Tierra de Gigantes (The Land of the Giants)

Our knowledge of the natural history of giant anoles, such as Anolis cuvieri, is relatively poor. Yesterday I had the opportunity to observe two events that are relatively rarely witnessed in this species. These included a male courting a female and a male-male agonistic interaction. Furthermore, the events occurred sequentially. First, a male began to display to a female and then chased her in an attempt to initiate copulation. As illustrated by the photo below, the copulation attempt progressed to the stage at which the male bites the nuchal region of the female while trying to orient his body properly.

A. cuvieri copulation attempt

Unfortunately for the first male, another male interrupted the copulation. Thus began a long period of aggressive displays between the males. This agonistic event included each male presenting laterally, dewlapping, head-bobbing, and opening its mouths with protruded tongue. Interestingly, neither male was willing to cede the "higher ground", resulting in both individuals constantly moving up their respective perches. Which lasted until both males reached the canopy of their respective perches, a point at which one of the males began to exhibit what appears to be submissive behaviors. These types of observations are clear reminders that "boots on the ground" are needed if we want to understand the complexity and diversity of social behaviors.

Males A. curvieri following the "higher ground" tactic 

     

   

Alsophis portoricensis foraging

Alsophis portoricensis is a rear-fanged colubrid and the largest of the two racers endemic to the island Puerto Rico. Anoles are a staple of their diet, as was the case for the male A. krugi in this photo. However, a close look at the picture shows that anoles are not a completely easy meal, and it is relatively common to find anoles biting the snake. In some instances with sufficient power to cause damage to the snakes, including loosing head scales. Also note, how the snake is trying to inject the venom (which is of the hemorrhagic type) by positioning the lizard at the rear portion of her jaw where the fangs are located.

Alsophis portoricensis feeding on A. krugi

         

Anolis occultus unveiled

Anolis occultus, a member of the twig ecomorph group, is one of the most secretive species of lizard found in Puerto Rico. However, today I was lucky enough to observe a male A. occultus drinking water from the surface of a leaf. It was an amazing few minutes; after a very light rain, the individual began to search for water droplets on the surface of leaves. This was done by slowly walking while searching for the droplets. Once a water droplet was found, the individual would approach it and very deliberately protrude his tongue to lick the droplet as shown by the picture below. 

Anolis occultus drinking water

As an "Experimental Natural Historian" these are the moments that remind me, why it is critical to be in the field as much as possible. It only took a few tiny rain droplets to elicit the behavior, immediately raising the question of how a lizard can sense such a few rain droplets.

Predation Green Iguana

Populations of the introduced green iguana (Iguana iguana) are becoming extremely dense in many areas of Puerto Rico, including the xeric region of Guanica the southwest region of the island. This trend is likely due in part to the fact that there are no large-bodied native lizards on the island and/or the diversity of potential predators is relatively low (i.e., limited to birds). However, as shown in the picture below, the birds of prey are beginning to exploit this "new" and extremely abundant resource. This juvenile iguana ended its days as the meal of a hawk, possibly a red-tailed hawk. The photo was taken at Guanica Dry Forest.   

Red-Tailied Hawk with a green iguana for feeding (his/her) nestlings

What are they eating? (via MANUEL LEAL)

Although we have visited our field sites in the Bahamas and Jamaica, the Chipojo Blog has been relatively quiet. I am currently in Puerto Rico and took the following picture yesterday, while working up in the canopy tower at El Verde. This is relatively common foraging behavior exhibited by A. stratulus. However, I have no idea what they are eating. Through my observations, I am convinced that they are not randomly "cleaning" everything off the leaves (i.e., once the individual is done, that area of the leaf appears cleaner than areas surrounding it). Instead, an individual will slowly walk over the leaf until it finds what he/she seems to be searching for before beginning to exhibit the behavior. Also, the lizards are not drinking water. There is no water on the surface of the leaves, and water-drinking is a very different behavior.

Visitors

This last Sunday, three groups of local 5th grade students visited our lab! The students were part of a 5th grade Scholars Day through Duke University and a non-profit called Playworks. Our lab was the "Science" portion of their tour, and allowed them to learn more about how research is conducted. We took this opportunity to discuss basic principles of color vision, cognition and communication in lizards. The highlight of the visit was a tour of the "lizard room" where they were able to see the lizards up-close and how behavioral experiments are conducted.

Maria showing a male anole and its dewlap

Students doing their best lizard poses. Some even displayed their "dewlaps!"

More than meets the eye

The online version of Dave's paper on signal modulation just came out in Animal Behaviour, here is link to the pdf. This study nicely illustrates the advantages of conducting behavioral experiments under natural conditions, which usually comes with plenty of bumps on the road. In the case of this study, those bumps included several years of trouble-shooting & hours of sitting in tropical downpours, waiting for those rare moments of sun to film the displays of A. gundlachi. Thus, KUDOS to Dave for sticking with the project and figuring out an elegant experimental design to demonstrate that distance between interactive individuals can account for differences in the amplitude of head-bob displays.

Two cameras, two lizards, and plenty of sun. Picture taken at El Verde, Puerto Rico

From an animal communication perspective, this study illustrates how sensory systems can constrain the physical properties of signaling displays and how signalers can modulate the physical properties of their signals to achieve signal efficacy. By integrating the motion detection parameters of the sensory system of A. gundlachi (see blog for discussion), Dave was able to demonstrate that lizards are producing displays within the motion parameters that would result in a higher probability of detection. As illustrated by the figure below, the majority of the displays were given within the predicted range for higher detection.

Correlation between receiver distance and maximum amplitude.  Most of the head-bobs displays have an amplitude that falls within the predicted values of maximal stimulation based on the sensory system of A. gundlachi (shaded region). 

The study also sheds light on an old observation that anoles give smaller displays as they interact at closer distance. This was previously suggested to result as a natural progression of social interactions.  Although the previous statement can certainly be one factor that contributes to small amplitude displays, the fact is that smaller displays are also necessary to maximize the probability of detection at the start of displays given at close-distance. Thus, changes in signal amplitude can also be driven by selection for efficacy of communication.  

Another equally interesting implication of Dave's findings is that lizards are capable of estimating the distance at which a potential receiver is located. Although this may not come as a surprise to anyone who has watched anoles, the mechanism(s) used to estimate distance, particularly at the scale used in this experiment, is an intriguing and unanswered question that we are currently working on.

The video shows a typical display of A. gundlachi, NOTE the highly stereotype head movements.