Total Morphological Comparison Between Anolis oculatus and Anolis cristatellus Figure 1 Dominican anole (Anolis oculatus) Figure 2 Puerto Rican crested anole (Anolis cristatellus) Nicholas Gill June 2015
Abstract The habitat of the native Dominican anole (Anolis oculatus) is currently being invaded by the aggressive and territorial Puerto Rican crested anole (Anolis cristatellus), which was accidentally introduced into the port of Roseau about a decade ago. I conducted an observational study in various locations on Dominica to compare and contrast the overall morphology of the two species. Certain morphological variables were not found to be significantly different, which could possibly explain how cristatellus was able to outcompete the local species in certain areas of the island. Hind limb length and mouth length was found to be significantly larger in oculatus than in cristatellus. It is possible that these traits are preferred in the inland forests of the island, where oculatus is still largely dominant. Introduction The Dominica anole (Anolis oculatus) was once the only species of anolis lizard on the Caribbean island of Dominica. Previously it had been able to thrive due to the lack of competition. The species vast polymorphism also allowed it to exist in a wide variety of environments across the island (Stenson et al. 2002). However, the situation changed when another anolis species, the Puerto Rican crested anole (Anolis cristatellus), was introduced through the port of Roseau a little over a decade ago (Reinhart 2010). Studies indicate that this aggressive and highly territorial species is currently supplanting the endemic anole population s habitat. One study suggested that oculatus prefer habitats with closed canopies, low perches, temperatures in the mid thirties (Celsius), and low elevations (Tarbox 2005). Likewise,
cristatellus prefers to perch on tree trunks close to the forest floor. In addition, cristatellus can survive well in urban environment, perching on fences and walls where trees are unavailable (McDaniel 2015). I intended this study to build on Tarbox s study by comparing differences in overall morphology between the two species. This information may be used in future studies to learn how these differences may have influenced their geographic and habitat distribution. Materials and Methods The study took place in several locations on Dominica from June 4-8. These places include the botanical gardens in Roseau, Archbold Tropical Research Center in Springfield, Emerald Pool National Park, and Cabrits National Park (See Figure 3). 10 Anolis oculatus and 11 A. cristatellus were caught by hand. I measured five variables for each anole using calipers or a ruler when calipers were unavailable. The five measurements were (1) snout-vent length, (2) hind limb length, (3) head Figure 3 Map of Dominica. Red X s indicate study locations. (Source: hoopermuseum.earthsci.carleton.ca) width, (4) head length, and (5) mouth length.
Results and Discussion Dr. Lacher performed a principal components analysis with the raw data shown in Table 1. This method allowed me to visualize trends in overall morphology while considering all five variables simultaneously (Figure 4). There was poor discrimination and lots of overlap between species. Table 2 shows that the two dimensions used in Figure 4 contain 92.7% of the variance among individuals. Location S-V length Hind limb length Head width Head length Mouth Length A. Oculatus Archbold Tropical Research Center 67 50 12.5 22 20 Archbold Tropical Research Center 37 25.5 8 14 11 Emerald Pool 84 60 15 35 21 Emerald Pool 50 42 10 16 13 Emerald Pool 60 41 10 19 14 Emerald Pool 69 53 13 25 18 Cabrits National Park 61.38 47.73 13.04 22.07 19.84 Cabrits National Park 63.17 51.20 14.36 24.57 19.33 Cabrits National Park 77.35 60.06 15.78 26.04 21.27 Cabrits National Park 48.31 33.70 8.01 18.56 10.76 A. Cristatellus Botanical Gardens 70 38.35 11.4 19.6 13.3 Botanical Gardens 39.3 25.9 8 13.5 8.9 Botanical Gardens 44 30.8 8.2 14.7 10 Botanical Gardens 65.5 49.6 12.2 24.55 16.3 Botanical Gardens 45 29.5 9.1 19.4 10 Botanical Gardens 46.1 30.6 9.7 17.6 11.7 Emerald Pool Visitor's Center 42 33 9 15 10 Cabrits National Park 41.85 32.86 6.81 13.96 9.71 Cabrits National Park 55.52 46.15 13.03 22.91 14.59 Cabrits National Park 58.28 49.15 12.71 22.03 13.76 Cabrits (Fort Shirley) 59.98 47.53 12.36 21.30 14.47 Table 1 Raw data. All quantities are in millimeters.
Figure 4 Principle components analysis results. Every point labeled 1.00 is oculatus. Every point labeled 2.00 is cristatellus.
Total Variance Explained Component Initial Eigenvalues a Extraction Sums of Squared Loadings Total % of Variance Cumulative % Total % of Variance Cumulative % 1 317.308 94.177 94.177 317.308 94.177 94.177 2 12.551 3.725 97.903 12.551 3.725 97.903 Raw 3 4.180 1.241 99.143 4 2.300.683 99.826 5.586.174 100.000 1 317.308 94.177 94.177 4.500 90.005 90.005 2 12.551 3.725 97.903.136 2.711 92.716 Rescaled 3 4.180 1.241 99.143 4 2.300.683 99.826 5.586.174 100.000 Extraction Method: Principal Component Analysis. a. When analyzing a covariance matrix, the initial eigenvalues are the same across the raw and rescaled solution. Table 2 Component Matrix a Raw Component Rescaled Component Dr. Lacher created the component matrix in Table 3 to separate 1 2 1 2 S-V length (mm) 12.870-2.319.984 -.177 Hind limb length 10.391 2.580.969.241 Head width 2.427.420.935.162 Head length 4.822 -.041.927 -.008 Mouth Length 3.816.581.927.141 Extraction Method: Principal Component Analysis. a. 2 components extracted. Table 3 individuals based on size and on having relatively large or small hind limbs. All variables loaded positively on the first axis, indicating that it separated individuals based upon size. The second axis plotted individuals with long hind limbs and larger mouths on the positive side. There were a larger number of oculatus in this region of the plot.
Table 4 shows the results of a series of multivariate tests that Dr. Lacher performed on my data. The Wilks Lambda test in particular indicated a significant difference in morphology between the two species when all five variables were considered. When the two species were compared, A. oculatus was found to have significantly longer hind limbs and a larger mouth than A. cristatellus (Table 5). Multivariate Tests a Effect Value F Hypothesis df Error df Sig. Pillai's Trace.961 73.907 b 5.000 15.000.000 Intercept Wilks' Lambda.039 73.907 b 5.000 15.000.000 Hotelling's Trace 24.636 73.907 b 5.000 15.000.000 Roy's Largest Root 24.636 73.907 b 5.000 15.000.000 Pillai's Trace.605 4.589 b 5.000 15.000.010 Species Wilks' Lambda.395 4.589 b 5.000 15.000.010 Hotelling's Trace 1.530 4.589 b 5.000 15.000.010 Roy's Largest Root 1.530 4.589 b 5.000 15.000.010 a. Design: Intercept + Species b. Exact statistic Table 4
Tests of Between-Subjects Effects Source Dependent Variable Type III Sum of Squares df Mean Square F Sig. S-V length (mm) 537.237 a 1 537.237 3.536.075 Hind limb length 408.737 b 1 408.737 4.108.057 Corrected Model Head width 15.874 c 1 15.874 2.540.127 Head length 68.967 d 1 68.967 2.772.112 Intercept Species Error Total Mouth Length 118.366 e 1 118.366 10.207.005 S-V length (mm) 67258.226 1 67258.226 442.630.000 Hind limb length 36963.920 1 36963.920 371.489.000 Head width 2580.887 1 2580.887 412.984.000 Head length 8727.860 1 8727.860 350.853.000 Mouth Length 4370.782 1 4370.782 376.923.000 S-V length (mm) 537.237 1 537.237 3.536.075 Hind limb length 408.737 1 408.737 4.108.057 Head width 15.874 1 15.874 2.540.127 Head length 68.967 1 68.967 2.772.112 Mouth Length 118.366 1 118.366 10.207.005 S-V length (mm) 2887.079 19 151.952 Hind limb length 1890.539 19 99.502 Head width 118.738 19 6.249 Head length 472.646 19 24.876 Mouth Length 220.323 19 11.596 S-V length (mm) 70262.834 21 Hind limb length 38977.104 21 Head width 2702.080 21 Head length 9215.408 21 Mouth Length 4651.016 21 S-V length (mm) 3424.316 20 Hind limb length 2299.275 20 Corrected Total Head width 134.612 20 Head length 541.613 20 Mouth Length 338.689 20 a. R Squared =.157 (Adjusted R Squared =.113) b. R Squared =.178 (Adjusted R Squared =.134) c. R Squared =.118 (Adjusted R Squared =.071) d. R Squared =.127 (Adjusted R Squared =.081) e. R Squared =.349 (Adjusted R Squared =.315) Table 5
The data were obtained from a small sample size, which could be a significant source of error. Also, there were times when calipers were unavailable and I had to measure with a ruler instead. This is another possible source of error since it is harder to measure accurately with a ruler. The age of the anoles is also a source of error since older anoles tend to be larger than younger ones. Conclusion A. cristatellus and A. oculatus appear to share several similarities in morphology, which could partly explain how cristatellus was able to invade oculatus territory. However, certain differences could also suggest why cristatellus has not yet become common in tropical forests at higher altitudes. From my own perspective, it appeared that cristatellus is mostly restricted to urban areas near sea level while oculatus still dominates most of the inland region. One possible explanation for this could be that longer legs allow oculatus to grip twigs and the like from farther away than cristatellus. The longer mouth of oculatus may be better suited for the diet found in the tropical forest than the shorter mouth of cristatellus. I suggest further studies on the relationship between geographic distribution and morphology to assess these hypotheses. Works Cited Reinhart, Sammy. A survey and discussion of the invasion of Anolis cristatellus and its effects on the endemic Anolis oculatus (2010). http://dominica.tamu.edu/student%20projects/dominica%20projects%20pdf%20copy/re inhart_sammy_2010.pdf
McDaniel, Dylan. Animal Diversity Web. Accessed March 28, 2015. http://animaldiversity.org/accounts/anolis_cristatellus/ Stenson, A. G., Malhotra, A., & Thorpe, R. S. (2002). Population differentiation and nuclear gene flow in the Dominican anole (Anolis oculatus).molecular Ecology, 11(9), 1679-1688. Tarbox, Geoffrey (2005). Habitat Selection by Dominica Anoles. http://dominica.tamu.edu/student%20projects/dominica%20projects%20pdf%20copy/ta rbox_geoffrey.pdf