Research Article |
Corresponding author: L. Lee Grismer ( lgrismer@lasierra.edu ) Academic editor: Uwe Fritz
© 2022 L. Lee Grismer, Kin Onn Chan, Robert E. Lovich, Jesse L. Grismer.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Grismer LL, Chan KO, Lovich RE, Grismer JL (2022) A statistical reanalysis of morphological differentiation among island night lizards (Xantusia riversiana) from the California Channel Islands. Vertebrate Zoology 72: 1-27. https://doi.org/10.3897/vz.72.e78092
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This study re-analyzes morphometric and meristic data among island night lizards, Xantusia riversiana, from the California Channel Islands of San Clemente, Santa Barbara, and San Nicolas in order to ascertain whether the implementation of different statistical methods can recover different results that could potentially alter biological interpretations. Our results concur with a recent previous study demonstrating that the three island populations differ morphologically from one another and that the San Nicolas Island population is the most divergent. Several important aspects, however, of the previous study depart significantly from those recovered here. Our analyses found sexual dimorphism within each population for both morphometric and meristic characters to be relatively uncommon whereas the previous study found nearly all characters to be sexually dimorphic for all island populations. The previous study also recovered significant differences among the three island populations for all morphometric characters whereas far fewer differences were recovered in the present study. Both studies found few significant inter-island differences among the meristic characters. The discordances between these two studies stem from differences in the a priori treatment of the raw character data and the different downstream statistical analyses and visualization techniques used on those data. This was particularly relevant with the use here of an allometric growth algorithm for size-correcting the morphometric data not used in the previous study and by treating all three populations as independently evolving groups. We did not conduct analyses where data from the San Clemente and Santa Barbara island populations were conflated based on their subspecific designation (X. r. reticulata) and then compared to data from the independently evolving San Nicolas Island population. This imprudent use of taxonomy violates the assumptions of statistical independence. We emphasize that explicit justification for the use of particular statistical analyses should occur in all studies—especially if the results bear on the implementation of effective and efficient resource management programs.
Meristics, morphometrics, statistical analysis, Xantusiidae
The California Channel Islands comprise the western limits of the California Borderlands—one of the most tectonically active areas in North America. Distributed across these eight islands are a suite of endemic vertebrates that continue to serve as exemplary models for studying the evolutionary patterns and processes of island radiations (
Considerable high-quality field research on the ecology of the three populations of Xantusia riversiana has been ongoing since the 1990’s (e.g.,
All morphological analyses were based on the raw data set of
We followed a standardized statistical protocol for morphological analyses (
Because preliminary Student t-tests on males and females from each island detected sexual dimorphism among some of the adjusted morphometric (see below) and raw meristic characters within each population, males and females were analyzed separately in all subsequent analyses unless stated otherwise.
One-way analyses of variance (ANOVA) were performed on the morphometric and meristic data sets to test for the presence of statistically significant interisland differences. If detected, Tukey HSD post hoc tests were performed to ascertain which pairs of island populations had significantly different mean values for which character(s). All statistical tests were performed at a significance threshold of α = 0.05. Boxplots for the discrete meristic characters and violin plots embedded with boxplots for continuous morphometric characters were generated to visualize the distribution of variation among the characters across islands. Summary statistics (mean, range, and ±1 standard deviation) were generated for all meristic and adjusted morphometric characters.
Separate and concatenated principal component analyses (PCA) of the morphometric and meristic data were employed to visualize and assess the degree of morphospatial separation among the three populations. PCA is an unsupervised analysis that does not group individuals a priori according to species/population/sex. A non-parametric permutation multivariate analysis of variance (PERMANOVA) from the vegan package in R (Oksanan et al. 2020) was used to determine if the centroid locations of each population in PCA were statistically different (
A subsequent supervised analysis, discriminant analysis of principal components (DAPC;
Comparing the results recovered here with those of
Overview. Overall, the following analyses found far less sexual dimorphism within all three island populations compared to that of
p-values for statistically significant mean differences in the characters among males and females from each island population based on Student t-tests. Character abbreviations are in the Materials and methods. Orange shaded cells denote sexually dimorphic characters reported by
Morphometric characters | SVL | HL | HW | HD | SNT | IO | FLL | HLL | PW |
San Clemente Island | 0.007 | 0.04 | 0.006 | ||||||
Santa Barbara Island | 0.03 | ||||||||
San Nicolas Island | |||||||||
Meristic characters | VS | GS | PA | FP | TL4 | ||||
San Clemente Island | 0.0005 | 0.009 | |||||||
Santa Barbara Island | |||||||||
San Nicolas Island | 0.02 | 0.004 |
Morphometric characters. All characters conformed to parametric test assumptions of normality (Shapiro-Wilk test, p < 0.05) and homogeneity of variances (F-test, p > 0.05). Student t-tests between the sexes for each character for each island population recovered minimal instances of sexual dimorphism (Table
Summary statistics of the adjusted morphometric characters for males and females from each island. Abbreviations are in the Materials and methods.
Santa Barbara Island | |||||||||
Females | SVL | HL | HW | HD | SNT | IO | FLL | HLL | PW |
Mean | 1.93 | 1.47 | 1.16 | 0.97 | 0.77 | 0.95 | 1.27 | 1.38 | 1.13 |
±1 sd | 0.03 | 0.03 | 0.02 | 0.02 | 0.01 | 0.02 | 0.01 | 0.02 | 0.02 |
Minimum | 1.88 | 1.41 | 1.12 | 0.93 | 0.75 | 0.89 | 1.23 | 1.34 | 1.09 |
Maximum | 2.00 | 1.51 | 1.19 | 1.01 | 0.80 | 0.99 | 1.30 | 1.42 | 1.16 |
n | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 |
Males | |||||||||
Mean | 1.91 | 1.47 | 1.16 | 0.97 | 0.77 | 0.95 | 1.27 | 1.37 | 1.12 |
±1 sd | 0.03 | 0.02 | 0.01 | 0.01 | 0.01 | 0.01 | 0.01 | 0.02 | 0.02 |
Minimum | 1.88 | 1.42 | 1.14 | 0.94 | 0.74 | 0.94 | 1.24 | 1.34 | 1.07 |
Maximum | 1.99 | 1.50 | 1.18 | 0.99 | 0.78 | 0.98 | 1.28 | 1.39 | 1.14 |
n | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 |
San Clemente Island | |||||||||
Females | SVL | HL | HW | HD | SNT | IO | FLL | HLL | PW |
Mean | 1.93 | 1.48 | 1.17 | 0.98 | 0.76 | 0.95 | 1.27 | 1.38 | 1.15 |
±1 sd | 0.03 | 0.02 | 0.02 | 0.01 | 0.01 | 0.02 | 0.01 | 0.02 | 0.02 |
Minimum | 1.89 | 1.44 | 1.13 | 0.95 | 0.74 | 0.91 | 1.23 | 1.32 | 1.11 |
Maximum | 1.99 | 1.50 | 1.20 | 1.01 | 0.79 | 0.99 | 1.30 | 1.41 | 1.17 |
n | 37 | 37 | 37 | 37 | 37 | 37 | 37 | 37 | 37 |
Males | |||||||||
Mean | 1.92 | 1.48 | 1.18 | 0.97 | 0.76 | 0.95 | 1.27 | 1.38 | 1.13 |
±1 sd | 0.03 | 0.01 | 0.02 | 0.01 | 0.01 | 0.02 | 0.01 | 0.01 | 0.02 |
Minimum | 1.86 | 1.44 | 1.13 | 0.95 | 0.74 | 0.91 | 1.24 | 1.35 | 1.07 |
Maximum | 1.96 | 1.50 | 1.20 | 0.99 | 0.80 | 0.99 | 1.30 | 1.40 | 1.17 |
n | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 |
San Nicolas Island | |||||||||
Females | SVL | HL | HW | HD | SNT | IO | FLL | HLL | PW |
Mean | 1.96 | 1.51 | 1.19 | 1.01 | 0.81 | 0.99 | 1.30 | 1.40 | 1.17 |
±1 sd | 0.04 | 0.01 | 0.01 | 0.02 | 0.01 | 0.02 | 0.01 | 0.02 | 0.02 |
Minimum | 1.88 | 1.48 | 1.17 | 0.94 | 0.78 | 0.95 | 1.27 | 1.38 | 1.14 |
Maximum | 2.02 | 1.53 | 1.22 | 1.05 | 0.83 | 1.03 | 1.32 | 1.44 | 1.23 |
n | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 |
Males | |||||||||
Mean | 1.94 | 1.51 | 1.20 | 1.01 | 0.80 | 1.00 | 1.29 | 1.41 | 1.16 |
±1 sd | 0.03 | 0.01 | 0.02 | 0.03 | 0.02 | 0.02 | 0.01 | 0.01 | 0.02 |
Minimum | 1.88 | 1.48 | 1.15 | 0.95 | 0.77 | 0.94 | 1.26 | 1.37 | 1.12 |
Maximum | 2.03 | 1.55 | 1.23 | 1.06 | 0.84 | 1.04 | 1.32 | 1.44 | 1.20 |
n | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 32 |
Summary statistics for the PCAs of the morphometric characters between the sexes for all island populations. Shaded cells denote characters bearing the highest loadings. Abbreviations are in the Materials and methods.
Santa Barbara Island | PC1 | PC2 | PC3 | PC4 | PC5 | PC6 | PC7 | PC8 | PC9 |
Standard deviation | 1.69102 | 1.19550 | 1.04608 | 0.96615 | 0.92217 | 0.85369 | 0.73535 | 0.58753 | 0.46733 |
Proportion of variance | 0.31773 | 0.1588 | 0.12159 | 0.10372 | 0.09449 | 0.08098 | 0.06008 | 0.03835 | 0.02427 |
Cumulative proportion | 0.31773 | 0.47653 | 0.59812 | 0.70183 | 0.79632 | 0.8773 | 0.93738 | 0.97573 | 1 |
Eigenvalue | 2.85955 | 1.42921 | 1.09428 | 0.93344 | 0.85040 | 0.72878 | 0.54075 | 0.34519 | 0.21840 |
SVL | –0.04170 | –0.00296 | –0.67114 | 0.71277 | –0.04750 | 0.18035 | –0.05392 | –0.04386 | –0.01301 |
HL | –0.39380 | –0.48365 | 0.17609 | 0.06467 | –0.13896 | 0.07244 | –0.35329 | –0.16653 | –0.63143 |
HW | –0.18974 | 0.59742 | 0.14656 | –0.00647 | –0.51300 | 0.23115 | –0.21042 | –0.46700 | 0.08125 |
HD | –0.42701 | 0.25634 | 0.26804 | 0.18657 | 0.01194 | 0.36080 | 0.06720 | 0.71408 | –0.02334 |
SNT | –0.27901 | –0.19318 | –0.44067 | –0.55613 | 0.11196 | 0.47275 | –0.23358 | 0.00431 | 0.30128 |
IO | –0.37182 | 0.03627 | 0.24164 | 0.20921 | 0.66420 | 0.12713 | 0.22999 | –0.47257 | 0.15728 |
FLL | –0.42503 | –0.29438 | 0.09280 | 0.16825 | –0.27118 | –0.48096 | –0.14584 | 0.05380 | 0.60559 |
HLL | –0.25193 | 0.46539 | –0.27478 | –0.14427 | 0.35194 | –0.52069 | –0.40356 | 0.11074 | –0.23135 |
PW | –0.40575 | 0.03958 | –0.29755 | –0.22530 | –0.25303 | –0.19111 | 0.72932 | –0.06200 | –0.24127 |
San Clemente Island | PC1 | PC2 | PC3 | PC4 | PC5 | PC6 | PC7 | PC8 | PC9 |
Standard deviation | 1.59879 | 1.26841 | 1.03970 | 0.96864 | 0.92026 | 0.78509 | 0.73735 | 0.65214 | 0.61932 |
Proportion of variance | 0.28402 | 0.17876 | 0.12011 | 0.10425 | 0.0941 | 0.06849 | 0.06041 | 0.04725 | 0.04262 |
Cumulative proportion | 0.28402 | 0.46278 | 0.58289 | 0.68714 | 0.78123 | 0.84972 | 0.91013 | 0.95738 | 1 |
Eigenvalue | 2.55614 | 1.60886 | 1.08097 | 0.93826 | 0.84687 | 0.61637 | 0.54368 | 0.42528 | 0.38356 |
SVL | –0.04292 | 0.02769 | –0.81357 | 0.11408 | –0.54945 | 0.03169 | –0.12149 | 0.06882 | 0.00945 |
HL | –0.44434 | –0.07742 | 0.05257 | –0.17015 | –0.24620 | –0.55029 | 0.62959 | 0.04986 | 0.05046 |
HW | –0.47163 | –0.13716 | 0.18281 | 0.33507 | –0.04872 | 0.00500 | –0.36073 | 0.24987 | 0.64660 |
HD | –0.26039 | –0.38202 | –0.37201 | –0.19276 | 0.51821 | –0.34490 | –0.31792 | –0.34259 | –0.06916 |
SNT | –0.31992 | –0.22811 | –0.03820 | –0.65677 | –0.02707 | 0.61308 | 0.08046 | 0.16703 | 0.04296 |
IO | –0.38463 | –0.30393 | 0.22019 | 0.40702 | –0.26597 | 0.26869 | –0.01530 | –0.39533 | –0.49607 |
FLL | –0.37658 | 0.46190 | 0.06652 | –0.10542 | 0.05073 | –0.16943 | –0.36567 | 0.44845 | –0.51304 |
HLL | –0.21043 | 0.61169 | 0.06194 | –0.23014 | –0.13959 | 0.04537 | –0.12398 | –0.65217 | 0.25001 |
PW | –0.27223 | 0.31075 | –0.32454 | 0.38408 | 0.52254 | 0.31364 | 0.44930 | 0.04858 | 0.02891 |
San Nicolas Island | PC1 | PC2 | PC3 | PC4 | PC5 | PC6 | PC7 | PC8 | PC9 |
Standard deviation | 1.55341 | 1.32003 | 1.04910 | 1.00828 | 0.97695 | 0.86045 | 0.68796 | 0.60170 | 0.44392 |
Proportion of variance | 0.26812 | 0.19361 | 0.12229 | 0.11296 | 0.10605 | 0.08226 | 0.05259 | 0.04023 | 0.0219 |
Cumulative proportion | 0.26812 | 0.46173 | 0.58402 | 0.69698 | 0.80303 | 0.88529 | 0.93788 | 0.9781 | 1 |
Eigenvalue | 2.41309 | 1.74249 | 1.10061 | 1.01662 | 0.95443 | 0.74038 | 0.47329 | 0.36204 | 0.19706 |
SVL | –0.01892 | 0.07381 | –0.16493 | 0.80586 | –0.55711 | 0.02425 | –0.06887 | 0.03681 | 0.02296 |
HL | 0.35249 | 0.29690 | –0.02701 | 0.15973 | 0.36469 | 0.62071 | –0.44842 | 0.15948 | 0.12868 |
HW | 0.35843 | –0.11165 | 0.48683 | –0.18608 | –0.46334 | 0.35612 | 0.00173 | –0.38893 | –0.30767 |
HD | 0.40005 | –0.49581 | 0.07878 | 0.17950 | 0.14547 | –0.08116 | 0.15353 | –0.24927 | 0.66502 |
SNT | 0.50436 | 0.09166 | –0.26741 | 0.07841 | 0.10880 | 0.07611 | 0.69665 | 0.25127 | –0.30572 |
IO | 0.37677 | –0.44900 | –0.11137 | 0.05868 | 0.08518 | –0.38884 | –0.49867 | 0.19769 | –0.44091 |
FLL | 0.24388 | 0.54309 | –0.03576 | 0.13027 | 0.20886 | –0.43466 | –0.07619 | –0.61780 | –0.08544 |
HLL | 0.35005 | 0.37398 | 0.16698 | –0.29929 | –0.40071 | –0.32767 | –0.10654 | 0.45112 | 0.37061 |
PW | 0.09239 | –0.05745 | –0.78472 | –0.37696 | –0.31225 | 0.16566 | –0.13943 | –0.26908 | 0.11614 |
Meristic Data | F statistic | R1 | p value | p adjusted |
Female | ||||
San Clemente vs Santa Barbara | 7.976328842 | 0.112380127 | 0.000119998 | 0.000359993 |
San Clemente vs San Nicolas | 17.8912351 | 0.210754798 | 2.00E-05 | 6.00E-05 |
Santa Barbara vs San Nicolas | 28.63042077 | 0.330489227 | 2.00E-05 | 6.00E-05 |
Male | ||||
San Clemente vs Santa Barbara | 3.38186634 | 0.074520213 | 0.020819584 | 0.062458751 |
San Clemente vs San Nicolas | 12.35061832 | 0.168377835 | 2.00E-05 | 6.00E-05 |
Santa Barbara vs San Nicolas | 13.11340086 | 0.242331856 | 2.00E-05 | 6.00E-05 |
Morphometric Data | F statistic | R1 | p value | p adjusted |
Female | ||||
San Clemente vs Santa Barbara | 4.256208897 | 0.063283509 | 0.001919962 | 0.005759885 |
San Clemente vs San Nicolas | 58.34675042 | 0.465482753 | 2.00E-05 | 6.00E-05 |
Santa Barbara vs San Nicolas | 60.41960349 | 0.510216229 | 2.00E-05 | 6.00E-05 |
Male | ||||
San Clemente vs Santa Barbara | 2.961345004 | 0.065864244 | 0.008879822 | 0.026639467 |
San Clemente vs San Nicolas | 66.56440768 | 0.521810189 | 2.00E-05 | 6.00E-05 |
Santa Barbara vs San Nicolas | 46.06328362 | 0.52907818 | 2.00E-05 | 6.00E-05 |
Sex Concatenated Data | F statistic | R1 | p value | p adjusted |
Female | ||||
San Clemente vs Santa Barbara | 5.50039433 | 0.080297265 | 2.00E-05 | 6.00E-05 |
San Clemente vs San Nicolas | 41.91324325 | 0.384831468 | 2.00E-05 | 6.00E-05 |
Santa Barbara vs San Nicolas | 47.10026177 | 0.448145999 | 2.00E-05 | 6.00E-05 |
Male | ||||
San Clemente vs Santa Barbara | 3.20096222 | 0.070816241 | 0.002359953 | 0.007079858 |
San Clemente vs San Nicolas | 40.67761809 | 0.400064624 | 2.00E-05 | 6.00E-05 |
Santa Barbara vs San Nicolas | 31.24682525 | 0.432501015 | 2.00E-05 | 6.00E-05 |
Total Concatenated Data | F statistic | R1 | p value | p adjusted |
M = male; F = female | ||||
San ClementeM vs Santa BarbaraM | 3.20096222 | 0.070816241 | 0.001679966 | 0.025199496 |
San ClementeM vs San NicolasM | 40.67761809 | 0.400064624 | 2.00E-05 | 0.000299994 |
San ClementeM vs San ClementeF | 3.646344679 | 0.05161406 | 0.000539989 | 0.008099838 |
San ClementeM vs Santa BarbaraF | 6.155707548 | 0.095949492 | 2.00E-05 | 0.000299994 |
San ClementeM vs San NicolasF | 48.90634901 | 0.440969786 | 2.00E-05 | 0.000299994 |
Santa BarbaraM vs San NicolasM | 31.24682525 | 0.432501015 | 2.00E-05 | 0.000299994 |
Santa BarbaraM vs San ClementeF | 4.747560753 | 0.091744629 | 8.00E-05 | 0.001199976 |
Santa BarbaraM vs Santa BarbaraF | 1.147106447 | 0.029302458 | 0.334293314 | 1 |
Santa BarbaraM vs San NicolasF | 35.92059215 | 0.460989722 | 2.00E-05 | 0.000299994 |
San NicolasM vs San ClementeF | 34.79306543 | 0.345193048 | 2.00E-05 | 0.000299994 |
San NicolasM vs Santa BarbaraF | 41.35669936 | 0.42047669 | 2.00E-05 | 0.000299994 |
San NicolasM vs San NicolasF | 1.897946171 | 0.03017501 | 0.054378912 | 0.815683686 |
San ClementeF vs Santa BarbaraF | 5.50039433 | 0.080297265 | 2.00E-05 | 0.000299994 |
San ClementeF vs San NicolasF | 41.91324325 | 0.384831468 | 2.00E-05 | 0.000299994 |
Santa BarbaraF vs San NicolasF | 47.10026177 | 0.448145999 | 2.00E-05 | 0.000299994 |
Using an ANOVA on the size-corrected SVLs and analyses of covariance (ANCOVAs) on all other size-corrected characters with size-corrected SVL as a covariate,
Meristic characters. All characters conformed to parametric test assumptions of normality (Shapiro-Wilk test, p < 0.05) and homogeneity of variances (F-test, p > 0.05). Student t-tests indicated that males from the San Clemente Island population have significantly more precloacal scales and femoral pores (PA and FP, respectively) than females (Tables
Summary statistics of the meristic characters for males and females from each island. Abbreviations are in the Materials and methods.
San Barbara Island | |||||
Females | VS | GS | PA | FP | TL4 |
Mean | 34.30 | 50.96 | 5.19 | 10.57 | 23.38 |
sd | 1.08 | 2.81 | 0.94 | 0.79 | 0.91 |
Minimum | 32 | 45 | 3 | 9 | 22 |
Maximum | 36 | 57 | 7 | 12 | 25 |
n | 28 | 28 | 28 | 28 | 28 |
Males | |||||
Mean | 34 | 50.91 | 5.08 | 10.75 | 22.92 |
sd | 1.21 | 2.35 | 0.79 | 0.45 | 1 |
Minimum | 32 | 45 | 4 | 10 | 22 |
Maximum | 36 | 54 | 6 | 11 | 25 |
n | 12 | 12 | 12 | 12 | 12 |
San Clemente Island | |||||
Females | VS | GS | PA | FP | TL4 |
Mean | 33.51 | 48.43 | 4.78 | 10.35 | 22.68 |
sd | 0.93 | 2.24 | 0.82 | 0.82 | 1.18 |
Minimum | 32 | 43 | 3 | 8 | 20 |
Maximum | 36 | 52 | 7 | 13 | 25 |
N | 37 | 37 | 37 | 37 | 37 |
Males | |||||
Mean | 33.34 | 48.63 | 5.59 | 10.81 | 22.31 |
Sd | 0.83 | 2.55 | 1.01 | 0.54 | 1.18 |
Minimum | 31 | 43 | 4 | 10 | 20 |
Maximum | 35 | 55 | 7 | 12 | 25 |
n | 32 | 32 | 32 | 32 | 32 |
San Nicolas Island | |||||
Females | VS | GS | PA | FP | TL4 |
Mean | 33.06 | 43.63 | 4.28 | 10.63 | 21.59 |
sd | 1.05 | 2.12 | 0.96 | 0.71 | 1.41 |
Minimum | 31 | 38 | 3 | 9 | 19 |
Maximum | 35 | 47 | 6 | 12 | 24 |
n | 32 | 32 | 32 | 32 | 32 |
Males | |||||
Mean | 32.90 | 45.06 | 4.94 | 10.61 | 21.45 |
Sd | 1.16 | 2.45 | 0.77 | 0.99 | 1.34 |
Minimum | 31 | 40 | 4 | 9 | 19 |
Maximum | 35 | 51 | 7 | 13 | 25 |
n | 31 | 31 | 31 | 31 | 31 |
Summary statistics for the PCAs of the meristic characters between the sexes for all island populations. Shaded cells denote characters bearing the highest loadings. Abbreviations are in the Materials and methods.
Santa Barbara Island | PC1 | PC2 | PC3 | PC4 | PC5 |
Standard deviation | 1.35020 | 1.06599 | 0.90096 | 0.80562 | 0.76149 |
Proportion of variance | 0.36461 | 0.22727 | 0.16235 | 0.1298 | 0.11597 |
Cumulative Proportion | 0.36461 | 0.59188 | 0.75422 | 0.88403 | 1 |
Eigenvalue | 1.82304 | 1.13634 | 0.81174 | 0.64902 | 0.57987 |
VS | –0.55376 | 0.08801 | –0.32068 | –0.39447 | 0.65358 |
GS | –0.47302 | –0.45275 | 0.21901 | –0.49266 | –0.52971 |
PA | –0.49439 | –0.29215 | 0.39941 | 0.67806 | 0.22568 |
FP | 0.16313 | –0.71739 | –0.65114 | 0.18451 | 0.02669 |
TL4 | –0.44562 | 0.43274 | –0.51545 | 0.32843 | –0.49052 |
San Clemente Island | PC1 | PC2 | PC3 | PC4 | PC5 |
Standard deviation | 1.23114 | 1.11417 | 0.91322 | 0.85415 | 0.82424 |
Proportion of variance | 0.30314 | 0.24828 | 0.16680 | 0.14591 | 0.13588 |
Cumulative Proportion | 0.30314 | 0.55142 | 0.71821 | 0.86412 | 1.00000 |
Eigenvalue | 1.51570 | 1.24138 | 0.83398 | 0.72957 | 0.67938 |
VS | –0.51197 | 0.24144 | –0.62982 | 0.38195 | 0.37018 |
GS | –0.52228 | –0.14062 | 0.66328 | 0.51605 | –0.03458 |
PA | 0.02325 | –0.72012 | –0.39511 | 0.30278 | –0.48281 |
FP | –0.37543 | –0.57778 | 0.03169 | –0.54629 | 0.47517 |
TL4 | –0.56888 | 0.26370 | –0.07919 | –0.44463 | –0.63473 |
San Clemente Island | PC1 | PC2 | PC3 | PC4 | PC5 |
Standard deviation | 1.35766 | 0.97754 | 0.94659 | 0.82394 | 0.79136 |
Proportion of variance | 0.36865 | 0.19112 | 0.17921 | 0.13578 | 0.12525 |
Cumulative Proportion | 0.36865 | 0.55977 | 0.73897 | 0.87475 | 1.00000 |
Eigenvalue | 1.84324 | 0.95558 | 0.89604 | 0.67888 | 0.62626 |
VS | 0.50525 | –0.20941 | 0.30917 | –0.65894 | 0.41362 |
GS | 0.50567 | –0.31369 | 0.08704 | 0.73085 | 0.32276 |
PA | –0.36679 | –0.60275 | –0.57665 | –0.11135 | 0.39651 |
FP | 0.33892 | 0.61644 | –0.63488 | –0.03456 | 0.31759 |
Using ANCOVA analyses with size-corrected SVL as a covariate,
Overview. The results below recover statistical morphometric and meristic differences among all three island populations using the data types separately or combined as summarized in Table
Female morphometrics. ANOVA and Tukey HSD post hoc analyses recovered a number of statistical differences between San Nicolas Island and the other two islands (Tables
(A) Principal component analysis, (B) discriminant analyses of principal components, and (C) comparative violin plots with embedded boxplots of morphometric characters among females from each island showing the range, frequency, mean (white dot), and 50% quartile (black rectangle) for each character. Violin plots are vertically oriented mirror-imaged frequency diagrams.
p-values for statistically significant mean values of the adjusted morphometric and raw mensural data between pairs of island populations determined from Tukey HSD post hoc tests following ANOVAs. Island abbreviations are SB = Santa Barbara Island, SC = San Clemente Island, and SN = San Nicolas Island. Character abbreviations are in the Materials and methods. Orange cells denote significant differences reported by
Morphometric data | |||||||||
Females | SVL | HL | HW | HD | SNT | IO | FLL | HLL | PW |
SN-SC | 0.009 | 0 | 1.43E-05 | 0 | 0 | 0 | 5.95E-07 | 0 | 1.83E-07 |
SB-SC | 0.011 | ||||||||
SB-SN | 0.021 | 7.59E-08 | 1.65E-08 | 4.01E-09 | 0 | 1.06E-07 | 2.27E-07 | 9.29E-10 | 1.03E-08 |
Males | |||||||||
SN-SC | 3.000463e-07 | 2.00E-07 | 9.58E-06 | 7.59E-09 | 4.84E-10 | 5.15E-10 | 8.69E-10 | 4.09E-08 | 3.00E-07 |
SB-SC | 0 | ||||||||
SB-SN | 4.837105e-10 | 2.25E-09 | 4.84E-10 | 5.26E-10 | 4.84E-10 | 6.35E-09 | 6.21E-10 | 3.99E-07 | 4.84E-10 |
Meristic data | |||||||||
Females | VS | GS | PA | FP | TL4 | ||||
SN-SC | 1.62E-01 | 4.85E-10 | 8.84E-04 | ||||||
SB-SC | 8.60E-03 | 1.52E-04 | 6.41E-02 | ||||||
SB-SN | 3.11E-05 | 4.84E-10 | 0.0007 | 4.07E-07 | |||||
Males | |||||||||
SN-SC | 7.31E-07 | 0.012 | 0.018 | ||||||
SB-SC | 2.14E-02 | ||||||||
SB-SN | 0.0078 | 4.10E-09 | 0.002 |
Using an ANOVA on the size-corrected SVLs and ANCOVAs on all other size-corrected morphometric characters with size-corrected SVL as a covariate,
Female meristics. ANOVA and Tukey HSD post hoc analyses recovered a number significant differences among the females from all islands for ventral and gular scales (VS and GS, respectively) and subdigital lamellae (TL4) (Tables
Comparisons of a priori treatment of raw morphological data and subsequent analyses and visualization techniques between
|
This study |
a priori treatment of raw data | a priori treatment of raw data |
log-transformation of morphometric data | |
Size correction of log-transformed morphometric data using geometric means | size-correction of raw morphometric data using the GroupStruct equation |
Size correction of meristic data presumably using geometric means | raw meristic data untreated |
Sexual dimorphism | Sexual dimorphism |
ANOVA on log-transformed SVLs | Student t-test for each island population |
ANCOVA on log-transformed morphometric data and meristic data with island and sex as factors and size-corrected log-transformed SVL as a covariate. | two-way ANOVA using species and sex as independent variables |
MANOVA including all islands | |
Inter-island differences | Inter-island differences |
MANOVA on log-transformed morphometric | one-way ANOVA and Tukey HSD post hoc analyses on adjusted morphometric data for each sex |
MANOVA on size-corrected log-transformed morphometric for each sex | |
Pillai’s Trace (V) calculated from MANOVA to estimate the effect size of the factors island, subspecies, and sex. | this analysis was not done |
MANOVA on size-corrected meristic data corrected for each sex | one-way ANOVA and Tukey HSD post hoc analyses on raw meristic data for each sex |
Data visualization | |
PCA on log-transformed morphometric data for each sex | PCA and DAPC on adjusted morphometric data for each sex |
PCA on size-corrected log-transformed morphometric data for each sex | |
PCA and DAPC on raw meristic data for each sex | |
PCA and DAPC on various combinations of concatenated data | |
LDA of log-transformed morphometric data for each sex | this analysis was not done |
LDA of size-corrected log-transformed morphometric data for each sex. | this analysis was not done |
Conducting ANCOVAs on the size-corrected meristic data set using the size-corrected SVL as a covariate,
Female concatenated data. The concatenated morphometric and meristic data set recovered the same pattern as the individual data sets of each data type but with even clearer separation of the San Nicolas Island population from the other island populations in the PCA (Fig.
Male morphometrics. The pattern of morphometric variation in the males mirrored that of the females. ANOVA and Tukey HSD post hoc analyses recovered a number of significant differences between San Nicolas Island and the other two islands for all characters (Tables
(A) Principal component analysis, (B) discriminant analyses of principal components, and (C) comparative violin plots with embedded boxplots of morphometric characters among males from each island showing the range, frequency, mean (white dot), and 50% quartile (black rectangle) for each character.
Using an ANOVA on the size-corrected SVLs and ANCOVAs on all other size-corrected morphometric characters with size-corrected SVL as a covariate,
Male meristics. The pattern of meristic variation in the males also mirrored that of the females. ANOVA and Tukey HSD post hoc analyses recovered a number of statistical differences among all islands for gular scales (GA), between the San Clemente and San Nicolas island populations and between the Santa Barbara and San Nicolas island populations for subdigital lamellae (TL4), between the San Clemente and San Nicolas island populations for precloacal scales (PA), and between the San Clemente and San Nicolas island populations for ventral scales (VS) (Tables
Conducting ANCOVAs on the size-corrected meristic data set using the size-corrected SVL as a covariate,
Male concatenated data. The PCA of the concatenated morphometric and meristic data from all islands was consistent with the individual PCAs and that of the female concatenated PCA in that it recovered complete separation of the San Nicolas Island population from the other island populations which overlapped greatly (Fig.
Concatenation of all data and both sexes. The PCA of the concatenated data set that included all characters, all islands, and males and females, recovered the same clear separation of the San Nicolas Island population from the other two island populations which again, showed complete overlap (Fig.
Discriminant analyses. We did not perform a linear discriminant analysis (LDA) on any data set as there were no questions as to the insular provenance of any sample. Furthermore, LDA does not always return the correct provenance probabilities (see
Differences between the results of
A priori treatment of morphometric data.
A priori treatment of meristic data. In their Table
Differences in morphometric analyses.
Differences in meristic analyses. In some of their analyses,
Sexual dimorphism.
Why statistics matter. Morphology is inextricably linked with many essential organismal functions, such as feeding and locomotion, which in turn influence ecological interactions, resource use, and ultimately, survival and reproduction (
Our data indicate that the majority of overall morphological variation among the three island populations is reflected in body shape (Figs
We presume that based on their log-transformed data corrected for size,
Our data demonstrated that lizards from the San Nicolas island population had relatively longer limbs than those from the other two islands. Many studies have shown that shorter limbs are more advantageous for locomoting through restrictive types of vegetation (e.g.
We thank Nicole E. Adams and Gregory B. Pauly for sharing the raw data upon which this paper was written. We thank them and William J. Mautz for sending us literature. We are grateful for the comments of Robert Fisher, Brad Hollingsworth, and Diego Arenas-Moreno and one anonymous reviewer. We are grateful to Charles A. Drost for many insightful conversations and emails regarding the natural history of Island night lizards. We thank Nicholas A. Hess for providing photographs of lizards from Santa Barbara Island.