles but they are not as visible. I made the mistake
      of buying a pair of greenies and released them in my backyard.
      Big mistake as the territory was already claimed by another
      male greenie. They fought until both were torn up and bloody.
      I have never seen the brown males fight. They just do a few
      pushups and move on. The greenies seem to be much more
      there territorial. I will also mention that during cool sunny
      weather I sometimes see more green than brown.

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